The Quebrada del Diablo Lower West Au deposit (Gualcamayo mining district, Argentina): A Carlin-type mineralization?

Accepted Manuscript The quebrada del diablo lower west au deposit (gualcamayo mining district, Argentina): A carlin-type mineralization? María Celeste D'Annunzio, Nora Rubinstein PII:

S0895-9811(18)30401-2

DOI:

https://doi.org/10.1016/j.jsames.2019.03.010

Reference:

SAMES 2135

To appear in:

Journal of South American Earth Sciences

Received Date: 1 October 2018 Revised Date:

11 February 2019

Accepted Date: 11 March 2019

Please cite this article as: D'Annunzio, Marí.Celeste., Rubinstein, N., The quebrada del diablo lower west au deposit (gualcamayo mining district, Argentina): A carlin-type mineralization?, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/j.jsames.2019.03.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT THE QUEBRADA DEL DIABLO LOWER WEST AU DEPOSIT (GUALCAMAYO MINING DISTRICT, ARGENTINA): A CARLIN-TYPE MINERALIZATION?

INGEOSUR- CONICET- Universidad Nacional del Sur- Departamento de Geología.

San Juan 670, Bahía Blanca. Argentina. 2

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María Celeste D´Annunzio1* and Nora Rubinstein2

IGEBa, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y

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Naturales, Pabellón 2, Ciudad Universitaria. Ciudad Autónoma de Buenos Aires. Argentina.

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* Corresponding author. E-mail address: [email protected]

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ACCEPTED MANUSCRIPT ABSTRACT The Gualcamayo Mining District (GMD), located in the eastern margin of the Argentinine Precordillera

includes three Au deposits: Amelia-Ines, Magdalena ,

oz. and resources of 2.3 million oz of Au.

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Quebrada del Diablo Main, and QDD Lower West. The district has reserves of 491,000

The metallogenetic analysis of these Au deposits based on previous work and new

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micro-geochemistry (LA- ICP- MS), isotopic (C-O-S-Pb) and fluid inclusions studies revealed that they are genetically linked and share many characteristics with Carlin-type

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deposits, including the lithology of host rocks, structural control, geochemical signatures, ore paragenesis, and dissolution and decarbonization processes. However, they also display some noticeable differences. The sulfur isotopic composition supports a genetic link between these gold deposits and the precursor magmatism of the

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porphyry-type mineralization of the area. Besides, the fluids responsible for the Au mineralization proved to be neutral to slightly alkaline and Au-saturated. The link with the magmatism along with the character of the mineralizing fluids and the presence of

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significant base metal sulfides allowed classifying the GMD deposits as “distal disseminated gold deposits” that occur on the distal edges of magmatic-hydrothermal

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systems.

Keywords: structural control, host rocks, character of the mineralizing fluids, magmatic origin, distal disseminated Au deposit, Precordillera.

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ACCEPTED MANUSCRIPT 1. Introduction The Gualcamayo Mining District (GMD) is located in the eastern margin of the Argentine Precordillera, immediately to the east of Principal and Frontal Cordillera (Fig. 1 a). This mining district is hosted by a complex structural block of

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Cambrian/Ordovician carbonate sequences deformed by the Andean E-W compression which formed the Precordillera.

The Precordillera is a narrow N-S trending thrust-and-fold belt of tectonically

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deformed clastic and carbonate rocks of lower to mid-Paleozoic age, overlain by

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Carboniferous and Permian marine and continental sequences, Triassic volcanic and continental deposits, and Tertiary continental redbeds. The outstanding stratigraphic feature of the Precordillera is the thick Cambrian and Lower Ordovician limestones (Keller et al., 1994), which contrast markedly with the coeval clastic and volcaniclastic successions present in the rest of the South American margin. These carbonate deposits

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and the presence of Laurentian trilobite faunas have led to consider the Precordillera, as a rifted-drifted microcontinent, accreted to the Gondwana margin at ~460 Ma (Rapalini

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and Cingolani, 2004).

During the Miocene, the Precordillera was affected by subduction-related

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deformation (Andean Orogeny) that generated a high-level fold and thrust belt with crustal shortening of 60 to 90%. Major N-S trending thrust faults horizontally displaced more than 100 km to the east lower Paleozoic rocks and superimposed over Tertiary continental redbeds (Jordan et al., 1993). The GMD includes three Au deposits Amelia-Ines, Magdalena (AIM), Quebrada del Diablo Main (QDD Main) and Quebrada del Diablo Lower West (QDD Lower West).. Mining operations at the district began in 2009 and during this year the mine produced 90,000 oz of Au. In 2017, Au production increased to 154,052 oz., and the 3

ACCEPTED MANUSCRIPT mine reached a total production of ~ 1,000,000 oz of Au. The district has reserves (proven and probable) of 491,000 oz. of Au and resources (measured and indicated) of 2.3 million oz of Au (Yamana Gold, 2017). Here, we present new micro-geochemistry (Laser ablation-inductively coupled

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plasma-mass spectrometry, LA- ICP- MS), isotopic and fluid inclusion data that allow building a metallogenic model for the QDD Lower West deposit. These data, together with the previous information referred to the other Au deposits of the mining district,

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provide new insights into the genesis of the whole Au mineralization in the GMD.

2. Regional geology.

The basement of the Precordillera is formed by non-outcropping Grenvillian metamorphic rocks in amphibolite and granulite facies (Kay and Abbruzzi, 1996).

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Overlying this metamorphic basement, there are Lower Cambrian and Lower Ordovician carbonate sequences deposited in a passive margin (Bordonaro, 1986). Tremadocian transgression of global extension (Keller, 1999) flooded the carbonate

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platform depositing limestones, skeletal wackestones, and packstones in a deep subtidal

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environment (Fig. 1 b).

A thick succession of Middle Ordovician black shales overlaid the carbonate

platform. From the Middle to the Upper Ordovician, a relative drop in the sea level on a global scale and an increase in the tectonic activity of the basin, allowed the deposition of slope sequences, formed by calcareous conglomerates (mega- turbidites), megaolistoliths and megabreccias deposited by debris and turbidity flows (Fig. 1 b) (Astini, 1998a).

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ACCEPTED MANUSCRIPT During Silurian and Devonian times there was a transgressive event that deposited a succession of mudstones and shales which was followed by the Chanic Orogeny (Upper Devonian to Lowest Carboniferous) that formed the Paganzo basin (Carrera et al., 2013) (Fig. 1 b).

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From the Middle to the Upper Miocene, arc magmatism produced a large volume of subvolcanic rocks that reflect the eastward expansion of the arc. The broadening of the arc is linked to the collision of the Juan Fernández Ridge (Yañez et

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al., 2001) that led to the shallowing of the subducting plate and a change in the character of the magmatism from calcoalcaline to adakitic as a result of the progressive crustal

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thickening beneath the arc (Kay and Mpodozis, 2002). All the sequence is covered by

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thick Pliocene and Pleistocene alluvial fan deposits (Ciccioli et al., 2010; Fig. 1 b).

Figure 1. a) Regional location of the Gualcamayo Mining District. b). Representative stratigraphic column of the Precordillera. The stratigraphic column was built based on 5

ACCEPTED MANUSCRIPT the works of Kay and Abbruzzi (1996), Bordonaro, (1986), Keller (1999), Astini (1998), Carrera et al., (2013), Yañez et al., (2001), Kay and Mpodozis (2002) and

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Ciccioli et al., (2010).

3. Tectonic framework

The prolonged history of convergence in the Gondwana Pacific margin resulted

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in several episodes of contractional, extensional, and strike-slip deformation. The overprinting relationships between different structures in the Precordillera preserve

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evidence of at least four deformational events that took place during Early Paleozoic, Early Permian, Late Permian–Middle Triassic, and Miocene–Holocene times (Giambiagi et al., 2011).

The Early Paleozoic tectonic history of the South American southwestern

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margin was mainly controlled by subduction and accretion of exotic terranes. At 29° S, this deformation is characterized by two orogenic events; the first one (Ocloyic Orogeny; Fig. 1 b) occurred during the Middle to the Late Ordovician and is linked to

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the collision of the Cuyania terrane (Vujovich et al., 2004) whereas the second one occurred during the Devonian (Chanic Orogeny; Fig. 1 b) as a result of the Chilenia

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accretion (Davis et al., 2000).

The late Paleozoic tectonic cycle began with the subduction inception along the

present continental margin after the collision of the exotic terranes. In Early Permian times, a widespread contractional event (San Rafael Orogeny, generated a wide orogenic belt that underwent an extensional collapse by the Late Permian (Mpodozis and Kay, 1990).

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ACCEPTED MANUSCRIPT From the Late Permian to the Early Cretaceous, there was a generalized extension in the Gondwana land forerunner to the fragmentation of this supercontinent and the opening of the South Atlantic Ocean. This extensional regime led to the

margin of Gondwana (Legarreta et al., 1993).

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formation of a set of rift basins with an overall N-NW trend, along the southwestern

By the Upper Cretaceous, when the Mesozoic extensional period was over, a major plate tectonic reorganization took place. By this time, the compressive Andean

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cycle and the inception of a subduction regime started (Mpodozis and Ramos, 1989). Particularly between 28 and 33° S) the collision of the Juan Fernández Ridge by 22 Ma

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led to the shallowing of the subducting plate. Owing to the onset of the slab flattening the orogenic front migrated eastward from the Principal Cordillera to the Frontal Cordillera. Between 15 and 9 Ma, the deformation shifted to the western Precordillera at a high propagation rate as a result of a peak of the shallowing (Ramos et al., 2002).

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Shortening started during the middle to late Miocene. The active thrust front is now located on the eastern border of the Precordillera, where Quaternary strata are involved

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in the Andean deformation (Giambiagi et al., 2003). 4. District geology and structure

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The oldest rocks of the GMD area are limestones (Los Sapitos and San Juan

Formations; Fig. 2 a) deposited in a marine platform during the Late Cambrian to the Early Ordovician (Albanesi et al., 1998). These rocks are overlain by a succession of Middle Ordovician shales and thick deltaic deposits of Upper Ordovician age consisting of coarse siliciclastic conglomerates and sandstones (Gualcamayo Formation and Trapiche Group; Fig. 2 a) (Astini, 1994). The lower Paleozoic deposits are discordantly overlaid by post-glacial transgressive facies and a long-lived sequence of paralic sedimentary facies (Paganzo Group; Fig. 2 a) (Limarino et al., 2010). 7

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Figure 2. a) Geology of the Gualcamayo mining district. b) Kinematic interpretation of the structure. c) Close up of (a) showing the location of the mineralized bodies. Taken from D´Annunzio et al. (2018).

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Figure 3.W- E cross-section showing the geological units, faults, and mineralized

(2009).

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bodies. See location in Figure 2 c. Compiled from Bruno (2005) and Soechting et al.,

The Paleozoic sequence is intruded by the Gualcamayo Igneous Complex (Fig. 2

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a and b), which consists of subvolcanic dacite intrusives with porphyry-type

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mineralization (Las Vacas, Varela, El Rodado and Dacitic dikes) of late Miocene age (D´Annunzio et al., 2018). Las Vacas dacite is a sill intruded shales and sandstones of Carboniferous-Permian age consisting of plagioclase, quartz, amphibole and biotite phenocrysts in a highly altered groundmass. It exhibits strong pervasive carbonatization and sericitization, carbonate veins and weak chloritic alteration. The Varela and El Rodado dacites intrude the Lower Paleozoic limestones developing irregular skarn halos. These rocks are composed of plagioclase, quartz, biotite, amphibole and pyroxene phenocrysts in a highly altered groundmass. They have moderate pervasive

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ACCEPTED MANUSCRIPT potassic alteration with an assemblage of K-feldspar, biotite, magnetite, and quartz and thin quartz ± K-Feldspar veins. The Dacitic dikes intrude the Paleozoic sequence with E-W and NW-SE strikes. These rocks are composed of plagioclase, quartz and biotite phenocrysts in a felsitic groundmass and display moderate pervasive and in veinlets

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potassic alteration with an assemblage of K-feldspar, biotite, quartz, and magnetite. Strong carbonatization occurs pervasively and in veinlets (D´Annunzio et al., 2018).

The dominant structure underlying the GMD area is a shallow east dipping

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detachment structure, which juxtaposes the upper part of the San Juan Formation against the Trapiche Formation. This structure is interpreted to be a back thrust of an

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age similar to the Andean west dipping thrusts (Simpson, 2006). NW-trending, Wvergent folds are common within the hanging wall of this detachment structure forming a unique structural domain compared to the lesser deformed W-dipping carbonates in the footwall. Besides, first order NWt-trending folds are overprinted by second-order E-

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W trending folds and related brittle faults. The later folding is interpreted to be the result of dextral rebound along pre-existing NW trending sinistral faults that extend as much as 300 meters outboard into thin-bedded limestones of the Upper San Juan Formation.

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This second-order folding produces a dome and basin geometry of carbonate beds along

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the southwest margin of the Varela Dacite (Simpson, 2006). In the GMD, the fault systems follow N-S, WNW-ESE, NNW- SSE, and NNE-

SSW trends (Fig. 2 a). The N-S trend is represented by overthrusts, with deflections in the strike. One of these main structures is the La Silla-Alaya overthrust which extends over 100 km and represents the thrust front of the Precordillera (Fig. 2). The La SillaAlaya overthrust (Fig. 2 a) is cut by the Tamberías fault, a sinistral strike-slip fault trending WNW/ESE (Fig. 2 a). The NNW-SSE trend is represented by high angle to

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ACCEPTED MANUSCRIPT vertical inverse faults, like Quebrada del Diablo.The NNE-SSW trend is formed by strike-slip faults. The kinematic analyses of the local structures reveal an E-W maximum stress

5. Au deposits of the Gualcamayo mining district

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(σ1) perpendicular to the N-S thrust front and an N-S maximum extension (see Fig. 2 b)

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As mentioned above, the GMD includes three Au deposits:AIM, QDD Main and

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QDD Lower West deposits. Geochemical signatures in GMD Au deposits include S, Au As, Sb, Hg, Tl, Ag, Ba ± W ± Te ± Se.

AIM (Figs. 2 a and c) is a NW-SE mineralized body consisting of veinlets and tectonic breccias. The veinlets and the cement of the breccias are formed by pyrite,

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chalcopyrite, sphalerite, arsenopyrite, Au, calcite, and quartz (Simpson, 2006) which overprints a skarn mineralization. The skarn involves a prograde paragenesis of quartzdiopside- garnet- magnetite with pyrrhotite and chalcopyrite which is partially replaced

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by a retrograde paragenesis of tremolite-phlogopite-feldspar- (quartz- smectite- zeolite) (Logan, 1999). It is located over a strike-slip fault to the north of QDD Main.

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QDD Main (Fig 2 a and b) is a Au deposit hosted mainly in carbonate breccias

and to a lesser extent in polymictic breccias forming a mineralized body elongated in the W- E direction, located over Quebrada del Diablo fault (Fig. 3). Three alterationmineralization stages have been recognized in QDD Main (Bruno, 2005). The first one (alteration) led to dolomitization, silicification, argilitization, decarbonization and carbon enrichment of the host rocks. The second stage (mineralization) is formed by pyrite- marcasite- orpiment- realgar- Au- arsenical pyrite- chalcopyrite- sphaleritegalena and cinnabar with calcite- barite- quartz and gypsum as gangue minerals. The 11

ACCEPTED MANUSCRIPT third stage corresponds to supergene alteration and is characterized by the presence of hematite and limonites. Based on the composition of the host-rock, the ore and alteration assemblages and the characteristics of hydrothermal fluids, this deposit has been interpreted as a “sediment-hosted disseminated Au deposit” (Bruno, 2005).

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QDD Lower West is a blind Au deposit with an ore grade of 2.85 g/ton (Yamana Gold, 2017) located at 500- 600 m of the surface, to the west of QDD Main (Fig. 2 a, b and Fig. 3). This mineralized body has an irregular shape with 500 m E-W in length,

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100 to 150 m in width and ~150 m in thickness ( Soechting et al., 2009 and references therein) with the mineralization hosted in the cement of tectonic breccias (D’Annunzio

Geology of QDD Lower West.

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6.

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et al., 2014; D’Annunzio and Rubinstein, 2013).

The mineralization of QDD Lower West occurs as tectonic breccia cement. These tectonic breccias have both a fluid- and a not fluid- assisted tectonic origin. The

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fluid-assisted tectonic breccias are polymictic and clast-supported (15 to 30 % matrix)

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and have no evidence of dissolution of carbonate clasts. The not fluid- assisted tectonic breccias are polymictic and matrix-supported. The matrix (> 80%) is composed mainly of syntectonic illite with preferential orientation, typical of fault rocks formed in a fragile tectonic regime (D’Annunzio et al., 2015, 2014). Three alteration-mineralization stages have been recognized in QDD Lower West (D’Annunzio et al., 2017) and are summarized in Figure 4.

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Figure 4. Paragenetic sequence of QDD Lower West deposit. *Inclusions in Pyrite.

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(Modified from D’Annunzio et al., 2017).

The first stage consists of a fine-grained (~50 µm) intergrowth of pyrite, arsenopyrite, chalcopyrite, and sphalerite mainly conforming the cement of the tectonic

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breccias (Fig. 5 a). This fine-grained intergrowth of sulfides also occurs in veinlets and

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massive aggregates in the host rock (Fig. 5 a).

Pyrite is the most abundant mineral and forms cubic or tabular crystals with

variable contents of Pb (1300- 1950 ppm) and Bi (1200- 1990 ppm) which were interpreted as submicroscopic inclusions of scheelite, galena, and sobiyite. Arsenopyrite occurs as isolated acicular crystals (<10 µm). Arsenopyrite occurs as isolated acicular crystals (<10 µm). Chalcopyrite appears in isolated crystals (~100 µm) and intergrown with pyrite. Sphalerite occurs as intergrowth with pyrite or as isolated crystals (>50 13

ACCEPTED MANUSCRIPT µm). Electron probe microanalyses (EMPA) of sphalerite revealed high values of Au (3800- 5900 ppm) and Pb (1600 ppm) which were interpreted as Au and galena submicroscopic inclusions. FeS moles% in sphalerite indicate intermediate sulfidation conditions for the fluids of this mineralizing stage (D’Annunzio et al., 2017). These

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sulfides are spatially associated with fine-grain sparitic calcite (I) and subordinate microcrystalline aggregates of quartz (I). Pyrite and chalcopyrite of the first stage are partially replaced by hypogene marcasite (Fig. 5 b) and covellite respectively suggesting

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an increase in fS during this mineralization stage (D’Annunzio et al., 2017).

Figure 5. First mineralization stage of QDD Lower West deposit. a) Drill cores showing

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different occurrences of first mineralization stage. Br: breccia with sulfide cement. Sul: fine-grained sulfides conforming massive aggregates in the host rock. Ven sul: veinlets consisting of fine-grained sulfides. b) Marcasite (Ms) replacing pyrite crystals (Py).

The second alteration-mineralization stage consists of barren zoned calcite veinlets (calcite II; Fig. 6 a; c, d, and e), whereas the third stage consists of fine-grain intergrowths (~5 to 10 µm) of calcite (III), adularia and quartz (II) in veinlets and minor hydrothermal breccias (Figs. 6 a and b). Irregularly distributed and spatially associated 14

ACCEPTED MANUSCRIPT with these minerals, there are radial aggregates of orpiment crystals (Figs. 6 e and f), aggregates or isolated crystals (< 50 µm) of realgar (Fig. 6 f) and crystals of calaverite

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and coloradoite (D’Annunzio et al., 2017).

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ACCEPTED MANUSCRIPT Figure 6. Second and third mineralization stage. a) Polished section and the location of b, c, and d. b) QEMSCAN mapping showing the distribution of the minerals of the second and third stage c) Cathodoluminescence image of growth zones in calcite II (Ca II). d) Cathodoluminescence image of third mineralization stage cutting calcite II. e)

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Orpiment veinlets (third stage) surrounding a calcite druse (second stage). f) Orpiment

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(Orp) and realgar (Re) crystals surrounded by calcite crystals (Ca) of the third stage.

7. Methods

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The LA-ICP-MS mapping was performed at the Chemical Fingerprinting Laboratory of Laurentian University, Ontario, Canada by using a Resonetics Resolution M-50 excimer Ar-F laser system with He-Ar carrier gas (ca. 650 ml/m He and ca. 800 ml/min Ar). A small volume of N2 (ca. 6 ml/min) was introduced to the sample-gas

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mixture via the smoothing device to enhance signal sensitivity and reduce oxide formation. The instrument was tuned with scans on GSE reference material (Jochum et al., 2011) at the beginning of each analytical session. Data reduction and production of

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trace element distribution maps were undertaken with the software Iolite v2.5 (Paton et

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al., 2011). The map was made by rastering the laser over the area and periodically analyzing a few reference materials (wavelength 193 nm; pulse duration 20 ns). Fluid inclusion studies included petrographic and micro-thermometric analysis

of seven samples (12QD700-141; 12QD710-146.20; 12QD738-86.40; 12QD724-64.20; 12QD717; 12QD705 and 12QD738). The only suitable mineral to perform the studies was Calcite II. The micro-thermometric analysis was conducted using a Linkam stage calibrated at −56.6°C, +0.0°C, and +300°C. The precision of the temperature measurements on the cooling and heating runs were ± 0.1°C and ± 2°C, respectively. A

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ACCEPTED MANUSCRIPT heating rate of less than 0.1°C/s was used for ice-melting temperatures. The heating rate for measurements of the temperature of homogenization was less than 1°C/s. For twophase fluid inclusions, the measurements recorded were the temperature of final ice melting (Tm ice) and the temperature of homogenization (Th). Salinity and density were

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calculated using the Microsoft Excel spreadsheet for interpreting micro- thermometric data from fluid inclusions HOKIEFLINCS_H2O-NaCl (Steele-MacInnis et al., 2012).

Two samples of sulfides from QDD Lower West (GLQD679 b and GLQD679 c)

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and two samples of sulfides from AIM (GL 71/13 and GL 73/13), were analyzed for sulfur isotopes at Activation Laboratories (Actlabs, Ontario, Canada). BaSO4 and

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sulfide samples were combusted to SO2 gas under ~10-3 torr of vacuum. The SO2 was inlet directly from the vacuum line to the ion source of a VG 602 Isotope Ratio Mass Spectrometer (Ueda, 1986). Sea WaterBaSO4 and FisherBaSO4 internal Lab Standards were run at the beginning and end of each set of samples (typically 25) and used to normalize

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the data as well to correct for any instrument drift. Isotope ratios are reported in the δnotation relative to CDT with a precision better than 0.2‰.

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One sample of limestone (GL02/11), one sample of hydrothermal calcite (11QD679) and one sample of dacitic dikes (09QD587) were analyzed for C and O

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isotopes (at Actlabs). For the analysis of C isotopes, approximately 2 to 5 mg of powdered sample was digested with anhydrous phosphoric acid in a Y tube reaction vessel at 25°C. The evolved CO2 was cryogenically distilled from the reaction vessel into a 6 mm Pyrex tube and flame sealed. The CO2 was then inlet to the ion source of a VG SIRA-10, stable isotope ratio mass spectrometer and analyzed for the 13C/12C ratio. Internal Lab Standards (‘Lublin’ carbonate) were run at the beginning and end of each set of samples (typically 20) and used to normalize the data as well as to correct for any instrument drift. The internal lab standard is periodically calibrated against the NBS 19 18

ACCEPTED MANUSCRIPT International Standard. All the results were reported in the per mil notation relative to the international PDB standard. Precision and reproducibility using this technique are typically better than 0.2 per mil (n= 10 internal lab standards). For the analysis of O isotopes, silicate and oxide samples were reacted with BrF5 at ~650°C in nickel bombs

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following the procedures described by Clayton and Mayeda (1963). The fluorination reaction converts O in the mineral(s) to O2 , which is subsequently converted to CO2 using a hot C rod. All reaction steps were quantitative. Isotopic analyses were

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performed on a Finnigan MAT Delta, dual-inlet isotope ratio mass spectrometer. The data were reported in the standard delta notation as per mil deviations from V–SMOW.

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Based on repeated analyses of our internal white crystal standard the external reproducibility was ± 0.19‰ (1 s). The value for NBS 28 was 9.61 ±0.10‰ (1 s). Four samples (ore from AIM -GL10/11-, limestone wall- rock -GL02/11-, dacitic dike -GL09QD587a- and ore from QDD Lower West -GL11QD679a-) were

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analyzed for Pb isotopes in Activation Laboratories (Actlabs). Pb was separated using the ion-exchange technique with Bio-Rad 1x8. Pb isotope compositions were analyzed on a Finnigan MAT -261 multi-collector mass spectrometer. The Pb isotope ratios

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measured were corrected for mass fractionation calculated from replicate measurements

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of Pb isotope composition in NBS SRM- 982 standards. The external reproducibility of lead isotope ratios (206Pb/204Pb =0.1%,

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Pb/204Pb=0.1%,

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Pb/204Pb=0.2%) on the 2σ

level has been demonstrated through multiple analyses of standard BCR-1 (Alberto and Alves, 2010).

8. Results 8.1.

LA-ICP-MS Mapping.

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ACCEPTED MANUSCRIPT The analysis performed in the ore paragenesis of the first mineralization stage (Fig. 7 a) shows the presence of Au (<280 ppm.) in calcite and pyrite (Fig. 7 b). Besides, Ag (< 1.1 wt. %) not spatially associated with Au was detected in calcite (Fig. 7 c). Sb (<14 ppm.) detected in the pyrite crystals (Fig. 7 d) probably corresponds to the

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sorbyite inclusions previously found by EPMA (D’Annunzio et al., 2017). As contents (<650 ppm) are randomly distributed in the pyrite crystals suggesting that it is an Asbearing pyrite (Fig. 7 e). The Pb anomalies (<260 ppm) (Fig. 7 f) observed in pyrite

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crystals correspond to galena inclusions detected by other analytical methods (see D’Annunzio et al., 2017). Anomalies of Zn (<3 wt. %) (Fig. 7 g) are irregularly

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distributed in calcite, which could correspond to small sphalerite disseminated crystals.

Figure 7. LA-ICP-MS Mapping. a) Mapped sample (Pyrite: Py; Marcasite: Ms, Calcite: Ca). d-i) Quantitative maps of the elements analyzed.

8.2. Fluid inclusion investigation

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ACCEPTED MANUSCRIPT 8.2.1. Fluid inclusion petrography: Fluid inclusions were clasified as primary, secondary or pseudo-secondary was carried out following the criterion of Roedder (1984). Prymary and pseudo-secondary inclusion were used in micothermetry. Additionally, based on the number, nature, and volume proportions of phases observed

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at room temperature fluid inclusions were classified according to the scheme of Nash (1976).

Primary inclusions are Type I and, liquid-rich and have a vapor bubble that

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occupies 20 to 30% of the inclusion volume. In general, these inclusions have an isolated distribution in calcite crystals and commonly have a negative crystal

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(rhomboid) shape or elongated regular shape.

Pseudo-secondary inclusions are Type I and, liquid rich and have a vapor bubble that occupies 20% of the inclusion volume. These inclusions are grouped along healed

irregular shapes.

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fractures that do not cut across the growth zone of the calcite crystals and have mainly

8.2.2. Microthermometry: The main characteristics of the fluid inclusions

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analyzed are summarized in Table 1.

Primary fluid inclusions mainly homogenized to vapor, with homogenization

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temperatures between 306.5 and 294.8 °C and fairly constant salinities (1.56- 1.58% NaCl eq.). A small group of primary inclusions homogenized to liquid with lower homogenization temperature (248.5°C) and slightly higher salinity (1.65 % NaCl equivalent). Pseudo-secondary inclusions homogenized to liquid with homogenization temperatures between 162.31 and 164.4°C and salinities between 1.17 and 1.37 % NaCl eq., i.e. lower than those of the primary inclusions.

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ACCEPTED MANUSCRIPT Host Mineral

Homogenization Salinity (wt % NaCl equiv) phase average (n)

Size of FI (µm)

Tice (°C) range

Th(°C) average (n)

8- 12

-0.9; -0.5

306.5 (3)

vapor

1.56 (3)

12-19

-1;-0.9

248.5 (2)

liquid

1.65 (2)

20- 30

-1.1; -0.5

294.8 (8)

vapor

1.58 (8)

8-20

-1; -0.2

164. 4 (7)

30-40

-1; -0.4

162.31 (7)

Primary Inclusions Calcite

Calcite

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Pseudosecondary Inclusions liquid

1.17 (7)

liquid

1.37 (7)

Table 1. Microthermometric data and the calculated salinities of fluid inclusions hosted

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in Calcite II from the QDD Lower West deposit. FI= Fluid inclusion; Tice= final ice

8.3. Isotopic investigation

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melting temperature; Th= homogenization temperature; n= number of measurements.

8.3.1. Oxygen and carbon. The results of O and C isotope analyses in unaltered limestone and hydrothermal calcite of QDD Lower West are reported in Table 2 along

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with data from Bruno (2005). The δ18O and δ13C values of unaltered limestones (Table 2) are typical of this type of rocks (between 18 and 32 δ18O‰, see Rollinson, 1993). In

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the δ13C vs. δ18O diagram (Fig. 8) the hydrothermal calcites from QDD Main and QDD Lower West define a trend suggesting the involving of decarbonatization processes

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along with carbonate dissolution in their genesis. Description

δ18OSMOW

δ13CVPDB

Limestone

Unaltered limestone wall- rock surface sample

20.2

-1.54

Limestone Hydrothermal Calcite Hydrothermal Calcite

Unaltered limestone wall- rock from a drill core QDD Lower West Sample (Calcite I) from a drill core Hydrothermal calcite of QDD Main from a drill core

21.7 to 24.9

-1.9 to 2.4

17.3

0.295

20.1- 23.1

0.7- 2.7

Sample

Rock

GL02/11 L*(n=13) GL 11 QD 679 Mx* (n=9)

Table 2. Isotope compositions (C-O) of the unaltered limestone wall-rock and hydrothermal calcite of the QDD Lower West and QDD Main deposits.*Data from (Bruno, 2005) ; n= number of samples.

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Figure 8. δ13C vs. δ 18O diagram from Rui-Zhong et al., (2002).

8.3.2. Sulfur. Table 3 shows the results of the sulfur isotope analysis along with the values of the hydrothermal fluid equilibrated with pyrite. The δ34SH2S calculated for

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the pyrite of the first stage of QDD Lower West was ~2 ‰, whereas that for QDD Main was between 0.2 and -1.8‰ and that, for AIM was between -2.9 and 0.91‰. These

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ranges, which are within the magmatic field (~ from -5 to 7‰, see Seal, 2006), suggest the involvement of magmatic sulfur.

Deposits

QDD Lower West AIM QDD Main* (n= 3)

δ34Sfluid δ34Smin

T (ºC) 305.5 - 294.8

248.5

-0.89

-1.79

-2.09

-0.75

-1.65

-1.95

0.25

-0.65

-0.95

0.29

-0.61

-0.91

-0.6 to1.4

-1.5 to 0.5

-1.8 to 0.2

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ACCEPTED MANUSCRIPT AIM* (n= 1)

-2.6

-1.7

-2.9

Table 3. δ34S values for sulfides of QDD Lower West, QDD Main and AIM deposits of Gualcamayo Mining District. The isotope fractionation in fluids was calculated according to Kajiwara and Krouse, (1971) at 305.5°C, 294.81°C and 248.5°.*Data from

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Bruno (2005); n= number of samples.

8.3.3. Lead. The Pb isotope ratios obtained for pyrite from QDD Lower West

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are slightly lower than those obtained for the dacitic dike. The AIM skarn had isotopic

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values between those of the limestone and the dike, which is typical of contact metamorphic rocks (Table 4). In the uranogenic diagram, the dacitic dike and the AIM skarn plot in the orogen curve while the pyrite of QDD Lower West plots between the orogen and the lower crust curves (Fig. 9 a). The orogenic diagram confirms the involvement of the lower crust in the genesis of the QDD Lower West ore (Fig. 9 b).

Sample GL10/11 GL02/11a GL09QD587a

Pb/204Pb (Figs. 9 a and

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

AIM

18.416

15.588

38.295

Wall- rock limestone

20.673

15.765

39.313

QDD Lower West Ore

17.914

15.519

37.688

Dacitic Dike

18.039

15.565

37.986

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GL11QD679a

206

Litology

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b).

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Limestone, on the other hand, shows an anomalous value of

Table 4. Lead Isotope Composition of QDD Lower West pyrite and the host rocks of the deposit.

24

Figure 9.

207

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Pb/204Pb vs.

206

Pb/204Pb (a) and

208

Pb/204Pb vs.

206

Pb/204Pb (b) diagrams

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showing the QDD Lower West deposit and its host rocks Pb-isotope evolution curves generated by the plumbotectonics model for the mantle (A), orogene (B), upper-crust contributed to the orogene (C), and lower crust contributed to the orogene (D) (from Zartman and Doe, 1981).

9. Discussion 9.1.

QDD Lower West deposit model 25

ACCEPTED MANUSCRIPT The structural analysis of the QQD Lower West deposit revealed that the emplacement of the tectonic mineralized breccias is controlled by the maximum stress direction (E-W). Therefore, we consider that the mineralization is controlled mainly by

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the structure. Besides, the mineralogical studies showed that the Au occurs as submicroscopic free Au inclusions in As-pyrite, pyrite, sphalerite and calcite in the first mineralizing

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stage and as Au tellurides together with realgar and orpiment in the third stage.

The pH of the mineralizing solutions in QDD Lower West has been estimated

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based on the texture and mineralogy of the gangue (D’Annunzio et al., 2017). For the first mineralizing stage the lack of dissolution in the carbonates could show a relatively neutral pH. The presence of adularia + calcite ± quartz suggests a pH relatively neutral (Moncada and Bodnar, 2012). Besides, previous work (D’Annunzio et al., 2017)

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revealed that the early mineralizing fluids had intermediate sulfidation conditions whereas the late mineralizing fluids had intermediate to high sulfidation conditions. On the other hand, fluid inclusion studies suggest that the hydrothermal fluids (second

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stage) had a low temperature (306.5- 248.5 °C) and very low salinity (~1.5 % NaCl wt.eq.). The correlation between homogenization temperature and salinity of the fluid

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inclusion studies revealed a cooling trend from high-temperature, high salinity fluids to fluids with lower temperature and lower salinity. This pattern was interpreted as a dilution process due to a mixing of magmatic and meteoric fluids. (see D’Annunzio et al., 2015). The C and O isotopic composition of the hydrothermal calcite of QDD Lower West suggests the involvement of carbonate host rock dissolution processes (Fig. 8) while the S isotopic shows that the S derives from a magmatic source. Besides, the Pb

26

ACCEPTED MANUSCRIPT isotope ratios suggest a magmatic source and also the involvement of an older (and less radiogenic) source for this metal, but different from the limestone wall-rocks. 9.2.

Genetic link between QDD Lower West, AIM and QDD Main

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The different Au deposits of the GMD show similarities that allow establishing a genetic link between them. QDD Lower West and QDD Main have E-W orientation (coincident with the Miocene maximum stress direction) which indicates that they have

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the same structural control (Fig. 10), and AIM, is emplaced in a NNW strike-slip fault (Fig. 2 c). The structural controls of the ore deposits of the GMD are coherent with the

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transtensive shear zone affecting the area (see Fig. 2).On the other hand, a similar origin for the three mineralized breccia bodies has not yet been elucidated. Whereas the QDD Lower West and the AIM breccias have a clear tectonic origin, the process that formed the QDD Main calcareous breccias is still under debate. Bruno (2005) proposed that

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they were formed by dissolution and collapse processes whereas Lynch et al. (2000) suggested a tectonic origin.

Regarding the ore paragenesis, AIM has the same ore paragenesis as the first

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mineralization stage of QDD Lower West whereas QDD Main has a paragenesis similar

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to that of the mineralization stages I and III of QDD Lower West, but with the sulfides partially to completely oxidized. Besides, AIM, QDD Lower West and QDD Main display similar isotopic

signatures. The S isotope ratios obtained for the three deposits are close to 0‰ suggesting a common magmatic origin for the S. In addition, the C and O isotopic compositions suggest the variable involvement of decarbonatization and carbonate dissolution processes in the genesis of the hydrothermal calcite of the ore deposits, with

27

ACCEPTED MANUSCRIPT carbonate dissolution probably predominating in QDD Lower West, which is consistent with its host rock (see Fig. 8). Although there are no geochronological constraints on the age of the GMD

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deposits, the age of mineralization could be constrained based on field relationships, S isotopes, and structural control. Considering that AIM deposit overprints the skarn wich is about the age of the Gualcamayo Igneous Complex (~9 Ma), it can be considered as the maximum age of the deposit. Besides, the magmatic signature of the S

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suggests a link with the magmatism of the area which also supports the Late Miocene

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age of the GMD Au mineralization. Moreover, the structural control of the

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mineralization confirms a Miocene age for the deposits.

Figure 10. Proposed genetic model for QDD Main and QDD Lower West Au deposits.

28

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9.3.

Comparison with Carlin-type deposits

Differences between the majority of Carlin-type deposits in Nevada, USA, and other

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Carlin-style deposits have led to a proliferation of terms, including Carlin-type, Carlinlike, Carlinesque, sedimentary rock-hosted Au deposits, and distal disseminated deposits, among others (Muntean and Cline, 2018). The four main clusters of Carlin-

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type Au deposits in Nevada -the Carlin trend, Getchell, Cortez, and Jerritt Canyonshare many features (Hofstra and Cline, 2000; Muntean et al., 2011). Hofstra and Cline,

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(2000) were the first to use these similar characteristics to formally define Carlin-type Au deposits of Nevada. Additionally, other two localities (southwest China and Yukon) have Carlin-style deposits that are the most similar to the Carlin-type deposits and are considered to be Carlin-type Au deposits (Muntean and Cline, 2018). Considering that the Au deposits of the GMD have some features of Carlin-type Au deposits and that

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there is a probable genetic link between them the most relevant aspects in the classification of Carlin-type deposits are analyzed below.

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Regarding the host rocks, of the GMD Au deposits the rocks associated with the mineralization are structurally controlled tectonic breccias in QDD Lower West and

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AIM and, carbonatic and pollimictic breccias in QDD Main. The calcareous clasts of both types of breccias are carbonatic marine sedimentary rocks of Cambrian to Ordovician age. The Nevada deposits are hosted in Paleozoic sedimentary rocks, consisting of limestones, siltstones, argillites, shales, and quartzites, deposited on the western margin of the North American craton. Chinese Carlin-type deposits occur in Paleozoic to lower Mesozoic limestones and bioclastic limestones deposited in sedimentary basins along margins of the Yangtze Precambrian craton (Peters et al.,

29

ACCEPTED MANUSCRIPT 2002). Both in Nevada and in China the geometry and the textures of orebodies are commonly parallel to structural features, such as folds and faults, which implies that and the mineralization has spatial and genetic relationship with the deformation. Many Carlin-type Au deposits associated with zones of tectonic deformation also contain

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zones of collapse and volume loss due to hydrothermal dissolution (Peters, 2004). Similar characteristics are observed in the GMD Au deposits where the shape of the breccia bodies are structurally controlled (see Fig. 2 b). Regarding the size of the

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deposits, the GMD Au deposits have an intermediate tonnage of Au (1-140 tonnes) between the Carlin-type deposits of China (1-80 tonnes; Rui-Zhong et al., 2002) and

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those ofNevada (1-1000 tonnes; Hofstra et al., 1999).

Regarding the ore and alteration assemblages, the residence of Au, the geochemical signatures, the ore paragenetic sequence, and the physicochemical conditions of the fluids, the GMD Au deposits exhibit some differences with typical

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Carlin-type deposits. The mineralization in Carlin-type Au deposits takes place in three stages: an early quartz-pyrite stage, a main auriferous quartz- arsenian pyrite-

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arsenopyrite stage, and a late quartz-calcite- realgar (orpiment)- cinnabar- stibnite ± tellurides barren stage. The ore paragenesis resulting from Au-undersaturated

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mineralizing fluids is characterized by auriferous, arsenian pyrite formed by sulfidation during the replacement of the calcareous host-rocks, where the majority of Au occurs in the form of Au+1 in As- Pyrite (Muntean and Cline, 2018). In QDD Lower West Au occurs as submicroscopic inclusions in arsenian-pyrite, pyrite, sphalerite, and calcite in the first quartz-poor mineralizing stage which in the third stage, Au occurs as tellurides associated to realgar-orpiment-adularia-quartz and calcite. The presence of free Au in the first mineralization stage of QDD Lower West implies that the mineralizing fluids were Au-saturated (Zhu et al., 2011). The occurrence of Au in late mineralization stage 30

ACCEPTED MANUSCRIPT could be related to the Au solubility, wich depends on the pH. The maximum solubility of Au at 250 °C occurs between pH ~7 and 8 (Moncada and Bodnar, 2012), which coincides with the neutral pH estimated for the first mineralization stage fluids. Under these pH conditions, part of the Au would have remained in solution and precipitated

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during the late mineralizing stage probably due to a high fTe (log fTe= -11; Lehmann et al., 1999) forming calaverite.

The geochemical signatures in the deposits of China include S, Au, As, Sb, Hg,

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Tl Ag and U (Rui-Zhong et al., 2002), while those in Nevada include S, Au As, Sb, Hg,

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Tl, Ag, and Ba ± W ± Te ± Se (Emsbo et al., 2003). The geochemical signature of the GMD Au deposits is similar to that of those of Nevada for the presence of Te. Regarding the pH of the mineralizing fluids of the Carlin-type deposits they are acidic enough (pH 4–6) to dissolve calcite and alter the silicates of the host rocks to

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illite or kaolinite (Rui-Zhong et al., 2002). In QDD Lower West the texture and gangue mineralogy indicate a neutral to slightly alkaline character of the mineralizing fluids, while in QDD Main the argillic alteration of the host rocks suggests that the fluids are

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acid such as in the Carlin type-deposits. The acidification of the fluids could be the

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result of the involvement of meteoric waters revealed by the fluid inclusion results. The C and O isotopic composition of the unaltered host rocks limestones from

the GMD is typical of marine limestones, although a few samples have lower δ18O values, a fact that likely reflects some interaction with hydrothermal fluids. In comparison with unaltered limestone, the hydrothermal calcite has in general lower δ18O and δ13C values that define two arrays: one with a nearly horizontal slope and one with a positive slope (Fig. 9). The horizontal data array suggests that the CO2 derived from the dissolution of marine limestone whereas the positive slope array may be

31

ACCEPTED MANUSCRIPT indicative of reactions between the host rocks and fluids containing CO2 derived from the oxidation of organic carbon, the atmosphere (δ13CCO2= -7‰), or both. These trends in C and O isotopes are also observed in the deposits of China and those of Nevada reflecting the same process and sources for C and O (Cline and Hofstra, 2000; Emsbo et

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al., 1999; Rui-Zhong et al., 2002).

The Nevada deposits have a variable involvement of sulfur derived from the Paleozoic sedimentary rocks. In the Chinese deposits, the range of δ34SH2S values (-24

SC

to 17 ‰) is very similar to that of diagenetic pyrite of the host rocks suggesting that the

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H2S derived from the dissolution of the diagenetic pyrite or organic sulfur (Rui-Zhong et al., 2002). The GMD Au deposits are spatially associated with Miocene intrusives and the S isotopic composition suggests a magmatic (probably deep, see Allégre, 2008) source. This magmatic signature of the fluids is one of the main differences between the

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GMD Au deposits and the Carlin-type deposits.

Despite the similarities between the GMD deposits and the Carlin-type deposits

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(structural control, host rocks, geochemical signature, C and O isotopic composition),

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the close genetic link with magmatism, the significant presence of base metal sulfides and the character of the hydrotermal fluids (neutral to slightly alkaline and Auundersaturated) suggest a different origin. These characteristics are typical of Carlinstyle deposits termed “distal disseminated Au ± silver deposit” (Muntean and Cline (2018) that occur on the distal edges of magmatic-hydrothermal systems, existing a a continuum between porphyry, skarn, pollymetalic and distal disseminated deposits (Carten et al., 1993; Johnston et al., 2008; Pierce and Bolm, 1995; Titley, 1993). Johnston et al., (2008) proposed a possible continuum between Carlin-type Au deposits

32

ACCEPTED MANUSCRIPT and distal disseminated deposits, controlled by the distance of the deposits to intrusions. This genetic link may have significant implications for the potencial exploration of porphyry related and Carlin type deposits.

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10. Conclusions The metallogenetic analyses of the GMD support a genetic link between the three Au deposits of this mining district. Although these Audeposits share many

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characteristics with Carlin-type deposits including the type of host rocks, structural control, geochemical signatures, ore paragenesis and evidences of dissolution and

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decarbonization processes, they also display noticeable differences regarding the close genetic link with magmatism which is suported by the S isotopic composition, the significant presence of base metal sulfides and the character of the hydrotermal fluids. Unlike Carlin-type deposits, where fluids are acidic, the mineralizing fluids of

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QDD Lower West are neutral to slightly alkaline. This results not only in a peculiar gangue mineral assemblage but also in the presence of Au mineralization in the late stage which is usually barren in the Carlin-type deposits. Moreover, in the Carlin-type

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deposits the mineralizing fluids are Au-undersaturated whereas in the GMD Au deposits

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the presence of native Au indicates that the mineralizing fluids are Au-saturated. The features of the GMD Au deposits thus allow us to classify them as “distal disseminated Au deposit”.

33

ACCEPTED MANUSCRIPT ACKNOWLEDGMENTS We thank M.A.S.A. (Minas Argentinas Sociedad Anonima, Yamana Gold) that authorized the field work, sampling and, financed the isotopic analysis. This project was

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funded by PIP 11220130100107 CO (CONICET) project, Argentina.

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The metallogenetic analysis of the Gualcamayo Mining District Au deposits revealed that they are genetically linked and share many characteristics with

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Carlin-type deposits, including the lithology of host rocks, structural control, geochemical signatures, ore paragenesis, and dissolution and decarbonization

•

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processes.

A noticeable difference with Carlin type deposits are :the sulfur isotopic

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composition that supports a genetic link between these gold deposits and the precursor magmatism of the porphyry-type mineralization of the area, and that the fluids responsible for the Au mineralization proved to be neutral to slightly alkaline and Au-saturated.

The link with the magmatism along with the character of the mineralizing fluids

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and the presence of significant base metal sulfides allowed classifying the GMD deposits as “distal disseminated gold deposits” that occur on the distal edges of

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magmatic-hydrothermal systems.