resolving the relative timing of au enrichment in volcanogenic...

©2016 Society of Economic Geologists, Inc.Economic Geology, v. 111, pp. 1495–1508

RESOLVING THE RELATIVE TIMING OF Au ENRICHMENT IN VOLCANOGENIC MASSIVE SULFIDE DEPOSITS USING SCANNING ELECTRON MICROSCOPY-MINERAL LIBERATION ANALYZER:

EMPIRICAL EVIDENCE FROM THE MING DEPOSIT, NEWFOUNDLAND, CANADA

Jean-Luc Pilote,1,† Stephen J. Piercey,1 Stefanie M. Brueckner,1 and David Grant2

1 Department of Earth Sciences, Memorial University of Newfoundland, 300 Prince Philip Drive, St. John’s, Newfoundland, Canada A1B 3X5

2 CREAIT Network, Memorial University of Newfoundland, 230 Elizabeth Avenue, St. John’s, Newfoundland, Canada A1C 5S7

AbstractThe metamorphosed Cambro-Ordovician Ming volcanogenic massive sulfide deposit in northern Newfound-land, Canada, is locally overlain by a unit consisting of mafic to intermediate medium- to coarse-grained volcan-iclastic breccia with up to 10 vol % sulfide clasts. Analysis via mineral liberation analyzer of two sulfide clasts, completed using scanning electron microscope observations, allowed the identification of a number of micro-scopic and submicroscopic electrum grains. These electrum grains occur in three types of textural settings: (1) free electrum grains with tellurides within gangue minerals, (2) inclusions of electrum with tellurides in pyr-rhotite grains, and (3) free electrum grains with base metals and tellurides interstitial between base metals and along cataclastic fractures in pyrite. These three textural settings are similar to those in the underlying massive sulfide orebodies that represent very likely sources to the sulfide clasts. The polylithic nature and angularity of the volcaniclastic fragments in the breccia suggest a postmineralization gravity-controlled debris flow proximal to its source. The mineral assemblage and textures of electrum in the sulfide clasts implies evidence in sup-port of syngenetic and predeformation Au introduction in the volcanogenic massive sulfide deposit, and argues against an orogenic overprint as the cause for Au enrichment.

IntroductionThe origin of Au enrichment in ancient Au-bearing, meta-morphosed massive sulfide deposits is controversial, with some workers arguing for a syngenetic, volcanogenic massive sulfide (VMS)-related origin (Hannington et al., 1986, 1999, 2005; Hannington and Scott, 1989; Larocque et al., 1993; Herzig and Hannington, 1995; Huston, 2000; Dubé et al., 2007a; de Ronde et al., 2011), whereas other workers argue that Au enrichment is due to a later, orogenic overprint on a preexisting massive sulfide deposit (e.g., Tourigny et al., 1989; Marquis et al., 1990; Yeats and Groves, 1998). In many depos-its, Au-bearing sulfide clasts in volcaniclastic rocks occur stratigraphically above massive sulfide orebodies (e.g., Horne: Kerr and Gibson, 1993, Doyon-Bousquet-Laronde: Dubé et al., 2007b; Mercier-Langevin et al., 2007, Mount Read Volca-nics: McPhie and Gemmell, 1992). The nature of Au, the min-eral associations, and textural relationships within these clasts provide important insight into the nature of Au emplacement and are central to understanding the origin of Au enrichment. If Au is found in veins crosscutting the clasts then it could be syngenetic and remobilized by later deformation, or due to orogenic introduction (i.e., equivocal). In contrast, if Au is found armored in sulfide clasts and has similar mineral assemblages and textures to associated massive sulfide min-eralization, it provides supportive evidence for syngenetic Au introduction.

This study was undertaken to examine a well-constrained, intermediate volcaniclastic unit containing sulfide clasts that immediately overlies part of the precious metal-bearing Ming VMS deposit, northern Newfoundland, Canada. The vol-caniclastic unit contains assay values of up to 0.9 g/t Au and

3.85 wt % Cu in the 2.6 m overlying the massive sulfide (L. Pilgrim, unpub. data, 2013); however, the nature of Au in this unit is yet unclear. Although numerous lines of evidence sup-port a syngenetic Au mineralization at the Ming deposit from recent work exclusively on the (semi-)massive sulfide orebod-ies (Brueckner et al., 2014, 2016), the results of this study and the nature of Au in this clast-rich unit provide an additional critical test of the syngenetic versus epigenetic origin for Au enrichment. Here we use for the first time on samples from a different unit at the Ming deposit the combination of scan-ning electron microscopy (SEM)-mineral liberation analysis (MLA) to identify the textural and mineralogical setting of Au-bearing minerals. This technique was used for three rea-sons: (1) it is an efficient technique, at relatively low cost, to examine thousands of mineral grains; (2) it provides a statisti-cally representative analysis of the mineral composition of a sample; and (3) it allows the ability to distinguish extremely fine grained or complexly intergown minerals at submicro-scopic levels that are often very difficult to determine using conventional reflected light microscopy and SEM. The MLA observations were supported by SEM-backscattered electron imaging and reflected light microscopy. The integration of field relationships with SEM-MLA, while focused on VMS deposits herein, provides a powerful tool for resolving fun-damental genetic problems in mineral deposits, regardless of deposit type.

The goals of this study are as follows: (1) the identification of Au-bearing minerals inside the sulfide clasts, (2) character-ization of their textures and mineral associations, and (3) com-parison with the previously studied underlying massive sulfide orebodies (Brueckner et al., 2014, 2016). These results are then utilized to provide insight into Au provenance within the clasts and to assess whether Au enrichment was syngenetic or epigenetic.

0361-0128/16/4428/1495-14 1495Submitted: November 18, 2015

Accepted: April 30, 2016

† Corresponding author: [email protected]

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Geologic Setting

The geology of the Ming deposit and its regional setting are well documented (Tuach and Kennedy, 1978; Hibbard; 1983; Castonguay et al., 2009; Skulski et al., 2010; Pilote and Piercey, 2013; Pilote et al., 2014, 2015), and only a brief sum-mary will be presented here. The Ming deposit is hosted by Cambro-Ordovician (c. 487 Ma) intermediate to felsic rocks of the informally named Rambler Rhyolite formation, Baie Verte Peninsula, Newfoundland (Fig. 1; Hibbard, 1983; Skul-ski et al., 2010). The Rambler Rhyolite formation forms the upper part of the Cambro-Ordovician Pacquet Complex, which is a partial ophiolite sequence (Hibbard, 1983) of the Baie Verte oceanic tract (van Staal, 2007). The Pacquet Com-plex is defined at its base by low Ti boninites, locally cogenetic felsic tuffs, and rhyodacitic flows (Hibbard, 1983; Piercey et al., 1997), and pillowed intermediate Ti boninites (Skulski

et al., 2010). The Pacquet Complex is unconformably over-lain by a sequence of the Early to Middle Ordovician Snooks Arm Group that includes, starting at its base, magnetite-rich siltstone, tholeiitic mafic volcanic rocks, volcaniclastic and epiclastic fragmental rocks, and calk-alkaline flows and frag-mental rocks (Skulski et al., 2010).

The Rambler Rhyolite formation is defined by a 6-km-wide and 2.5-km-thick folded dome-shaped sequence of quartz-phyric rhyodacite, quartz-bearing intermediate to felsic tuff, and tuff breccia (Tuach and Kennedy, 1978; Hibbard, 1983; Skulski et al., 2010; Pilote et al., 2015). The Ming deposit, located in the upper part of the Rambler Rhyolite forma-tion, consists of five orebodies that are hosted within variously hydrothermally altered intermediate to felsic volcanic rocks with a large portion of volcaniclastic rocks. The five orebodies are from the northwest to the southeast: 1807 zone, 1806 zone, Ming North, Ming South and Lower Footwall zone (Fig. 2).

Mineralized lens, projected to surface

RockyPond

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England’s Brook

To Baie Verte

To La Scie

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Intermediate Ti-boninite, island arc tholeiitic pillow basalt, pillow breccia

Rambler Rhyolite formationRhyodacite, rhyolite massive flows, volcaniclastic(ca. 487 Ma)

Pacquet Complex (Cambrian-Ordovician)

Pillow basalt, gabbro

Mafic tuff, epiclastic rocks, turbidites

Pillow basalt

Rhyolite (467 Ma), and felsic tuff (470 Ma)

Mafic epiclastic rocks, black shale

Snooks Arm Group (Ordovician)

Burlington Granodiorite (434 - 430 Ma)Granodiorite

Cape Brulé Porphyry (430 Ma)Intrusions

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Highway and highway number414

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564000m E 566000m E 568000m E

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Fig. 1. Simplified geologic map of the Pacquet Complex with the Ming VMS deposit (modified after Skulski et al., 2010). The different orebodies of the Ming deposit are projected to surface.

SCIENTIFIC COMMUNICATIONS 1497

They consist of strata-bound semimassive to massive sulfides lenses and underlying discordant sulfide stringers. The orebod-ies are Cu rich, precious metal bearing, and locally enriched in Zn. The sulfide assemblage in the orebodies is complex and is dominated by a pyrite-chalcopyrite assemblage with minor pyrrhotite-sphalerite and traces of galena-arsenopyrite ± cubanite ± sulfosalts ± tellurides ± (sulfo-)antimonides ± pre-cious metals ± oxides (Brueckner et al., 2014, 2016).

Three generations of mafic to intermediate intrusive rocks are present in the deposit. They each have distinctive litho-geochemical signatures and are interpreted to be genetically related to the mafic rocks of the Snooks Arm Group cover sequence (Pilote and Piercey, 2013; Pilote et al., 2014, 2015).

Alteration, deformation, and metamorphism

The Rambler Rhyolite formation, host to the Ming deposit, is overprinted by Cambro-Ordovician moderate to intense hydrothermal alteration (Pilote et al., 2014, 2015). Rocks overlying the massive sulfide orebodies of the Ming deposit postdate mineralization and, therefore, escaped ore-related hydrothermal alteration. Silurian-Devonian upper green-schist- to lower amphibolite-grade metamorphism affected all rocks of the Rambler Rhyolite formation and Snooks Arm Group (Gale, 1971; Tuach and Kennedy, 1978; Castonguay et al., 2009, 2014), including at least four phases of regional deformation (D1 to D4), with D2 (and M2) being the most intense (Tuach and Kennedy, 1978; Hibbard, 1983; Caston-guay et al., 2009, 2014). Late syn- to post-D2 biotite, epidote, actinolite, carbonate, and sulfides porphyroblasts overprint structural fabrics defined by various assemblages predomi-nantly composed of quartz-sericite-chlorite-epidote-sulfides, observed within and outside the deposit (Gale, 1971; Tuach and Kennedy, 1978).

Geology of diamond drill hole RMUG13-205

Sulfide clasts described in this study are found in diamond drill hole RMUG13-205, which was drilled in the up-plunge

section of the 1807 zone as part of an underground definition drilling program (Fig. 2). The thickness for each unit reported here is core length and a stratigraphic column is shown in Fig-ure 3. The hole intersects a 3.5-m-thick semimassive to mas-sive sulfide horizon with significant Cu, Ag, and Au contents (11.7 wt % Cu, 17.6 g/t Ag, and 2.7 g/t Au over 0.45 m), and is a critical drill hole to test the source of Au in the clasts given the elevated Au grades.

Composition and texture of the sulfides within the (semi-)massive sulfide horizon vary from medium-grained equigran-ular pyrite to coarse-grained, subrounded pyrite supported by a chalcopyrite- and galena-bearing matrix (Fig. 4a). The sulfide horizon contains up to 10 vol % intermediate volcanic rock fragments that are quartz altered (Fig. 4b). The strati-graphy immediately below the (semi-)massive sulfide horizon is cut by a 6.5-m-thick dark green, chlorite-carbonate-altered, fine-grained gabbro separating the intermediate volcanic rocks from the sulfides. Two intervals of sulfide veins are enclosed within the gabbro with contacts at moderate angle to core axis. A similar mafic dike is repeated at depth. The two dikes enclose a 3.5-m-thick section of fine-grained, actinolite-chlorite-altered, intermediate coherent volcanic rock. Both dikes show chilled margins with the volcanic rock and sulfides.

The (semi-)massive sulfide horizon is immediately overlain by a 10-m-thick unit of polymictic breccia of mafic to inter-mediate composition. This unit contains clasts of different compositions including sulfide-bearing clasts and dark purple to medium gray quartz and quartz-sericite hydrothermally altered coherent rhyodacite clasts (Fig. 4c, d). The sulfide clasts locally make up to 10 vol % of the rock, whereas the rhyodacitic clasts make up to 30 vol %. The majority of the sulfide and silicate clasts are subrounded to subangular, vary in size (≤5 cm), and stretched coaxial to the main stretching lineation related to D2. The matrix of the breccia is fine to medium grained, foliated, and is composed of an assemblage of recrystallized quartz-plagioclase-epidote-chlorite, with bio-tite grains forming postkinematic porphyroblasts, commonly

1807

1806

Ming North

Ming South

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+100 m

0 m -100 m -200 m -300 m -400 m -500 m -600 m

-700 m

-800 m

-900 m

Down plunge

RMUG13-205

18071800718

1806 1

UTM Northing Grid

N

Fig. 2. Map outlining all the orebodies (red = massive sulfide; black = Lower Footwall zone) forming the Ming deposit. Diamond drill hole RMUG13-205 is located in the up-plunge section of the 1807 zone. The gray shaded area represents the approximate maximum known extension of the sulfide-rich volcaniclastic unit. The gray lines and circles represent drill hole traces and collars, respectively.


overprinting chlorite (Fig. 4e). The biotite porphyroblasts are related to younger Silurian-Devonian (or younger) regional metamorphism. Late quartz-epidote veins with traces of sulfide minerals cut the rock and are transposed into the dominant foliation (Fig. 5a). These postmineralization, but predeformation, veins are also found crosscutting the under-lying massive sulfide orebodies, the felsic footwall rocks, and the mafic dikes and only locally contain traces of pyrite and chalcopyrite (Pilote and Piercey, 2013; Pilote et al., 2014). Gold- and Te-bearing minerals have yet to be found within these veins.

The volcaniclastic unit is overlain by a thin (≤1 m), dark purple to black, magnetite-rich siltstone (Fig. 4f). This sedi-mentary horizon represents the base of the Snooks Arm Group and can be traced throughout the deposit (Skulski et al., 2010; Pilote et al., 2015).

Methodology and SamplingSulfide clasts (samples 205-2 and 205-3) were extracted from the volcaniclastic unit in RMUG13-205 (Fig. 5a, b). The clast

samples were collected from drill core, 8.4 and 9.0 m above the upper part of the (semi-)massive sulfide zone. A chip from sample 205-2 was cut parallel to the elongation of the clast, whereas 205-3 was cut perpendicular to the elongation. The chips were then mounted in 40-mm-diam resin rounds to obtain a maximum surface area for analysis, followed by polishing and the application of a carbon coating. The MLA measurements were made on an FEI MLA 650 field emis-sion gun SEM instrument at the CREAIT Centre, Memorial University. Operating conditions included an accelerating voltage of 15 kV and an imaging scan speed of 8 μsec. The use of 15 kV allows for a better resolution during analyses and for imaging on grains of small dimensions (≤5 μm). The mounds were measured in extended backscattered electron (XBSE) mode using a beam current of 10 nA and a spot size of 5.77. X-ray analyses were triggered for a BSE range of 125 to 255; each X-ray measurement was acquired for 10 msec on a 0.5- × 0.5-mm frame with a resolution of 1,000 dpi. Data reduction was performed on MLA Data View (FEI) software version 3.1.4.683.

Ash/

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Dark grey finely laminatedmagnetite-rich siltstone

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Mafic sulfide-bearing polymicticconglomerate to breccia

py-ccp-sp-gn massive sulfidewith intermediate volcanic rockfragments

Chlorite-altered coherent intermediatevolcanic flow

Fine-grained equigranular gabbrowith minor ccp-py veins

205-2 Sample location

Foot

twal

l

Legend

Fig. 3. Stratigraphic section of diamond drill hole RMUG13-205, 1807 zone. Locations of the two samples collected, 205-2 and 205-3, are shown in black stars. Mineral abbreviations: act = actinolite, biot = biotite, carb = carbonate, ccp = chalcopy-rite, chl = chlorite, ep = epidote, gn = galena, mgt = magnetite, py = pyrite, qtz = quartz, ser = sericite, sp = sphalerite, sul = sulfide.


A B

DC

E F

4 cm

gn

ccpsppy

3 cm

quartz alteredrhyodacite

quartz alteredrhyodacite

biotite

Fig. 4. Representative and selected drill core photographs of diamond drill hole RMUG13-205. (A). Massive sulfide consist-ing of rounded, coarse-grained pyrite with interstitial chalcopyrite, galena, and sphalerite. (B). Massive sulfide consisting primarily of fine-grained pyrite-chalcopyrite ± sphalerite with quartz-altered aphanitic rhyodacite fragments. (C). Polymictic medium- to coarse-grained intermediate breccia with rhyodacite and sulfide clasts. Note the elongation of the clasts due to deformation. (D). Subangular quartz-altered rhyodacite in the fine-grained portion of the volcaniclastic unit. (E). Close-up of the fine-grained matrix of the breccia consisting of quartz-plagioclase-sericite-biotite ± epidote ± chlorite. Note the biotite porphyroblasts. (F). Magnetite-rich siltstone overlying with a sharp contact the volcaniclastic unit. Mineral abbreviations are as in Figure 3.

A

B

A

B

Fig. 5. Photographs of samples (A) 205-2 and (B) 205-3 selected for SEM-MLA analyses.


Sulfide Mineralogy and Mineral Textures of the Sulfide Clasts

Sulfide mineralogy

The clasts are composed of a pyrite > chalcopyrite > sphaler-ite > quartz ≈ biotite assemblage that makes up more than 99.9 wt % of the samples (Fig. 6). The remaining percentage consists of an assemblage of tellurides (coloradoite, hessite, and Bi telluride), sulfides (arsenopyrite, galena, and pyrrho-tite), oxides (cassiterite and hematite), and precious metals (mercurian electrum). The wide range of trace mineral phases identified in both samples with MLA underlines the complex ore mineralogy in the sulfide clasts. Table 1 shows results of MLA with values normalized to the abundance of the afore-mentioned list of trace minerals to emphasize their relative abundances. Gold-bearing minerals were identified with MLA as minerals with >10 wt % Au. Brueckner et al. (2014, 2016) showed that Au occurs predominantly as (mercurian)

electrum at the Ming deposit orebodies. Although the relative abundances of Ag, Au, and Hg in electrum vary (Brueckner et al., 2014, 2016), distinctions between the different Au-bearing alloys in the clasts are difficult to make using MLA due to the semiquantitative detection and the small size of the analyzed grains. Hence, the term electrum is used in this paper for all Au-bearing phases that have >10 wt % Au.

Precious metal textures

Textures of electrum in the sulfide clasts can be divided into: (1) free electrum with telluride grains within gangue minerals (Fig. 7a); (2) inclusions of electrum with telluride in pyrrho-tite (Fig. 7b); and (3) free electrum grains with base metals and tellurides interstitial between base metals and along cata-clastic fractures in pyrite (Fig. 7c, d).

The first texture is characterized by the contact of elec-trum with gangue phases (quartz, biotite, chlorite, epidote, albite, and white micas) where quartz is the predominant

Table 1. Result Data of Trace Minerals from MLA in Samples 205-2 and 205-31

RMUG13-205-2 RMUG13-205-3

Sample Particle Grain Particle GrainMineral Formula Wt % Area (%) Area (μm2) count count Wt % Area (%) Area (μm2) count count

Arsenopyrite FeAsS 0.29 0.29 63.00 23 23 9.29 10.13 1,532.00 60 60Cassiterite SnO2 0.15 0.13 28.25 4 4 0.18 0.17 26.25 9 9Coloradoite HgTe 0.93 0.71 154.00 12 12 0.89 0.73 110.50 9 9Electrum (Au,Ag) 6.20 3.11 678.50 98 98 14.60 7.93 1,198.50 140 144Galena PbS 3.13 2.53 552.50 185 185 6.41 5.61 847.75 128 128Hematite Fe2O3 0.63 0.73 159.50 25 25 0.02 0.03 4.25 3 3Hessite Ag2Te 29.49 23.73 5,181.75 406 411 54.01 47.07 7,116.50 771 783Pyrrhotite Fe1-XS 15.82 21.04 4,593.00 1,592 1,597 5.68 8.18 1,237.00 809 811Bi-telluride (Bi,Te) 37.32 29.26 6,390.00 218 220 3.81 3.23 488.75 69 70Unknown2 6.04 18.48 4,034.50 1,754 1,770 5.11 16.92 2,557.50 1,272 1,295

Total 100.00 100.00 21,835.00 4,317 4,345 100.00 100.00 15,119.00 3,270 3,312

1 The list excludes major minerals (pyrite, chalcopyrite, sphalerite, quartz, and biotite) present in the samples and values listed (area (%) and wt %) are nor-malized to 100% relative to the total area covered by the minerals (in μm2)

2 Minerals with unknown X-ray spectra

Breccia(groundmass)

sulfide-rich clastqtz-carb

veinccp

py

qtz+bio

A 5 mm

B 5 mm

qtzccp

py

Fig. 6. Mosaic of part of the backscattered electron (BSE) images associated with MLA scans. Samples (A) 205-2 and (B) 205-3. The yellow crosses represent locations of electrum grains in the samples. Mineral abbreviations are as in Figure 3.


gangue mineral in contact with electrum (Fig. 7a). The silicified horizon capping and the stringer zone underlying the 1806 zone also show this type of precious metal texture (Fig. 8a, b; Brueckner et al., 2014, 2016). However, within the sulfide clasts, electrum is also in contact with epidote.

Moreover, hessite and Bi telluride, and fine-grained chalco-pyrite are proximal to electrum in the hosting gangue mineral assemblage of the clasts. The origin of this texture is unclear and is either attributed to syngenetic deposition or internal remobilization during Silurian-Devonian deformation and

A

C

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D

el

el

el

qtz

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ccp

ccpccp

elsp

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ep

py

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Fig. 7. Backscattered electron images and energy dispersive X-ray (EDX) scans by SEM on samples 205-2 and 205-3 for selected elements. Semiquantitative EDX images show intensities (or relative abundance) of the measured elements. (A). BSE (left) image showing electrum occurring with Bi telluride (BiTe), hessite with gangue minerals such as chlorite, quartz, and epidote. EDX (right) scan of Ag and Au highlighting electrum (i.e., Au = lime green) and hessite (i.e., Ag = red). (B). BSE (left) image showing the occurrence of electrum, enclosed in pyrrhotite, with hessite in contact with chalcopyrite and pyrite. Hessite also occurs along a fracture in a recrystallized pyrite grain. Both chalcopyrite and pyrrhotite are enclosed in pyrite. EDX (right) scan of Ag and Au highlighting electrum (Au = green) and hessite (Ag = red). Note the light red color of electrum indicating a high Ag content. (C). Free grains of electrum coexisting with chalcopyrite and recrystallized pyrite. (D). Brittle fractures in pyrite filled with electrum and sphalerite in close spatial associations with chalcopyrite. Mineral abbreviations are as in Figure 3 with the addition of: hes = hessite.


metamorphism (Brueckner et al., 2016). Although uncom-mon, epidote is also found as a gangue mineral, together with quartz, muscovite, and carbonate minerals in the 1807 mas-sive sulfide orebody (Fig. 8c, d).

In the second texture, electrum occurs with hessite in pyr-rhotite (Fig. 7b). This texture is the most abundant precious metal texture in the observed sulfide clasts. Electrum occurs either as single grains or is intergown in pyrrhotite. In the 1806 and 1807 zones, electrum inclusions within pyrrhotite are accompanied by arsenopyrite and (sulfo-)antimonides, respectively (Fig. 8e; Brueckner et al., 2014, 2016). The ori-gin of these inclusions is unclear and may be the result of late syngenetic decomposition (Dobbe, 1991) or (metamorphic) internal replacement (Cook, 1996).

The last precious metal texture observed in the sulfide clasts of the hanging wall of the 1807 zone is also observed in the underlying orebodies (Fig. 8f). The occurrence of precious metals interstitially between (recrystallized) base metals and/or along cataclastic fractures in pyrite (Fig. 7c) is the product

of internal remobilization/metamorphic liberation of previ-ously syngenetically deposited precious metal phases dur-ing Silurian-Devonian metamorphism (Laroque et al., 1993; Wagner et al., 2007; Brueckner et al., 2014, 2016).

DiscussionVolcaniclastic rocks locally overlying the Ming VMS deposit are enriched in Au and provide critical insight into the origin of Au enrichment in this deposit. Two main models have been invoked for Au enrichment in VMS deposits: syngenetic (e.g., Hannington et al., 2005) and epigenetic (e.g., Evans, 1999). In the clast-rich unit, mineral assemblages and ore textures involving electrum provide key information required to test the syngenetic versus epigenetic origin of Au enrichment.

First, the distribution of electrum and spatially closely asso-ciated minerals (e.g., tellurides) in the volcaniclastic unit are confined to the sulfide clasts. This is shown in sample 205-2 where an MLA scan was done beyond the extent of the sulfide clast, into the host breccia and a crosscutting quartz-carbon-ate vein (Fig. 6b). The input of precious metals from an exter-nal source such as an epigenetic style of Au enrichment (e.g., orogenic) would typically result in the deposition of Au within structurally controlled veins and/or fabrics, commonly discor-dant to and/or associated with carbonate alteration (Dubé and Gosselin, 2007). However, the clasts and their surrounding within the 10-m volcaniclastic unit show no evidence of such features. The presence of electrum exclusively within the clasts and the absence of an overprint of ore-forming fluid-related alteration is, therefore, direct evidence for Au enrich-ment prior to the deposition of the volcaniclastic unit.

In Table 2, precious metal textures observed in sulfide clasts and (semi-)massive sulfides of the Ming deposit are compared. Although there are minor differences, the majority of textures and assemblages found in the clasts are remarkably similar to the (semi-)massive sulfide ores (Fig. 8). Furthermore, the hydrothermally altered rhyodacitic clasts exhibit similar alteration as the underlying footwall rhyodacitic host rocks, together with the sulfide clasts that are spatially restricted and proximal to massive sulfide mineralization, suggest a prove-nance from the immediately underlying host assemblage and mineralization, respectively. Collectively, the above results support a syngenetic, rather than an epigenetic, origin for pre-cious metal-enrichment at the Ming deposit.

Au-Te association

The presence of tellurides in VMS deposits is generally uncommon, nevertheless, the presence of Te-bearing min-erals in close textural association with Au-bearing minerals has been recorded in some metamorphosed VMS deposits (e.g., Abitibi, Egypt, Urals; see Appendix 1). Telluride coex-istence with native gold/electrum, and the possible processes of transport and deposition, have also been documented in modern hydrothermal systems in volcanic arcs and back-arc basins (Watanabe and Kajiruma, 1994; Moss and Scott, 2001; Hannington et al. 2005), and some well-preserved ancient unmetamorphosed VMS deposits (Maslennikova et al., 2008; Maslennikov et al., 2009). Tellurium in VMS deposits is likely to be of magmatic origin as it is commonly mineralogically and chemically associated with other magmatophile volatiles (e.g., Bi, Tl, Sn, Se), and less likely from the hydrothermal

mia

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apy

gn

Fig. 8. Textural variations of precious metals in Ming massive sulfide ore-bodies. (A). Electrum with pyrrhotite, sphalerite, and miargyrite (AgSbS2) on chalcopyrite margin with contact to gangue (semimassive sulfides, 1806 zone). (B). Electrum marginal on chalcopyrite on contact to (silicic) gangue and adjacent to sphalerite and unknown AgCuFeS phase (silicified horizon in contact with massive sulfides, 1806 zone). (C). and (D). Gangue miner-als in pyrite-sphalerite-chalcopyrite-rich massive sulfide under transmitted (cross-polarized) and reflected light, respectively (1807 zone). (E). Electrum as inclusions in and marginal to pyrrhotite, together with arsenopyrite and sphalerite (silicified horizon in contact with massive sulfides, 1806 zone). (F). Electrum with galena and spatially close sphalerite and Bi telluride (BiTe) between recrystallized pyrite (massive sulfides, Ming South zone). Mineral abbreviations: apy = arsenopyrite, cal = calcite, ccp = chalcopyrite, el = elec-trum, ep = epidote, gn = galena, gud = gudmundite, mia = miargyrite, mus = muscovite, po = pyrrhotite, py = pyrite, qtz = quartz, sp = sphalerite. Pho-tomicrographs in (A), (B), and (F) are after Brueckner et al. (2016) and (E) from Brueckner et al. (2014).


leaching of the host rocks (Patten et al., 2015). Tellurium is predominantly transported in the vapor phase (Afifi et al., 1988; Grundler et al., 2013), but the transport as minor aque-ous species can also occur, but is rarer (Zhang and Spry, 1994; McPhail, 1995). Transport conditions of Te in VMS deposits occur in a wide range of temperatures (Maslennikov et al., 2009), and typically in reduced fluids with a high ƒTe2/ƒS2 ratio (Affifi et al., 1988; Zhang and Spry, 1994). Decrease of the lat-ter ratio, increase of the ƒO2, and/or decrease of temperature due to seawater mixing leads to the precipitation of telluride minerals and/or substitution of S2– by Te2– in other minerals (e.g., galena and arsenopyrite; Afifi et al., 1988; Zhang and Spry, 1994; Brueckner et al., 2016). Syngenetic coprecipita-tion of Au and Te is suggested in some metamorphosed and unmetamorphosed VMS deposits of the Urals, as illustrated by the elevated Au contents (up to 5.2 wt % Au) in telluride minerals (Vikentyev, 2006).

The intimate relationship between Au and Te at the Ming deposit is demonstrated by the close spatial association between electrum and tellurides such as hessite and Bi telluride. Much like electrum, the tellurides are restricted to the sulfide clasts and the underlying (semi-)massive sulfide orebodies (Brueck-ner et al., 2014, 2016). Moreover, orogenic Au deposits hosted in the Snooks Arm Group that occur 10 to 20 km north of the Ming deposit have distinct ore mineralogy and associated alteration mineral assemblages (e.g., Fe carbonate, albite, and leucoxene alteration; Evans, 1999; Ramezani, 2000), which are absent at the Ming deposit (Brueckner et al., 2014; Pilote et al., 2015). In particular, the syndeformation quartz-carbonate veins hosting native Au in the orogenic Au deposits are devoid of tellurides (Copeland et al., 2015), and formed at higher temperatures (480°C > T >250°C; Ramezani et al., 2000) than the hydrothermal fluids that precipitated electrum (T ≈ 260°–300°C) and tellurides (<260°C) in the Ming deposit (Brueck-ner et al., 2016). Although the Ming deposit was affected by extensive deformation and upper greenschist-lower amphibo-lite metamorphism, the tellurides and electrum reside in the orebodies and sulfide clasts are spatially associated with one another, and exhibit no evidence of postmineralization enrich-ment (Brueckner et al., 2016). Therefore, a syngenetic Au-Te enrichment of the Ming VMS deposit is proposed.

Genetic implications

The polylithic nature and angularity of the volcaniclastic frag-ments in the breccia are suggestive of a postmineralization

gravity-controlled debris flow rather than a pyroclastic flow (Fig. 9; McPhie et al., 1993). It is difficult to determine the specific cause of this flow but considering (1) the formation of nearby intermediate to rhyolitic dome(s) and synchronous mafic volcanic rocks based on the current understanding of the regional geologic setting (Castonguay et al., 2009; Skulski et al., 2010), (2) volcanic architectural reconstruction of the deposit (Pilote et al., 2015), and (3) the chaotic paleoseafloor topography in this type of environment (Ross and Mercier-Langevin, 2014), collapse of at least part of the marginal domal edifice may have triggered mass flow of volcanic debris. During this process, the sulfide clasts were derived from the underlying (semi-)massive sulfides along its flow path and incorporated into the volcaniclastic unit (Fig. 9). The sul-fide clasts are relatively small (≤5 cm across) compared to sulfide-rich debris flows overlying other VMS deposits (e.g., Bousquet-Laronde; Dubé et al., 2007b, Mercier-Langevin et al., 2007), and one could argue that with permissive dynamic properties of the host mass flow, small-size fragments, includ-ing the sulfide clasts, can travel substantial distances (i.e., kilo-meters; McPhie et al., 1993). However, the sulfide-rich breccia is restricted to the 1806 and 1807 zones of the deposit; hence, supporting proximal deposition rather than distal. The pres-ence of associated altered volcaniclastic fragments (mostly silicified; Fig. 4d) in this unit is also an indication of the prox-imity of the source. Moreover, owing to textural resemblance of the electrum grains and associated minerals with textures observed in the underlying orebodies, it is extremely likely that the sulfides originated from the 1806 and/or 1807 zones.

Despite the pervasive and multistage deformation that affected rocks in and associated with the Ming deposit (Cas-tonguay et al., 2009; Pilote et al., 2014), and the presence of pre- to postdeformation quartz-epidote-bearing veins cutting the volcaniclastic unit, electrum is restricted to the sulfide clasts. The mineral associations and textural characteris-tics of electrum are in agreement with the genetic model of Brueckner et al. (2014, 2016) in which precious metals were introduced into the hydrothermal system and deposited syn-genetically to form the Ming VMS deposit.

The application of MLA

The application of MLA allowed answering the fundamental questions outlined in this study in a relatively short analyti-cal time frame (~20 h) and significantly minimized the poten-tial for operator bias and human error (e.g., overlooking the

Table 2. Comparison of Precious Metal Textures Observed in Sulfide Clasts in the Stratigraphic Hanging Wall and in (Semi-)Massive Sulfide Horizons of the Ming Deposit

Sulfide clasts of the (Semi-)massive sulfide orebodiesPrecious metal texture hanging wall (this study) (Brueckner et al., 2014, 2016)

Free electrum grains in chalcopyrite with contact to other sulfides x xFree electrum grains on sulfide-gangue contact x xElectrum as product of tennantite decomposition - xPrecious metals between grain boundaries of recrystallized pyrite/arsenopyrite and/or x x along cataclastic fractures in pyrite/arsenopyriteElectrum with arsenopyrite - xElectrum as inclusion in pyrrhotite partly accompanied by arsenopyrite or (sulfo-)antimonides x x

Notes: x = texture observed, - = texture not observed


presence or underestimating the amount of microscale pre-cious metal grains due to grain size), and at a much higher resolution compared to common petrographic or exploratory SEM methods (Sylvester, 2012). Although, the application of MLA is often focused in applied mineralogy and metal-lurgical processes (Fandrich et al., 2007; Ford et al., 2011; Kelvin et al., 2011), heavy mineral provenance in sediments and sedimentary rocks (Lowe et al., 2011; Tsikouras et al., 2011), and till and stream-sediment prospecting (Wilton and Winter, 2012), it can be a strategic tool when used with basic geologic, stratigraphic, mineralogical, and alteration observa-tions to solve genetic problems in the study of metallic min-eral deposits.

ConclusionsThe use of SEM-MLA on natural samples has shown to be an effective analytical method in detecting a range of submi-croscopic minerals, such as electrum. Hence, we were able to

identify all the electrum grains (≥0.5 μm in size) present on cut surfaces of sulfide-bearing clasts that are part of a volcan-iclastic unit, immediately overlying the Cambro-Ordovician Ming VMS deposit. The SEM-MLA results show that elec-trum is restricted to the sulfide clasts and share similar textural characteristics and mineral associations with the underlying 1806 and 1807 massive sulfide orebodies. The presence of electrum in the sulfide clasts with textural and assemblage features identical to underlying massive sulfide mineralization is evidence for syngenetic and pre-deformation Au enrich-ment in the VMS deposit. The results herein also illustrate how SEM-MLA can be a useful tool to provide information to answer genetic questions on how mineral deposits form.

AcknowledgmentsThis research has been financially supported by a Natu-ral Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, by the NSERC-Altius Industrial

C) Final stage (deposition of sediments and dike intrusions feeding the cover sequence)

B) Debris flow stage (incorporation of sulfide fragmentsinto the volcaniclastic rock)

18061807

Mafic flows andepiclastic rocks

Mafic dikes

Magnetite-rich siltstone

Flow direction

18071806

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Fig. 9. Genetic model for the origin of the massive sulfide clasts in a submarine volcaniclastic mass flow deposit overlying the Ming VMS deposit. See text for explanation.


Research Chair in Mineral Deposits supported by NSERC, Altius Resources Inc., and by the Research and Development Corporation of Newfoundland and Labrador. We sincerely thank Rambler Metals and Mining PLC for drill core access and logistical support during the project. Editorial and scien-tific review by P. Spry and D. Huston greatly improved the manuscript.

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e

an

d ar

e as

soci

ated

with

Gai

sk d

epos

it C

u≥Zn

(Au,

Ag)

L

ower

-Mid

dle

Ura

l Mts

., R

ussi

a U

nkno

wn,

but

tellu

rium

(Te)

, tet

rady

mite

chal

copy

rite

-ten

nant

ite-

(“U

ralia

n ty

pe”)

Dev

onia

n

ex

tens

ivel

y

(Bi 2T

e 2S)

, tel

luro

bism

uthi

te

te

trah

edri

te-g

alen

a

defo

rmed

(Bi 2T

e 3),

petz

ite (A

g 3A

uTe 2

),

Pyrr

hotit

e-be

arin

g or

es c

arry

sylv

anite

((A

g, A

u)2T

e 4),

only

min

or a

mou

nts

ofSa

fyan

ovsk

C

u≥Zn

(Au,

Ag)

L

ower

-Mid

dle

Ura

l Mts

., R

ussi

a U

nkno

wn,

but

kren

neri

te (A

g 3A

uTe 8

),

te

lluri

des

de

posi

t

(“U

ralia

n ty

pe”)

Dev

onia

n

no

n-de

form

ed

C

alav

erite

(AuT

e 2),

D

iscr

ete

Au-

, Ag-

and

Te-

mon

tbra

yite

(Au,

Sb) 2

Te3)

,

min

eral

s ro

utin

ely

form

fine

Uch

aly

depo

sit

Zn>C

u (A

u, A

g)

Low

er-M

iddl

e U

ral M

ts.,

Rus

sia

Unk

now

n, b

ut

em

pres

site

(AgT

e),

in

clus

ions

insi

de p

yrite

,

(“

Ura

lian

type

”)

D

evon

ian

exte

nsiv

ely

m

uthm

anni

te (A

uAgT

e 2),

chal

copy

rite

, and

sph

aler

ite

defo

rmed

voly

nski

te (A

gBiT

e 2)

Man

y te

lluri

des

cont

ain

A

ltaite

, hes

site

and

tetr

adym

ite

so

lid-s

olut

ion

gold

Uze

lgin

sk

Zn>C

u (A

u, A

g)

Low

er-M

iddl

e U

ral M

ts.,

Rus

sia

Unk

now

n

are

rela

tivel

y co

mm

on

depo

sit

(“

Ura

lian

type

”)

D

evon

ian

Nat

ive

tellu

rium

, Au-

and

A

u-A

g-te

lluri

des

are

rare

Yam

an-K

asy

C

u≥Zn

(Au,

Ag)

Si

luri

an

Ura

l Mts

., R

ussi

a A

bsen

t; lo

cally

B

i-Pb

tellu

ride

s, a

ltaite

(PbT

e),

Dis

sem

inat

ed te

lluri

des

in d

rusy

N

ovos

elov

et a

l. (2

006)

depo

sit

(“

Ura

lian

type

”)

ze

olite

faci

es

he

ssite

, (A

g 2Te

), st

uetz

ite

ch

alco

pyri

te w

ithin

sph

aler

ite-

an

d re

fere

nces

(Ag 7

Te4)

, syl

vani

te (A

gAuT

e4),

c

halc

opyr

ite-p

yrite

-mar

casi

te

th

erei

n M

asle

nnik

ov

em

pres

site

(AgT

e), c

erve

lleite

chim

neys

et a

l. (2

009)

and

(Ag 4

TeS)

, nat

ive

tellu

rium

O

verg

row

th o

f var

ious

tellu

rim

refe

renc

es th

erei

n

(T

e), t

ellu

rium

oxi

des,

min

eral

s pa

rtly

obs

erve

d

Pb

-Ag-

Au

tellu

ride

s Pb

-Ag-

Au

tellu

ride

s (s

ylva

nite

,

stue

tzite

, hes

site

, em

pres

site

,

and

elec

trum

) par

tly a

s m

icro

-

incl

usio

ns in

cha

lcop

yrite

Cer

velle

ite in

out

er p

art

(c

halc

opyr

ite z

one)

of m

arca

site

-

chal

copy

rite

-sph

aler

ite c

him

neys

as s

epar

ate

grai

ns, i

nter

grow

n

with

hes

site

-col

orad

oite

, or

thin

vein

lets

in s

ylva

nite

-hes

site


Bab

aryk

dep

osit

Cu-

Zn-P

b-A

u±ba

rite

Pa

loez

oic

Ura

l Mts

., R

ussi

a U

nkno

wn

Cer

velle

ite-li

ke s

ulfo

tellu

ride

Su

lfote

lluri

de is

in b

ande

d py

rite

- N

ovos

elov

et a

l. (2

006)

(“K

urok

o ty

pe”)

chal

copy

rite

-sph

aler

ite-b

orni

te-

ga

lena

ore

s w

ith te

nnan

tite-

te

trah

edri

te a

s an

hedr

al to

elon

gate

d gr

ains

with

in g

alen

a

A

t the

gal

ena-

born

ite g

rain

bou

ndar

y

A

ssoc

iate

d m

iner

als

are

chal

coci

te,

st

rom

eyer

ite a

nd e

lect

rum

Seve

ro-

Cu-

Zn-P

b-A

u±ba

rite

Pa

leoz

oic

Ura

l Mts

., R

ussi

a U

nkno

wn

Hes

site

(Ag 2

Te),

Te

lluri

des

occu

r in

pol

ymet

allic

N

ovos

elov

et a

l. (2

006)

Uva

ryaz

hsko

e

(“

Kur

oko

type

”)

cerv

elle

ite (A

g4Te

S)

or

e do

min

ated

by

spha

leri

te-

an

d re

fere

nces

de

posi

t

ga

lena

-ten

nant

ite-t

etra

hedr

ite

th

erei

n

with

trac

es o

f ele

ctru

m-n

ativ

e

silv

er-a

rgen

tite

Cer

velle

ite is

ass

ocia

ted

with

elec

trum

, nat

ive

silv

er a

nd a

cant

hite

Min

g de

posi

t C

u(-A

u)

Cam

bro-

A

ppal

achi

an M

ts.,

U

pper

H

essi

te (A

g 2Te

), al

taite

Te

lluri

des

pref

eren

tially

with

gal

ena

Bru

eckn

er e

t al.

(201

6)

O

rdov

icia

n

New

foun

dlan

d,

gr

eens

chis

t/

(PbT

e), t

sum

oite

(BiT

e),

±

spha

leri

te ±

pyr

rhot

ite w

ithin

and

this

stu

dy

C

anad

a

low

er

un

nam

ed B

i-tel

luri

de

(s

emi-)

mas

sive

sul

fides

am

phib

olite

(Bi 3T

e 2)

Tellu

ride

s ne

ver

in d

irec

t

faci

es

cont

act w

ith e

lect

rum

Um

Sam

iuki

Zn

-Cu-

Pb-A

g Pr

ecam

bria

n E

aste

rn D

eser

t,

Gre

ensc

hist

C

erve

lleite

(Ag 4

TeS)

, Te

lluri

des

are

asso

ciat

ed w

ith

Hel

my

(199

9)

depo

sit

E

gypt

faci

es

he

ssite

(Ag 2

Te)

sp

hale

rite

-pyr

ite-c

halc

opyr

ite-

ga

lena

- bor

nite

-ten

nant

ite-

te

trah

edri

te-e

lect

rum

Cer

velle

ite o

ccur

s as

sm

all,

su

bhed

ral g

rain

s cl

ose

to m

argi

ns

of

hes

site

and

gal

ena

or a

s

incl

usio

ns in

sph

aler

ite, w

here

it is

asso

ciat

ed w

ith s

ilver

-ric

h bo

rnite

-

chal

copy

rite

-hes

site

Mat

taga

mi L

ake

Zn

-Cu

Arc

hean

Su

peri

or P

rovi

nce,

G

reen

schi

st

Alta

ite (P

bTe)

, hes

site

Te

lluri

des

occu

rrin

g in

spe

cific

T

horp

e an

d H

arri

s

depo

sit

Q

uebe

c, C

anad

a

faci

es

(A

g 2Te

), (a

ntim

onia

n an

d

“t

ellu

ride

zon

e” p

artly

with

in

(1

973)

ferr

oan)

mat

taga

mite

mas

sive

sul

fide

ore

and

part

ly

((

Co 0

.65,

Fe 0

.37,

Sbx) 1

Te2)

,

with

in c

hlor

itize

d po

rphy

ritic

tellu

r-an

timon

y (S

b 1.9Te

3),

m

etar

hyol

ite

un

know

n A

g-Sb

tellu

ride

Te

lluri

des

occu

rrin

g w

ith

(A

g 0.9Sb

Te2)

spha

leri

te-c

halc

opyr

ite-

co

balti

te-p

yrrh

otite

-pyr

ite

RE

FE

RE

NC

ES

Bru

eckn

er, S

., Pi

erce

y, S

., Pi

lote

, J.-L

., L

ayne

, G.,

and

Sylv

este

r, P.

, 201

6, M

iner

alog

y an

d m

iner

al c

hem

istr

y of

the

met

amor

phos

ed a

nd p

reci

ous

met

al-b

eari

ng M

ing

depo

sit,

Can

ada:

Ore

Geo

logy

R

evie

ws,

v. 7

2, p

. 914

–939

.H

elm

y, H

.M.,

1999

, The

Um

Sam

iuki

vol

cano

geni

c Zn

-Cu-

Pb-A

g de

posi

t, E

aste

rn D

eser

t, E

gypt

: A p

ossi

ble

new

occ

urre

nce

of c

erve

lleite

: Can

adia

n M

iner

alog

ist,

v. 3

7, p

. 143

–154

.M

asle

nnik

ov, V

.V.,

Mas

lenn

ikov

a, S

.P.,

Lar

ge, R

.R.,

and

Dan

yush

evky

, L.V

., 20

09, S

tudy

of t

race

ele

men

t zon

atio

n in

ven

t chi

mne

ys fr

om th

e Si

luri

an Y

aman

-Kas

y vo

lcan

ic-h

oste

d m

assi

ve s

ulfid

e de

posi

t (s

outh

ern

Ura

ls, R

ussi

a) u

sing

lase

r ab

latio

n-in

duct

ivel

y co

uple

d pl

asm

a m

ass

spec

trom

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PMS)

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ic G

eolo

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111–

1141

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ovos

elov

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Zayk

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Yako

vlev

a, V

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2006

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ulfo

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s fr

om v

olca

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assi

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amor

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min

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resolving the relative timing of au enrichment in volcanogenic...

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