Trace element heterogeneity in molybdenite fingerprints stages of mineralization

    Trace element heterogeneity in molybdenite fingerprints stages of mineralization C.L. Ciobanu, N.J. Cook, C.R. Kelson, R. Guerin, N. Kalleske, L. Danyushevsky PII: DOI: Reference:

S0009-2541(13)00125-3 doi: 10.1016/j.chemgeo.2013.03.011 CHEMGE 16843

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

8 September 2012 4 March 2013 8 March 2013

Please cite this article as: Ciobanu, C.L., Cook, N.J., Kelson, C.R., Guerin, R., Kalleske, N., Danyushevsky, L., Trace element heterogeneity in molybdenite fingerprints stages of mineralization, Chemical Geology (2013), doi: 10.1016/j.chemgeo.2013.03.011

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ACCEPTED MANUSCRIPT Trace element heterogeneity in molybdenite fingerprints stages of mineralization

Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences,

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C.L. Ciobanu1,2,*, N.J. Cook1,2, C.R. Kelson3, R. Guerin1, N. Kalleske1, L. Danyushevsky4

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University of Adelaide, S.A., 5005, Australia Deep Exploration Technology Cooperative Research Centre (CRC-DET), University of Adelaide, North

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Terrace, SA 5005, Australia Dept. of Geology, State University of New York College at Potsdam, Potsdam, NY, 13676, U.S.A. CODES, University of Tasmania, Hobart, Tasmania, Australia

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Abstract

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4 March 2013

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Revised manuscript submitted to: Chemical Geology

Variations in molybdenite trace element chemistry represent a tool for discriminating discrete events in young magmatic-hydrothermal systems and constrain the role of granites in older greenstone belt-hosted gold systems. We show that besides Re and W (typical lattice-bound elements), molybdenite also concentrates chalcophile elements with chalcogenide affinity such as Bi, Pb, and Te (CEs). Elements from the latter group form nano- to micron-scale inclusions which also attract Au and Ag incorporation. High concentration of all elements is attributable to lattice-defects and coherent intergrowths between

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Corresponding author. E-mail address: [email protected]. Telephone: (+61) 405 826 057

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ACCEPTED MANUSCRIPT molybdenite and CEs-minerals rather than polytypism. If both groups are used, trace element patterns are useful for interpreting superimposed ore-forming processes. We test the validity of this hypothesis by carrying out Laser-Ablation Inductively-Coupled Mass Spectroscopy spot analysis and element mapping

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on high-Re molybdenite from the Tertiary Au deposit at Hilltop (NV, USA) and the Archean Boddington Cu-Au deposit (Western Australia). Whereas W and CEs are affected by both deformation and interaction

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with subsequent fluids, Re is only affected by the latter. An epithermal overprint at Hilltop, recorded in a grain with a CE-rich halo surrounding a core with Re-W oscillatory zoning, upgrades Re content and is

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also traceable by measurable Au. At Boddington, granite-derived fluids dilute Re in precursor

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molybdenite, but both granite and orogenic deformation assist CEs-mineral coarsening and Au release.

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(216 words)

Keywords: Molybdenite; Laser-Ablation Inductively-Coupled Mass Spectroscopy; Element mapping;

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Bismuth; Gold.

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1. Introduction

Molybdenite, the primary source of Mo is long known as a carrier of rhenium (Re) and tungsten (W) where both elements are incorporated in the molybdenite lattice due to similar ionic radii of Mo4+ (0.65Å), Re4+ (0.63Å) and W4+ (0.66Å). One mechanism explaining Re enrichment in molybdenite is expansion of the crystal lattice via polytypism (Frondel and Wickman, 1970), which is attractive since there are only two molybdenite polytypes: hexagonal (2H) and the rarer rhombohedral (3R). McCandless et al. (1993) have confirmed that the 3R polytype is often richer in Re. Newberry (1979) emphasized the correlation between crystal structure and chemistry noting that other elements (W, Nb, V, Fe and Ti) are also concentrated in the 3R polytype. Drawing attention to associations of 3R molybdenite and Biminerals in several deposits and the structural-chemical parameters for analogous MSn layer compounds, 2

ACCEPTED MANUSCRIPT he discussed how Bi and Sn could also be incorporated. Blevin and Jackson (1998) used Laser-Ablation Inductively-Coupled Mass Spectroscopy (LA-ICP-MS) to measure a number of elements in molybdenite containing both Bi-mineral inclusions and Au. They found that such chalcophile elements and associated

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abundance could potentially be a valuable petrogenetic tool.

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chalcogens (Pb, Bi, Te; CEs) are indeed present and were the first to propose that variation in their

Such ‘dirty’ molybdenite can be used as a petrogenetic tool and Norman et al. (2004) point at the

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usefulness of molybdenite trace element data, notably including Bi, for fingerprinting mineralization in

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granitic settings. This idea, however, has not been widely followed-up and the focus on molybdenite has instead shifted to geochronological applications using Re-Os isotopic ratios (Stein et al., 2001a). The

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method offers a mineralization age and is robust with respect to subsequent overprinting (e.g., Stein et al.,

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2003; Selby and Creaser, 2004) except when subject to supergene alteration (McCandless et al., 1993).

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Heterogeneity within single grains of molybdenite has been documented by Aleinikoff et al. (2012). In their samples from Hudson Highlands, NY, distinct cores and rims, each with distinct trace element

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compositions, give different Re-Os ages suggestive of a process of dissolution/reprecipitation following peak high-grade regional metamorphism. Coexisting molybdenite grains giving different Re-Os ages had earlier been reported by Requia et al. (2003) who considered this resulted from temporally separated pulses of molybdenite deposition in the Salobo iron oxide copper-gold deposit, Brasil, and by Wilson et al. (2004) for the Cadia porphyry system, New South Wales, Australia.

LA-ICP-MS spot analysis and mapping are used to document and interpret patterns of trace element distribution in refractory sulfides such as pyrite and arsenopyrite (e.g., Cook et al., 2009, 2013; Large et al., 2009; Sung et al., 2009; Thomas et al., 2011; Ciobanu et al., 2012; Winderbaum et al., 2012), but have not been applied to molybdenite which readily undergoes ductile deformation. Here, we use such methods 3

ACCEPTED MANUSCRIPT to document the behavior of a wide range of trace elements in Re-rich molybdenite attributed to porphyry-style mineralization from two deposits of very different geological settings and ages. Our aim is to demonstrate how petrographic study can be corroborated by trace element analysis of molybdenite to

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pinpoint the sequence of ore-forming events in complex geological environments. These two deposits were selected for study since previous work has, in both cases, clearly shown the influence of successive

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events.

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2. Deposits sampled

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The ~2 MOz Hilltop Au deposit, Lander County, NV, U.S.A. (Fig. 1a, b), is one of several Au±Ag±base metal deposits within the northern Shoshone Range. Collectively, these deposits formed

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throughout the continuum between low-temperature (epithermal) and higher-temperature (porphyry)

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regimes (Kelson et al., 2005, 2008). Hilltop contains mineralization of both types where porphyry-related Mo+Cu±Au-bearing quartz veins are spatially, temporally, and genetically associated with mostly felsic

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Eocene intrusive igneous rocks and occur only within or adjacent to these rocks. Conversely, younger epithermal Au±Ag±base metal mineralization is mostly confined to a tectonic breccia (“main zone”) composed of highly-fractured Ordovician Valmy Formation and Eocene intrusive igneous rocks. Kelson et al. (2005, 2008) give a weighted Re-Os molybdenite mean age of 40.23±1.7 Ma for Hilltop molybdenite (n=4).

Epithermal mineralization at Hilltop occurs as open-space filling and fracture-fill hosted by both the Ordovician Valmy Formation (chert, quartzite, siltstone, and argillite) and by Tertiary intrusive rocks (feldspar and quartz-feldspar porphyry, granodiorite and diorite) that occur as small plugs, dikes, and sills. The Ordovician host rocks were bleached and recrystallized, presumably by the igneous rocks 4

ACCEPTED MANUSCRIPT associated with molybdenite mineralization, prior to fracturing and the Au mineralization event. Most of the Au and base-metal mineralization at Hilltop is localized within a megabreccia (“main zone”) between two sub-parallel, N-trending, W-dipping faults. No molybdenite is observed within the main zone.

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Molybdenite-bearing quartz veins are only observed in the igneous rocks and in Valmy Formation rocks adjacent to them. The Valmy Formation rocks stratigraphically below the main zone are unaltered and

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unmineralized; igneous rocks are truncated by the main zone faults.

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The ages of molybdenite and primary igneous biotite (Ar/Ar; 39.7-38.5 Ma) from Hilltop igneous

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rocks are given by Kelson et al. (2008); Re-Os ages are slightly older than the Ar/Ar ages, probably due to the difference in closure temperatures between the two isotopic systems. Molybdenite-bearing quartz

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veins, or pieces of them, are not been observed in direct contact with main zone Au and base-metal mineralization, but main zone mineralization frequently incorporates bleached and recrystallized Valmy

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Formation rocks; this alteration presumably occurred during emplacement of the igneous rocks associated

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with molybdenite precipitation. It should be noted that the molybdenite mineralization at Hilltop cannot strictly be classified as porphyry-type mineralization, since the usual alteration assemblages associated

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with porphyry systems and the typical (volcanic) host rocks are absent.

The World-class Boddington Cu-Au-(Mo) deposit, (>26 MOz Au, >1.2 Mt Cu) is located within Archean felsic to intermediate igneous rocks belonging to the northern part of the Neoarchean (27142665 Ma) Saddleback Greenstone Belt (SGB) in the South West Terrane of the Yilgarn Craton, Western Australia (Fig. 2). The whole-rock geochemistry of the SGB suggests formation in an island arc setting (e.g., Wilde and Pidgeon, 2006). The main magmatic activity in the SGB appears to have taken place during two distinct episodes (2714-2696 and ~2675-2665 Ma; Wilde and Pidgeon, 1986, 2006; Allibone et al., 1998). Syntectonic granitic intrusion at 2650-2630 Ma; Wilde and Pidgeon, 1986; Nemchin and Pidgeon, 1997; Qiu and Groves, 1999) postdates terrane amalgamation and a later post-tectonic, “A-type” 5

ACCEPTED MANUSCRIPT granite event (at ~2612 Ma) is also recorded at Boddington (Allibone et al., 1998) and elsewhere across

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the South West Terrane (Korsch et al., 2011).

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The Boddington deposit lies within a steeply-dipping sequence of metamorphosed and faulted sedimentary, felsic, and mafic volcanic and pyroclastic rocks. Strong structural and lithological control of

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the mineralization is evident. Ores are characterized by a pronounced Au-Cu-Mo-Te-Bi association within a generally low-sulfide system. Molybdenite is associated with disseminated and veinlet Cu-Au

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mineralization in shears, faults and altered rocks (mainly diorite) located along a 4 km-long NNW-SSE-

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striking zone between two major ductile shear zones.

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Genetic models for the deposit have shifted from an Archean porphyry (Symons et al., 1990; Roth and Anderson, 1993) to an orogenic Au system (Allibone et al., 1998). Stein et al. (2001b) provide two

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distinct Re-Os ages for Boddington molybdenite (2707±17 Ma and 2623±9 Ma), supporting a two-stage

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model combining the two. The error on the second age overlaps with SHRIMP U-Pb zircon dating of the barren Wourahming granite (2611±3 Ma; Allibone et al., 1998) which intrudes southeast of the deposit.

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McCuaig et al. (2001) attributed the second Re-Os age to intrusion-related Au-Cu rather than orogenic Au. This latest interpretation, based on the intrusion-related model of Thompson and Newberry (2000) has been taken up by other authors (Baker et al., 2005; Duuring et al., 2007; Hart 2007) noting that Boddington does indeed at least some of the characteristics of an intrusion-related gold deposit. If correct, Boddington would represent the first well-documented occurrence of an Archaean Au-Cu deposit associated with post-tectonic granitoids. Despite this, evidence of extensive reworking of pre-existing mineralization and trace-element signatures in molybdenite from diorite and granite favor a multi-stage genesis (Kalleske, 2010; Guerin, 2011). Re concentrations show three distinct populations, attributed to porphyry-style (hundreds of ppm) and orogenic- and granite-related systems (<1 to a few ppm, respectively, discriminated from one another by different Co-Ni, CEs and Au signatures). Hagemann et 6

ACCEPTED MANUSCRIPT al. (2007) sought to support an intrusion-related model by addressing melt and fluid inclusions in the Wourahming monzogranite. The study demonstrated trapping of saline, Cu-Au-Bi-W-enriched magmatic hydrothermal fluids during cooling of the stock. Hagemann et al. (2007) did not rule out other metal

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sources but took the data to indicate that the monzogranite contributed at least some metals.

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The three molybdenite-bearing samples from Hilltop (HT-X, 97-10 106.1, and 97-12 339) studied here were collected from two different core holes. Figures 1c and 3a-d show the locations of these samples and

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their spatial relationships to intrusive igneous and siliceous/siliciclastic host rock types, and to main zone

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mineralization. The Mo-bearing quartz veins also contain chalcopyrite, Au-bearing arsenian pyrite and

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carbonate minerals (Fig. 3a).

In both deposits, a pronounced association between Bi-(Pb)-chalcogenides (tetradymite group and

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aleksite series), galena and molybdenite is recognized (Kelson et al., 2005; Kalleske, 2010; Guerin, 2011; Fig. 3d-f). This is especially pronounced at Boddington where Bi-(Pb)-chalcogenides are common

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components of molybdenite; sub-5 μm-sized grains of gold are also seen between molybdenite lamellae

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(Fig. 3e). By comparing and contrasting trace element patterns in molybdenite, we address the question of whether grain-scale heterogeneity reflects deposit evolution.

3. Methodology

3.1 Laser-Ablation inductively-Coupled Mass Spectroscopy

An Agilent 7500 mass spectrometer coupled with a UP-213 laser-ablation system (CODES, University of Tasmania) was used to acquire the molybdenite trace element data. The following isotopes were 7

ACCEPTED MANUSCRIPT measured: 207

Bi,

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Fe,

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Co,

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Ni,

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Cu,

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Zn,

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As,

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Se,

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Ag,

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Sn,

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Sb,

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Te,

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W,

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Re,

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Au,

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Pb,

U. The in-house standard STDGL2b2 was used for calibration (Danyushevsky et al., 2011).

Standards were analyzed at 10 Hz laser frequency and 80 μm spot size whereas measurement of

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molybdenite spots was performed at 5 Hz laser frequency and 35 μm spot size. Pre-ablation at 1 Hz was undertaken to clean the surface of contaminants. Measured isotopes were selected to avoid isobaric and

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polyatomic interferences. Analysis time for each sample was 90 s, comprising a 30 s measurement of background with laser off, and 60 s measurement with laser-on. An integration time of 0.02 s was used

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for each of the elements measured. The raw analytical data in each spot analysis is plotted as a time-

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resolved depth spectrum and the integration times for background and sample signal selected. Counts for each element are then corrected for instrument drift and converted to concentration values using known

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values of a major element in the analyzed minerals as an internal standard - in this case Mo. Standards

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were run before and after each set of spots on a given sample (typically 12-20 unknowns).

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The uncertainty on the Mo concentration in standard STDGL2b2 is ~6% (Danyushevsky et al., 2011). As Mo is used as the internal standard, this uncertainty is propagated to all other elements analyzed. The

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uncertainty for the Re concentration in the standard is less well constrained but is likely to be of a similar magnitude. The extent of possible Re/Mo fractionation during ablation of molybdenite relative to the standard has not been assessed in detail but is unlikely be >20% based on the results of Danyushevsky et al. (2011). We thus estimate the total error on reported Re concentration to be 20-40%. Consistent with this, the measured Re concentrations are at the same order of magnitude as published Re concentrations for samples from both Hilltop and Boddington as measured by Carius-tube methods during the course of Re-Os dating. Irrespective of any errors introduced in analysis, the observed small difference between those concentration values and the ones given here (our values tend to be somewhat higher) is also consistent with substantial variation between grains in the same sample and between samples from the same locality. 8

ACCEPTED MANUSCRIPT The element maps of molybdenite in the Hilltop sample (Fig. 5) were made using the same LA-ICPMS instrumentation. Images were acquired using laser frequency of 10 Hz, laser beam size of 6 μm, and a

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rastering rate of 10 μm/sec. Further details of the methodology are given by Large et al. (2009). Element

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maps on the Boddington material (Figs. 6-8) were acquired with a laser beam sizes of 10 μm, 8 μm and

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15 μm, respectively.

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3.2 Focused Ion Beam – Scanning Electron Microscopy and Transmission Electron Microscopy

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A FEI-Helios nanoLab Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM) system at

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Adelaide Microscopy was used to prepare samples for Transmission Electron Microscopy (TEM) foils. Methodology is described by Ciobanu et al. (2011). The TEM study was performed on a Philips 200CM

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instrument operated at 200 kV. The instrument is equipped with a double-tilt holder and Gatan digital

4. Results

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camera. Measurements on the diffractions were performed using DigitalMicrograph™ 3.11.1.

4.1 Molybdenite trace element heterogeneity

Molybdenite in analyzed samples from both deposits (Table 1) occurs mostly as up to cm-sized, lamellar or knotted aggregates as well as single wispy lamellae with common kinks and folding. Inclusions of Bi-minerals and galena in molybdenite, often associated with native gold at Boddington, are ubiquitous throughout the samples (Fig. 3d-f). LA-ICP-MS spots were selected to characterize homogeneous areas, targeting key intragranular fabrics to understand variation with respect to textures. 9

ACCEPTED MANUSCRIPT Molybdenite shows highly-variable Re concentrations (Tables 2 and 3), ranging from tens to >1,000 ppm (Fig. 4a). At Hilltop, one sample (HT-X) has much higher Re than the others and one of the grains

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stands out in having a coarse lamellar core surrounded by a finer-grained halo cemented by chalcopyrite, silicates and with inclusions of Bi-chalcogenides and galena. Re concentration is low in the inner core but

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higher and irregular in the outer part of the core whereas the halo has remarkably constant values. Other grains in this sample have still higher Re values. Tungsten is typically tens of ppm but up to 195 ppm in

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the grain halo. CEs range up to several hundred ppm; Bi, Pb and Te are high in the grain halo; Au is

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highest (mean 4.8 ppm) in HT-X. At Boddington, Re variation is highest in molybdenite from the granite aureole and W varies across two orders of magnitude in all samples. Mean Bi and Pb concentrations are

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hundreds of ppm but Te is lower and displays greater variance; Au is up to 360 ppm but typically tens of ppm. In both deposits, Au+Ag and CEs show a good correlation (Fig. 4b). Time-resolved LA-ICP-MS

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depth profiles reflect two groups of elements in the flatness (Re, Se and in most cases W) or noisiness

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CEs (Fig. 4d).

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(CEs) of the profiles (Fig. 4c). In Boddington molybdenite, W can also show irregular profiles similar to

Mapped grains were selected to reflect core-halo relationships at Hilltop and response to deformation and replacement at Boddington. Trace element maps for Fe, Cu, CEs and U outline the core-halo texture at Hilltop (Fig. 5). Re displays oscillatory zonation in a depleted inner-core and no marked boundary between the outer-core and halo whereas W shows an inverse pattern to Re in the core and is highest in the halo. Expected Re depletion in areas highest in Cu is not seen due to pervasive molybdenite inclusions in chalcopyrite.

All element maps of Boddington samples show grain-scale heterogeneity. Patterns in part of a lamella framed by kinks (Fig. 6) show that Re is homogeneously distributed, thus contrasting with W which is 10

ACCEPTED MANUSCRIPT concentrated in kinks and along grain margins. CE-rich spots correlate with cleavages and kinks; Au and Ag show clustered spots along the CE patterns. Similar patterns are seen in a strongly-deformed molybdenite aggregate (Fig. 7) where W is enriched in intensely-deformed domains but Re is unaffected.

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A distinct Re pattern is mapped on a lamella from a diorite sample in the granite aureole (Fig. 8), 200 m below the sample mapped in Fig. 7. Here, molybdenite features a subtle fine-grained texture when

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adjacent to chalcopyrite infilling cleavages and both Re and W are concentrated along such replacement

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zones.

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4.2 Crystal-structure and defects

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Whether local trace element enrichment correlates with polytypism in molybdenite was checked by preparing TEM foils from the core-halo grain using the FIB-SEM approach (Ciobanu et al., 2011) and

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method. Slices were cut at site-specific locations (Fig. 9a): (i) outer core; and (ii) halo. High-resolution FIB-SEM imaging shows nanoporosity associated with hairpin folding (Fig. 9b) and sub-μm galena

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inclusions (Fig. 9c) in foil (i). CE-mineral inclusions reach several μm in the halo (Fig. 9d). Electron

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diffraction patterns indicate the presence of the 2H polytype only in both foils (Fig. 9e). Abundant nanoscale fractures with terminations sealed by curvature of lattice fringes in both foils may be associated with stacking disorder (Fig. 9f and inset).

5. Discussion

Complementary to previous studies that have distinguished co-existing generations of molybdenite with implications for evaluation of deposit evolution (Requia et al., 2003; Wilson et al., 2004; Aleinikoff et al., 2012), the data presented here shows trace element patterns that can be used to interpret 11

ACCEPTED MANUSCRIPT mineralization histories. CEs are an important element set for constraining either deformation or replacement, particularly when compared with the patterns of lattice-bound elements in the same grain. The persistent noisiness of CEs signals, contrasting with Re, Se and (largely) W in both study cases,

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suggests a distinct trapping and release behavior for inclusion- and lattice-bound elements. However, W can behave differently to other lattice-bound elements under similar conditions, e.g., the ragged signal

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mimicking CEs (Fig. 4d).

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Observed CEs-minerals and their size down to nanoscale, indicate that whichever the trapping

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mechanism, CEs readily coarsen out of smaller impurities attached to crystal defects or lamellae surfaces. Crystal-chemical modularity in Bi-chalcogenides (Cook et al., 2007a, b; Ciobanu et al., 2009) may

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‘attract’ coherent intergrowths with other minerals that have mixed-layer configuration such as molybdenite. The presence of the 2H polytype in both outer-core and halo molybdenite from Hilltop

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indicates that cell expansion by polytypism is not sensitive to order-of-magnitude differences in trace element concentrations, even if observed stacking disorder can be related to lattice-scale defects initiating

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2H→3R transformation. Sub-grain-scale trace element trapping mechanisms discussed for molybdenite

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(Stein et al., 2003) extend to the nanoscale. Furthermore, crystal-structural defects and porosity enhance fluid infiltration and promote incorporation, nucleation and coarsening of impurities.

Despite the abundance of such defects, element maps and narrow concentration ranges in 2 of the 3 cases from Boddington show Re remains locked in molybdenite under deformation. However, in the granite aureole, Re-enrichment along replacement boundaries suggests local dissolution and reprecipitation of molybdenite behind the chalcopyrite-forming reaction front. Replacement by coupled dissolution-reprecipitation reaction (CDRR; Putnis, 2002) results in a precipitate that, even if the same as the parent mineral, will differ in trace element concentration and grain size (fine-grained Re-rich molybdenite in this case). 12

ACCEPTED MANUSCRIPT A stronger case for trace element redistribution via CDRR, expressed best by Re, is Hilltop. The steady, inwards advance of reaction is seen in formation of a well-defined molybdenite halo with

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characteristic fine-grain size and constant Re concentration. The waning stage, when coupling between dissolution and reprecipitation is no longer sustained, is seen as replacement limited to cleavages and

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fractures (erratic Re values in the outer-core). These relationships point to a second molybdenite

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generation, also carrying a CE-U signature, as a halo superposed onto a preserved oscillatory-zoned core.

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Second stage molybdenite at Hilltop suggests an epithermal overprint linked to an unidentified Rerich, Mo-Cu-Au-bearing intrusion, or upgrading precursor molybdenite. Such superposition of events is

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common in young, relatively short-lived magmatic-hydrothermal systems.

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Understanding the molybdenite history from an Archean Au deposit within a granite-greenstone belt

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such as Boddington, centers on the genetic role of granite as a metal source or heat engine. We show here that early, Re-rich molybdenite, attributable to the porphyry-stage, occurs across the deposit. Unlike W

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and CEs, Re is largely unaffected by the orogenic event. Granite-derived fluids, although low in Re (as seen in granite-hosted molybdenite itself; Kalleske, 2010; Guerin, 2011) affect pre-existing diorite-hosted molybdenite, resulting in an overall dilution but retention of higher Re along replacement zones.

The analytical datasets from both deposits show that molybdenite concentrates Au and Ag if CEs are present. This can be attributed to the role of Bi-chalcogenides as scavengers and hosts for Au (Ciobanu et al., 2009b). Coarsening of CEs-minerals with subsequent Au release and reconcentration are attributable to both orogenic and granitic events at Boddington. Aleinikoff et al. (2012) have demonstrated that multiple ages can be measured in individual molybdenite grains. Here we show that such zonation is also reflected by trace element distributions. 13

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Acknowledgements

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The authors thank Newmont Asia-Pacific for supporting two B.Sc. Hons. projects at Boddington, and Barrick

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Cortez for their support with the Hilltop material and for permission to publish. AMMRF is thanked for use of the FEI Helios nanoLab Dual Beam FIB/SEM system. Partial funding for this project was provided by the Department

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of Geology, State University of New York at Potsdam. We appreciate the helpful comments from the two

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anonymous journal reviewers, as well as those of Marc Norman on an earlier version of this manuscript. This is

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TRaX contribution no. 288.

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geochronologic evidence for two alkali porphyry ore-forming events in the Cadia District, New South Wales, Australia. Economic Geology 102, 3-26.

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Figure captions Figure 1. (a) Location of the Hilltop area in central Nevada within the Battle Mountain – Eureka trend and (b) geological sketch map of the Hilltop area (both maps after Kelson et al., 2005, 2008). (c) Simplified drillcore sections for the two holes from the Hilltop prospect (J and L) sampled in this study. Figure 2. Simplified geological map of the Boddington deposit (after Newmont, unpublished data and Kalleske, 2010) with sample locations marked. The inset shows the location of the deposit in the South West terrane of the Yilgarn Craton.

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ACCEPTED MANUSCRIPT Figure 3. Photographs illustrating petrographic aspects of molybdenite in the sample suite. (a) Hilltop sample HTX consisting of porphyritic quartz-feldspar rock containing molybdenite-bearing quartz veins and thin veinlets of Au-bearing arsenian pyrite. Molybdenite grains analyzed are highlighted in the black circle. (b) Hilltop

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sample 97-12 consisting of molybdenite-bearing quartz veins in porphyritic quartz-feldspar rock. Analyzed

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molybdenite grains are highlighted in the black circle. (c) Coarse aggregate of molybdenite lamellae with abundant intergrowths and inclusions of chalcopyrite (Hilltop, sample HT-X) (d) Coarse, felted aggregates of

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molybdenite containing inclusions of Bi-(Pb)-chalcogenides and galena (Hilltop, sample 97-12). (e) Coarse aggregate of molybdenite lamellae containing inclusions of Bi-(Pb)-chalcogenides and native gold along the

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molybdenite cleavage planes (Boddington, sample 25RG). (f) Detail of larger molybdenite lamellae containing

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inter-lamellar inclusions of Bi-tellurides (Boddington, sample 60RG). Figure 4. (a) Plot showing Re variation in the samples. Individual spot analyses are arranged in decreasing order

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within each sample or zone; mean sample concentrations (ppm) are given in red. Mean Re in the BRIM sample (97-10; 90 ppm) agrees well with concentrations given by Kelson et al. (2005) (51 ppm; n=2). Similarly, values

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given here compare with those reported by Stein et al. (2001b) for Re-rich molybdenite at Boddington (328, 344, 435, 832 ppm). Abbreviations: BRIM - intrusive matrix breccias; Fds – feldspar; Qz – quartz. (b) Positive

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correlation between (Au+Ag) and CEs (Bi+Pb+Te) for all samples. (c) and (d) Representative time-resolved

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LA-ICP-MS depth profiles for molybdenite from Hilltop and Boddington to show noisiness for CEs (and W on (d)) contrasting with flat signals for lattice-bound elements. Element concentrations (ppm) are given in brackets. Figure 5. Reflected light photomicrograph and LA-ICP-MS element maps for molybdenite in HT-X (Hilltop). Scales represent 10n counts-per-second values. Cp–chalcopyrite, Qz–quartz. Figure 6. Reflected light photomicrograph and LA-ICP-MS element maps for molybdenite in RG10 (Boddington). Scales represent 10n counts-per-second values. Contoured areas are Au-Ag-rich spots. Figure 7. BSE image and LA-ICP-MS element maps for molybdenite in sample 25NK (Boddington). Scales represent 10n counts-per-second values. Scales represent 10n counts-per-second values. Figure 8. Reflected light photomicrograph and LA-ICP-MS element maps for molybdenite in sample 27RG (Boddington). Scales represent 10n counts-per-second values. Note abundant inter-lamellar inclusions of chalcopyrite. 19

ACCEPTED MANUSCRIPT Figure 9. (a-d) Secondary electron FIB images of molybdenite showing location of FIB cuts (a), nanopores in hairpin fold (b) and CE-mineral inclusions ranging in size from several hundred nm (c) to μm (d). (e) Selected area of electron diffraction (SAED) on [1100] zone in molybdenite showing the c spacing (12Å) characteristic

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Hilltop, Northern Shoshone Range, Battle Mountain-Eureka Trend, Lander County, NV, USA (Au-Ag-Cu-Mo; Eocene)

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drillcore L, 339.3 Qz-Fds ft porphyry

margin Qz veins, coarsegrained

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drillcore intrusive along QzJ, 106.1 matrix breccia veins; fineft (BRIM) grained

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cm-size sheared veinlet of molybdenite Cp

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Southern altered Diorite diorite(Clz, Deep, Qz, Ab, Chl)

mm- to cm sized patches

Bi, Bitell

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97-12

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25NK

10RG

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Boddington, Yilgarn Craton, Western Australia (Cu-Au-Mo; Archean)

S pit (RL108 m)

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Southern altered diorite mm- to cm Diorite (Bt, Qz, Fds, sized Deep, Mu, Chl, Stb) patches, Cp drillcore WBD105 900002

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replacement along cleavages/

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lamellae boundaries

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Abbreviations: Ab - albite; Au - gold; Bi - native bismuth; Bitell - Bi-tellurides; Bt - biotite; Cp - chalcopyrite; Cb cubanite; Chl - chlorite; Clz - clinozoisite; Fds - feldspar;

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Gn - galena; Mu - muscovite; Po - pyrrhotite; Py - pyrite; Qz - quartz; Sph - sphalerite; Stb - stilbite; Stn stannite; Ttd - tetradymite

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Trace element heterogeneity in molybdenite fingerprints stages of mineralization

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Molybdenite carries significant concentrations of chalcophile elements (Bi, Pb, Te) Intragrain heterogeneity reflects multiple stages of growth Trace element chemistry of molybdenite is valuable for petrogenetic interpretation Molybdenite containing chalcophile elements can be an attractor for gold

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Highlights

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