Geological Fieldwork 2010, Paper

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1 Atlin Placer Gold Nuggets Containin ng Mineral and Rock Matter: Implications for Lode Gold Exploration by M.G. Mihalynuk, T.K. Ambrose 1, F.A.M. Devine 2 and S.T. Johnston 1 KEYWORDS: placer gold, intrusive-related gold, Atlin, Yellow Jacket, gold deposit, listwanite, quartz-carbonate- Creek, Otter Creek, Snake Creek, Quartz Creek, Ruby Creek, Pine Creek, Fear Creek, anthropogenic gold mariposite alteration, Surprise Lake batholith, Boulder nuggets INTRODUCTION Placer gold deposits are derivedd from lode gold bedrock sources. Thus, gold placers are a first-order exploration vector for lode gold deposits. In many placer camps lode gold sourcess remain elusive as past attempts to trace placer mineralization upstream to lode source have not met with success. However, an oft-overlooked and effective means for establishing a geological context for lode gold source is to look for diagnostic mineral matter thatt is intergrown with placer gold. Non-quartz mineral or rock matter is of most use as quartz is ubiquitous. In Atlin placer camp of northwestern British Columbia (Figure 1), this methodology was applied to juvenile placer depositss of Fear Creek (Sack and Mihalynuk, 2004) and diagnostic mineral intergrowths of gold with thorite and cassiterite were identified. These minerals provide an unambiguous genetic linkage to U, Th and Sn-enriched Surprise Lake batholith (Figure 2). Such a relationship is not surprising given that most of productive placer streams in Atlin camp have at ir headwaters Surprise Lake batholith or its rmal metamorphic halo (Figures 2, 3). However past exploration efforts in Atlin placer camp have followed ultramafic-associated lode gold deposit model, focusing on widely distributedd altered ultramafic rocks. Indeed, within entire Atlin camp, only significant lode gold production is from altered mafic-ultramafic rocks at Yellow Jacket deposit. As a test for utility of searching for more subtle intrusive-related lode gold sources of gold, this project 1 University of Victoria, Victoria, BC 2 Merlin Geosciences Inc., Atlin, BC This publication is also available, free of charge, as colour digital files in Adobe Acrobat PDF format from BC Ministry of Forests, Mines and Lands website at empr.gov.bc.ca/ /Mining/Geoscience/PublicationsCat alogue/fieldwork. Figure 1. Location of Atlin placer camp in northwestern British Columbia. looked for intrusive-related mineral matter intergrown withh gold in nuggets recovered from five established placer streams in Atlin camp. So far, we have been unable to repeat results of Sack and Mihalynuk (2004) in any or placer stream. We find no clear signal of an intrusive-related source for placer gold, nor do we see evidence of a listwanite lode gold source. In at least one placer stream, gold is intergrown with graphitic argillite in stream bed. LODE GOLD EXPLORATION Persistent exploration efforts in placer camps can succeed in revealing lode gold sources. One recent success story is at margin of historic Klondike district (Lowey, 2006), where a resurgence of lode gold Geological Fieldwork 2010, Paper

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3 Figure 3. A view to north over placer operation on Snake Creek. In middle background are disturbed white gravels of Boulder Creek which are largely derived from underlying Surprise Lake batholith. Light-coloured slopes on horizon are underlain by Surprise Lake batholith. exploration has followed discovery of White Gold deposit in 2008 (Burke and Lewis, 2010). About 450 km to south, lode gold exploration on outskirts of Atlin has been on-goingg for more than a century. It was not until 2008, however, that viable lode gold production was demonstrated by Prize Mining Corporation (n Muskox Minerals Corp.) with liberation of kg of gold (doré with unspecified fineness) from a 2880 tonne bulk sample of Yellow Jacket deposit (Figure 2; Dandy and Price, 2010). Yellow Jacket deposit The Yellow Jacket deposit has a historical resource estimate of tonnes at g/t Au (non-ni compliant; Schroeter and Pinsent, 2000). Lode gold is hosted in a faulted and altered mafic-ultramafite rock package beneath Pine Creek valley floor, near location of first claims staked within placer camp in Ash (1994) recognized quartz-carbonate- was mariposite (chrome mica)-gold association and successful in extracting mariposite from which undisturbed radiogenic argon release spectra could be obtained 2. These releasee spectra provided ages of ~ Ma, consistent with cooling following tectonic 1 Placer workings that were about 50 years old were reported by early placer miners ca These old workings weree probably of Russian origin, and indicate that low-grade placers were known in Atlin camp by ca. 1850, prior to discovery of rich placer deposits on Pine Creek (Bilsland, W.W. (1952): Atlin, : The story of a gold boom; British Columbia Historical Quarterly, Volume 16, Numbers 3 and 4; Reprinted in 1971 by Atlin Centennial Committee, 63 pages with pictorial supplement) University of Victoria, Victoria, BC. 2 Subsequent attempts (by MGM) to acquire undisturbed spectra from mariposite collected from altered ultramafic rocks elsewhere in Atlin region have not been successful. emplacement of Atlin ophiolite and accretionary complex and intrusion of Fourth of July batholith which cuts and immediately postdates mplacement fabrics (Mihalynuk et al., 2004). Thus, Ash (2001) concluded that gold deposition was genetically related to ophiolite emplacement and that The placers are considered to be derived from quartz lodes previously contained within ophiolitic crustal rocks" (page 25). More recent work at Yellow Jacket deposit has shown that highest gold grades are associated with a darkk green, fine-grained andesite (Dandy and Price, 2010). This unit is described by Dandy and Price (2010) as forming irregular pods and slivers and containing 10-15% quartz phenocrysts along with hornblende±biotite and/ /or plagioclase. These authors attributed concentration off auriferous vein material to competency contrast between andesite and enclosing altered ultramafite. The brittle andesite shattered and spaces thus formed between fragments were n flooded with carbonate and auriferous quartz. Dandy and Price (2010) also believe that Yellow Jacket gold mineralization to be related to Pine Creek fault which cuts ophiolite mplacement fabrics of Ash (1994). Gold mineralization and associated alteration and veining assemblagess that occur along Pine Creek fault are, refore, related to a younger mineralizing event, of as-yet undetermined age. Late-stage mariposite, which is spatially related to gold mineralization along Pine Creek fault zone, could display reset ages or could record age of mineralizing event. Furr analysis of mariposite in areaa needs to be undertaken to confirm timing relationships of local gold mineralization and alteration events. It is possible that re are multiple stages of mariposite-forming alteration within ultramafic rocks in region, related to different structural and intrusive events. If gold mineralization at Yellow Jacket is typical of lode gold sources for most of placer gold in Atlinn camp, n quartz, carbonate, pyrite and mariposite should occur as intergrowths with some of impure goldd nuggets. In our investigation of nuggets from five placer creeks we have observed quartz, carbonate and evidence of weared pyrite and or minerals/alloys, but so far, no mariposite. COMPOSITION OF IMPURE NUGGETS Results of our study of impure nuggets are grouped according to composition of impurity. Here we present observations of rock matter, carbonate, pyrite and mercury. Nuggets that are an intergrowth of gold and rock weree obtained from Otter, Snake and Boulder Creeks. At this point in study, compositional analysess have been attempted only for rock within Otter Creek nuggets. Geological Fieldwork 2010, Paper

4 Analytical Methods All elemental analyses reported here are semi- quantitative. They have been obtained using University of Victoria Advanced Microscopy Facility Hitachi S scanning electron microscope (SEM) fitted with a Bruker Quantax energy-dispersive x-ray spectroscopy (EDX) system. Operating conditions were optimized for EDX analysis of both points and fields on grains. Working distance was set to approximately 15mm with a beam voltage of 20kV. Samples were mounted to aluminum or carbon stubs with eir carbon tape or carbon paste. One sample from upper Otter Creek required a carbon coating as it was not sufficiently conductive. Otter Creek nuggets Rock matter intergrownn with gold in nuggets recovered from upper portions of Otter and Snake creeks appears to be of local derivation. In both cases hostrock is fine-grained sediment that has been recrystallized. Like most placer workings, Zogas operation on upper Otter Creek recovers gold from both pay gravels and regolith (Figure 4). Processing of regolith is primarily for detrital gold that has worked its way down into cracks within fractured and weared bedrock. Rock scraped from placer pit in 2010 was black, graphitic and phyllitic argillite, commonly with quartz veins less than 1 cm thick. We know of two recently recovered nuggets with pieces of graphitic argillite attached. The smaller of se, donated to us for analysis, is shown in Figure 5. Scanning Electron Microscope and Energy Dispersive Spectral analysis revealed nothing unexpected. The mineral matter is mainly quartz, with some Mg-Fe-bearing mineral, probably chlorite, and graphite(?); however, because it was necessary to coat nugget with conductive carbon, carbon analyses are meaningless. Figure 4. Aerial overview of Zogas operation on upper Otter Creek. Thee deep part of pit at right is excavated down to bedrock. Figure 5. Gold intergrownn with graphitic phyllite like thatt of local creekk bedrock (about 1.5 cm across). Snake Creek nuggets Near top end of Snake Creek, a small dredging operation has recovered nuggets of gold intergrown with rock (Figure 6). The rock matter appears to be recrystallized fine grained siliceouss sediment. Unfortunately we were unable to obtain a sample for analysis. Outcrops of rmally altered cherty argillite and argillaceous chert located within drainage basin at an elevation above dredging operation represent bedrock from which gold was likely derived. Boulder Creek nugget A nugget of gold intergrown with very dark green to black rock was obtained from Boulder Creek. No elemental analysis of rock has yet been performed. It appears monomineralic, has a hardness of ~3 (Figure 7), and is probably chlorite or serpentine. Figure 6. Nugget from Snake Creek is gold intergrown with rock which is like rmally-altered argillaceous chert comprising bedrock upstream. Scale divisions are millimetres. 68 British Columbia Geological Survey

5 Figure 7. Photomicrograph of intergrown gold and rock nugget from Boulder Creek. The homogeneous rock matter was easily scratched to produce furrow. Scale markings are millimetres. Figure 8b. SEM photomicrograph of sponge-like fissures created by mercury attack of a Boulder Creek nugget. Gold with mercury Gold nuggets with conspicuous light silver patches were recovered from Quartz and Boulder creeks, and are common in placer gold recovered from Pine Creek. This patchy silver colouration is known as mercury staining (Figures 8a, b, c). SEM-EDX analyses of silver patches confirmed that y are mixtures of mercury and gold. While silver patches are distinct, y have diffuse margins. At high magnification, surface of gold appears fissured (Figure 8b) as a result of amalgam removed following attack by mercury. Figure 8c. EDX analysis of a broader field of view includes quartz and Fe-Mg mineral, probably chlorite. Mercury wass commonly used in most old placer camps to aid in recovery of fine gold from heavy separates, and staining from introduced sources of mercury is well known in Pine Creek, especially near old Discovery showing. Modern knowledge of cumulative toxicity of mercury in humans and environment has resulted in severe curtailment of its use in all but least regulated of nations. Naturally occurring native mercury is also common in most placer camps and we argue below that mercury etching seen on nuggets from Boulder Creek could be a natural phenomenon rar than anthropogenic. Figure 8a. Nuggets from Quartz Creek with silver patches. Width of top nugget is about 1.5 mm. PSEUDO-BIOGENIC GOLD One gold nugget from Boulder Creek was analyzed at highh magnification to reveal a network of ovoid and filiform morphologies which resemble bacteriaa (Figure 9). Although evidencee for gold mobility in biofilms has Geological Fieldwork 2010, Paper

6 Figure 9. Pseudo-biogenic features on surface of a gold nugget from Boulder Creek. EDX analysis shows field of view to be Au with minor Ag. recently been demonstrated for gold nuggets in Australia (Reith et al., 2010), textures that we observed are probably artefacts. Identical textures were discovered by John Watterson of US Geological Survey during an SEM study of acid-cleaned gold nuggets from Lillian Creek in Alaska. These textures were found to be widespread in Alaskan placer deposits prompting suggestion that bacteria was widely implicated in Alaskan gold nugget formation (Watterson, 1992). Watterson later retracted this hyposis after he was able to reproduce pseudo-biogenic structures with a combination of amalgam attack and nitric acid leaching (Watterson, 1994). Treatment of nuggets with nitric acid is a common practice in many placer camps and is specifically used to remove mercury staining. We suspect that Boulder Creek nuggets have NOT been subjected to nitric acid etching because such treatment leaves gold bright and shiny; whereas Boulder Creek nuggets are dull and partly Fe-oxide stained. These nuggets may have been subjected to low ph fluids formed naturally through aqueous oxidation of pyrite which generates sulphuric acid. Cubic pits within some of gold nuggets imaged suggest that pyrite, once intimately intergrown, has been removed by oxidation or mechanical action. Neverless, it is difficult to rule out possibility that acid cleaned nuggets were subsequently stained. Such staining can occur in a few days if nuggets are left in waterlogged sluice concentrates toger with rusting magnetite and pyrite. MINERAL INCLUSIONS IN GOLD Mineral inclusions within gold nuggets of Atlin placer camp have been identified on basis of crystal morphology and composition as determined by semi- include: quantitative EDX analyses. Identified minerals chlorite, Mg-calcite or dolomite, cerussitee (PbCO 3 ), a range of Fe-oxides/hydroxides and quartz. Except for cerussite, se minerals are ubiquitous within veins and country rocks of Atlin camp. Cerussitee is a common alteration product of galena and its occurrence in gold nuggets may indicate that gold streaks occur within sulphide veins. Auriferous polymetallic veins at old Atlin Ruffner mine, located 5 km northwest of Boulder Creek headwaters (Figure 2), are associated with carbonate-altered lamprophyre dikes which cut Fourth of July batholith and are, in turn, cut by Surprise Lake batholith.. Ore produced from 1916 to 1981 from Ruffner had an averagee grade of 5% combined lead and zinc, 6000 grams silver and 0.42 grams gold (MINFILE, 2006). Quaternary ice-flow Blyth, 1993) and opposite to that required to direction is to northwest (Levson and carry ice-scoureto Boulder Creek drainage. However, skarn materials from Atlin Ruffner deposit mineralization at South and Silver Diamond prospects (MINFILE, 2006), located at margin of Surprise Lake batholith in Boulder Creek drainage (Figure 2, locations 1 and 2), is reported to contain galena. Gold is not reported as a commodity in se prospects, but intervening Sunbeam occurrence (Figure 2, location 3) purportedly returned high gold assays (MINFILE, 2006). Anthropogenic nuggets For more than 100 years placer miners have been living andd working along placer streams of Atlin camp. During much of this time waste materials were discardedd into bush or burned. Trash burning remains most common form of garbage disposal. In mid- barrels and in last few decades, electronic circuitry might have been added s plastics would have been introduced to burning Somee of samples that we analyzed are likely burning barrel nuggets. One nugget from Boulder Creek is primarily composed of lead, probably originally part of a lead acid battery. Anor Boulder Creek nugget is a mixture of plastic and gold, perhaps product of a smelting mishap, an accidental cabin fire, or a melted electronics component.. A nugget from Snake Creek is relativelyy pure gold with a splattered droplet of lead-tin- industry prior to widespread replacement of lead-based solders with tin-silver-copper±antimony alloys. It is likely zinc alloyy (Figure 10) of type used in electronics product of a melted circuit board. SUMMARY Analysis of intergrown rock and gold nuggets in Otter andd Snake creekss show that in each case rock matter can be explained by a local bedrock source. In Otter Creek at least, a local source is consistent with angular, gold rich colluvium within pay gravels (Levson, 1992). A rock-gold nugget from Boulder Creek could be product off altered lamprophyre dike, but this hyposis needs to be tested with furr analyses. If so, it might have been sourced from mineralization similar to that of e nearby Atlin Ruffner mine where auriferous 70 British Columbia Geological Survey

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8 Nanoparticle factories: Biofilms hold key to gold dispersion and nugget formation; Geology, Volume 38, pages Sack, P.J. and Mihalynuk, M.G. (2004): Proximal goldcassiterite nuggets and composition of Fear Creek placer gravels: clues to a lode source near Atlin, B.C.; in Geological Fieldwork 2003, BC Ministry of Energy, Mines and Petroleum Resources, Paper , pages Schroeter, T.G. and Pinsent, R.H. (2000): Gold production, resources and total inventories in British Columbia ( ); BC Ministry of Energy, Mines and Petroleum Resources, Open File , 96 pages. Watterson, J.R. (1992): Preliminary evidence for involvement of budding bacteria in origin of Alaskan placer gold; Geology, Volume 20, pages Watterson, J.R. (1994): Artifacts resembling budding bacteria produced in placer-gold amalgams by nitric acid leaching; Geology, Volume 22, pages British Columbia Geological Survey