Fluid evolution of the Neoarchean Pampalo orogenic gold deposit (E Finland): Constraints from LA-ICPMS fluid inclusion microanalysis

    Fluid evolution of the Neoarchean Pampalo orogenic gold deposit (E Finland): Constraints from LA-ICPMS fluid inclusion microanalysis Fusswinkel, Thomas Wagner, Grigorios Sakellaris PII: DOI: Reference:

S0009-2541(16)30682-9 doi: 10.1016/j.chemgeo.2016.12.022 CHEMGE 18192

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

7 October 2016 8 December 2016 16 December 2016

Please cite this article as: Fusswinkel, Wagner, Thomas, Sakellaris, Grigorios, Fluid evolution of the Neoarchean Pampalo orogenic gold deposit (E Finland): Constraints from LA-ICPMS fluid inclusion microanalysis, Chemical Geology (2016), doi: 10.1016/j.chemgeo.2016.12.022

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Fluid evolution of the Neoarchean Pampalo

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orogenic gold deposit (E Finland):

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constraints from LA-ICPMS fluid inclusion microanalysis

Department of Geosciences and Geography, Division of Geochemistry and Geology, University of

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Tobias Fusswinkel1*, Thomas Wagner1, Grigorios Sakellaris2

Helsinki, Gustaf Hällströmin katu 2a, FI-00014 University of Helsinki, Finland 2

Malmi geoconsulting, Pohjoisranta 14 A 30, FI-00170 Helsinki, Finland

*corresponding author: [email protected]

Submitted to: Chemical Geology Date: 7 December 2016

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ACCEPTED MANUSCRIPT Abstract The sources of fluids and precious metals in orogenic gold deposits remain controversial, and a key

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question is whether auriferous magmatic-hydrothermal fluids are a decisive ingredient for economic

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orogenic gold mineralization. Contributions of magmatic-hydrothermal fluids have been proposed based on indirect arguments such as isotope data or emplacement of granites coevally with gold

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deposits. By contrast, complete elemental concentration data for orogenic gold fluids, which could be

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used to compare them directly with those of magmatic-hydrothermal fluids, are not yet available. This study reports the major and trace element chemistry of fluid inclusions from the Neoarchean Pampalo

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orogenic gold deposit, located in the Hattu schist belt in Eastern Finland. Three successive generations of gold-mineralized and barren veins provide the paragenetic and chronological framework for

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establishing the fluid evolution history of the Pampalo deposit. Based on petrography,

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microthermometry and LA-ICPMS fluid inclusion microanalysis, five major fluid types were

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identified. The data shows that all fluid types, despite characteristic differences, carry the signature of orogenic gold fluids, i.e., low- to intermediate-salinity, presence of CO2, CH4 or N2, high S contents, enrichment in B, As, Sb and W, and low Pb and Zn concentrations. Gold concentrations vary significantly between the oldest (ca. 0.6 µg/g) fluid related to the zones with highest gold grades in the deposit and later fluids types (< 0.03 µg/g) unrelated to gold mineralization. This shows that gold mineralization was caused by episodic input of exceptionally gold-rich fluids and not by continuous and long-lasting hydrothermal activity. Comparing the fluid composition data from Pampalo with the well-established major and trace element characteristics of magmatic-hydrothermal fluids, the potential role of magmatic fluids in orogenic gold systems can be quantitatively evaluated. Mass balance calculations using metal concentrations as well as Cl/Br ratios demonstrate that the composition of the early Au-rich fluid at Pampalo is not compatible with any significant magmatic-hydrothermal fluid 2

ACCEPTED MANUSCRIPT contribution, and that all fluids at Pampalo are metamorphogenic in origin. Molar Cl/Br ratios as low as 9-20 are found in some of the fluid types, which groups them among the most Br enriched crustal fluids

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reported so far. The exceptional Br enrichment is inconsistent with an evaporative origin but instead

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points to Br uptake from organic matter in metasedimentary rocks during metamorphism and fluid production. Taken together, the selective Au enrichment of only the earliest fluid type is well explained

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by metamorphic fluid production from Au-rich source rock lithologies, without any significant

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contributions from magmatic-hydrothermal fluids.

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Keywords: Orogenic gold deposit, hydrothermal system, fluid inclusions, LA-ICPMS, metamorphic

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fluid, magmatic-hydrothermal fluid, Cl/Br ratios

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ACCEPTED MANUSCRIPT 1. Introduction Orogenic gold deposits contain about one third of the global gold reserves (Frimmel, 2008). They are

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characterized by large variability in their geological age (from Precambrian to Phanerozoic), host

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rocks, mineralization style, and inferred pressure-temperature conditions of formation (Phillips and Powell, 1993; Gebre-Mariam et al., 1995; Groves et al., 1998; Goldfarb et al., 2001; Goldfarb and

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Groves, 2015). Despite the differences, orogenic gold deposits have a number of common features

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which distinguish them from other types of gold deposits. These include their metal inventory (enriched in Ag, As, K, Sb ± B, Bi, Hg, Mo, Se, Te and W but low in base metals), their relative timing in the

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evolution of the orogenic belts (mostly syn- to post-peak metamorphic), their formation from low salinity, H2O±CO2±CH4±N2±H2S fluids, their structural control by second- or third-order splays of

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crustal-scale shear zones located in fore- or back-arc settings, and their formation in broad thermal

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equilibrium with their host rocks as indicated by alteration assemblages (Kerrich and Fryer, 1981;

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Gebre-Mariam et al., 1995; Groves et al., 1998; Goldfarb et al., 2001; Goldfarb et al., 2005; Goldfarb and Groves, 2015).

While the major component characteristics of the ore-forming fluids in orogenic gold deposits are well established and broadly similar for all deposits, the ultimate source of the ore fluids and the precious metals is not well understood and controversially debated (Phillips and Powell, 1993; Ridley and Diamond, 2000; Groves et al., 2003; Goldfarb et al., 2005; Phillips and Powell, 2010; Goldfarb and Groves, 2015). Many studies conclude that metamorphic fluids are the key ingredients for orogenic gold ore formation (Phillips and Groves, 1983; Pettke et al., 2000; Phillips and Powell, 2010; Yardley and Cleverley, 2013) and that fluid production and precious metal uptake are the consequence of devolatilization reactions in metasedimentary or mafic metavolcanic rocks (Pettke et al., 2000; Elmer et al., 2006; Pitcairn et al., 2006; Phillips and Powell, 2010; Pitcairn et al., 2014; Goldfarb and Groves, 4

ACCEPTED MANUSCRIPT 2015). These are either driven by prograde dehydration reactions at the greenschist- to amphibolite facies transition (Phillips and Powell, 2010) or alternatively by pressure drops during uplift, causing

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isothermal overstepping of dehydration reactions (Yardley and Cleverley, 2013). A major controversy

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remains whether formation of some Neoarchean gold deposits hosted in higher-grade metamorphic rocks can be explained by a metamorphic fluid model, because the geochronological data for ore

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mineralization and metamorphic events are not in good agreement with each other (Goldfarb and

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Groves, 2015).

As a distinct alternative to the purely metamorphic fluid model, major contributions of Au-rich

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magmatic-hydrothermal fluids exsolved from syntectonic granitic plutons have been suggested for a large number of orogenic gold deposits. Such magmatic-hydrothermal contributions to the fluid

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systems are often inferred based on the close spatial and/or temporal association of gold deposits with

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felsic intrusions (e.g. Wall et al., 2004; Li et al., 2015; Song et al., 2015), stable or radiogenic isotope

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data (Burrows et al., 1986; Wang et al., 1993; Krienitz et al., 2008; Zoheir and Moritz, 2014; Molnár et al., 2016a) and noble gas and halogen data that carry a magmatic signature (Fu et al., 2011; Kendrick et al., 2011a). As a third alternative, essential contributions of mantle-sourced fluids to orogenic gold deposit forming crustal hydrothermal systems have been proposed (Groves, 1993; Zhang et al., 2008; Goldfarb and Santosh, 2014). Mantle-derived fluids have been discussed as a possible driver for gold deposit formation in the Jiaodong peninsula in the eastern part of the North China block, where robust structural and geochronological evidence appears to conflict with metamorphic and magmatichydrothermal models (Goldfarb and Santosh, 2014). Taken together, most of the contradicting evidence for metamorphic devolatilization versus magmatic-hydrothermal fluid sources is based on equivocal mineralogical, isotopic, and fluid inclusion major component and halogen data (Goldfarb and Groves, 2015). Despite extensive research, the major 5

ACCEPTED MANUSCRIPT and trace element composition of ore-forming fluids related to orogenic gold mineralization is still largely unknown, yet could provide important and more direct evidence for the fluid sources. Advances

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in laser-ablation inductively coupled plasma-mass spectrometry (LA-ICPMS) now permit complete

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analysis of individual fluid inclusions including rock-forming elements, ore metals, sulfur, boron and the halogens (Günther et al., 1997, 1998; Heinrich et al., 2003; Allan et al., 2005; Guillong et al.,

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2008a; Seo et al., 2011). This multi-element in-situ technique has been applied to hydrothermal fluids

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from a diverse range of geological environments including magmatic-hydrothermal, unconformityrelated and metamorphic fluids (Audétat et al., 2000; Landtwing et al., 2005; Heijlen et al., 2008; Seo

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et al., 2012; Fusswinkel et al., 2013; Marsala et al., 2013; Miron et al., 2013; Rauchenstein-Martinek et al., 2014, 2016; Wagner et al., 2016). However, very little data is available for orogenic gold systems,

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with the first incomplete dataset only recently reported by Garofalo et al. (2014).

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This work reports the results of a detailed fluid inclusion study of the actively mined Pampalo

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orogenic gold deposit, located in the Neoarchean Hattu schist belt in Eastern Finland. Building on structural and mineralogical criteria, we establish the relative time sequence of different quartz vein types at the deposit. In combination with fluid inclusion petrography, this provides the basis to link successive stages of vein formation with the chemical evolution of the hydrothermal system derived from microthermometry, Raman spectroscopy and LA-ICPMS analysis of fluid inclusions. The fluid chemistry dataset comprises concentrations of 24 major and trace elements, including S, Cl and Br as well as consistent gold concentrations for some of the fluid inclusion assemblages. The data demonstrate a progressive change of the fluid composition with time, which is visible in the major fluid components and the concentrations of many major and trace elements. The data permit to identify an early gold ore fluid, a modified rock-reacted ore fluid and two later types of barren metamorphic fluids unrelated to gold mineralization. The fluid composition data are used to evaluate potential fluid sources 6

ACCEPTED MANUSCRIPT and, in conjunction with mass balance calculations, to quantitatively assess the possible contributions

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of magmatic-hydrothermal fluids to the gold mineralizing fluid system at Pampalo.

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2. Geological setting 2.1. The Hattu schist belt, Eastern Finland

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The Hattu schist belt forms part of the Ilomantsi complex, located in the western part of the

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Neoarchean Karelian craton in eastern Finland (Fig. 1). The Ilomantsi complex includes a suite of granitoid bodies surrounding a supracrustal greenstone belt sequence, which in turn is subdivided into

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the western Kovero and the eastern Hattu schist belts (Sorjonen-Ward and Luukkonen, 2005). The Hattu schist belt comprises predominantly feldspathic epiclastic rocks, intermediate to felsic volcanics

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and volcaniclastics, and smaller volumes of mafic to ultramafic volcanics and banded iron formations.

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The Hattu schist belt hosts one of the best preserved Archean supracrustal sequences in Finland, and

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the belt has retained a high degree of stratigraphic coherence and even some primary lateral facies variations (Sorjonen-Ward, 1993b). This made it possible to reconstruct the original depositional environment of the volcano-sedimentary rocks, which consisted of two overlapping subvolcanic complexes situated in occasionally emergent, but mostly turbidite-dominated basins (Sorjonen-Ward and Luukkonen, 2005). The supracrustal units were intruded by a series of intermediate to felsic rocks (tonalites and granodiorites). Their emplacement ages effectively overlap with the depositional ages of the supracrustal rocks within an age range of 2754–2726 Ma based on the available U-Pb zircon age data (Vaasjoki et al., 1993; Sorjonen-Ward and Luukkonen, 2005; Huhma et al., 2012). Slightly younger (ca. 2700 Ma), highly evolved, tourmaline-muscovite leucogranites occur in the northern and southern part of the schist belt and record reworking and partial melting of older continental crust. This

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ACCEPTED MANUSCRIPT interpretation is further corroborated by the presence of detrital zircons within the sedimentary units, dated at ages up to 3180 Ma (Vaasjoki et al., 1993).

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On the regional structural scale, the schist belt is thought to have undergone dextral

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transpressive deformation, resulting in steeply dipping, generally upward-facing structures (SorjonenWard and Luukkonen, 2005). Deformation and significant hydrothermal alteration within the Hattu

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schist belt are closely linked to the emplacement of granitic bodies, particularly a series of biotite

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tonalites that intruded as sub-concordant sheet-like bodies into already tilted sedimentary units. Hydrothermally altered mineral assemblages record subsequent deformation and metamorphic

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overprint at greenschist to lower amphibolite facies conditions with a peak temperature of 550 ± 50 °C

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and estimated pressures of 3–5 kbar (Kojonen et al., 1993; O’Brien et al., 1993).

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2.2. Gold mineralization in the Hattu schist belt

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Gold exploration in the Hattu schist belt started as early as 1982 and was triggered by anomalous arsenic concentrations found in till geochemical data. This was followed by systematic mapping and drilling campaigns, which ultimately led to the discovery of several gold prospects. Currently, the Pampalo deposit (termed Ward deposit in older maps and publications) is actively being mined, with smaller scale mining and test mining going on in several other prospects along the schist belt. The total remaining gold resources within the belt are estimated at 5500 kg, with the Pampalo deposit amounting to about half of the total tonnage (Sorjonen-Ward et al., 2015). Gold mineralization has been identified in almost all rock types found within the Hattu schist belt, indicating that rheological and geochemical contrasts were essential for focusing fluid flow and gold deposition in response to fluid-rock interaction (Sorjonen-Ward et al., 2015). Many of the gold prospects (Hosko, Rämepuro, Muurinsuo, Elinsuo, Kivisuo and Korvilansuo; Fig. 1) within the Hattu 8

ACCEPTED MANUSCRIPT schist belt are hosted by metasedimentary rocks, mostly greywackes intercalated with conglomerates, banded iron formations and rare mafic or felsic volcaniclastics (Nurmi et al., 1993). They underwent

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pronounced sericitization, biotitization and tourmalinization and, with the exception of the Hosko

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prospect, are commonly intruded by mineralized felsic porphyry dikes. The Pampalo deposit is hosted by volcaniclastic metavolcanic rocks and mineralized porphyry dikes, and the host rocks and structural

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setting will be described in more detail below. The Kuittila deposit is entirely hosted within a tonalite

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body (termed Kuittila tonalite), while the Kelokorpi deposit is hosted by the Kuittila tonalite and by metasedimentary rocks in the footwall (Nurmi et al., 1993). Throughout the entire schist belt, Kojonen

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et al. (1993) identified three major ore mineral assemblages, although not all of them are present in every deposit. An early Mo-W-(Au) stockwork mineralization is followed by a main Au-sulfide-

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telluride mineralization which predominantly occurs in veins. The latest Au-telluride-sulfide

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1993; Molnár et al., 2016a).

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mineralization has been mostly described as disseminations in mineralized host rocks (Kojonen et al.,

The timing of gold mineralization relative to the metamorphism in the schist belt is not well established. Sorjonen-Ward et al. (2015) summarize all available data, which suggest that peakmetamorphic growth of garnet porphyroblasts and dynamic recrystallization of gold-bearing quartz veins overprint and therefore post-date gold mineralization and associated hydrothermal alteration of wall rocks. However, for the Pampalo zone, they conclude that the nature of structural control, the timing of gold introduction, and the sources of the auriferous fluids remain poorly constrained. In the northern part of the schist belt, extensive potassic alteration of clastic, volcaniclastic and subvolcanic rocks has been interpreted as synvolcanic, although there is no evidence for gold introduction by epithermal or porphyry systems and the alteration features are clearly older than the deformation events related to gold mineralization (Sorjonen-Ward, 1993b). However, the presence of 9

ACCEPTED MANUSCRIPT scheelite, molybdenite, tourmaline, and bismuth-tellurium bearing minerals is a common feature of gold mineralization throughout the Hattu schist belt, particularly of mineralized areas in close

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proximity to tonalites (Kojonen et al., 1993). Sorjonen-Ward et al. (2015) discuss how this could be

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interpreted in terms of contributions of magmatic-hydrothermal fluids to the ore-forming hydrothermal system. They conclude that the available isotope data from gold mineralization are in agreement with,

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but do not prove this hypothesis. However, they also highlight that Au-bearing veins in the Kuittilla

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tonalite clearly postdate older molybdenite-scheelite veins, and that the spatial association between magmatic-hydrothermal features and gold mineralization does not necessarily provide robust evidence

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for a temporal and genetic link.

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2.3. The Pampalo gold deposit

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The Pampalo deposit (Fig. 2) is hosted by diverse host rock types within a well-established

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stratigraphic context (Sorjonen-Ward et al., 2015). The lowermost stratigraphic unit in the vicinity of the deposit comprises turbiditic greywackes and conglomerates of the Tiittalanvaara Formation, which are overlain by a banded iron formation marking the base of the Pampalo Formation. The next stratigraphic unit is formed by barren dolerites and basalts (metabasalt, MB) showing a distal carbonate-pyrrhotite alteration and ubiquitous barren quartz-calcite-tourmaline-pyrrhotite veins. These metabasalts are overlain by an intermediate metavolcaniclastic rock, termed andesitic tuff (abbreviated AT) in drilling reports and the older literature, which hosts the bulk of the gold mineralization as disseminations

and

as

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quartz+K-feldspar+pyrite

veins.

The

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metavolcaniclastic rock is intruded by equally mineralized felsic porphyry (FP) dikes along the Pampalo

shear

zone.

Both

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types

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extensive

alteration

(quartz+K-

feldspar+biotite+sericite+carbonate). To the east, the next higher stratigraphic unit comprises a bimodal 10

ACCEPTED MANUSCRIPT suite of ultramafic talc-chlorite schists, thought to have formed from komatiitic protoliths. They are intercalated with gold mineralized vitreous felsic units, possibly representing former rhyolitic flows, as

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well as mineralized felsic porphyry dikes (Sorjonen-Ward et al., 2015).

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The Pampalo deposit has a NE-SW directed structural orientation (Fig. 2), which is highly anomalous compared to the general NW-SE structural trend of the surrounding area within the Hattu

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schist belt (Fig. 1). Sorjonen-Ward (1993b) explain this feature by placing the Pampalo deposit in the

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toe of a contractional, oblique sinistral strike slip duplex (Juttuhuuhta duplex). This local structure is also interpreted to have caused the apparent thickening of the upper units of the Pampalo Formation

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through tectonic repetition by fault imbrication, following detachment along the basal banded iron formations. The final structural layout of the deposit was then caused by variations in the far-field

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stress configuration, resulting in a progressive transition from contractional to oblique extensional

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deformation (Sorjonen-Ward et al., 2015).

2.4. Quartz vein generations at Pampalo The complex structural history of Pampalo which records multiple deformation events is reflected by several generations of quartz veins recording ductile, brittle-ductile and brittle deformation. Field observations make it possible to classify three major vein types and place them into a relative time sequence, based on their structural context as well as their gangue and ore mineralogy. Early quartz+Kfeldspar+calcite+pyrite±gold veins are closely associated with boudinaged feldspar-porphyry (FP) dikes and occur within the main, northerly trending (about 40°) foliation of the andesitic tuff (AT; Figs. 3A, 3B, 3F) and, to a lesser extent, the mafic metavolcanic rock (metabasalt, MB) to the west. The feldspar-porphyry dikes are strongly silicified, often highly mineralized and constitute one of the main gold ores (Fig. 3E). The quartz veins appear to be roughly coeval with these feldspar-porphyry dikes or 11

ACCEPTED MANUSCRIPT slightly younger (Fig. 3A). Most importantly, some of these early quartz veins extend into low-strain zones between boudins of the feldspar-porphyry dike or transect these boudin necks (Fig. 3A). Where

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this is the case, the boudin necks sometimes contain open cavity infills of the same mineral assemblage

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that is observed in the quartz veins, i.e. quartz+K-feldspar+calcite+pyrite and late muscovite+chlorite. The open cavities contain euhedral quartz crystals up to 10 cm in size which commonly have smoky

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colored cores and more colorless overgrowths (inset in Fig. 3A). They occasionally contain inclusions

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of K-feldspar crystals on growth zones (Fig. 4A), demonstrating the genetic link with the early quartz veins.

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A second younger type of quartz veins (termed tension gash quartz in the following sections) is present as en echelon sets of tension gashes developed at high angles to the direction of the main

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foliation. Mineralogically, these veins are more diverse, but generally contain the assemblage

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quartz+calcite+tourmaline+pyrite. This vein type is also ubiquitous in the metabasalt unit to the W of

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the deposit, where pyrrhotite is the dominant sulfide mineral instead of pyrite. The third and latest major vein type has formed at high angles to the main foliation, and the vein structures suggest vein formation in response to brittle deformation. These veins are mostly composed of quartz, but can contain small amounts of fluorite, galena and sphalerite. They frequently show open space fillings and sometimes cavities with tabular clear euhedral quartz crystals of up to 5 cm in size (Fig. 3A). A fourth and rather rare vein type was occasionally observed. It is present as very late fracture fillings related to brittle deformation occurring at high angles to the direction of the main foliation. These veins contain a quartz+calcite+gypsum+muscovite assemblage. The presence of gypsum but not of anhydrite was confirmed by Raman spectroscopy.

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ACCEPTED MANUSCRIPT 3. Samples and methods 3.1. Samples

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The samples used in this study are a subset of a larger body of samples covering gold mineralizations

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and their host rocks from the entire Hattu schist belt. The Pampalo samples were collected from the underground mine at various depth levels (levels: -112 m, -210 m, -420 m, -455 m, -475 m, -515 m)

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and supplemented with drill core samples from production and exploration drill holes (drill cores:

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PAM-322, PAM-817, PAM-820, PAM-822). The sample set covers the three major vein types in all important host rock lithologies. In total, 25 representative doubly polished fluid inclusion sections were

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prepared from 37 individual samples. Table 1 contains descriptions of all samples.

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3.2. Fluid inclusion studies

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The relative chronology of fluid inclusion assemblages (FIA) and their relation to the host minerals

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were established by detailed fluid inclusion petrography. Fluid inclusion assemblages are defined as groups of coeval fluid inclusions trapped on temporally constrained petrographic features such as growth zones or healed fractures (Goldstein and Reynolds, 1994). Consequently, with the exception of special cases such as phase separation, there should be no internal chemical variation within an FIA, which provides the basis for critical evaluation of analytical data obtained from microthermometry, Raman spectroscopy and LA-ICPMS. The fluid inclusion assemblage approach was consistently applied in this study, and fluid inclusion data reported in tables and figures are assemblage averages along with their one sigma standard deviation. Fluid inclusions are further classified into main types and abbreviated using their room temperature phase assemblage, with capital letters indicating the presence of aqueous liquid (Lx, the numerical subscript denoting the volume percentage of aqueous liquid), carbonic liquid (Lc), solid (S) and vapor (V) phases. 13

ACCEPTED MANUSCRIPT The fluid inclusion studies were carried out on doubly polished 500 µm thick sections. Following petrography, the thick sections were cut into rectangular wafers to fit into the sample cells of

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the microthermometry stage and the LA-ICPMS ablation cell. Microthermometry was carried out using

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a Linkam THMSG-600 heating-freezing stage mounted on a Leica DM2500P petrographic microscope. Synthetic H2O-CO2, H2O-NaCl and pure H2O fluid inclusion standards were used for daily calibration

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of the heating-freezing stage to the H2O-CO2 Q3 quadruple point (-56.6 °C), the H2O-NaCl eutectic (-

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21.2 °C), the melting temperature of ice (0.0 °C), and additionally the critical temperature of H2O (374.1 °C) if homogenization experiments were conducted on the same day. Phase transition

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temperatures were monitored at heating rates of 0.5 °C/min. The reproducibility of microthermometric freezing experiments is estimated as 0.1 °C and that of heating runs as 2 °C.

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Bulk fluid compositions were calculated using the programs BULK (aqueous fluids) and Q2 or

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ICE (aqueous carbonic fluids) from the Fluids and Clathrates software packages, respectively (Bakker,

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1997, 2003). The programs Q2 or ICE were used depending on the observed phase transitions in aqueous-carbonic inclusions. The Q2 program can only be used when final clathrate dissolution occurs in the presence of an aqueous liquid and coexisting carbonic liquid and vapor phases (i.e. Q2 equilibrium conditions). In some H2O-dominated FIA with only small amounts of the carbonic phase, the homogenization of the carbonic phase could not be observed, meaning that Q2 clathrate dissolution conditions could not be assumed. In such cases, the program ICE provides an alternative way of calculating inclusion salinity from the clathrate dissolution temperature, the ice melting temperature and the estimated phase proportions. For all calculations involving carbonic phases, the composition was assumed to be pure CO2, based on the microthermometric observation that the final dissolution of the solid carbonic phase was only slightly lowered relative to the triple point of pure CO2 (-56.6 °C). This demonstrates that the concentrations of other volatile components are very small, and will not 14

ACCEPTED MANUSCRIPT detectably affect the calculated densities. The programs Q2 and ICE only calculate the salinity of the aqueous phase, which then needs to be corrected using phase proportions to arrive at the salinity and

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Na concentration of the bulk fluid. This is important, because the carbonic phase may contain elevated

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concentrations of volatile species, while being essentially Na-free. Representative isochores based on

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bulk fluid inclusion compositions were calculated with the computer program LonerAP (Bakker, 2003).

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3.3. Raman spectroscopy

Laser Raman spectroscopy was carried out with a Dilor Labram II instrument equipped with a 488 nm

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solid-state diode laser, and a Renishaw InVia Reflex spectrometer with 532 and 688 nm laser wavelengths at the University of Tübingen, Germany. The Raman instruments were calibrated daily

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using a silicon and a calcite standard. All analyses were carried out using 100x magnification

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objectives. Raman spectra were acquired by triple accumulation of 20 s integrations per spectral

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window. Qualitative Raman spectroscopy was performed on several fluid inclusions for each FIA. Atmospheric contributions of N2 were assessed by regular measurements of the pure quartz host and found to be minor in relation to the N2 signals found in some of the fluid assemblages.

3.4. LA-ICPMS analysis of fluid inclusions and data treatment Fluid inclusion LA-ICPMS microanalysis was performed at the University of Helsinki with an Agilent 7900s quadrupole ICPMS equipped with a high-sensitivity s-lens ion lens configuration, and coupled to a Coherent GeoLas Pro MV 193 nm excimer laser ablation system. A fast washout small-volume 1 cm³ ablation cell (designed at ETH Zurich) was used to ensure high sensitivities and lowest possible limits of detection for transient fluid inclusion signals. The ICPMS system was tuned daily to maximize signal-to-background ratios, ensure low oxide production rates (ThO/Th < 0.4 %) and optimal ablation, 15

ACCEPTED MANUSCRIPT transport and plasma conditions (U/Th ratio about 1.00 ± 0.02). The instrument accuracy was checked daily by replicate measurements of the composition of NIST 613 synthetic glass standards using NIST

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611 as the external reference material. The determined accuracies for the concentrations of major and trace elements are within the reported range of preferred values and their uncertainties (Spandler et al., 57

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2011). The only element for which higher deviations were observed is

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are listed in Table 2.

Fe. The analytical conditions

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Prior to drilling to the target fluid inclusion with a 10 Hz laser repetition rate and the desired ablation spot size, the first 10-20 µm of the quartz host were carefully pre-drilled at lower repetition

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rates using a step-wise increase in ablation spot sizes from 4 µm up to the target diameter (typically 90 to 160 µm, depending on the fluid inclusion size). This approach made it possible to yield 70-80 %

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successful ablations with a quantifiable ICPMS signal. In order to avoid depth-dependent elemental

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(Guillong and Pettke, 2012).

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fractionation effects, the depth-to-diameter ratios of the ablation craters were always kept well below 2

The SILLS software package (Guillong et al., 2008b) was used for LA-ICPMS data reduction. All elements except Cl and Br were quantified against NIST SRM 611 (Spandler et al., 2011) as external standard. The halogens were externally standardized against the natural scapolite standard Sca17 (Guillong et al., 2008a; Seo et al., 2011). Absolute concentration values were then calculated by internal standardization against Na concentrations determined by microthermometry. Most fluid types at Pampalo contain significant amounts of K, as well as Ca in Type B fluid inclusions, so that the NaCl equivalent salinity (NaCleqv) is not identical to the true fluid salinity. However, true Na concentrations for internal standardization can be reasonably well approximated by mass balancing using cation/chlorine ratios from LA-ICPMS signals, as described by Allan et al. (2005). The method is implemented in the SILLS data reduction software, and the cations used for mass balancing were Na 16

ACCEPTED MANUSCRIPT and K for all fluid inclusion types except the calcium-rich Type B fluid inclusions, for which Ca was included into the mass balance. Limits of detection were calculated using equation (6) in Pettke et al.,

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(2012), which yields concentration of an element at the 95% confidence level. The method is

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implemented in the SILLS software package.

The ICPMS detector drift was corrected for by bracketing series of about 20 fluid inclusion

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ablations with measurements of the external standards. The lengths of the integration intervals of fluid

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inclusion signals were defined based on the return of the Cl signals to their background levels. The signals of other elements were only considered significant if the signal peak was synchronous with the

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Na signal. Host correction was carried out by assuming a pure SiO2 host and by closely bracketing fluid inclusion signals to minimize uncertainties for elements that possess high background count rates and

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that are absent from the quartz host, e.g. S, Cl and Br (Fig. 7A; Guillong et al., 2008a). This approach is

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critical because the count rates for such elements remain at background noise levels during ablation,

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while the count rates for all other elements diminish as ablation proceeds due to deepening of the ablation pit and decreasing amounts of ablated material. Consequently, the element/Si ratios for these elements increase continuously, resulting in systematically increasing apparent host background concentrations. Using unsuitable time intervals which do not closely bracket fluid inclusion signals for host correction would consequently underestimate the concentrations of S, Cl and Br in the fluid inclusions.

4. Results 4.1. Fluid inclusion petrography Initial fluid inclusion petrography was performed on all 25 thick sections to establish a basic inventory of fluid inclusion types and to document their main petrographic features. These results provided the 17

ACCEPTED MANUSCRIPT basis for selection of the most promising and representative samples, which were then more extensively

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characterized petrographically and in which fluid inclusion assemblages were defined.

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4.1.1. Early boudin neck quartz infill and early quartz veins

Nine sections of the earliest quartz vein type were initially screened (PAM-03, PAM-09, PAM-15,

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PAM-17, PAM-19, PAM-21, PAM-32, PAM-33, PAM-822_80.55), out of which three sections of

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euhedral quartz crystals from boudin neck infills and one drillcore sample of a massive early quartz vein (PAM-822_80.55) were selected for further fluid inclusion studies.

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The euhedral quartz crystals commonly exhibit well-developed growth features, providing an excellent basis for robust petrographic analysis of the relative time sequence of different fluid inclusion

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assemblages (Figs. 4A, 4B). Primary (P) fluid inclusions aligned on the growth zones are usually

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irregularly shaped and do not exceed sizes of 5–15 µm. They are two-phase aqueous inclusions with

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the vapor phase occupying about 10–15 vol.% of the inclusion contents (L90V–L85V). Pseudosecondary (PS) FIA terminating at growth zones are ubiquitous and often contain large inclusions of 50–100 µm and more in size (Fig. 4C). PS inclusions have subregular to perfectly negative crystal shapes (Fig. 6A, B) and, like the P inclusions, are L90V–L85V two-phase aqueous fluid inclusions. Raman spectroscopy demonstrates the presence of N2 in PS fluid inclusions. Secondary FIA commonly occur on healed fractures that extend from the inner part of the crystals to the terminating outer crystal faces. They contain fluid inclusions with subregular to negative crystal shapes up to 100 µm in size, in some cases even larger (Figs. 4D, 6C, 6D). In addition to aqueous liquid and vapor in proportions similar to those of the other fluid types in the same samples, the secondary FI often contain very small solids that were identified as calcite crystals by Raman spectroscopy (Fig. 6D). Although they were not observable in all inclusions of the respective FIAs, they are still considered to be true daughter crystals because 18

ACCEPTED MANUSCRIPT subsequent LA-ICPMS analysis revealed elevated and consistent calcium concentrations throughout these FIA (see section 4.4), regardless whether calcite crystals were optically visible. Thus, the phase

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assemblage of this fluid type is L90SV–L85SV.

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By comparison, the massive drill core quartz sample PAM-822_80.55 is characterized by a more complex set of crosscutting fractures containing abundant secondary fluid inclusion assemblages.

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They are truncated by later fracture fillings containing virtually inclusion free quartz along with calcite

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and tourmaline. The FI are irregular to subregular in shape, with sizes reaching up to 60 µm. No daughter phases have been observed and the phase proportions are very consistent throughout all FIA

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with about L90V, similar to the primary and secondary FIA in the euhedral quartz crystal samples.

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4.1.2. Tension gash quartz

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Quartz from the tension gash veins is usually massive and lacks clear internal petrographic features

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such as growth zones or crystal faces. Five samples of tension gash quartz veins were studied (PAM06, PAM-08, PAM-13, PAM-817_34.2, PAM-822_68.3). Some were affected by partial recrystallization, obliterating most original fluid inclusion information. Out of the 5 samples, six thick sections of the sample PAM-08 which contained the best preserved fluid inclusions were selected for further study. In this sample, small linear groups of fluid inclusions are arranged on larger-scale linear arrays transecting the entire thin section, resembling micro-scale en-echelon tension gash fractures (Figs. 5A to 5C). The three-phase aqueous-carbonic fluid inclusions hosted by these structures are usually rather small (5-35 µm) and irregular and have highly variable phase proportions ranging from L5LcV to L95LcV (Fig. 6E). The linear arrays are cut by sets of healed fractures, which in turn can be subdivided into two distinct age groups. The petrographically earlier type of fractures is characterized by closely spaced, 19

ACCEPTED MANUSCRIPT linear or slightly curved trails of fluid inclusions extending into the host mineral on both sides of the main fracture (Figs. 5A, 5B, 5D). They closely resemble feather veins, which form where tension

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gashes grade into more continuous veins (Passchier and Trouw, 2005). The feather veins host

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subregular three-phase aqueous-carbonic fluid inclusions that are up to 40 µm in size (Figs. 5D). The phase proportions within these FIA are more uniform and range from L80LcV to L90LcV (Fig. 6F). The

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later fracture set lacks feather veins and hosts aqueous, subregular to regular fluid inclusions rarely

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exceeding 25 µm in size. They constitute the youngest fluid type within the tension gash quartz samples. The fluid inclusions contain predominantly aqueous liquid and many of the smaller inclusions

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do not show a vapor bubble. The phase proportions of this late fluid type are therefore L95V–L100 (Fig.

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4.1.3. Late extensional quartz veins

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6G).

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Out of three samples from the extensional quartz veins (PAM-01, PAM-07, PAM-822_111.5), the euhedral quartz sample PAM-01 was selected for further study. The quartz crystal contains abundant primary fluid inclusions located on growth zones (Fig. 5F, 5G). The inclusions have generally regular to perfectly negative crystal shapes with sizes between 15–100 µm. They are two-phase aqueous inclusions with phase proportions of around L90V (Fig. 6H). Pseudosecondary inclusion trails terminate at the growth zones (Fig. 5H) and host not so regularly shaped but similarly large fluid inclusions of identical phase proportions. Secondary fluid inclusions occur on healed fractures transecting the entire crystal (Fig. 5F, 5G, 5I). Their petrographic properties are indistinguishable from those of the primary and pseudosecondary inclusion types.

4.2. Fluid inclusion microthermometry and Raman spectroscopy 20

ACCEPTED MANUSCRIPT The microthermometric data, in conjunction with the petrographic observations, make it possible to define a number of systematically different fluid types within the three major quartz vein generations at

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Pampalo. The microthermometric properties presented below are average values of FIA. The complete

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fluid inclusion dataset is available in the Electronic Supplementary Material, and a summary is presented in Table 3. In total, 600 fluid inclusions in 65 FIA were studied by microthermometry.

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Homogenization experiments were not carried out on all of them, because even small amounts of CO2

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gas in an inclusion may result in rapid build-up of overpressure and decrepitation upon heating, which could have rendered entire wafers unusable for subsequent LA-ICPMS analysis. Therefore,

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homogenization experiments were only conducted on smaller FI from selected FIA after successful

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LA-ICPMS analysis.

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4.2.1. Early boudin neck quartz: Fluid inclusion Types A and B

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Primary and pseudosecondary FIA in the early boudin neck quartz constitute the paragenetically and petrographically oldest fluid type at Pampalo, termed Type A fluid inclusions. Type A fluid inclusions are aqueous with no microthermometric indication for the presence of significant amounts of a carbonic phase. Raman spectroscopy shows the presence of N2 gas within the vapor phase. Ice is the only solid phase observable during heating of frozen inclusions. Type A fluid inclusions show very consistent final ice melting temperatures between -5.0 and -3.0 °C (average: -4.0 ± 0.6 °C), corresponding to salinities of 5.0 to 8.0 wt. % NaCleqv (6.6 ± 1.0 wt.% NaCleqv). Type A inclusions homogenize into the liquid phase (LV→L) at temperatures of 160 ± 5 °C. Type B fluid inclusions are aqueous L90SV–L85SV with minor amounts of N2 in the vapor phase, hosted by secondary FIA in the early boudin neck quartz. They often contain solid daughter crystals of calcite (see section 4.4). Like Type A fluid inclusions, they are characterized by ice being 21

ACCEPTED MANUSCRIPT the only solid phase to melt during heating, but at markedly lower final melting temperatures of -10.9 to -6.4 °C (-8.7 ± 1.6 °C), corresponding to salinities between 9.9 and 15 wt.% NaCleqv (12.5 ± 2.4 wt.

4.2.2. Tension gash quartz: Fluid inclusion Types C and D

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PT

%).

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Aqueous-carbonic fluid inclusions occur in both the linear arrays of micro-scale en echelon tension

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gashes as well as in the feather veins. Their microthermometric properties are identical, which is consistent with the close genetic link between tension gashes and feather veins (Passchier and Trouw,

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2005). Therefore, all aqueous-carbonic FIA were grouped together into fluid inclusion Type C and further subdivided into Type C-I for micro-scale tension gash-hosted FIA and Type C-II for feather

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vein-hosted FIA. Type C-I fluids are three-phase aqueous-carbonic inclusions showing highly variable

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phase proportions (L5LcV to L95LcV), possibly caused by phase separation. This interpretation is

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supported by consistent microthermometric data for both H2O-rich and H2O-poor inclusions within one assemblage. After freezing and reheating, the solid carbonic phase melts in a narrow temperature interval between -57.3 and -56.6 °C (-56.9 ± 0.3 °C), very close to the triple point of pure CO2 gas, indicating that CO2 is the dominant carbonic component in these FIA. This is confirmed by Raman spectroscopy, which shows that the vapor phase is composed of CO2 and minor N2. Because the inclusions are three-phase at room temperature, they must follow a heating path through the Q2 point where CO2 clathrate dissolves (Diamond, 2001). In pure H2O-CO2 fluids, Q2 is located at 9.8 °C while in Type C-I fluid inclusions the dissolved salts in the aqueous solution depress clathrate melting temperatures to around 6.8 ± 0.3 °C. Taking into account the phase proportion estimates, these clathrate dissolution temperatures translate into bulk inclusion salinities of 4.6 to 5.3 wt.% NaCleqv (4.8 ± 0.2 wt.%) in the H2O-rich inclusions and 0.7–1.7 wt.% NaCleqv (1.2 ± 0.5 wt.%) in the dominantly carbonic 22

ACCEPTED MANUSCRIPT inclusions. Within assemblages, the variation in measured clathrate dissolution temperature can vary by approximately 1.5 °C, due to difficulties in accurately observing this phase transition in small

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inclusions. Upon further heating, the carbonic phase homogenizes into the liquid phase (LcV→Lc) at

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temperatures between 24 to 28 °C (25.6 ± 1.5 °C). Total homogenization temperatures could not be determined for Type C-I inclusions because the inclusions would invariable decrepitate at temperatures

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around 250 °C. Type C-II fluid inclusions are hosted by feather veins (Figs. 5A, 5B). They are H2O-

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dominated, three-phase aqueous-carbonic inclusions. Phase proportions are uniform within assemblages, providing no evidence for phase separation.

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Type D inclusions are hosted by petrographically late healed fractures. They are two-phase at room temperature with high proportions of aqueous liquid (L95V). There is no microthermometric

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D

evidence for the presence of a carbonic phase, though Raman spectroscopy reveals very weak signals of CO2, CH4 and H2 gas. Ice is the only solid to melt upon heating, and ice melting occurs at

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temperatures between -3.2 to -2.2 °C (-2.7 ± 0.4 °C) corresponding to salinities between 3.7 to 5.2 wt.% NaCleqv (4.5 ± 0.6 wt. %).

4.2.3. Late extensional quartz veins: Fluid inclusion type E Type E fluid inclusions are two-phase at room temperature and microthermometry did not yield any indication for the presence of additional volatile gas species. Thus, even though Raman spectroscopy sometimes reveals trace amounts of CO2 and CH4 in vapor bubbles of the largest inclusions, this inclusion type is virtually purely aqueous. Ice is the only phase to melt during heating. Final melting occurs at temperatures between -3.3 to -2.5 °C (-2.8 ± 0.2 °C) corresponding to salinities of 4.2 to 5.5 wt.% NaCleqv (4.6 ± 0.4 wt.%). Total homogenization (LV→L) occurs at temperatures around 150 ± 5 °C. These microthermometric properties were measured for all inclusions within the studied sample, 23

ACCEPTED MANUSCRIPT irrespective of their primary, pseudosecondary or secondary nature (Fig. 5f, g). Therefore, fluid

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inclusion Type E comprises FIA from all three petrographic types.

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4.3. Fluid inclusion LA-ICPMS

We imposed rigorous quality criteria to define successful ablations. The most important criterion was

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stable and steady ablation of host quartz for at least several seconds before and after inclusion breach

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(Figs. 7A, 7B), without fracture formation (leakage of fluid) (Fig. 7C) at the bottom of the ablation pit or explosive opening of inclusions (spallation) (Fig. 7D), because both processes may lead to

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incomplete sampling of inclusion contents. Fracturing of quartz can result in partial fluid leakage into the sample along newly formed crack surfaces extending away from the ablation pit, which results in

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very long, drawn out signals with low intensity. Spallation usually results in uncontrolled opening of

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the inclusion cavity and explosive fluid release, which may not ensure complete vaporization of

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inclusion contents. In contrast to the findings of Allan et al. (2005), we found that both processes significantly alter individual inclusion element/Na ratios compared to other inclusions from the same FIA, yielding highly anomalous concentration values. These were consequently considered to be analytical artifacts and excluded from the final dataset. Not all fluid inclusions characterized by microthermometry were suitable for analysis by LAICPMS, for example because of their small size or because they were located too close to the surface of the wafer, which makes them more prone to spallation. In total, about 450 inclusions were ablated, of which 310 yielded data meeting the quality criteria defined above. Table 4 and Figure 8 show average elemental concentrations for all analyzed FIA. The full dataset of individual measurements is available as Electronic Supplementary Material.

24

ACCEPTED MANUSCRIPT All analyzed fluid inclusions show elevated concentrations of a suite of elements that is inferred to be characteristic of orogenic gold ore fluids, i.e. K, As, B, Sb and W. At the same time, significant

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systematic differences are observed between samples from the different vein types and the previously

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established fluid inclusion types.

Type A and Type B fluid inclusions contain the highest sulfur concentrations of all studied

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inclusions, up to 2000 µg/g in some assemblages. The average sulfur content is lower in Type B fluid

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inclusions compared to Type A inclusions. The variation within FIA is most likely caused by the analytical challenge of accurately analyzing S by LA-ICPMS (Guillong et al., 2008a; Seo et al., 2011).

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These studies showed that quadrupole ICPMS systems are capable of adequately reproducing S concentrations in synthetic fluid inclusions with uncertainties of about 30 % (relative standard

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deviation) using NIST SRM 610 or NIST SRM 611 as external standard. This uncertainty includes the

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possible influence of 16O16O interfering with 32S, low signal-to-background ratios due to high 32S blank

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count rates and the uncertainties in S concentrations in the external standard materials. In light of these uncertainties in the range of 30 % which are higher than for the other elements, we have reported the S concentration data in table 4 to the next significant digit. The relative differences between S concentrations in the different fluid types, however, are not affected by the somewhat larger uncertainties of S analysis.

Type A fluid inclusions have also other characteristics that distinguish them from the later Type B fluids within the same samples. The most striking feature is that elevated Au concentrations only occur in Type A FIA. They range from below the limits of detection to values as high as 0.6 µg/g in some assemblages. Internally, most FIA show rather consistent Au concentrations, but there are exceptions in which the Au concentration values differ rather significantly between individual FI of the same FIA. The significance of the variability between and within FIA is addressed in a separate section 25

ACCEPTED MANUSCRIPT below. The concentrations of As and W in Type A inclusions (700–7000 µg/g and 1.5–10 µg/g, respectively) are consistently higher than those in Type B inclusions (about 100 µg/g As, < 1 µg/g W).

40

Ar35Cl and

40

Ca35Cl. Due to their

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may have been somewhat affected by polyatomic interferences of

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The relative differences in As concentration levels are considered robust even though the 75As isotope

higher Cl and Ca contents, Type B inclusions should be more affected by these interferences than Type

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A inclusions, which is opposite of the concentration differences that were found. In fact, the low As

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concentrations in Type B fluids might be even slightly overestimated due to interference effects. The boron concentrations are more variable in Type B inclusions, but on average they are lower than those

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of Type A inclusions.

Type B inclusions show distinctly higher Ca and Sr concentrations, as expected from the

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presence of calcite daughter crystals. The Cl/Br molar ratios in Type A and Type B inclusions vary

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from 25 up to 150 for some Type B FIA, with average values of about 42 and 63 for Type A and Type

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B, respectively. All other fluid types at Pampalo are even more enriched in Br, but it has to be emphasized that molar Cl/Br of 63 and less already place them among the lowest ever reported values for crustal hydrothermal fluids (Yardley 2013). The aqueous-carbonic Type C-I inclusions are compositionally more diverse than the other types. Figure 8 shows average liquid and vapor compositions for the two Type C-I FIA in separate columns (labelled L and V). If phase separation indeed is the cause for the highly variable phase proportions in these inclusions, then it would be accompanied by significant preferential elemental fractionation into the liquid or vapor phases (Heinrich et al., 1999). This could likely account for the observed compositional differences between the liquid and vapor dominated inclusions of C-I FIA (Fig. 8). However, at present the dataset is too small to allow for reconstruction of apparent liquid-vapor fractionation coefficients. In addition, the analytical errors are considered rather large due to the 26

ACCEPTED MANUSCRIPT generally small size of the inclusions and the low proportion of aqueous liquid in them, resulting in very weak LA-ICPMS signals. By contrast, Type C-II inclusions are compositionally more

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homogenous and they show some differences in their trace element inventory compared to Type C-I

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inclusions, possibly related to the effects of phase separation causing liquid-vapor fractionation and mineral precipitation. These differences include Ba and W concentrations of 3–90 and 4–6.5 µg/g in

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Type C-II fluid inclusions, while both elements were below the limit of detection in all Type C-I fluid

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inclusions. Type C-II inclusions also appear more enriched in Br and have very consistent Cl/Br molar ratios of 15-28, while Type C-I and Type D inclusions have somewhat higher Cl/Br ratios between 46

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and 111. Compared to Type C-II inclusions, the aqueous Type D fluid inclusions are characterized by somewhat lower Li and higher Sr concentrations, while Cu and W concentrations are below the limits

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of detection (<0.3 and <0.2 µg/g, respectively).

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Type E fluid inclusions show very little compositional variability throughout all studied FIA,

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but they are notably different from Type A inclusions. They share some features with Type B inclusions, such as sulfur concentrations of < 600 µg/g and Au concentrations below the limits of detection (0.01–0.03 µg/g). However, in terms of their Ca, Sr, Sb and Ba concentrations they are clearly distinct from Type B inclusions. The most striking feature of Type D inclusions is the very consistent and quite low Cl/Br molar ratio that ranges between 9 and 35.

4.3.1. Significance of the Au concentration data The Au concentrations in the auriferous Type 1 fluid inclusions do still show some variation between FIA and within some FIA, despite the rigorous data quality assessment and data screening. Gold was not detected in all inclusions of any given FIA, even though the limits of detection in all inclusions where gold was not detected were lower than the Au concentration values in inclusions where gold was 27

ACCEPTED MANUSCRIPT reliably detected. However, we do not consider the magnitude of within-assemblage variations as violation of the fundamental assumption of constant composition of a given fluid inclusion assemblage,

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but rather a consequence of analytical limitations (Rauchenstein-Martinek et al., 2014). Fluid inclusion

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ICPMS signals are invariably transient, lasting only a few seconds with count rates varying by up to 8 orders of magnitude. This effect is even more pronounced for elements that have low concentrations

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like Au, whose count rates often return to blank levels even faster. The gold peaks are therefore only

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defined by a few data points for most fluid inclusion signals analyzed. In fact, the maximum of the Au signal peaks in our time-resolved LA-ICPMS signals is often defined by only one single data point

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(Fig. 7A), with the adjacent earlier and later data points recording Au count rates which are lower by several orders of magnitude.

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Every single data point in time-resolved LA-ICPMS signals represents one full sweep cycle

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through the mass range, during which each mass is analyzed for the duration of the corresponding

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dwell time (see Table 2). The data points are therefore spaced by time increments that represent the length of one full sweep cycle, which in this study was about 300 ms. The dwell time on mass 197 (Au) was 60 ms, which means that during the acquisition of each data point, only 20 % of the total sweep cycle time was spent on measuring Au. Therefore, ultra-trace elements with short signal durations will be inherently prone to unrepresentative sampling due to the sequential detection of elements (see also Pettke et al., 2012). If the arrival of the bulk of Au is not exactly in phase with the ICPMS detector measuring mass 197, some (or most) of the Au ions will not be detected, resulting in rather low count rates or even completely missing the Au signal. This effect can easily account for the magnitude of the variations between and within FIA and for the fact that gold was not detected in some fluid inclusions from a given FIA. We note that we have detected Au in 38 out of a total number of 173 Type 1 fluid

28

ACCEPTED MANUSCRIPT inclusions, i.e. in 22 % of all inclusions analyzed. This percentage is equal to the fraction of

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measurement time (dwell time for Au) spent on Au during a full sweep cycle.

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5. Discussion 5.1. Production and chemistry of metamorphic fluids

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Metamorphic fluids are essential drivers for mass transfer in accretionary and collisional orogenic belts

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(Bickle and McKenzie, 1987; Connolly and Thompson, 1989; Ferry and Dipple, 1991) and considered as key ingredients in the formation of orogenic Au deposits (Kerrich and Fryer, 1979; Kerrich and

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Fyfe, 1981; Phillips and Groves, 1983; Goldfarb et al., 1988; Groves et al., 2003; Phillips and Powell, 2010; Goldfarb and Groves, 2015). The chemical composition of metamorphic fluids has been

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characterized through a number of conventional (Banks et al., 1991; Mullis 1975; Yardley et al., 1993;

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Mullis et al., 1994; Mullis 1996) and some more recent Raman spectroscopic and LA-ICPMS fluid

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inclusion studies (Scambelluri et al., 2004; Marsala et al., 2013; Miron et al., 2013; RauchensteinMartinek et al., 2014, 2016). The compositions of rock-buffered metamorphic fluids change systematically along prograde Barrovian pressure-temperature paths from subgreenschist- to amphibolite-facies conditions (Rauchenstein-Martinek et al., 2016), further supporting the widely accepted model that the systematic increase in temperature and pressure drives progressive devolatilization reactions of hydrous silicates, carbonates and sulfides (Norris and Henley, 1976; Oliver, 1996; Pettke et al., 1999; Phillips and Powell, 2010; Tomkins, 2010). The Paleozoic Monte Rosa Gold District in the northwestern Alps is a prominent example where gold mineralization at shallow crustal levels could successfully be linked to prograde metamorphic devolatilization of calcschists at greater depth (Pettke et al., 2000). However, Yardley and Cleverley (2013) point out that formation of large orogenic gold deposits from prograde metamorphic fluids is not easily reconciled 29

ACCEPTED MANUSCRIPT with the devolatilization model, because the gold deposits form very quickly and require extremely high fluid fluxes (e.g. Goldfarb et al., 1991) that exceed the fluid production rates that can be attained

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under conditions of slow prograde heating. They suggest that overstepping of dehydration reaction

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curves in response to isothermal pressure drops during uplift would result in much higher fluid production rates and could also explain the retrograde metamorphic timing of many orogenic gold

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deposits.

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Irrespective of the geodynamic process that produces metamorphic fluids, they are typically low- to intermediate-salinity aqueous or aqueous-carbonic fluids and the major cations are dominated

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by Na, K and lesser Ca, while characteristic minor components are other alkaline and alkaline-earth metals, As, Sb, B and sometimes S (Yardley et al., 1993; Scambelluri et al., 2004; Marsala et al., 2013;

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Miron et al., 2013; Rauchenstein-Martinek et al., 2014, 2016; Wagner et al., 2016). The concentrations

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of the base metals Pb and Zn show a remarkable correlation with both temperature and fluid salinity in

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a wide range of crustal fluids, including metamorphic fluids, magmatic-hydrothermal fluids and lowtemperature basinal brines (Yardley, 2005). This has been interpreted in terms of equilibrium solubility control, where both Pb and Zn are dominantly transported by metal-chloride species with ligand numbers from 1 to 4 (Wood and Samson, 1998; Yardley, 2005). The concentrations of Pb and Zn are typically rather low, on the order of 1–10 µg/g, but they can reach values as high as several hundreds of µg/g in more saline fluids (Yardley 2005, 2013). Sulfur concentrations broadly correlate with metamorphic grade, reaching up to 2000 µg/g in amphibolite-facies fluids (Rauchenstein-Martinek et al., 2014, 2016). Gold concentrations in these high grade Alpine metamorphic fluids are very low (< 0.03 µg/g), however, and several orders of magnitude lower than predicted by solubility calculations. Therefore, Rauchenstein-Martinek et al. (2014) have concluded that gold availability in the source rocks rather than solubility is the limiting factor for gold concentrations in metamorphic fluids. Many 30

ACCEPTED MANUSCRIPT studies which proposed metamorphic models for the formation of orogenic gold deposits have emphasized the importance of Au-enriched source rocks. They suggested that sulfide-rich

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metasediments would be the best source rock candidates for Phanerozoic orogenic gold deposits

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(Glasson and Keays, 1978; Pitcairn et al., 2010; Tomkins, 2010; Large et al., 2011), whereas mafic/ultramafic metavolcanics are the most likely sources for Archean deposits, because sufficiently

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thick metasedimentary supracrustal rock sequences underlying the orogenic gold deposits are often

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absent (Kerrich and Fryer, 1981; Groves et al., 1987; Phillips et al., 1987; Elmer et al., 2006; Goldfarb

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and Groves, 2015).

5.2. Ore fluids in orogenic gold systems and controls on aqueous Au transport

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Based on the results of conventional fluid inclusion studies, the hydrothermal fluids forming orogenic

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gold deposits from a wide range of geological environments and throughout Earth history lie in a rather

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narrow compositional window (Ridley and Diamond, 2000; Groves et al. 2003; Goldfarb and Groves, 2015). They are typically low salinity (4–7 wt. % NaCleqv) aqueous-carbonic and aqueous fluids in the system H2O-NaCl-CO2-CH4-N2. The proportions of some of the components vary considerably between deposits (Ridley and Diamond, 2000; Garofalo et al., 2014; Goldfarb and Groves, 2015), but this may at least partly reflect processes at the depositional site such as phase separation in response to pressure fluctuations (Sibson et al., 1988) or modifications via wallrock reactions (Phillips and Groves, 1983; Neall and Phillips, 1987; Williams-Jones et al., 2009). Carbon dioxide is commonly viewed as an essential constituent of orogenic gold fluids because the CO2-HCO3- equilibrium acts as a buffer that maintains pH close to the window of maximum gold solubility up to at least 350 °C (Phillips and Evans, 2004). Gold solubility in low-salinity fluids is effectively controlled by reduced gold bisulfide complexes such as AuHS0 and Au(HS)2- over a wide 31

ACCEPTED MANUSCRIPT range of pressure-temperature conditions (Stefánsson and Seward, 2004; Williams-Jones et al., 2009), and therefore sulfur loss is considered as the most effective mechanism that causes gold precipitation

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(Phillips and Groves, 1983; Neall and Phillips, 1987; Williams-Jones et al., 2009). By contrast, the base

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metals Pb and Zn are transported as chloride complexes and reduced sulfur acts as a very effective precipitant for these metals (Wood and Samson, 1998, Yardley, 2005). This different complexing

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behavior explains the chemical decoupling between gold and base metals and the extreme scarcity of

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Pb and Zn in orogenic gold deposits (Phillips and Powell, 2010; Goldfarb and Groves, 2015).

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5.3. Fluid types at Pampalo and their relation to gold mineralization Despite the considerable compositional diversity of fluid inclusion types at Pampalo, they all carry the

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hallmarks of orogenic gold fluids. From the earliest to the latest fluids, and irrespective of their

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dominantly aqueous or aqueous-carbonic composition, they all have elevated concentrations of a suite

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of elements which also characterize metamorphic fluids, i.e. K, B, As, Sb, S and W, and which are typically enriched in orogenic gold deposits. The concentrations of the dominantly chloride complexed alkali and alkaline earth metals K, Rb, Cs and Ca (Wood and Samson, 1998) vary as a function of bulk salinity (Figs. 9A, 9B, 9D), although there are some exceptions which will be discussed further below. A comparable salinity dependence can be observed for boron (Fig. 9C), especially for the earliest Type 1 fluids.

5.3.1. Early quartz vein and boudin neck fluids: Type A and Type B fluid inclusions Elevated gold concentrations were only measured in Type A fluid inclusions, which is in good agreement with the highest sulfur concentrations being present in this fluid inclusion type. The average Au and S concentrations in Type A FIA show a good positive correlation (Fig. 9F), which demonstrates 32

ACCEPTED MANUSCRIPT that Au was dominantly complexed and transported as bisulfide species in the Pampalo ore fluids. Gold was consistently below the limits of detection (0.01 to 0.03 µg/g in most FIA) in all other fluid

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inclusion types (Fig. 8), suggesting that the concentrations lie in the range that has been reported for

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metamorphic fluids unrelated to gold deposits in the Alpine orogenic belt (Rauchenstein-Martinek et al., 2014). Reliable data for Au concentrations in natural hydrothermal fluids are still rather scarce,

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particularly for orogenic gold fluids. Garofalo et al. (2014) report concentrations of 0.5 µg/g to 5 µg/g

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for the world-class Sigma gold deposit in the Abitibi Greenstone Belt, Canada, although for a small number of individual inclusions. Pettke and Diamond (2015) review the Au concentration data for

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porphyry Cu deposits and conclude that reliable data are in the sub-µg/g range. They suggest that Au concentrations in orogenic gold fluids should be in a similar range, well below 1 µg/g. Lastly, Goldfarb

D

and Groves (2015) point out that gold mineralizing hydrothermal systems need not necessarily have

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formed from gold saturated fluids, if fluid focusing and precipitation are sufficiently efficient. We can

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therefore conclude that gold concentrations of up to 0.6 µg/g found in Type A fluids at Pampalo characterize this early fluid as a viable gold ore fluid candidate, and that gold introduction is therefore linked to the formation of the quartz+K-feldspar veins and boudin neck infills. This interpretation is strongly supported by deposit-scale ore grade distribution data, which demonstrate that these early vein sets and boudin neck infills have the highest gold grades of all ore types at Pampalo. Type B fluids occur in the same early quartz veins and boudin neck infills which also host Type A fluids, but they are characterized by elevated concentrations of Ca and Sr and lower W, As and S (Fig. 8; Fig. 9B). Comparable fluid compositions are not present in the later quartz vein generations (i.e., tension gash veins and extensional quartz veins), suggesting that Type B fluids were not trapped much later than Type A fluids and are part of the same hydrothermal system. If this interpretation is accepted, the Type B fluid could represent a modified ore fluid that has extensively reacted with the 33

ACCEPTED MANUSCRIPT Ca-rich intermediate and mafic wall rocks of the main ore zone at Pampalo. Fluid-rock reactions would have resulted in K- and Na-metasomatism of the mafic and ultramafic wall-rocks and uptake of Ca and

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Sr by the fluid. This is consistent with the observed K- and Na-alteration of plagioclase in all major

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rock types at Pampalo (Kojonen et al., 1993) and the lower K and higher Ca and Sr concentrations in Type B fluids (Figs. 9B, 9E). The commonly observed tourmalinization and scheelite precipitation in

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the wall rocks would account for the lower concentrations of B and W in Type B fluid (Figs. 8, 9C).

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Wall rock sulfidation (as shown by the increase of the modal abundance of pyrite in altered wall rocks) would then have caused sulfur loss from the fluid and triggered efficient Au precipitation in the wall

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rocks, in good agreement with elevated Au grades in laterally extensive alteration envelopes

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surrounding the major ore zone.

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5.3.2. Tension gash related fluid: Type C and Type D fluid inclusions

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Variable phase proportions in aqueous-carbonic Type C-I fluid inclusions suggest that phase separation has occurred during formation of the tension gash quartz veins, possibly in response to localized pressure drops when the tension gashes opened. In contrast, C-II fluid inclusions occur in feather veins and have not undergone phase separation. Phase separation and sulfur loss to the carbonic vapor phase is an efficient mechanism of gold precipitation and has been invoked as a driving force for the formation of many orogenic gold deposits (Sibson et al., 1988; Weatherley and Henley, 2013). Despite such apparently favorable physical conditions, the compositional data for the Type C fluid inclusions show that this fluid had a rather low gold potential, most likely linked to the low dissolved sulfur concentrations (Figs. 8, 9f). The same applies to the aqueous Type D fluid inclusions which also occur in tension gash quartz.

34

ACCEPTED MANUSCRIPT 5.3.3. Late extensional quartz veins: Type E fluid inclusions Type E fluid inclusions occur in quartz veins associated with late stage barren brittle faulting and

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fracturing. Their S concentrations are as low as in Type B fluids and Au is consistently below the limits

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of detection of < 0.03 µg/g (Figs. 8). However, for Type E fluid inclusions we interpret this as a primary feature rather than to be the result of fluid-rock interaction. This is because Type E inclusions

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do not have elevated Ca and Sr concentrations and all sequentially trapped Type E FIA have essentially

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the same composition. Type E fluid inclusions therefore constitute a separate fluid type. The most striking feature of Type E fluids are their elevated Sb concentrations, which are much higher than those

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of all earlier fluid types. This supports the conclusion that Type E fluids are unrelated to the major gold mineralization at Pampalo and represent a late stage in the evolution of the hydrothermal system.

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Antimony anomalies are typical for orogenic gold systems (Kerrich and Fryer, 1981), and the highest

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Sb concentrations are found in shallow systems formed at the low pressure-temperature end of the

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spectrum of orogenic gold systems (e.g. Groves et al., 1998). In fact, it has been suggested that certain types of Sb-Hg mineralizing epithermal systems (Kuskokwim, Alaska, USA; Pinchi Belt, British Columbia, Canada) are shallow manifestations of orogenic style gold mineralization formed at greater depths (Groves et al., 1998). The latest extensional quartz veins hosting Type E fluids may therefore record a late stage of erosion and uplift of the Hattu schist belt, which brought the deposit into a low PT hydrothermal environment in the upper part of the crustal-scale shear zone.

5.4. Pressure-temperature conditions of gold mineralization The pressure-temperature conditions of formation of the three successive vein types at Pampalo are difficult to estimate. The P-T conditions and even the timing of ore formation relative to peak metamorphism are not well constrained (Sorjonen-Ward et al., 2015). Peak conditions were estimated 35

ACCEPTED MANUSCRIPT at 550±50 °C and 3–5 kbar (Kojonen et al., 1993; Sorjonen-Ward et al., 2015) based on phase equilibria of ore and alteration mineral assemblages. Kojonen et al. (1993) suggested that the main

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stage of gold formation in the Hattu schist belt has occurred at temperatures of 250-350 °C, based on

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textural observations and sulfide mineral assemblages. Conversely, microstructural data showed that porphyroblast growth and dynamic recrystallization have post-dated gold mineralization in the

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Kelokorpi prospect, leading to the interpretation that ore formation in the entire schist belt occurred

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during prograde metamorphism (Nurmi et al., 1993). The fluid inclusion data demonstrate that the hydrothermal system was still active during the retrograde part of the metamorphic cycle, but they do

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also show that the main gold mineralization is related to the earliest stage of hydrothermal fluids entering the structure. Unfortunately, the fluid inclusion data to not provide additional robust

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constraints on the P-T conditions of gold mineralization. The isochores of the early Type A fluid

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inclusions intersect the inferred peak metamorphic pressures (3–5 kbar) at temperatures between 320

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and 410 °C (Fig. 10). However, because ore formation did not necessarily coincide with the attainment of the peak pressure conditions, these temperatures give only an upper limit on the mineralization temperatures.

5.5. Fluid sources and the possible role of auriferous magmatic fluids The elemental composition data, notably the elevated concentrations of Au and S, identify the early Type A fluid as the principal gold ore fluid. The chemical characteristics of this fluid can now be compared to those of typical metamorphic and magmatic-hydrothermal fluids, making it possible to identify likely fluid sources for the gold mineralizing system at Pampalo. A distinct feature recognized in many orogenic gold deposits is the general absence of Pb and Zn base metal mineralization (Kerrich and Fryer, 1981; Phillips and Powell, 2010; Goldfarb and Groves, 2015). In orogenic gold systems, Pb 36

ACCEPTED MANUSCRIPT and Zn are commonly not enriched over average crustal values, whereas enrichment factors of Au can easily be in excess of 1000 (Phillips and Powell, 2010). The Pampalo deposit is no exception in this

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regard (Nurmi et al., 1993) and no Pb and Zn sulfides occur in the early quartz+K-feldspar veins and

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boudin neck infills. In the mineralized host rocks, both galena and sphalerite occur in small amounts, but are generally rare in all gold prospects of the Hattu schist belt (Kojonen et al., 1993). Pb tellurides

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do occur in mineralized host rock at Pampalo, but principal component analysis of lithogeochemical

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data of hydrothermally altered rocks and their fresh least altered precursors showed that Pb and Zn are not affected by significant mass transfer during alteration (Bornhorst and Wilkin, 1993; Kojonen et al.,

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1993). This demonstrates that both elements were not supplied in significant quantities by the hydrothermal fluids and that rather S and Te addition via fluid-rock reaction caused formation of the Pb

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and Zn sulfides and Pb tellurides. The compositions of all fluid types at Pampalo are in excellent

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agreement with these conclusions, with the concentrations of Pb and Zn being mostly below the limit

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of detection and only rarely exceeding 1 µg/g (Table 4; Fig. 8). These very low concentrations contrast with the high Pb and Zn concentrations on the order of several 100s to 1000s µg/g which have been reported for magmatic-hydrothermal fluids (Rusk et al., 2004; Yardley 2005, 2013; Audetát et al., 2008; Williams-Jones et al., 2010).

5.5.1. Base metals in magmatic-hydrothermal fluids Magmatic-hydrothermal fluids exsolving from crystallizing melts are the essential drivers for formation of a wide range of related ore deposit types, and extensive research has led to a firm understanding of their compositional features and the physical and chemical processes which control them (Hedenquist and Lowenstern, 1994; Campbell et al., 1995; Audétat et al., 1998; Heinrich et al., 1999; Migdisov et al., 1999; Rusk et al., 2004; Pokrovski et al., 2008; Zajacz et al., 2010). When fluid saturation is 37

ACCEPTED MANUSCRIPT reached during crystallization of a shallow-level intrusion, fluids begin to exsolve and fluid-soluble incompatible elements partition into the fluid phase (e.g. Hedenquist and Lowenstern, 1994). In

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shallow crustal settings, fluid pressure build-up causes episodic release of large volumes of fluids,

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which ascend and undergo pressure drops causing phase separation into coexisting hypersaline liquid and low-density vapor. Phase separation results in element partitioning and ultimately leads to the large

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diversity of fluid compositions that form different types of magmatic-hydrothermal ore deposits, such

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as epithermal, Carlin-type, Porphyry-Cu and skarn deposits (e.g. Heinrich et al., 1999; Landtwing et al., 2005; Sillitoe, 2010; Williams-Jones et al., 2010). The salinity of exsolving magmatic fluids is mostly a

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function of pressure, resulting in a wide range of salinities observed in single-phase, supercritical magmatic fluids (Cline and Bodnar, 1991). Zajacz et al. (2008) studied element partitioning between

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melts and magmatic-hydrothermal fluids and found that partitioning coefficients (Dfluid/melt) of Pb and

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Zn into the fluid increase linearly with chlorinity from around 10 in low-salinity fluids to 50-60 in

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highly concentrated brines. Neither melt composition nor oxygen fugacity exert any major influence on the partitioning coefficients.

These results are consistent with compositional data of other magmatic fluids, which are generally characterized by high mPb/mCl and mZn/mCl ratios (Yardley, 2005). The transport capacity for Pb and Zn in magmatic-hydrothermal brines can be increased even further through precipitation of sulfides such as pyrite and chalcopyrite, provided that the fluids initially have an excess of Fe over reduced S. Sulfide precipitation results in a sulfur-deficient fluid, which can retain high Pb and Zn concentrations down to temperatures as low as 200 ºC (Heinrich et al., 2004). The transport capacity of Pb and Zn in magmatic-hydrothermal fluids is further enhanced by the increasing dissociation of the HCl0 species in response to cooling, which makes a fluid that evolves along a fluid-buffered path increasingly more acidic and capable of keeping Pb and Zn in solution (Tagirov et al., 1997). The small 38

ACCEPTED MANUSCRIPT relative solubility differences between Cu, Zn and Pb sulfides then cause the typical metal zonation observed in magmatic-hydrothermal systems surrounding shallow-level felsic intrusions (Audétat et al.,

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2000; Sillitoe, 2010; Williams-Jones et al., 2010). All available experimental and analytical data

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demonstrate that magmatic-hydrothermal fluids derived from a wide spectrum of melt compositions are characterized by high chlorine normalized Pb and Zn concentrations, from the earliest stages of

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magmatic fluid saturation to the waning stages of distal skarn formation. Table 5 lists a compilation of

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literature data on Pb and Zn concentrations in magmatic-hydrothermal fluids, metamorphic and

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orogenic gold fluids.

5.5.2. Pb and Zn as proxies for fluid sources and gold-enrichment processes

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The well-established chemical characteristics of magmatic-hydrothermal fluids make it possible to

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quantitatively evaluate possible contributions of such fluids to the gold ore fluids at Pampalo. This, in

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turn, permits to discriminate whether magmatic-hydrothermal fluids are necessary ingredients for potent orogenic gold mineralizing systems. Large-scale mixing of metamorphic and magmatichydrothermal fluids released from granitic intrusions thought to be coeval with the orogenic gold deposits has been postulated for many Archean and Phanerozoic orogenic gold provinces, including the Hattu schist belt (Walshe et al., 2003; Neumayr et al., 2008; Molnár et al., 2016a). Figure 11A illustrates a compilation of the chlorinity normalized molar concentrations of Zn and Pb (i.e., mZn/mCl and mPb/mCl) of magmatic-hydrothermal fluids, of normal metamorphic fluids unrelated to orogenic gold deposits and of the early gold-rich and the later gold-poor fluid types from Pampalo. The figure also shows fluid compositions produced by mixing between barren metamorphic fluids and metal-rich evaporite-related fluids (Klemm et al., 2004). The difference between gold-rich and gold-poor fluids from Pampalo is remarkably small, while the Pb and Zn concentration levels of all Pampalo fluids are 39

ACCEPTED MANUSCRIPT even lower than those of normal metamorphic fluids. Conversely, all magmatic-hydrothermal fluids ranging in bulk salinity from 0.9 to 10 wt.% NaCleqv. have much higher concentrations of Pb and Zn, Pb

and

Zn

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which is even more evident when chlorine normalized values are used (Table 5). The

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concentrations of single-phase magmatic-hydrothermal fluids have been used to calculate mixing lines between a magmatic and a metamorphic end-member fluid (Fig. 11B). For the magmatic end-member,

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we have used a representative composition from the Bingham Canyon porphyry Cu deposit (Seo et al.,

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2012), while for the metamorphic end-member we have used the late-stage Au-poor fluid from Pampalo. The calculated mixing lines show that because of the large concentration differences between

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the two fluid end-members, even very small contributions of magmatic-hydrothermal fluids to a metamorphic fluid system would result in elevated concentrations of Pb and Zn in the mixed orogenic

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gold fluid, well above the values that have been analyzed for the Au-rich Type A fluid at Pampalo.

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Only if the proportion of magmatic-hydrothermal fluids is around 1 mass-%, the calculated Pb and Zn

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concentrations remain within the concentration range of Type A fluids. Reliable Au concentrations of early single-phase magmatic-hydrothermal fluids are not well established, but all available natural and experimental data indicate concentrations that do not exceed about 1 µg/g (Gammons and Williams-Jones, 1997; Seo et al., 2012; Pettke and Diamond, 2015). The low Au concentrations of around 0.03 µg/g in the late barren Type E fluids at Pampalo closely approximate those of typical metamorphic fluids, supported by the large body of Au concentration data from the Alps (Rauchenstein-Martinek et al., 2014) which lie in the same range. Mass balance would then dictate that producing an ore fluid with Au concentrations of 0.6 µg/g through mixing of metamorphic and magmatic-hydrothermal fluids would require that the magmatic end-member would contribute more than 50 % to the total fluid volume. This cannot be reconciled with the very low concentrations of Pb and Zn in the gold-rich fluid at Pampalo and not with fluid volumes that could be 40

ACCEPTED MANUSCRIPT produced from crystallizing shallow intrusions as compared to volumes released by metamorphic devolatilization. In conclusion, the fluid chemistry data for the gold-rich ore fluid at Pampalo, in

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conjunction with the mass balance calculations, demonstrate that magmatic-hydrothermal fluids were

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insignificant for this Archean orogenic gold mineralizing system and the data therefore fully support a

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metamorphic fluid model.

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5.6. Implications of the Cl/Br data

The Pampalo fluids are characterized by high concentrations of Br relative to Cl, leading to

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exceptionally low molar Cl/Br ratios in the range of 9–150, well below the value of modern day seawater (molar Cl/Br: 650). Halogens behave largely conservative during fluid-rock interaction, and

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therefore provide excellent tracers for fluid sources (Rittenhouse, 1967; Carpenter, 1978; Banks et al.,

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1991). The most important geological process that can produce fluids with very low Cl/Br ratios is

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evaporation beyond halite saturation, which results in passive Br enrichment due to Cl precipitating as halite (Carpenter, 1978). However, low Cl/Br ratios in crustal fluids were also described from geological settings which have no record of evaporite formation. In these cases, the low Cl/Br ratios of the fluid inclusions have been explained by fluid-rock interaction with Br-rich organic matter in shelf sediments, which are known to be strongly enriched in Br and I (Channer et al., 1997; Kendrick et al., 2011b; Marsala et al., 2013; Rauchenstein-Martinek et al., 2016). The halogen signature of the Pampalo fluids is difficult to explain by an evaporative origin, because the high Br concentrations would require unrealistically high degrees of evaporation (more than 99%) followed by extreme dilution in order to create low-salinity fluids with low Cl/Br ratios. Therefore, derivation of the Br from organic matter in clastic sedimentary rocks appears more plausible. Fluid inclusion data from ironoxide deposits in the Barberton greenstone belt (South Africa) demonstrated the presence of Br41

ACCEPTED MANUSCRIPT enriched hydrothermal fluids, which were interpreted as the product of interaction with halogen-rich organic matter as early as in Mesoarchean times (Channer et al., 1997). The data were taken as

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evidence that Mesoarchean seawater contained higher absolute concentrations of dissolved halogens,

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and that Br and I were selectively fractionated from Cl by deposition of organic matter rich sediments. This process would result in a decrease of Br content and increase in molar Cl/Br in the seawater over

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geological time, from Archean values around 405 to the modern day ratio of 650.

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The Cl/Br data of the Pampalo fluids show systematic differences between the gold-rich Type A fluids and the later barren fluid types. The Cl/Br ratios of the Au-rich Type A fluids are slightly higher

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than those of all barren fluid types (Fig. 8). Molar Cl/Br ratios in magmatic-hydrothermal fluids range from around 500 (close to modern-day seawater) to much higher, more Cl enriched compositions (up to

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about 1600) in some Sn-W mineralized granitic systems (Campbell et al., 1995; Yardley 2013). The

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highest values were interpreted to reflect crustal contamination (Campbell et al., 1995; Irwin and

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Roedder, 1995; Banks et al., 2000; Kendrick et al., 2001; Nahnybida et al., 2009; Seo et al., 2011). Due to very high absolute Br concentrations in the metamorphic fluids at Pampalo, mass balance dictates that any mixing process capable of creating the observed shifts in molar Cl/Br between barren Type E and Type A ore fluids would have required substantial additions of Br-poor magmatic-hydrothermal fluids, in the range of 60 % or more (Fig. 12). This is again in disagreement with the constraints from the Pb and Zn concentration data. Some authors have tentatively proposed that magmatic-hydrothermal fluids in convergent geodynamic settings inherit seawater Cl/Br ratios (Seo et al., 2011), which then could have resulted in more Br-enriched magmatic-hydrothermal fluids during the Archean because the seawater Cl/Br ratios were lower (Channer et al., 1997). If Archean seawater was more Br-rich, then magmatic-hydrothermal fluids derived from recycled seawater would have had an even weaker effect

42

ACCEPTED MANUSCRIPT on the Cl/Br ratios of the gold-mineralizing hydrothermal system at Pampalo, and would have required

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input of unrealistic proportions of magmatic-hydrothermal fluids (Fig. 12).

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5.7. Sources of fluids and precious metals for the Pampalo gold deposit

In line with our interpretation, Molnár et al. (2016b) identify early types of quartz hosted fluid

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inclusions in the Pampalo gold deposit, corresponding to Type A and B fluid inclusions of this study,

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as being of Archean age. Based on Pb, S and B isotope data, they further conclude that Archean gold mineralization was affected by local gold remobilization during the Paleoproterozoic Svecofennian

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orogenic cycle (Molnár et al., 2016a, b). Using the fluid compositional data presented in this study, it is possible to reconstruct the sources for the primary Archean gold ore fluids. The trace element

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composition and the strong Br enrichment of all hydrothermal fluids recorded at the Pampalo deposit

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clearly point to a metamorphic origin via devolatilization reactions in organic-rich metasedimentary

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rocks. Contrary to the great majority of Archean orogenic gold deposits, the Pampalo deposit is underlain by thick sequences of lithologically variable, turbiditic supracrustal metasedimentary rocks (Sorjonen-Ward, 1993b), considered as the essential Au source for many Phanerozoic deposits (Phillips and Powell, 1993; Pitcairn et al., 2006; Large et al., 2011; Pitcairn et al., 2014). In conjunction with the robust mass balance constraints arguing against significant input of magmatic-hydrothermal fluids, the metasedimentary rocks of the Hattu supersequence are likely candidates for the source of the fluids recorded at Pampalo. The compositional differences between the successive fluid types are then likely to be a consequence of fluids originating from different parts of the supracrustal pile in response to changes in pressure-temperature conditions along the metamorphic cycle. The observed differences between the fluid types cannot be just the consequence of different degrees of fluid-rock interaction

43

ACCEPTED MANUSCRIPT along the fluid flow paths or close to the depositional site, because this would not have affected their Cl/Br ratios.

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There is mounting evidence that even normal metamorphic fluids have rather high Au-

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mineralization potential, and that the limiting factor for forming orogenic gold deposits from metamorphic fluids is not Au solubility, but rather Au availability in the source rocks (Rauchenstein-

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Martinek et al., 2014). Consequently, the observation that the fluid inclusion history of the Pampalo

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deposit records just one gold-enriched ore fluid but multiple later compositionally distinct but barren fluid types points to relatively short-lived production of Au ore fluids from metamorphic

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devolatilization of specialized source rocks. By contrast, during most of the metamorphic P-T path only

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6. Conclusions

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normal Au-poor fluids were produced which had only very limited gold mineralization potential.

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Fluid inclusions of the Pampalo orogenic gold deposit record a long-lasting fluid evolution in a sequence of three major quartz vein generations. Compositionally, the fluids range from low- to intermediate-salinity aqueous to low-salinity aqueous-carbonic types. All fluids are characterized by elevated concentrations of a suite of elements considered typical for orogenic gold deposits (B, As, Sb, S, W and Au), but at the same time they have very low concentrations of Pb and Zn. The field, petrographic and fluid inclusion data made it possible to identify the earliest preserved Type A fluid as the likely gold ore fluid. Both primary and pseudosecondary fluid inclusions in the earliest gold-rich quartz+K-feldspar veins have Au concentrations up to 0.6 µg/g which are positively correlated with high S concentrations on the order of several 1000 µg/g. By contrast, all later fluid types preserved in younger veins have much lower Au concentrations of < 0.03 µg/g, comparable to normal metamorphic fluids unrelated to economic gold mineralization. 44

ACCEPTED MANUSCRIPT The fluid composition data make it possible to quantitatively evaluate a magmatic-hydrothermal versus purely metamorphic model for orogenic gold fluids. Experimental and fluid inclusion data show

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that magmatic-hydrothermal fluids are always characterized by high Cl-normalized Pb and Zn

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concentrations. By contrast, the Cl-normalized Pb and Zn concentrations in all fluid types at Pampalo are extremely low, which strongly argues against magmatic contributions to the gold mineralizing

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hydrothermal system. Mass balance calculations demonstrate that even modest contributions of

in much higher concentrations of these metals.

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magmatic-hydrothermal fluids to such an extremely Pb and Zn poor fluid system would have resulted

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The Cl/Br ratios of the Pampalo fluids are exceptionally low (molar Cl/Br: 9–150), and place the Pampalo fluids among the most Br-enriched crustal fluids reported so far. The Cl/Br data provide

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additional mass balance constraints that rule out significant contributions of magmatic-hydrothermal

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fluids, which have invariably much higher Cl/Br ratios. The unusually low Cl/Br ratios of the Pampalo

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fluids make an evaporative Br source highly unlikely, but derivation of Br from interaction with organic-matter in metasedimentary rocks during metamorphic fluid production explains the data better. The enrichment of Au in only the earliest fluid type related to the main gold mineralization at Pampalo is consistent with a model in which most of the fluids were extracted from Au-poor source regions, while golden peaks were produced when Au-enriched source rock types episodically released fluids. Our results demonstrate that gold ore fluids can be effectively produced without any contributions from magmatic-hydrothermal fluids through metamorphic devolatilization. Substantial contributions of magmatic-hydrothermal fluids are not compatible with the fluid chemistry of the gold ore fluids, suggesting that there is no genetic link between granitic intrusions and orogenic gold deposits in Archean greenstone belts.

45

ACCEPTED MANUSCRIPT Acknowledgments This study was made possible by funding from the Academy of Finland, project number 266180. We

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would like to thank Endomines Oy for their outstanding and generous support during field work

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campaigns and sampling, particularly Janne Vehmas, Ida Eriksson, Jani Rautio and Markus Ekberg. Furthermore, we thank Helena Korkka for providing excellent thin and thick sections for the fluid

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inclusion study as well as Melanie Keuper for assistance with the Raman analyses. We would also like

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as well as David Hilton for the editorial handling.

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to thank Iain Pitcairn and an anonymous reviewer for their constructive comments on this manuscript,

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ACCEPTED MANUSCRIPT Figure captions

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Fig. 1: Geological map of the Neoarchean Hattu schist belt, Ilomantsi district, Eastern Finland.

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Redrawn and modified after Sorjonen-Ward (1993a).

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Fig. 2: Geological map of the area close to the Pampalo orogenic gold deposit. Redrawn and modified

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after Sorjonen-Ward (1993a).

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Fig. 3: Field and hand specimen photographs of the different vein types and generations occurring at Pampalo, presented in sequence of increasingly younger age. A) Boudins of mineralized felsic

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porphyry (FP) dike in less competent metavolcaniclastic rock (andesitic tuff, AT). Early Au

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necks. The inset shows an euhedral quartz crystal from a boudin neck infill mineralization. B) Felsic porphyry (FP) boudin in andesitic tuff (AT). C) Barren quartz tension gash vein in metabasaltic (MB) host rock. D) Late extensional quartz vein crosscutting andesitic tuff (AT) at high angle relative to the foliation. E) Typical felsic porphyry (FP) ore sample with abundant disseminated pyrite and Au. F) Mineralized andesitic tuff (AT) showing pervasive foliation.

Fig. 4: A) High-resolution overview photograph of the thin section of a growth-zoned quartz crystal (sample PAM-32) from boudin necks in felsic porphyry dike, related to the early quartz+K-feldspar veins. Note the K-feldspar crystal on one of the successive growth zones. B) Map of fluid inclusion assemblages (FIA) showing primary (P) fluid inclusion assemblages, pseudosecondary (PS) FIA on healed fractures as well as secondary (S) FIA on microcracks crosscutting growth zones and earlier 61

ACCEPTED MANUSCRIPT trails with pseudosecondary FIA. The primary (P), pseudosecondary (PS) and secondary (S) fluid inclusion assemblages are highlighted with red, blue and green lines. C) Microphotograph of

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pseudosecondary fluid inclusion trails, clearly terminating at the boundary of the innermost growth

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zone. D) Microphotograph of a secondary fluid inclusion trail, extending into the outer part of the

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crystal.

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Fig. 5: A) High-resolution overview photograph of the thin section of a tension gash hosted quartz sample (PAM-08b). Note the presence of micro-scale tension gashes and related feather veins

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throughout the entire crystal. B) Petrographic interpretation of the structural and relative time relations of fluid inclusion assemblages in this sample. The micro-scale tension gashes host the petrographically

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oldest aqueous-carbonic fluid inclusions. A later set of healed fractures hosts similar aqueous-carbonic

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inclusions, while the latest fracture set hosts purely aqueous inclusions. The primary (P),

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pseudosecondary (PS) and secondary (S) fluid inclusion assemblages are highlighted with red, blue and green lines. C) Microphotograph of a micro-scale tension gash. D) Microphotograph of a feather vein hosting aqueous-carbonic fluid inclusions. E) Late stage healed fracture hosting aqueous fluid inclusions. F) High-resolution overview photograph of the thin section of a quartz crystal from a late stage extensional vein (PAM-01). G) Petrographic interpretation of the relative time relations of fluid inclusion assemblages in this sample. The primary (P), pseudosecondary (PS) and secondary (S) fluid inclusion assemblages are highlighted with red, blue and green lines. H) Pseudosecondary fluid inclusion assemblage terminating at a growth zone hosting abundant primary aqueous fluid inclusions. I) Pseudosecondary fluid inclusion trail hosting numerous large aqueous fluid inclusions.

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ACCEPTED MANUSCRIPT Fig. 6: Fluid inclusion microphotographs of the different fluid types. A) Pseudosecondary type A fluid inclusion assemblage. Note the regular appearance and well developed negative crystal shape of the

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inclusions (sample PAM-19b). B) Detail of a large type A fluid inclusion (sample PAM-19b, FIA 10).

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C) Secondary type B fluid inclusion assemblage (sample PAM-32, FIA 01). D) Type B fluid inclusion with a small daughter crystal of calcite (labelled Cal) to the right of the vapor bubble (sample PAM-32,

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FIA 01). E) Aqueous-carbonic type C-I fluid inclusions hosted in a micro-scale tension gash. Note the

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highly variable phase proportions within the FIA (sample PAM-08b). F) Secondary aqueous-carbonic type C-II inclusion with clearly visible three-phase assemblage of carbonic vapor (V), carbonic liquid

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Lc surrounded by aqueous liquid (L) (sample PAM-08b, FIA 12). G) Secondary aqueous type D fluid inclusion assemblage, with numerous very small inclusions (sample PAM-08b, FIA 06). H) Aqueous

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type E fluid inclusion assemblage in a quartz crystal from a late stage vein (sample PAM-01, FIA 01).

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Fig. 7: Time-resolved fluid inclusion LA-ICPMS count rate spectra. Time intervals used for signal integration and host correction are indicated by red and black bars labelled “FI signal” and “host”. A) Good ablation of a type A fluid inclusion. All element signals are perfectly synchronous with the Na peak used for internal standardization. The Au concentration in this fluid inclusion is 0.43 µg/g. B) Good ablation of a type B fluid inclusion. Note the elevated Ca and significantly lower W count rates. C) Crack formation (leaking) during ablation of a type A inclusion, resulting in incomplete sampling of the inclusion contents. Note the long, drawn out, low intensity signal due to ablation of the fluid filled fracture plane. D) Explosive breach (spallation) of an inclusion close to the sample surface. The inclusion content is expelled instantly as soon as the laser is switched on. Note the irregular signal shapes and lack of synchronous peaks for different elements. Element/Na ratios in this ablation were highly anomalous compared to the remainder of data from this FIA, and the analysis was discarded. 63

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Fig. 8: Summary of LA-ICPMS data, sorted by fluid inclusion types. Each symbol represents one FIA.

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Closed symbols represent measured average values with standard deviation of the respective FIA. The

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Cl/Br is plotted as molar ratio. The column for Type C-I comprises two FIA which are further subdivided into average liquid (L) and vapor (V) compositions reflecting the presence of L- and V-rich

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inclusions in Type C-I FIA.

Fig. 9: Correlation diagrams of fluid compositions. Each symbol represents the average value of one

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FIA and colors denote the different fluid types. A) Average K concentrations plotted as function of bulk salinity. B) Average Ca concentrations plotted as function of bulk salinity. C) Average B

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concentrations plotted as function of bulk salinity. The data for Type A fluid are located on a clear

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trend, while data for Type B fluid plot off this trend. D) Average Rb plotted as function of bulk salinity.

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E) Average Sr concentration plotted as function of Cs concentration, showing the strong enrichment of Sr in Type B fluids. F) Average Au concentrations plotted as function of S concentrations. The data for Type A FIA show a clear positive correlation.

Fig. 10: Calculated isochores (red lines) of the most Au rich Type A fluid compositions. The liquidvapor coexistence curve and the locus of the critical point were constructed for a binary H2O-NaCl mixture with 5 wt. % NaCleqv, using the equations in Driesner and Heinrich (2007). The isochores were constructed using the program LonerAP (Bakker, 2003). The two horizontal lines indicate the upper and lower limits of the peak metamorphic pressure estimate for the Hattu schist belt. Fluid inclusion trapping temperatures between these pressure conditions would have been in the range of 290–410 °C.

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ACCEPTED MANUSCRIPT Fig. 11: A) Comparison between chlorine-normalized Pb and Zn concentrations of Pampalo fluids (barren Type E and gold ore Type A fluid) and those of fluids from metamorphic and early magmatic-

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hydrothermal fluids. The data of Klemm et al. (2004) are from Alpine Pb-Zn deposits and record a

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transition from barren aqueous-carbonic metamorphic fluids to Zn- and Pb-rich mixed fluids that record ingression of evaporite-derived fluids. B) Results of mixing calculation between barren Type E

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Pampalo fluid and model magmatic-hydrothermal fluid (based on data from the Bingham Canyon

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porphyry Cu-Au deposit). Magmatic-hydrothermal contributions as low as 1 % would have a profound effect on the Pb and Zn concentrations of the mixed fluid, and their concentrations would well exceed

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those measured in the Pampalo fluid inclusions.

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Fig. 12: Results of mixing calculation between barren Type E Pampalo fluids and a range of different

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magmatic-hydrothermal fluids, labelled A through C. The calculations were done for average molar

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Cl/Br ratios of magmatic-hydrothermal fluids adopted from the compilation of Kendrick and Burnard (2013), and a model Archean magmatic-hydrothermal fluid that had a lower Cl/Br ratio. The grey shaded fields were constructed by assuming salinities from 3 to 12 wt.% NaCleqv for each magmatichydrothermal fluid composition. The horizontal line represents the average molar Cl/Br ratio of a Type A gold ore fluid inclusion assemblage (PAM-33-I, FIA 02) at Pampalo. In order to generate the Cl/Br ratio of the gold ore fluid by mixing between the barren metamorphic and a magmatic-hydrothermal fluid, 60 – 90 % of magmatic-hydrothermal fluid would be required.

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ACCEPTED MANUSCRIPT Table 1: List of studied samples. Sample names printed in bold were used for fluid inclusion study. Host rock abbreviations are FP = feldspar porphyry, AT = andesitic tuff (intermediate volcanic rock), MB = metabasalt (mafic volcanic rock) Sample

Description

Host rock

Hand specimens

PAM-10 PAM-11 PAM-12 PAM-13 PAM-14 PAM-15 PAM-16 PAM-17 PAM-18 PAM-19 PAM-21 PAM-22 PAM-23 PAM-24 PAM-25 PAM-26 PAM-27 PAM-28 PAM-29 PAM-30 PAM-31 PAM-32 PAM-33

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PAM-08 PAM-09

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PAM-06 PAM-07

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PAM-05

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PAM-04

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PAM-03

Euhedral tabular quartz crystal from a late stage extensional vein Mineralized feldspar porphyry, strongly silicified. Abundant disseminated pyrite and lesser amounts of tourmaline Early quartz-calcite-pyrite vein, cutting mineralized intermediate metavolcanic rock (andesitic tuff) with disseminated pyrite and minor tourmaline Mineralized feldspar porphyry, strongly silicified. Crosscut by thin quartz veins. Abundant disseminated pyrite and lesser amounts of tourmaline Mineralized feldspar porphyry, strongly silicified. Abundant disseminated pyrite and tourmaline Massive, barren tension gash quartz. No host rock in sample Late stage quartz-calcite extensional vein, strongly recrystallized with abundant pyrite and galena Massive barren quartz tension gash vein in mineralized feldspar porphyry Mineralized intermediate metavolcanic rock (andesitic tuff) with disseminated pyrite. Minor deformed veinlets containing quartz and calcite Intermediate metavolcanic rock (andesitic tuff), strongly deformed. Thin quartz seams with pyrite and chalcopyrite. Very strongly silicified feldspar porphyry with abundant disseminated pyrite Feldspar porphyry, silicified and containing abundant disseminated pyrite Tension gash quartz-tourmaline vein in mafic metavolcanic rock (metabasalt) Milky quartz-pyrite-chalcopyrite-chlorite veinlet, partially recrystallized in strongly silicified feldspar porphyry Early quartz-K-feldspar-pyrite-muscovite vein in strongly silicified and pyritized feldspar porphyry Early foliation-parallel, quartz-calcite-pyrite vein from contact between intermediate metavolcanic rock (andesitic tuff) and feldspar porphyry Early foliation-parallel, pure quartz vein in intermediate metavolcanic rock (andesitic tuff) Early quartz-k-feldspar-calcite-pyrite vein with late chlorite in strongly silicified feldspar porphyry with abundant disseminated pyrite Boudin neck infill in feldspar porphyry, close to contact to mafic metavolcanic (metabasalt). Large euhedral smoky quartz crystals, coated by fine grained chlorite Quartz-K-feldspar-calcite-pyrite vein in mafic metavolcanic rock (metabasalt), close to contact with mineralized feldspar porphyry dike. Very late stage vein infill of quartz, calcite, gypsum and muscovite in silicified feldspar porphyry. Very late stage quartz-calcite-gypsum-muscovite vein infill in silicified feldspar porphyry Very late stage vein infill of quartz, calcite, gypsum and muscovite in silicified feldspar porphyry. Intermediate metavolcanic (andesitic tuff) with disseminated pyrite. Barren feldspar porphyry, no Au mineralization. Strongly silicified, but distinctly less disseminated pyrite or tourmaline compared to mineralized samples. Mineralized feldspar porphyry with more disseminated pyrite than PAM-26 Mineralized intermediate metavolcanic (andesitic tuff), with abundant disseminated pyrite Very late stage vein infill of quartz, calcite, gypsum, muscovite and pyrite Small quartz-calcite-pyrite vein in strongly silicified feldspar porphyry Euhedral calcite crystal on K–feldspar, overgrown by muscovite. From early boudin neck infill in mineralized feldspar porphyry. Boudin neck infill in feldspar porphyry. Large euhedral smoky quartz crystals coated by fine grained chlorite Boudin neck infill in feldspar porphyry, close to contact to mafic metavolcanic (metabasalt). Large euhedral smoky quartz crystals intergrown with K-feldspar and coated by fine grained chlorite

AC CE P

PAM-01 PAM-02

n/a FP AT FP FP n/a n/a FP AT AT FP FP MB FP FP AT/FP AT FP FP MB FP FP FP AT FP FP AT n/a FP FP FP FP

Drill core samples PAM-817_34.2 PAM-820_67.5 PAM-822_68.3 PAM-822_80.55 PAM-822_111.5

Recrystallized tension gash quartz-tourmaline vein in mafic metavolcanic rock (metabasalt) Recrystallized early quartz-pyrite vein in pyrite-rich intermediate metavolcanic rock (andesitic tuff) Tension gash quartz-calcite-tourmaline-pyrrhotite vein in mafic metavolcanic rock (metabasalt) Quartz-K-feldspar-calcite vein in mineralized feldspar porphyry Late stage extensional quartz vein in intermediate metavolcanic rock (andesitic tuff) with disseminated pyrite

MB AT MB FP AT

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ACCEPTED MANUSCRIPT Table 2: Summary of instrumental parameters used for fluid inclusion LA-ICPMS analysis.

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Coherent GeoLas Pro MV excimer 193 nm Small volume aluminium cell (1 cm³) 12 – 15 J/cm² (adjusted for each sample) 10 Hz Circular spots. Adjusted using a mask slider array with sizes of 16, 24, 32, 44, 60, 75, 90, 120, 160 µm. Ablation spot sizes were chosen based on inclusion size and in order to maintain depth-to-diameter ratios <2 (see text). Most inclusions were shot using spot sizes > 60 µm 1.1 l/min

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Dwell times

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ThO/Th U/Th Isotopes measured

Agilent 7900s 15 l/min 0.85 l/min 1600 W Tuned and checked daily by measurement of NIST 613 against NIST 611 synthetic glass standards Tuned to < 0.4 % Tuned to ~ 100 % 7 Li 11B 23Na 24Mg 29Si 32S 35Cl 39K 44Ca 49Ti 55Mn 57Fe 63Cu 66Zn 75As 81Br 85Rb 88Sr 107 Ag 121Sb 133Cs 137Ba 182W 197Au 208Pb 5 ms, except: Na (10 ms) S, Cl, Br, Ag (20 ms) Au (60 ms)

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ICPMS Plasma gas flow (Ar) Auxiliary gas flow (He) RF power Accuracy and sensitivities

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Carrier gas flow (He)

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Laser system Wavelength Ablation chamber Energy density for quartz ablation Repetition rate Crater size

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ACCEPTED MANUSCRIPT Table 3: Fluid inclusion assemblage petrographic, microthermometric and Raman spectroscopic data. 1Method of calculation, see section 3.2 for details. 2Lc not visible due to high H2O proportions, presence inferred based on microthermometry. Petrography

Early quartz veins & boudin necks 1 33 PS PAM-19a-I 2 13 PS 3 3 S 4 3 PS 5 7 PS 6 3 PS 7 5 S

Phase Tm(Scarb) Tm(ice) Tm(Cla) Th(CO2) Th(tot) NaCleq Density Method1 (°C) (°C) (°C) LV-L(°C) LV-L (°C) (wt. %) (g/cm³) props. L90V L90V L95V L90V L90V L90V L95V

-4.6 -4.5 -6.4 -4.6 -5.0 -4.8 -6.5

Gas species FI type

PT

n

7.4 7.3 9.9 7.4 8.0 7.7 7.4

0.94 0.94 1.01 0.94 1.00 0.95 0.94

BULK BULK BULK BULK BULK BULK BULK

(N2, H2) (N2, H2) n/a n/a (N2, H2) n/a n/a

A A B A A A B

14.0 13.7 6.7 6.5 11.6 11.4 14.2

0.97 0.98 0.94 0.94 0.97 0.99 0.97

BULK BULK BULK BULK BULK BULK BULK

(N2, H2) (N2, H2) (N2, H2) (N2, H2) n/a n/a n/a

B B A A B B B

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FIA

SC

Sample

1 2 3 4 5 6 7

14 13 17 15 1 1 10

S S PS PS S S S

L90(S)V L90(S)V L90V L90V L90(S)V L90(S)V L95(S)V

-9.8 -9.2 -4.1 -4.0 -7.7 -7.5 -10.0

PAM-19b-I

6 7 8 9 10 11

14 6 6 5 5 6

PS PS PS PS PS PS

L90V L90V L95V L90V L90V L95V

-4.0 -3.4 -4.6 -3.2 -4.1 -4.8

6.5 5.6 7.4 5.3 6.7 7.7

0.94 0.93 1.00 0.93 0.94 1.00

BULK BULK BULK BULK BULK BULK

(N2, H2) (N2, H2) n/a n/a n/a n/a

A A A A A A

PAM-32

1 2 3 4

7 9 8 2

-10.9 -4.6 -4.5 -10.0

15.2 7.4 7.3 14.8

0.99 0.94 0.94 0.98

BULK BULK BULK BULK

(N2, H2) (N2, H2) (N2, H2) n/a

B A A B

PAM-33-I

1 2 3 4 5 6

6 11 14 6 16 5

PS PS PS PS PS P

L90V L90V L90V L90V L90V L90V

-3.2 -3.3 -3.1 -3.8 -3.3 -3.2

5.3 5.5 5.2 6.2 5.5 5.3

0.93 0.95 0.93 0.94 0.93 0.93

BULK BULK BULK BULK BULK BULK

n/a (N2, H2) (N2, H2) n/a (N2, H2) n/a

A A A A A A

1 2 3 4 5 6 7

21 16 9 20 11 11 14

PS PS PS PS PS S? PS

L90V L90V L90V L90V L90V L90V L90V

-3.2 -3.1 -3.1 -3.0 -3.1 -4.6 -4.0

5.3 5.2 5.2 5.0 5.2 7.4 6.5

0.93 0.93 0.93 0.93 0.93 0.93 0.94

BULK BULK BULK BULK BULK BULK BULK

(N2, H2) (N2, H2) (N2, H2) (N2, H2) n/a n/a

A A A A A A A

2 3 4 6 7 8 9 10 11

5 12 5 10 5 13 5 10 4

S S S S S S S S S

L90V L90V L95V L90V L90V L90V L90V L90V L90V

-4.4 -4.4 -4.2 -4.7 -4.6 -4.7 -4.0 -4.0 -4.7

7.1 7.1 6.8 7.6 7.4 7.6 6.5 6.5 7.8

0.94 0.94 0.94 0.95 0.94 0.95 0.94 0.94 0.95

BULK BULK BULK BULK BULK BULK BULK BULK BULK

n/a n/a n/a n/a n/a n/a n/a n/a n/a

A A A A A A A A A

PAM-822_ 80.55

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PAM-33-II

S L90(S)V PS L90V PS L90V S L90V

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PAM-19a-II

160

81

ACCEPTED MANUSCRIPT

5 16 13 5 6 8 4 9 7

P P PS S PS PS P PS PS

L85V L90V L90V L90V L90V L90V L90V L90V L90V

-56.6 -56.6 -56.8

-5.4 -5.4 -2.2 -2.8 -3.2 -2.5 -2.7 -2.6 -2.8 -3.0 -2.5 -3.3 -2.9 -2.6

26 26 24 24 28 n/o 24.2 27 n/o

150

Gas species FI type n/a n/a

A A C-I C-I C-I C-I C-I C-I C-I C-I C-I D D D

4.6 0.7 5.3 1.7 4.5 4.9 4.8 4.8 5.0 3.7 4.6 5.2

0.91 0.78 0.93 0.78 0.97 0.97 0.93 0.97 0.98 0.92 0.98 0.98

Q2 CO2, (N2) Q2 CO2, (N2) Q2 CO2, (N2) Q2 CO2, (N2) Q2 n/a ICE n/a Q2 n/a Q2 n/a ICE n/a BULK (CH4, H2,N2) BULK (CO2,CH4,N2) BULK n/a

4.2 4.5 4.4 4.7 5.0 4.2 5.5 4.9 4.4

0.87 0.93 0.93 0.93 0.93 0.92 0.93 0.91 0.93

BULK BULK BULK BULK BULK BULK BULK BULK BULK

PT

-5.0 -5.5

6.5 6.5 6.6 6.6 7.2 7.3 6.6 7.0 7.0

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-56.7 -56.7 -57.3 -57.3

SC

P* L95LcV P* L10LcV P* L80LcV P* L10LcV PS* L95LcV PS* L95(Lc)V2 PS* L80LcV PS* L95LcV PS* L95(Lc)V2 S L95V S L95V S L95V

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7 10 3 6 6 12 9 13 4 8 4 4

Phase Tm(Scarb) Tm(ice) Tm(Cla) Th(CO2) Th(tot) NaCleq Density Method1 (°C) (°C) (°C) LV-L(°C) LV-L (°C) (wt. %) (g/cm³) props. L90V -4.3 7.7 0.95 BULK L95V -4.1 6.7 0.99 BULK

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Petrography 10 S 03 S

D

12 13 Tension gash veins 1 (L) PAM-08b 1 (V) 2 (L) 2 (V) 9 11 12 13 14 4 6 8 Late extensional veins 1 PAM-01 2 4 7 8 9 10 11 12

n

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FIA

(CO2, CH4) (CO2) n/a (CO2) (CO2) n/a n/a n/a n/a

E E E E E E E E E

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Sample

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ACCEPTED MANUSCRIPT Table 4: Average elemental concentrations (µg/g) and molar Cl/Br of studied fluid inclusion assemblages (FIA). Average LOD values (calculated using Eq.6 in Pettke et al., 2012) are given for FIA in which no FI yielded concentrations above the LOD. LOD for S were rounded up to the next significant digit (see text for details). FIA n Type

PAM-19b-I

PAM-32

PAM-33-I

K

Ca

Mn

150 2000

46800 1700

360

2.2 <5.1

37 3200

46100 2300

210 <0.45 <5.5

1.7 <0.25

0.54 1800

56900 1800

370 <0.09 <1.1

2.2 <0.05

490 1100

42800 1800

580

6.5 <1.3

3.2 <0.06

38 1300

42600 1200

190

0.38 <4.4

A

7 3

A

65 740 24100 56 610 20700

8 5

A

10 2

69

1.6

0.19

<0.02

69

0.21 <0.06

<0.01

1.1

66

0.33 <0.004

0.14

150

<0.01

0.03

27

0.24

0.17

49

6.8 <0.003 <0.007

58

4.7

0.27 <0.09 4.3 <0.03

78800 850 8500 <0.47 <6.1

1.6 <0.3

490 2700

9.6

110

0.1

99900 2400 8100

4.2 <0.1

25 1500

32

500

0.04

24 <20 128600 2200 7200 <0.63 <8.1 <0.08 <0.27

75

540

3.7 <2.3

SC

RI

13

0.81 5.1

0.95 7.6 0.43

12

0.16

2.4

110 10800

25

190

2

51

5.8 <0.05 1200 1900

24

4.7 <0.005

0.42 9.2 <0.01

<5.2 <0.17

<2

1.9 <0.09

890 1400

20

2.3 <0.008

1.3 8.6 <0.01

7.2 <3.8

2.3 <0.17

3.7 1800

20

530

59 1400

91800 1100 29200 <8.3 <120 <1.8 <5.2 <7.4 2600

7.3

1500

<0.55

37 2100

38600 1900

560 <0.22 <2.9

1.8 1100 2200

18

6

0.37

1.9 2200

36800 1800

<19 <0.67

<8

1.7 <0.43 1400 1500

16

1.4

0.74

3.2

96

3 <0.23 1100 2100

15

13

0.07

6.2 5.1

3.3 <0.11 6.1

3.8 2700

40900 2400

0.77 3000

36100 1900

150

75600 1000 8900

630

440

2

0.11 <0.07 7.9 7.8

12

0.66 7.3

9.8 <0.85

40

11 <0.06

7.4

0.33

<0.04

55

0.35

3.6

0.04

<0.03

50

0.19

5.6

<0.01

<0.03

65

1.4 <0.008

<0.02

32

16 <0.04 <0.006

<0.02

103

7.2 <0.006

<0.01

108

3.8 <0.005

<0.01

93

0.79 <0.03 <0.005

<0.01

103

40500 1800

670 <0.27 <3.7

2.6 <0.21

960 1400

14

34

11 2

A

53 760 29100

670 1600

91000 1700 1800 <0.16 <2.2

4 <0.14

890 6500

12

33

1 3

B

4.4 <0.16

35 2700

47

830

<0.02 <0.08

16

2 3

A

33 210 46900 28 850 25800

680

21

7.9

<0.02 <0.08

10 <0.03

3 3

A

<0.01

4 2

B

1 4

A

32600 2300

370 <0.28 <3.6

1.9 <0.15 1100

26 880 26600 20 620 27300

0.77 2200

45600 2000 1700

1.5 <3.1

2.2 <0.13 1100 1100

18

49

0.77 1700

45700 1900

1.6 <3.5

2.1 <0.13

850 1000

16

71

27400 2300

0.75 2200 1500

2 5

160 450 19100 210 500 19700

34 1300

A

10 1400

32600 2500 2300

3 8

A

200 490 19000

40 1800

29000 2000

950

30 <0.04 <0.49 95

4 3 5 10

A

1 12

A

2 11

A

3 4

A

4 13

A

19 4.9 <0.02

2.1 7.5

0.07 <0.08 6.4

1.4

19

10

0.3

6.2 9.4

0.33

7.7

0.02

0.92

41

38

5.2

2.4 1900

950

18

12

0.03

9.4 5.7

0.96

7.5

0.54

0.41

77

2.8 <1.3

2

0.84 2100

970

17

12

0.73

11 7.3

0.38

8.1

0.12

0.32

67

0.22

10

80

0.1

260 1700

76 3200

<0.37

0.09

A

18 <4.1

95

<0.17

5.2

<11 <0.38 <4.4

560 123300 2700 10000

0.26

0.13

46900 1700

<0.02

6.2

3.5 <0.03 <0.005

740

49

1.7

<0.03 <0.19 9.2 <0.07 <0.04

94 920 27900 59 660 25000

A

PAM-33-II

38

2.3 <0.002 <0.005

3.5

400 1400

NU

6 12

37

0.1 <0.004

0.03

1.8 <0.24

MA

210 <17 23200

59

0.83 6.5

1.4 <0.03 5.7 <0.01

B

B

1.5 <0.02

Au

8.8

1 6

6 3

0.12

5.1 <0.009

W

14

23 730 29100 1400 1400

62 35400

Cl/B r

Ba

410 1400

B

85

Pb

Cs

11 <0.004

7 3

B

770 1800

Sb

13

6 3

5 3

Ag

780 3400

A

4 11

Sr

0.25 <0.14 6.9 <0.04

95 1000 30100 17 720 29400

A

Rb

0.29

A

61 650 24500 59 650 24100

1.2

Br

5.1

5 3

B

2

As

7.5

A

A

Zn

15

4 2

3 9

Cu

16

A

2 3

Fe

810 2800

2 9

160 230 44500 160 330 47000

Cl

PT

S

CE

PAM-19a-II

99 830 27100 92 700 27800

Mg

PT ED

Li B Na Early quartz veins & boudin necks A PAM-19a-I 50 860 27800 1 17

AC

Sample

150 620 22900 <0.01 4800 87 1400 250 460 19900

38700 2000 1200 <0.08 <0.99

1.3 <0.04 1800 2800

16

<0.00 1.2 <0.005 <0.01 7.5 7

6.9 <0.002 <0.003

31

28600 2300

62

6.2

1.9

3.9 2300 1200

27

16

0.93

26

13

1.2

8.5

0.03

0.74

54

94 330 19100 300 210 19600

93 2200

34900 2300

570

3.7 <2.1

2.6

0.59 2300 2100

20

18

0.25

7.8 7.6

0.79

9.3

0.18

1.1

37

12 1800

32400 1100

370

1.8 <1.3

1.5

0.98 2000 2300

11

8.2

0.12

7.7 3.5

0.42

9.9

0.05

2.9

32

530 430 19000 <0.02 1100 49 1200 250 360 18900

25800 1900 22200 1100

1.2

<4.8 <0.14 <1.5 55

0.95

25

1.5 <0.07 1900

850

19

7.9 <0.008

31

8

0.37

8.3

0.04

0.19

68

1.5

770

11

11

10 5.2

0.43

8.6

0.11

2.2

65

2.1 1700

0.17

83

ACCEPTED MANUSCRIPT Sample

FIA n Type Li

5 6

A

6 6

A

B Na Mg S 140 560 18900 <0.02 1500 16 2300 150 700 27800

Ca Mn Fe 51 <0.14 <1.5

Cu 6.8

Zn As Br 0.67 2000 1700

Rb 18

Sr 13

Ag 0.01

Sb Cs 9.6 7.6

Ba 0.68

W Au 7.4 <0.002

54600 1700

95 <0.11 <1.4

2.8

1.7 1100 2400

16

8.9

0.94

1.4

0.44

2.7

0.03 <0.006

300 460 24200

51

63 3800

40700 1800

480

0.7 <1.9

2.5 <0.09 1500 1900

19

6.8

0.17

2.4 7.7 <0.02

6.9

0.59

48

53 240 19 1100 28300 23 820 23700 <0.04 <20

57000 2200

130 <0.2 <2.8

4.2 <0.12 5500 5400

17

160

<0.01

24

11

22

4.8 <0.005 <0.009

40800 2500

160 <0.67 <8.6

3.9 <0.38 4400

710

28

150

<0.04

22

19

10

9.2

6.8 8.2

6

PAM-822_

6 6

A

80.55

9 1

A

10 5

A

360 <0.39 <5.1

1.1 <0.23 4800 4300

13

89

<0.02

A

85 360 21 600 23700 29 900 28700 <0.07 <20

50900 2500

11 4

32100 2700

<20 <0.6 <7.7 <0.1 <0.29 6700 3600

15

120

0.55

90

12

18

12 2 Tension gash veins PAM-08b 1L 3

A

18 1000 28200

4.2 <10

41400 2700

<13 <0.39 <5.2

20

110

<0.02

56

11

14

8700 100 2500 76

<32 <1.6 <54 <1.8

3.1 0.57

0.14 0.14

<0.11 <0.13

36 1.8 <0.18 <0.17 36 1.2 <0.33 <0.23

2L 3

C-I 22 640 14200 <0.16 250 C-I <0.83 240 2700 <0.29 250 C-I 190 1300 20000 <0.09 <20

8500 760

<12 <0.39 <5.7 <0.1

2V 6

C-I

7300 140

<13 <0.45 <6.3 <0.12

6 3

D

8 2

D

170

55 17500

290 <30

28000 420

RI

SC

NU

1.2 <0.21 7000 3700

<19 <22

0.72 <1.1 0.72 1.2

MA

82 590 6200 <0.08 <20 41 620 710 98 16900

111 74

0.31 1500

310 <0.06 <0.008

<0.03 <0.12 1.7 <0.04 <0.04

<0.01

<0.02

62

0.41

360

1.9 <0.009

<0.03 <0.15 2.7 <0.07 <0.03

<0.01

<0.03

46

<0.05 <0.24 1.5 <0.09 <0.1

<0.03

0.26

49

<0.08 <0.35

<0.04

<0.09

27

<0.02

<0.06

15

6.1 <0.006

<0.01

28

690

3.2

15

<0.05 <0.23 2.5

46

0.41

300 1100

7.3

32

<0.02 <0.07 4.7

91

0.32 1200 1600

4.7

24

<0.03

29 2.6

3.4 <0.05

<0.02

<0.05

22

3.6

18

<0.23

1.1 2.4

52

6.6

<0.11

<0.25

19

<18 <0.62 <6.5 <0.09

0.42 1100 1800 <0.05

16

<0.03 <0.16 1.8

<0.1 <0.03

<0.01

<0.01

25

<15 <0.44 <5.9

190 910 19100 <0.04 <20

20200 810

120 640 15000 <0.05 <10 99 560 16400 200 670

28300 2000

CE

16600 500

25900 1800

AC

E

<0.11 <0.15

870 2300

14 1 C-II Late extensional veins E PAM-01 1 3

11 5 12 1

<0.05 <0.06

0.55

3

10 2

25

<12 <0.24

<16 <0.55 <6.8

9 7

<0.02

130

15900 150

E

20

5.9 <0.008

4.4

130 330 18800 <0.1 1100 160 390 18500 <0.41 <130

E

<0.03

0.79 <1.5 3600

12 5 C-II 13 11 C-II

8 6

27

<0.01

<17 <0.29 6.1

E

<0.02

5.5

42800 460 2400 <1.1

<8 <0.22 <3.9

7 3

3.9 <0.009

64

13600 710

E

130

2.7

15600 560

4 11

<0.04

0.6 <0.99 1300

340 280 17000 <0.17 <20 8.9 260 140 560 18700

E

180 76

24

<0.01

<11 <0.19

9 3 C-II 11 7 C-II

2 12

980 650

7

1.3

37

630 <0.78

PT ED

1V 1

PT

7 8

A

Pb 0.89

Cl/B r

Cl K 27800 2100

340 <0.79

62 <3.9

<45

2.9

1

510 2000

2 <0.15 <0.18 4

2.2

0.45

660 4000

18

87

<0.04

99 8.9

13

12

<0.02

<0.04

16

3.3

7.9

2

0.2

670 4100

18

82

0.29

80

11

16

13 <0.007

0.43

14

85 <0.78

<10

1.2

1.8

620 3100

22

75

0.13

90

14

17

12

<0.03

<0.06

20

690 <0.29 <3.9 <0.06

0.2

450 5200

18

28

<0.02

13

11

27

23

<0.01

<0.02

18

<6

86 190 100 740 15700 83 1100 17800 <0.03 <10

27600 2200

160 730 17200 <0.04 1.5 130 900 14900

60

24900 2200

40 <0.24

40

1.7

0.18

820 2600

24

69

<0.02

120

13

11

14 <0.008

0.19

22

650

24700 2100

<28 <0.79

<11

2.1

0.6

470 1600

24

73

<0.09

95

12

18

11

<0.1

35

140 400 20200 <0.02 <10 11 <16 140 780 18200

37600 2000

<4 <0.13 <1.7

2.1

0.08

700 9000

21

64

<0.01

47 9.5

12

8.4 <0.004 <0.008

E

31900 1800

<18 <0.52 <6.9

2.9

0.52

570 5200

26

90

<0.06

100

14

19

12

<0.02

<0.03

14

E

120 890 16000 <0.03 <10

25600 1700

<0 <0.43 <5.2

1.3 <0.24

630 4100

26

96

<0.03

130

16

16

16

<0.02

<0.03

14

1.5 2100 2200

42500 970

<0.03

9

Average compositions by fluid type Type A Type B Type C-I Type C-II

120 670 24200 99 360 36200

93 1900 250 1000

98 690 10800 <0.16 8.9 190 490 18400

40700 2000

530

2

30

92000 1700 10400

7.6

<21

2.9

2.9 <0.9

17

30

0.57

14

8

3.8

5.9

0.17

0.98

42

250 3300

22

530

0.08

3.1

10

5.5

0.33

<0.03

0.38

63

250

6800 270

<28 <1.1

<13

0.72

0.64

960

230

1.9

0.14

<0.08

36 1.9 <0.16 <0.12

<0.03

<0.08

67

680

16400 550

200 <1.2

<15

4

0.54

800 1800

4.7

21

<0.07

15 2.8

<0.03

<0.08

21

48

5.6

84

ACCEPTED MANUSCRIPT Sample

FIA n Type

Type D

440

B Na 77 17200

Mg 170

S 620

Cl K Ca Mn 35400 440 1500 <0.94

Type E

120 750 16800

75

390

29900 1900

3.3

24

1.9

Zn As Br 0.7 <1.2 2500

Rb 3.6

Sr 97

0.5

22

74

620 4300

Ag Sb Cs Ba W <0.07 <0.3 1.8 <0.12 <0.14 0.21

86

12

17

13

Au <0.04

Pb 0.26

Cl/B r 32

<0.02

0.31

16

CE

PT ED

MA

NU

SC

RI

PT

270

Fe Cu <14 <0.24

AC

Li

85

ACCEPTED MANUSCRIPT Table 5: Compilation of average Pb and Zn contents in single phase magmatic-hydrothermal and metamorphic fluid systems, given as absolute and chlorinity-normalized values. The average composition of the early and late Pampalo fluids is given for reference. Asterisks indicate where analytical limit of detection values were used to constrain maximum values. Zn [µg/g]

Reference Min

Magmatic-hydrothermal fluids Bajo de Alumbrera (porphyry Cu-Au)

Ulrich et al. 2001

Max

Pb [µg/g]

Average

Min

log(mZn/mCl) log(mPb/mCl)

Max

Average

PT

Locality

200 1300 700 ± 540

90

1600

500 ± 730

-2.01 ± 0.3 -2.85 ± 0.56

420 1100 590 ± 160

190

1000

480 ± 150

-2.20 ± 0.1 -2.78 ± 0.11

Seo et al. 2012 Rusk et al. 2004, 2008

28

120 ± 43

3

58

21 ± 12

-2.41 ± 0.2 -3.71 ± 0.25

El Teniente (porphyry Cu-Mo)

Klemm et al. 2007

25 1600 360 ± 410

8

240

73 ± 67

-2.30 ± 0.23 -3.52 ± 0.27

Questa (porphyry Mo)

Klemm et al. 2008

860

2320 14800 Williams-Jones et al. 2010 Audétat et al. 2008

Rito del Medio Pluton (not mineralized)

Klemm et al., 2004

AC CE P

Pampalo fluids Type A (PAM-33-I FIA 02 average) Type E (PAM-01 FIA 01 average)

150 11 ± 16.3

D

Mixed metamorphic-evaporitic fluids (Binn valley)

Rauchenstein-Martinek et al. 0.04 2014, 2016; Miron et al. 2013 Klemm et al., 2004

TE

Metamorphic fluids (Binn Valley)

MA

Audétat et al. 2008

Metamorphic fluids Alpine fluids

8300 ± 490 3 5300 820 ± 1800 740 1800 1400 ± 430 19 690 410 ± 320

NU

Mole Granite (Sn-W)

Audétat et al. 2008

this study this study

SC

190 2490 770 ± 610

El Mochito (Zn-Pb-Ag skarn)

Stronghold Granite (Zn-Pb-Cu)

220

RI

Bingham Canyon (porphyry Cu-MoAu) Butte (porphyry Cu-Mo)

0.9 0.08

4.8 1.8

280 ± 220

-2.07 ± 0.26 -2.99 ± 0.32

300

16

7530 3100 ± 3000

-1.20 ± 0.28 -2.45 ± 0.61

0.2

1700

270 ± 540

-2.21 ± 0.28 -3.14 ± 0.49

130

540

240 ± 130

-1.71 ± 0.09 -2.98 ± 0.23

13

250

130 ± 100

-2.36 ± 0.55 -3.26 ± 0.41

0.01

420

4.6 ± 20.8

-3.74 ± 0.6 -5.19 ± 0.72

32

5

-2.68

-3.99

6550

10400

-1.75

-2.06

2.2 ± 2 0.004 0.51 0.7 ± 0.5 <0.01* <0.06*

0.4 ± 0.1 0.04*

-4.39 ± 0.3 -5.79 ± 0.9 -5.08 ± 0.4* -6.62 ± 0.1*

86