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ABSTRACT

AN ELECTRON MICROSCOPY INVESTIGATION OF AND ASSOCIATED FROM ROUND , NEVADA

by Michelle Burke Details regarding the mineralization patterns of gold in epithermal systems are poorly understood. The of interfaces of gold with dominant minerals such as quartz and adularia is not constrained. A refined understanding of gold microtextures and the interface between gold and associated minerals could provide insight into the details of gold growth and mineralization and may contain indicators of gold concentration mechanisms. Furthermore, a refined understanding of the interface may explain variation in extraction efficiency and may enable enhancement of recovery methods. Macrocrystalline samples from Round Mountain, Nevada were analyzed using field emissions scanning electron microscopy (FESEM) and focused ion beam (FIB) milling assisted electron microscopy (TEM). Results suggest that the two dimensional growth mechanism is dominant and that colloidal gold and nanoparticles are present at the interface and may play an important role in the formation of these deposits.

AN ELECTRON MICROSCOPY INVESTIGATION OF GOLD AND ASSOCIATED MINERALS FROM ROUND MOUNTAIN, NEVADA

A Thesis

Submitted to the Faculty of Miami in partial fulfillment of the requirements for the degree of Master of Science Department of and Environmental Science by Michelle Burke Miami University Oxford, Ohio 2015

Advisor ______Dr. Mark P. S. Krekeler

Reader ______Dr. John Rakovan

Reader ______Dr. Hailiang Dong

Table of Contents

List of Figures …………………………………………..…………………………... iv 1. Introduction ……………………………………………………………………..…….1 1.1 Gold ………………………………………………………………..…...…………1 1.2 Quartz ……………………………………………………………...... ………….1 1.3 Round Mountain Gold Mine………………………………………………………2 1.3.1 History and Location………………………………………………………...2 1.3.2. Geologic Setting…………………………………………………………….3 1.3.3 The Deposit …………………………………………………………………3 1.4 : Cyanide ………………………………………..4 1.5 Low Sulfidation (Adularia-Sericite) Epithermal Deposits ……………………….5 2. Purpose ………………………………………………………………………..………6 3. Materials and Methods………………………………………………………………..7 3.1 Samples …………………………………………………………………………...7 3.2 Scanning Electron Microscopy …………………………………………………...8 3.3 Transmission Electron Microscopy ………………………………………………8 3.4 Focused Ion Beam Milling ……………………………………….……………….9 4. Results ……………………………………………………………………………….10 4.1 SEM Results …………………………………………………………………….10 4.2 TEM Results ………………………………………………………………….....12 5. Discussion …………………………………………………………………………...14 5.1 FESEM Observed Growth Textures …………………………………………….14 5.2 The Gap at the Interface ………………………………………………………....15 5.3 Nanospherules …………………………………………………………………...17 5.4 Other Phases of Interest………………………………………………………….18 ii

5.5 Gallium Interference in FIB Prepared Samples ……………….………………...19 5.6 Implications for Cyanide Leaching………………………………………………19 5.7 Sequence of Mineralization ……………………………………………………..20 6. Conclusions ………………………………………………………….……………....21 7. References ……………………………………………………………………...……23 8. Figures ……………………………………………………………………………….27 9. Appendices …………………………………………………………………………..50 9.1 Appendix A: Supplementary SEM………………………………………………50 9.2 Appendix B: Supplementary TEM………………………………………………52

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List of Figures Figure 1: Location Map………………………………………………………………….27 Figure 2: The Round Mountain Mine Open Pit…………………………………………28 Figure 3: Macrocrystalline Samples…………………………………………………….28 Figure 4: Focused Ion Beam Milling……………………………………………………30 Figure 5: 2D Growth…………………………………………………………………….31 Figure 6: Triangular and Hexagonal Terrace Morphologies……………………………32 Figure 7: Hopper Crystals……………………………………………………………….33 Figure 8: Granular/ Texture………………………………………………………...34 Figure 9: SEM Gap at the Interface……………………………………………………..35 Figure 10: Quartz SEM………………………………………………………………….36 Figure 11: Adularia SEM………………………………………………………………..37 Figure 12: Ag SEM Elemental Maps……………………………………………………38 Figure 13: Gold TEM…………………………………………………………………....39 Figure 14: Nanoparticles Bright Field TEM…………………………………………….40 Figure 15: Nanoparticles Apparent Diameter Histogram……………………………….41 Figure 16: Nanoparticles in Other Phases……………………………………………….42 Figure 17: STEM Ag Nanoparticles Elemental Maps………………………………..…43 Figure 18: STEM Elemental Maps……………………………………………………...44 Figure 19: STEM Elemental Maps II……………………………………………………45 Figure 20: STEM Elemental Maps - Au, Ag, and Cu…………………………………...46 Figure 21: Linear Thermal Expansion Graph…………………………………………...47 Figure 22: Gallium STEM Maps………………………………………………………..48 Figure 23: Mineralization Sequence Interpretation………………………………….….4

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1. Introduction

1.1 Gold

Gold has long been valued for its unreactive nature leading to its classification as a “noble ”. It is one of the few that generally remains in its native state at the Earth’s surface and is commonly alloyed with other metals such as Cu, Fe, Sn, Ag, and the metalloid Te. Gold was commonly used for coins, jewelry, and for monetary reserves, but has now found its way into more modern industries such as electronics and . It currently stands as an $82.6 billion worldwide (Ridley 2013). Gold is fairly rare, with a crustal abundance of 0.004 ppm (Ridley 2013). For a deposit to be considered an economic source of gold it has to occur in ore grades with at least 5-10 ppm, concentrated at least 1200 times relative to its average crustal abundance (Ridley 2013). As reserves of known deposits of gold begin to decline, it becomes critical to find new deposits and more innovative methods for gold extraction. Gold prices have risen in the last decade, peaking in 2011 at over 1800 USD/oz. While they have declined since then, currently resting at ~1200 USD/oz., gold prices are higher than they have been in the past making gold a very valuable metal, second only to . With gold prices currently so high, extraction of gold from low grade is economic and exploration is underway in many countries to locate new deposits. Furthermore many mines have begun re- processing old in hopes of recovering more gold.

1.2 Quartz

Although far less economic of overall importance as an ore , quartz is an important mineral known in epithermal environments that is believed to deposit throughout gold mineralization. 13 different textures have been identified for low quartz in epithermal environments, many of them visible in hand specimen or under a petrographic (Dong et al. 1995). Common among these textures are bladed, crustiform, colliform, moss, massive, mosaic, and feathery. Each texture reveals information on the conditions of quartz formation, including indications of whether it is a primary texture, a recrystallization

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texture, or replacement texture, and whether the texture likely had a silica gel precursor or if boiling was a factor in formation (Dong et al. 1995; Etoh et al. 2002; Moncada et al. 2012). Furthermore, zoning of quartz crystals and trace elements and fluid inclusions contained in quartz can be detected using cathodoluminescence and electron microprobe and can provide information on the fluid conditions and chemistry at the time of crystal growth, such as conditions and other parameters that affect precipitation (Rusk and Reed 2002; Takahashi et al. 2008; Rusk et al. 2011). For this reason, quartz is an important mineral to fully understanding gold deposition in epithermal and other hydrothermal precious metal systems.

1.3 Round Mountain Gold Mine

1.3.1 History and Location

Nevada is, and has been, a major producer of gold for the . The Round Mountain gold mine is one source of Nevada gold. The mine is located in Nye County, Nevada approximately 200 miles northwest of Las Vegas (Figure 1). The nearby of Hadley and Carvers are home to many of the mine workers, and the small of Tonopah to the south is a popular tourist destination for visitors looking to learn more about the area’s rich mining history.

Gold production first began at Round Mountain in 1906, and between 1906 and 1969 350,000 ounces of gold was produced (Hanson 2006). Large scale commercial operations began in 1977. The Smoky Valley Common Operation (SVCO) owns the mineral and surface rights to the mine. The SVCO was originally controlled by the Range Co. (50% interest), Felmont Oil Co. (25% interest), and Case Pomeroy Co. (25% interest). In 1984 the Homestake Mining Company acquired 25% interest, which was increased to 50% interest in 2000. In 1985 Echo Bay Mines Inc. acquired the remaining 50% interest. Corporation merged with Homestake Mining Company in 2001 and Corporation merged with Echo Bay in 2003 so that the mine is currently a joint venture between Barrick and Kinross where Kinross is the operating partner.

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1.3.2 Geologic Setting

The Round Mountain Gold Mine is located in the Great Smoky Valley at the base of the Toquima Mountain Range, the result of Basin and Range extension. Underlying the deposit are meta-sedimentary rocks of Cambrian to Permian age, including argillite, phyllite, schist, quartzite, , and siltstone, which have been deformed and metamorphosed by Cretaceous age granitic plutons that are now exposed in the pit of the mine. These are in turn overlain by Oligocene age rhyolitic ash flow tuffs that stemmed from calderas in the Toquima and Toiyabe Ranges. The main source of this tuff is believed to be the proposed Round Mountain Caldera of the southern Toquima Range, now buried.

1.3.3 The Deposit

The Round Mountain gold deposit is a low sulfidation epithermal deposit, also known as an adularia-sericite type deposit, hosted in the volcanic rocks of the proposed Round Mountain Caldera in the Toquima Range. Gold mineralization includes electrum in conjunction with quartz, iron oxides, pyrite, and adularia in dominantly vein morphologies with some disseminated deposits in the Toquima range ash flow tuffs, dated at approximately 26 Mya using 40Ar-39Ar dating (Henry et al. 1997). Associated with a large granitic intrusion are small veins of ore in the form of huebnerite which has been mined since the early 1900’s when they were discovered.

The Round Mountain Mine is a large open pit mine (Figure 2). Several different geologic units are mined for ore at Round Mountain and the following classification is information provided by Kinross via Dave Emmons, April 2015, also available in the 2005 Round Mountain Mine Technical Report (Hanson 2006). The top- unit mined is a densely-welded rhyolite tuff, which acted as a cap for the upwelling hydrothermal fluid during gold mineralization (type 1). Quartz and adularia veinlets are present in this unit, and gold mineralization is largely fracture controlled. A second major ore host is a non-welded to poorly-welded pumice tuff that is porous and permeable, and is mined for disseminated gold (type 2). A transitional unit of moderately welded tuff between the two is a minor ore host (type 9). A moderate source of gold

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is a moderately welded lithic-rich tuff which often includes pieces of underlying Paleozoic rocks and sometimes contains laminations (type 3). Paleozoic meta-sedimentary rocks including phyllite, quartzite, and limestone are a major source of ore (type 4). Mineralization in these rocks is mainly fracture controlled and some of the high-grade veins found at Round Mountain came from this unit. The Stebbins Hill unit, a silicified proposed hydrothermal eruption breccia, is another gold-bearing unit (type 33). A unit of Quaternary is mined for placer gold (type 52). Other non-producing units at the mine include another unit of Quaternary alluvium (types 51) and Cretaceous (type 8).

1.4 Gold Extraction- Cyanide Heap Leaching

Round Mountain is an open-pit mine using the cyanide heap leach method for gold extraction. In 1990 this facility was producing an estimated 400,000 oz/yr. with an average recovery of 75-80% (Marsden and House 2006). Production peaked in 2003 at 785,300 oz and has declined since then, currently resting around 325,652 oz in 2013. From 1977 to 2013 the mine is estimated to have produced nearly 14 million oz of gold. Cyanide leaching is currently the conventionally used method for extracting gold because of its low startup cost, low operational cost, and high efficiency at extracting gold. Most mining operations their extraction process to better suit their specific ore and gangue mineral assemblage to maximize recovery. There are still several problems with cyanide leach extraction. Cyanide is environmentally un-friendly. It has been shown to cause contamination and is toxic to human beings in large quantities, and can be damaging to biological organisms as well (e.g. Salkowski and Penney 1994; Eisler and Wiemeyer 2004). In of cyanide to and from mines there have been several reported spills resulting in high environmental impact (Amegbey and Adimado 2003).There has been extensive research into the availability and attenuation of cyanide in mine tailings (Oudjehani et al. 2002, Zagury et al. 2004, Rao and Reddy 2006). Numerous studies have focused on finding alternatives to cyanide leaching, but thus far it remains the dominant extraction method in large scale commercial operations (Muir and Alymore 2004, Senanayake 2004, Hilson and Monhemius 2006).

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Extraction efficiency of gold and related metals from ore using the cyanide heap leach method has been the subject of much scrutiny and inquiry in the past several decades, and it has been shown that there are a number of factors that can decrease the efficiency of gold from the cyanide heap leach method (Marsden and House 2006; Bartlett 2007). Before being exposed to cyanide, gold ore is often milled or crushed to liberate it from the surrounding ore and create surfaces and pathways for the cyanide solution to interact with and dissolve the gold. Particle size of this liberated gold is important as a higher surface area (smaller particle size) to a shorter time that the gold needs to react with CN- to fully dissolve. Certain gangue minerals such as oxides, silicates, metals, and other gangue materials can either prevent the cyanide from reaching and dissolving the gold, or slow the gold dissolution process resulting in decreased yield (Marsden and House 2006). For example, certain copper and iron minerals associated with some gold ores will form complexes with cyanide during leaching leading to competitive consumption of cyanide, decreasing the recovery of gold or increasing the amount of CN- that must be used in the extraction process, raising cost and decreasing efficiency (Marsden and House 2006, Lopez et al. 2014). While some of these gangue minerals leach in cyanide solutions slower than others and not all gangue minerals hinder dissolution of gold by cyanide, this is still a major problem that plagues many mines utilizing cyanide leaching where copper and iron are present as these metals are less economic than gold and far more abundant (Logsdon et al. 1999, Lopez et al. 2014).

1.5 Low Sulfidation (Adularia-Sericite) Epithermal Deposits

General models for low sulfidation epithermal deposits exist (e.g. Sillitoe 1993; White and Hedenquist 1995; Sillitoe 1997). Collectively these models indicate that low sulfidation epithermal deposits occur at depths of 1-2 km, temperatures around 200oC-300oC, and are closely related to magmatic centers at a distance of 2-10 km. The longer distance for the fluid to travel results in a greater buffering of the fluid from the wall as it ascends resulting in near neutral to slightly alkaline pH’s. Since low sulfidation epithermal environments are mainly reduced, occurs in sulfides as S(II). At these temperatures and pH’s the main complex for - gold transportation in solution in these deposits is the complex Au(HS)2 . The precipitation of

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gold in these hydrothermal fluids is thought largely to be controlled by the destabilization of this complex during boiling of the fluid when the confining pressure drops as the fluid approaches the surface, and CO2 and H2S are portioned into the vapor phase leading to fracturing of the surrounding rock creating new migration pathways (Kesler 2005). This typically leads to the formation of vein deposits, unless the host rocks are more porous creating disseminated deposits.

Some studies have shown that colloidal gold and electrum may play a key role in the formation of many high concentration gold deposits (Saunders 1990; Saunders 1995). Both in materials science and ore geology, gold nanoparticles and colloidal gold have been widely researched topics, and much is now known on the properties, both chemical and physical, of colloidal gold (Williams-Jones et al. 2009; Hough et al. 2011). Emphasis has been placed on the role that colloidal gold and silica play in the generation of mesothermal and epithermal gold deposits, and studies have shown that silica may precipitate initially as an amorphous gel and later crystallize to form quartz, and that gold colloids are stable up to 300-350oC when protected by colloidal silica (Frondel 1938; Saunders 1990; Herrington and Wilkinson 1993). There remains much to be done in investigating the role that nanoparticles play in the formation of gold deposits.

2. Purpose

The goal of this project is to gain better insight into gold mineralization from epithermal deposits through a thorough investigation of naturally occurring interfaces between gold and other associated phases. No work has been done investigating natural examples of these interfaces. It is unclear to what extent the extraction of gold is hindered by the minerals and their associated textures on or near the interface. These phases, if present, may reduce yield by reacting differently with extraction chemicals or blocking extraction chemicals from interacting with the gold. Furthermore, the nature of the interface may identify new mechanisms of gold mineralization which could become critical to the understanding of conditions of gold growth. In addition, this study seeks to build on existing literature to help constrain the role that gold

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nanoparticles play in the formation of epithermal gold deposits through an investigation of nanoparticulate textures using high resolution imaging techniques.

Integrated electron microscopy investigations of gold in epithermal systems in Nevada utilizing field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) techniques have not been carried out. Such investigations involving high resolution backscatter detection (BSD) imaging, dispersive spectroscopy (EDS) in spot and mapping mode, selected area electron diffraction (SAED), and near atomic scale structural imaging of these systems have great promise for understanding gold mineralization in epithermal systems in unprecedented detail. A detailed electron microscopy investigation will allow for the testing of the following hypotheses:

(1) Specific, systematic microscale and nanoscale textures exist in epithermal Nevada gold deposits that will enable refinement of the understanding of the sequence or nature of gold mineralization in these systems.

(2) Unrecognized complexities at the gold-quartz interface exist in these deposits and may impact extraction efficiencies of gold in the cyanide heap leach process.

3. Materials and Methods

3.1 Samples

For this investigation five macrocrystalline gold samples from Round Mountain, Nevada were analyzed, purchased from the reputable mineral dealer John Betts Fine Minerals. Although these samples did not have specific geologic context they are very representative of macrocrystalline gold of Round Mountain. Samples ranged in size from 8-23 mm in length and exhibited a range of morphologies (Figure 3). Two of the samples displayed a dendritic morphology. In April 2015 new samples were collected directly from the Round Mountain Gold Mine from various gold-bearing units. During that visit, two macrocrystalline samples were generously donated by Terry Jennings and the geology staff at the Round Mountain Gold Mine,

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and Dave Emmons loaned several high-grade samples for use in this and further studies. Preliminary data from those samples is also included in this study in the appendices.

3.2 Scanning Electron Microscopy

The scanning used in this study was a Zeiss Supra 35 VP field emission scanning electron microscope (FESEM). FESEM was utilized to collect macro-scale and micro-scale textural data and gain chemical information. This FESEM is equipped with an EDAX Genesis 2000 energy dispersive spectrometry (EDS) detector with a detection limit for most elements of approximately 0.1 wt %. Elemental mapping using EDS was utilized to better understand the of elements at or near the interface. Images were collected in secondary electron (SE2) mode, backscatter detection (BSD) mode, and also in variable pressure

(VPSE) mode where N2 is the compensating gas to help prevent charging for uncoated, nonconductive sample materials such as quartz and adularia. Samples were prepared for scanning electron microscopy by mounting individual macrocrystalline samples to aluminum stubs using carbon sticky tabs.

3.3 Transmission Electron Microscopy

Transmission electron microscopy (TEM) allowed for near-atomic to atomic-scale analysis of gold, quartz, and adularia separately as well as the interface between the different phases. The TEM utilized in this study is a JEOL 200kV JEM-2100 instrument housed in Miami University’s Center for Advanced Microscopy and Imaging (CAMI). The TEM is outfitted with a Gatan 833 Orius camera, a JEOL BF STEM detector, a Gatan HAADF STEM detector, a Gatan GIF Tridiem post-column energy filter, EELS/EFTEM, and a Bruker Quantax 200 STEM XEDS with a SDD detector. Elemental mapping in scanning transmission electron microscopy (STEM) was also utilized to analyze areas of interest. TEM and STEM images were collected in bright field at a 200kV accelerating voltage. Collection time for STEM elemental maps varied from 20-60 minutes. Energy dispersive spectroscopy (EDS) was used to analyze the composition of the interface. Selected area electron diffraction (SAED) was also utilized to ascertain if a 8

preferred crystallographic orientation exists between the two phases, which can provide insight into the atomic-scale reactions at the interface.

3.4 Focused Ion Beam Milling

For TEM preparation, samples need to be reduced to a 3 mm circle grid containing areas that are atomically thin (<100 nm) and thus able to be imaged using transmitted electrons. For the purpose of this study, samples were embedded in Spurs hard resin to reinforce the quartz- gold samples, and samples were mechanically using a diamond-tipped saw blade. Samples were then imaged in the SEM to select areas of interest for milling and TEM analysis. Focused ion beam (FIB) milling is a technique that uses a beam of gallium ions to cut a small rectangular foil (Figure 4) approximately 15x 10 x 0.150 μm (dimensions may vary), which can then be directly mounted onto a TEM grid or further milled using an argon ion mill to further minimize distortions (Wirth 2009).

The FIB used in this study was a single beam instrument housed at the GeoForschungsZentrum (GFZ) in Potsdam, . FIB milling was done in September of 2014 under the direction of Anja Schreiber, manager of the TEM sample preparation laboratory at the GFZ. Samples were first carbon coated to prevent charging under the Ga+ beam. A platinum strip was deposited using platinum gas over the surface of the site designated for the FIB foil and x’s were milled into the ends of the stripe for computer software recognition. Samples were milled at 30 kV until trenches had been milled in front and behind the FIB slice. Then the sample was tilted 45o and the bottom and the sides of the sample were cut free and then tilted back to cut the foil completely free from the surrounding material. The sample was then removed from the FIB and an ex-situ micromanipulator consisting of a glass needle pulled to a fine point was used to remove the FIB foil and mount it to a Cu grid backed by a holey carbon film.

A large volume of literature exists on damage induced during FIB milling. It has been shown that gallium can contaminate the surface and top ~10 nanometers of FIB foils and that samples containing certain elements such as Fe and Cu can to the formation of new Ga

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complexes (Yu et al. 2006; Kiener et al. 2007). Furthermore, it has also been shown that other materials, especially silicon, can be made amorphous by FIB milling (McCaffrey et al. 2001; Rubanov and Munroe 2004). Despite gallium contamination, all TEM preparation methods induce some form of damage to the sample, especially soft materials such as gold, and the site specific selection and low mechanical deformation of FIB milling makes it the preferred method for many earth and materials science studies as well as for gold and the gold-silicate interface TEM preparation for this study. For the purpose of this study samples were prepared using both FIB milling or argon ion milling to determine which produced better quality images and to better constrain the affects that FIB milling with the Ga+ ion beam has on sample textures and chemistry.

4. Results

4.1 SEM Results

FESEM analysis of the surface of samples of macrocrystalline gold from Round Mountain commonly shows a dominantly pervasive microtopography (~60%) of steps and terraces (Figure 5). Terrace widths vary from approximately <0.5 µm to approximately 15 µm. Step heights appear to be fairly consistent despite terrace width and the heights are estimated to be <0.1 µm. These steps and terraces often form localized topographic highs or islands, many of which are polygonized with a hexagonal or triangular morphology (Figure 6). Some islands form on branch-like structures extending out from the bulk of the sample in a slight radial , while others are present at the surface. Polygonized terraces vary in extent and may occur as only a few steps to tens to hundreds of steps. Rounded islands are also present in other areas of the sample. Island distribution is concentrated in less than half of the sample, and island density can be as high as 10 microtopographic highs in a 30 µm2 area. Typically associated with these microtopographic highs are microtopographic depressions of similar morphology and distribution. Areas where islands are more rounded typically exhibit a higher island density than areas where polygonized islands are found. Island/depression width and height are fairly

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consistent for features in close proximity, though in polygonized regions topographic highs extend farther than depressions.

Less prevalent (~3%) regions of hopper surfaces are also present in areas adjacent to the step/terrace texture within the Round Mountain samples (Figure 7). Hopper surface areas are narrow, ranging in width from 2-10 µm, although they may extend for 35-45 µm in length. Individual hopper surfaces typically exhibit either a square or rectangular morphology, although diamond–shaped hopper surfaces have also been noted. In some instances, the edges of hoppers exhibit a jagged and saw-teeth-like morphology. Width of individual hopper surfaces ranges from approximately 0.5 µm to 3 µm. Most hoppers only extend to a height of a few steps, though some larger ones contain several steps.

A texture common in the Round Mountain samples are irregular surfaces of gold/electrum, commonly adjacent to the step/terrace with a granular, and in some cases pitted, texture (Figure 8). This texture makes up approximately 30% of the surface area of the Round Mountain samples and is less common than the step/terrace texture. The granular texture may occur in localized areas surrounded by the steps/terraces or on its own. In some cases there is a clear, sharp boundary between the step/terrace texture and this irregular granular texture, but in other cases there is a gradual transition between the two textures.

A vein texture, comprising approximately ~7% of the sample’s surficial texture and only found in regions of the granular texture, is also present (Figure 8). Veins vary in width from 0.2 µm to ~2 µm, and thicker veins typically exhibit small-scale polygonized, elongate (1-2 µm) islands. Thinner veins appear to be composed of anhedral crystals and are typically slightly rounded. Veins can extend from 3 µm to as long as 50 µm. Veins may be isolated or interconnected, and in some cases, veins may form branches.

FESEM indicates in some regions of sample material a gap is present at the interface between quartz and gold (Figure 9). The gap varies in width but is typically no wider than 1-2 µm. The gap typically does not extend the entire length of the grain it surrounds but is disrupted by areas in which the quartz and gold appear to be in contact. Such gaps are more prevalent in locations where larger (>15 µm) quartz crystals are present. Careful examination of this gap

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shows some euhedral edges, sometimes with steps. Based off of FESEM alone it is unclear to what lateral extent the gap extends.

Quartz often occurs in fractured masses, although in a few instances subhedral to euhedral quartz crystals occur (Figure 10). Although quartz is not very prevalent in these macrocrystalline gold samples, pockets of quartz grains as wide as 40-50 µm and lengths of >100 µm have been observed. These pockets often contain varying sizes of quartz grains from 2-10 µm, although the majority of the quartz grains are of the smaller distribution. Euhedral crystals of a mineral rich in K and Al, interpreted as the potassium feldspar, adularia, have also been noted to interface with the gold (Figure 11).

Chemical data collected using energy dispersive spectroscopy (EDS) in FESEM from Round Mountain samples indicates that some of the gold occurs in solid solution with minor amounts of Ag as electrum, although some gold is relatively pure and contains little silver or other metals. One texture in particular contains small (<0.5 µm) particles that elemental mapping confirms are Ag. The surrounding surface of gold contains only traces of Ag implying that the Ag in these particles may have been leached from the surface and reprecipitated nearby (Figure 12). Trace amounts of Fe have also been detected in Round Mountain samples. EDS also confirms the presence of Si and O corresponding with quartz and also K and Al indicative of adularia, presenting an additional interface of interest.

4.2 TEM Results

TEM analysis of FIB milled samples reveals a number of micro and nano-textures at the quartz-gold and adularia-gold interface and within the minerals themselves. In all examples investigated, the surface of gold at the interface is commonly euhedral, and is consistent with the numerous examples of step-terrace textures identified using FESEM (Figure 13). These steps typically have a height of approximately 0.1-0.5 µm. Adjacent step heights are consistent in some areas but vary in others. The step length can also vary from approximately 0.1-0.5 µm, and step length in the same area is also not necessarily consistent. SAED patterns of bulk gold yield

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diffraction patterns with discrete reflections as well as rings around some reflections. D-spacings for the rings around individual reflections are measured at 0.233 nm.

Nanoparticles appear to be common in the gold of the Round Mountain samples (Figure 14). SAED patterns have concentric rings consistent with multiple crystals randomly oriented. Planar spacings calculated from the rings are 0.234, 0.203, 0.142, 0.122, and 0.092 nm, which are all consistent with the gold or electrum lattice. These nanoparticles range in size from 2-10 nm, but on average their apparent diameter is 4-5 nm, although larger nanoparticles have been observed (Figure 15). Nanoparticles occur as circular to oblong spherules that sometimes overlap one another in the 2-dimensional cross section view of the TEM images. Atomic-scale structural resolution of the nanoparticles is visible in the higher magnification images of the Round Mountain gold. Nanoparticles are present both as inclusions in other phases such as titanium oxides, a2:1 layer silicate mineral rich in Al and K likely illite or sericite, and adularia, and also in bulk concentrations separate from other phases (Figure 16). The material between nanoparticles is dominantly silica or aluminum with an amorphous texture. The area of this texture was too small to test for crystallinity with SAED. Elemental mapping in STEM mode indicates that the nanoparticles that occur as inclusions in other phases at or near the interface are Ag dominant (Figure 17). In other regions of the sample, nanoparticles are dominantly electrum or gold.

In some sample regions, STEM analysis shows that Si, O, Al, and K correlate, indicating that adularia is present (Figures 18 and 19). Adularia shows abundant growth defects in the form of dislocation structures (Figure 13B). These structures are approximately 0.05-0.4 µm in width and can extend for 10’s of µm’s in an intertwining manner. STEM suggests that the gold and quartz or, adularia, are not always interfacing, and in some cases a gap may exist between the two phases. As these samples were embedded in resin during sample preparation the gap was likely filled with resin and preserved throughout TEM sample preparation. Spur’s resin is a polymer resin made up of lighter elements that are less readily detected using EDS. Carbon and oxygen have both been detected in this gap. Gallium deposition from FIB milling is present in each of the FIB milled samples. Gallium occurs in low concentrations throughout the FIB foils and is concentrated at the edges where the sample was irradiated with the ion beam.

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FIB prepared samples are thin enough to easily make out the carbon holey film underneath, and carbon appears in EDS spectra and STEM elemental maps collected from many of the samples. STEM mapping also identifies Cu, Fe, and Ag at the interface. Cu and Au show a strong correlation, and it appears that there is a zonation between Au-Cu rich areas and Ag rich areas (Figure 20). The zone of Ag depletion extends to a depth of approximately 200 nm beneath the surface of the Au and typically extends the entire length of the Au grain, which varies from a 800 nm to >1 µm. This compositional difference is common in samples investigated and the gradation is visible as a boundary in the TEM images. STEM also indicates that Fe is present at the interface, both in localized concentrations and also in solid solution with Au.

5. Discussion

5.1 FESEM Observed Growth Textures

Microtopographic and microtextural analysis of macrocrystalline gold collected using FESEM on samples from Round Mountain suggests that gold growth is dominated largely by the two dimensional nucleation and spread growth mechanism. Step and terrace textures are characteristic of two dimensional growth and nucleation and are interpreted to be growth rather than dissolution textures. Polygonized islands often have a hexagonal morphology implying that the {111} form is dominant. Some islands exhibit rounding indicative of a higher kink density and higher supersaturation which typically occurs near the transition between two dimensional growth and continuous or rough growth (Price 1991).The higher island density for rounded islands is consistent with models of crystal growth where the rate of two dimensional nucleation is higher at increased fluid supersaturation levels near the growth roughening transition (Sunagawa 1999). Hopper surface zones occur as short transitions between adjacent areas of steps. A possible explanation for this is a brief change in fluid chemistry during growth, possibly related to silver or copper concentrations. A second potential explanation is simply fluid heterogeneity during growth leading to higher levels of supersaturation at different areas in the fluid. The saw-tooth-like edges that some of the hopper surfaces exhibit suggest that the center of the hoppers began filling in during growth. Hopper growth, a morphology known to occur at

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saturation levels near the two dimensional-rough growth boundary where growth is preferential around the edges and corners of the crystals, further supports the increased fluid supersaturation levels near this growth morphology transition (Sunagawa 1999). These higher levels of fluid supersaturation imply a rapid reduction of Au in complexation to its solitary native state leading to suddenly high levels of Au supersaturation. This ties in to the epithermal gold deposition model for Round Mountain proposed by Sander and Einaudi (1990) that suggests dilution and cooling of the upwelling hydrothermal fluid with ambient upon fracturing of the wall rock led to a destabilization of the gold-transporting complex and thus fluid supersaturation of gold and gold deposition.

The granular texture could be interpreted a number of ways. This texture could be the result of continuous or rough growth above the supersaturation levels where smooth or two dimensional growth is dominant. As the granular texture is present in areas where the two dimensional step and terrace texture is absent, another potential explanation for this texture is growth interference or competitive growth. This implies that gold either grew against a pre- existing phase, such as quartz or adularia, or that co-deposition of another phase at the time of gold mineralization led to competition for available space to grow. The presence of micro-scale veins within this granular texture supports the growth interference model, implying that there were cracks or crevices in this pre-existing phase in which gold mineralized, in some cases unobstructed as some of the microveins also depict a step and terrace morphology. This granular texture could also indicate possible late stage dissolution. This idea is consistent with distinct and separate Ag particles observed at the surface of gold during FESEM analysis. As Ag is more readily leached than gold a dissolution stage could explain remobilization of silver from the surface of gold and reprecipitation of the silver in the form of small particles nearby similar to mobility described by Krupp and Weiser (1992).

5.2 The Gap at the Interface

Thermal contraction may be a possible mechanism for explaining the gap at the interface between gold/electrum and quartz or adularia, however the linear thermal expansion coefficients

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for gold, silver, adularia (K feldspar), and quartz are 14, 18, 17 and 0.5 *10-6 m/m oK respectively (Nix and MacNair 1941; Rosannolrz and Duor-nv 1941, Nix and MacNair 1942; Lager et al. 1982). Gaps observed at the gold – silicate mineral interface in SEM are commonly between 2 and 7 µm wide. Assuming a cooling temperature range of 300oC, which is more than adequate for low sulfidation epithermal deposits such as Round Mountain, and an initial model length of 100 µm, thermal contraction from cooling is <1% (Figure 21). Thus thermal expansion alone is not sufficient to explain the gap between the two phases.

The gap at the interface between quartz and gold may indicate dissolution of an early phase that may have played a role in mineralization by aiding in the precipitation of gold and electrum. For example, , which is not easily preserved, is known to occur in epithermal gold systems, and could help explain how gold is concentrated. Round Mountain mine staff indicated during a visit in April 2015 that Type 33 or Stebbins Hill rocks are enriched in mercury, although this unit is essentially above all ore units except the placer and alluvium. No trace of a remnant mercury phase or other mobile phase such as sulfate or chloride minerals was detected during FESEM analysis. The gap may also have resulted from mechanical or chemical separation of the mineral phases during any preparation done to the sample prior to its sale, though textural evidence does not support this. The lack of evidence of a precursor phase and the fact that thermal expansion/contraction alone cannot explain the gaps suggests the gaps simply reflect remnant porosity.

TEM analysis of silicate mineral - Au interfaces also show gaps. In these images the interface has been filled in with resin from the embedding sample preparation process and the texture is undisturbed owing to FIB sample preparation. STEM elemental mapping is consistent with there being a gap between adularia and gold at the interface. The elemental components of the resin are polymers composed of lighter elements not readily detectable by EDS which accounts for the lack of metal element data reported for these areas during STEM mapping. The gold adjacent to the gaps is commonly euhedral indicating that it grew unobstructed in these gaps following the two dimensional nucleation and spread growth microtopography identified in FESEM, supporting two dimensional growth at the nano-scale. In all of the TEM images no trace of mercury or other mobile phase components such as sulfur or chloride was detected.

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5.3 Nanospherules

A major result of this study is that nanospherules of native (gold- silver, electrum) are common in this deposit. This is a nano-scale texture that has not been previously recognized in low sulfidation epithermal gold deposits or other gold deposits globally, although the presence of gold nanoparticles of supergene origin and in refractory minerals such as arsenian pyrite, have been noted. As some of the nanoparticles are overlapping and connected this could suggest that they began aggregating, and these particles may have acted as nucleation surfaces to enable gold growth. Previous work by Burke and Krekeler has shown nanospherules to occur in some and silver from the Upper Peninsula of Michigan (Burke and Krekeler 2013, Burke and Krekeler in preparation).

One interpretation of this nanoparticle texture is an inclusion texture of gold or silver nanoparticles that formed at depth and were brought to the surface in the ascending hydrothermal solution and were then incorporated into more massive gold during bulk mineralization. STEM analysis of selected areas of nanoparticles in the Round Mountain samples suggests that the nanoparticles, at least in some cases, are Ag-rich. These nanoparticles could be the result of remobilization of silver from the surface of the gold, similar to the texture observed with the larger Ag particles in FESEM. STEM mapping shows that in some cases, the surface of the gold is depleted in Ag below detectable levels while Cu is relatively concentrated at the surface of the gold. Ag is more prevalent a couple hundred nanometers below the surface in the bulk of the gold. These zones of Ag depletion and Cu enrichment are commonly correlated to being adjacent to gaps between gold and quartz or adularia, and it is possible that these gaps played a role in providing a pathway for fluid to interact with the surface of the electrum and leach the Ag from the surface. Ag nanoparticles are also found in the quartz and adularia adjacent to the interface suggesting that electrum was deposited first and then silver was leached from the surface and incorporated into quartz or adularia as inclusions as they formed. Ag nanoparticles can also be found in the resin filling the gap at the interface, implying that some particles were adhered to the surface of the gold and became embedded in the resin during the embedding process. Dissolution and leaching of Ag from the surface of electrum is a possible explanation, consistent with FESEM observed texturally distinct late stage Ag growth.

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In other regions of the sample, gold and electrum nanoparticles occur as inclusions in other mineral phases such as titanium oxides, illite/sericite, and adularia. In these regions the nanoparticles remain isolated and have not aggregated together. In areas of the sample where nanoparticles are dominant, the nanoparticles have aggregated together, although some may remain isolated surrounded by a matrix of amorphous Al or Si. In SAED patterns collected from regions of the sample where the nanoparticle texture is present, circles of the same reciprocal space are sometimes centered on some of the spots, indicating secondary diffraction of the primary diffracted electron beam as a result of layers or films of nanoparticles underneath a single crystal. The thickness of this nanoparticle layer appears to determine the intensity and abundance of which these secondary rings appear. Otherwise nanoparticle regions yield polycrystalline patterns that are consistent with spacings for Au-Au and Au-Ag.

5.4 Other phases of Interest

The zonation between Cu, Au, and Ag at the interface has not been previously reported. This chemical texture might indicate changes in fluid composition throughout mineralization from a more Ag-rich to a more Cu-rich solution. Initially, the mineralizing solution was enriched in Ag, which was gradually depleted and eventually replaced by Cu. Copper phases are not known to be common in low sulfidation epithermal deposits. Macrocrystalline gold from Round Mountain is well recognized as being dominantly electrum. However, the complexities of Ag-Au-Cu solid solution and textural relationships have not been previously recognized.

Correlation between Al, Si, K, and O in STEM elemental mapping strongly suggests that adularia, a potassium feldspar mineral common in low sulfidation epithermal deposits, is present along with the quartz, and may present an additional interface of interest. In some of the FIB foils investigated, adularia appeared to be more abundant than quartz, although this could be the result of sample size bias as quartz was proven to be more abundant in FESEM analysis and quartz is typically the dominant gangue mineral in low sulfidation epithermal deposits and often forms in close association with gold.

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5.5 Gallium Interference in FIB Prepared Samples

A number of studies have been conducted looking into the affects of FIB milling on metallic samples and gallium induced contamination and the textures it produces (Lian et al. 2006; Kiener et al. 2011). It has been shown that gallium can be deposited on the surface and implanted at a depth of several tens of nanometers beneath the surface, and that FIB milling can lead to amorphization of the sample (McCaffrey et al. 2001; Rubanov and Munroe 2004; Kiener et al 2007). Areas irradiated by the Ga+ beam typically show higher concentrations of gallium contamination that can affect composition and texture while unirradiated areas show only minor contamination (Yu et al. 2006). For this reason, our areas of analysis were constrained to unirradiated areas. Furthermore, although FIB milling has shown gallium interference to be a potential issue with certain materials, a search of the literature reveals that FIB milling is not known to produce a nanospherule texture like the ones observed in this study.

STEM mapping confirms the presence of gallium contamination and deposition from the FIB milling process, though the Ga appears to be more distributed throughout the sample and concentrated along the edges where the Ga+ beam was directly applied rather than in discrete localized concentrations or nanoparticles (Figure 22). Given gold’s unreactive nature and a lack of known gold-gallium complexes, it seems unlikely that the nanoparticle texture could result from gallium deposition during FIB milling, and a more likely explanation is that the nanoparticles are inherent to the sample. As FIB milling allows for site specific milling and minimal sample preparation it remains the preferred method for prepping a mixture of hard and soft phases to analyze the interface despite the minimal gallium induced contamination detected during analysis.

5.6 Implications for Cyanide Leaching

Several of the textures observed in FESEM and TEM may have important implications in explaining gold extraction efficiencies for the cyanide heap leach method used at the Round Mountain Gold Mine. The gap at the interface between gold and associated gangue minerals could provide a pathway for cyanide accessibility during the leaching process. Through this

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conduit the cyanide solution could penetrate deeper into the ore and the higher surface area of exposed gold would increase recovery and decrease extraction time, depending on the degree and depth to which the gap occurs. As Round Mountain leaves much of the ore it processes though cyanide leaching un-milled as a result of the high cost of milling for large volumes of ore this could explain recovery efficiency for Round Mountain for un-milled ore.

The presence of copper and iron at the interface may present important implications for the extraction process as Cu and Fe, metals of a much less economic status, have been shown to compete with gold for cyanide during leaching reducing extraction efficiency (Marsden and House 2006; Lopez et al. 2014). This would increase the amount of cyanide necessary to add to fully maximize gold recovery thus increasing cost for the extraction process, or result in gold that remains unleached decreasing recovery efficiencies. Furthermore, the rate at which gold is leached from ore may be affected as not all of these metals leach at the same rate, and Cu or Fe in solid solution at the surface of the gold grains could slow dissolution. This would increase the time that the ore would need to be left on the heap to maximize recovery or result in increased levels of gold in mine tailings.

In this study nanoparticles have shown to be an important component of gold in macrocrystalline Round Mountain samples. Gold nanoparticles as inclusions in other mineral phases such as adularia, quartz, titanium oxides, and illite/sericite represent an additional refractory component. As the cyanide solution’s accessibility is hindered by the presence of these minerals, these nanoparticles likely remain unleached. Inclusions of gold nanoparticles are abundant in these associated minerals phases and could represent a large component of gold that is unable to be recovered from ore without further processing.

5.7 Sequence of mineralization

Mineralization sequences between quartz and gold/electrum in epithermal systems are variable and complex. Quartz or a silica precursor such as silica gel, , or chalcedony often deposits first, especially in veins, and then may recrystallize to form quartz or some other more stable silica phase (Saunders 1994; Dong et al. 1995). In this regard colloidal silica particles that

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formed at depth may play an important role in quartz growth (Herrington and Wilkinson 1993; Saunders 1990, 1995). Quartz as a precursor mineral could have provided a nucleation surface for gold/electrum mineralization, and evidence of quartz as having mineralized first has been observed in Round Mountain samples, though in this study no textural evidence for an amorphous silica precursor was detected (Figure 23). Similar to colloidal silica, colloidal gold has been shown to contribute to gold growth, and as colloidal gold particles collide they form near-instantaneous metallic bonds, aggregating together often dendrites in vein environments (Weitz and Oliveria 1984; Saunders 1994). As boiling begins to occur the higher energy environment could promote the collision of gold nanoparticles leading to nanoparticle aggregation. When the hydrothermal fluid supersaturation levels with respect to gold increase, two dimensional nucleation and growth of gold occurs as evident by two dimensional growth textures observed in Round Mountain samples. Gold particles present in the hydrothermal solution having formed at depth and then aggregated together may provide additional nucleation surfaces to facilitate gold growth from solution. At the time of two dimensional growth it is likely that co-deposition of both gold and quartz growth occurs, as evident by pockets of quartz crystals surrounded by gold and small inclusions of gold embedded in quartz.

6. Conclusion

FESEM results suggest that gold growth at Round Mountain is dominated by the two dimensional nucleation and spread growth mechanism with lesser amounts of hopper growth also present. Two dimensional growth textures include steps and terraces that often form polygonized or rounded islands representative of two dimensional nucleation and spread. This is indicative of higher levels of fluid supersaturation at the time of mineralization. Also present is a granular, more massive texture of gold/electrum that sometimes includes surficial veins that could be the result of rough growth at supersaturation levels that exceed the transitional boundary between smooth and rough growth. Another plausible, but less favorable, explanation given other textures observed in the samples from Round Mountain is that this represents a dissolution texture resulting from possible late stage dissolution. FESEM analysis also suggests that in some cases there is a gap at the interface between gold and quartz or gold and adularia. The gap at the 21

interface could have important implications for cyanide accessibility during gold recovery using the conventional cyanide heap leach method, as much of the ore at Round Mountain is un-milled prior to leaching so a gap at the interface between gold and other gangue minerals could provide increased access of the cyanide solution to the gold during dissolution.

TEM analysis of FIB milled grids further supports the concept that there is a gap at the interface. As no traces of a remnant phase have been detected at the interface and thermal contraction alone is not sufficient to explain the presence of this gap, it is likely that this gap is inherent to the sample and marks remnant porosity. Gold at the interface is commonly euhedral displaying a nanometer-scale version of the step/terrace textures observed in FESEM. Nanoparticles are present in the gold and could represent an important stage in gold mineralization. Cu and Ag correlate with Au at the interface, although Ag is more concentrated a few hundred nanometers beneath the surface of the gold and the edges of the gold grains are depleted in Ag. Silver nanoparticles have been observed both in the surrounding silicate phases and also in the resin that filled in the gap during the embedding sample preparation process. Iron is also present at the interface as confirmed by STEM elemental mapping. Copper and iron have been detected at the interface using STEM elemental mapping, and the presence of these two metals may act to reduce cyanide extraction efficiency by competing with Au for available cyanide. While further analysis is necessary to thoroughly characterize this interface, these results suggest that the interface between gold and quartz and other related minerals is far more complex than previously thought, and that textures present at the interface can help explain mineralization history and can also have implications for gold recovery efficiency for macrocrystalline ore.

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8. Figures

Figure 1 – Location Map

TTooiiiyyaabbee Round Mountain Gold Mine RRaannggee

TTooqquuiiimmaa RRaannggee

Figure 1) Map of the Round Mountain Gold mine, nestled in the Smoky Valley between the Toiyabe and Toquima Ranges in Nye County, Nevada. Inset map shows location. Map was created in ESRI’s ArcGIS using the World Imagery basemap. Sources: Esri, DigitalGlobe, Earthstar Geographics, CNES/Airbus DS, GeoEye, USDA FSA, USGS, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User

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Figure 2 – The Round Mountain Mine Open Pit

Figure 2) View from an observation point of the Round Mountain Gold Mine open pit. Image was taken in April of 2015 on a visit to the mine. The Toquima Mountain Range is visible in the background.

Figure 3 – Macrocrystalline Samples

Figure 3) Images of the macrocrystalline gold samples from Round Mountain used in this study. The top and bottom samples exhibit a dendritic morphology. Most samples also contained abundant quartz, although a couple samples analyzed such as the image second from the top was mainly gold and electrum with only minor inclusions of other phases present. The scale bar in the images is in millimeters.

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Figure 4 – Focused Ion Beam Milling

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Figure 4) Micrographs showing the FIB process: A) Polished surface of Round Mountain gold imaged in FESEM in BSD mode. Dark areas of the sample are atomically lighter phases (quartz, adularia), and brighter areas of the sample correspond to atomically heavier phases (gold, electrum) that cause greater scattering of electrons. B) Carbon coated sample with platinum stripe marking where the FIB slice will be cut. X’s cut into the ends of the stripe are for computer software recognition during milling. C) Tilted view of partially cut FIB slice after the material in the back and front has been milled away. D) Same tilted view from previous image but the sample has been thinned and more detail is visible. E) Top view of FIB slice cut free of surrounding material and ready for removal via an ex-situ micromanipulator. F) The gallium ion beam can be used to label or write messages on samples such as this one surrounding an empty FIB milled hole. G) FIB slice mounted to a copper TEM grid backed by holey carbon using an ex- situ micromanipulator. FIB slice is typically placed in center of the grid, but this one went into one of the grid boxes at the top by accident. H) Magnified view of the FIB slice (circled) resting on the holey carbon in the copper grid, ready for TEM analysis.

Figure 5 – 2D growth

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Figure 5) SEM secondary electron micrographs of: A) Polygonized growth islands with a hexagonal morphology in Round Mountain gold. B) Step and terrace textures dominating the surface of the Round Mountain gold, although irregular growth texture is present in lower left hand corner of image. C) Rounded growth islands and depressions indicative of the growth roughening transition at higher supersaturation levels. Island density in this area is higher than those observed in other regions of the samples. D) Two dimensional layered growth steps with a few small islands showing the extent of this texture.

Figure 6- Triangular and Hexagonal Terrace Morphologies

Figure 6) SEM photomicrographs collected in secondary electron mode of polygonized layered growth textures indicative of the {111}. A) Steps and terraces surrounding a polygonized depression with a hexagonal morphology. Subsequent steps mimic the six-sided morphology of the base of the depression. B) Region of a sample exhibiting a triangular morphology. C) Another example of a polygonized hexagonal depression.

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Figure 7 – Hopper Crystals

Figure 7) A) SEM micrograph of a hopper surface adjacent to two dimensional growth steps. Sharp boundary between the 2D growth texture and the irregular granular texture is visible on the left of the image. B) Hopper surface also adjacent to areas of two dimensional growth textures. Edges of these hoppers are rough showing a triangular morphology indicative of growth roughening. The depression on the right is more rounded with a slight hexagonal morphology while the steps and terraces on the left are polygonized. Transition between steps and terraces to hopper growth is more distinct on the left of the micrograph and may represent a grain boundary. C) Another region of a Round Mountain sample showing abundant hopper growth amidst layered growth textures. D) Higher magnification view of hopper crystals from image C. Some hopper crystals have triangular edges and may have begun filling in while others have straight edges.

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Figure 8 – Granular/Vein Texture

Figure 8) SEM micrographs of irregular granular surfaces of gold/electrum. A-C) Branch-like veins surrounded by granular texture in Round Mountain gold. In B the surface of gold is smeared slightly due to mechanical deformation. Transition to step/terrace texture is visible on the outer portions of the micrographs in B and C. D) Magnified view of a vein exhibiting the step/terrace texture surrounded by granular texture indicating that it likely grew unobstructed in a crack of crevice of another phase.

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Figure 9 – SEM Gap at Interface

Figure 9) SEM photomicrographs of gaps commonly observed at the interface between quartz and gold. A) BSD image of gold (bright material) surrounded by quartz (dark material). Some interface areas exhibit a gap while in other areas the two phases appear to be in contact. Bright spots in the quartz (red circles) indicate small particles of gold buried just beneath the surface of the quartz. B) Higher magnification view of the gap around the edge of a euhedral quartz crystal up against gold showing the step/terrace texture. C) BSD image of subhedral quartz crystals sandwiched between gold. In many areas there is a gap between the two phases. D) Higher magnification image of the gap from the previous image. Gold exhibits the step/terrace texture implying that it grew unobstructed in the gap.

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Figure 10 – Quartz SEM

Figure 10) SEM photomicrographs of quartz from Round Mountain samples. A-B) Quartz often occurs in pockets in the gold as subhedral to anhedral masses. C) On rare occasions quartz occurs as euhedral crystals such as the two in this BSD photomicrograph. The quartz crystal on the left looks like gold has grown around it implying that it formed prior to or at the same time of gold growth. D) Occasionally quartz grains exhibit signs of deformation such as the conchoidal fracture on this quartz grain imaged in VPSE mode to prevent charging.

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Figure 11 – Adularia SEM

Figure 11) Adularia crystals are fairly common in Round Mountain samples, though not as abundant as quartz. This photomicrograph collected in VPSE mode is one example of a euhedral adularia crystal that has been fractured. Corresponding EDS spectra confirms the presence of Si, O, Al, and K indicating the composition of a potassium feldspar.

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Figure 12 – Ag SEM Elemental Maps

Figure 12) SEM BSD photomicrograph of a region of the sample containing a cluster of small particles surrounded by gold and quartz. Elemental maps of this region identify the small particles as dominantly silver (center of the photomicrograph) surrounded by darker areas of Si and O corresponding with quartz and gold with lesser amounts of silver in solid solution with the gold. This texture is likely the result of silver leaching from the surface of the electrum and reprecipitating nearby. 38

Figure 13 – Gold TEM

Figure 13) TEM micrographs of gold/electrum at the interface. Gold is commonly euhedral showing a step and terrace morphology. Corresponding electron diffract patterns for each image show a faint single crystal net with rings around some of the reflections. These rings are of the same reciprocal space and have spacings of 0.233 nm consistent with (100) reflection for Au and Ag. The rings likely correspond with multiple illumination sources generated by secondary diffraction of the originally diffracted beam by stacked crystals. The wavy texture visible in the bottom corner of the micrograph on the far left could be the result of damage from the FIB milling process.

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Figure 14 – Nanoparticles Bright Field TEM

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Figure 14) TEM micrographs of nanospherules observed in Round Mountain gold. A). Polycrystalline SAED pattern collected from image B. Spacings of concentric rings(0.234, 0.203, 0.141, 0.122, and 0.092 nm moving from the central ring out) are consistent wih those of Au-Au and Au-Ag atom pairs. B) Nanospherules with visible atomic structure in many of them. Some of the nanospherules have aggregated together and are overlapping.Darker region of the sample correspond with the underlying carbon film and do not represent grain boundaries, as evident by the atomic structures that transcend the boundary C) Nanoparticles in the sample, some of which have aggregated and others which remain isolated and locked in surrounding phase of sericite-illite D-F) High and low magnification images of nanospherules at the edges of gold grains. E also contains adularia.

Figure 15 – Nanoparticles Apparent Diameter Histogram

Figure 15) Size distribution histogram of the apparent diameter of nanoparticles observed in the photomicrograph from figure 14B. Particles range from 3-8 nm in this portion of the sample but on average are 4-5 nm in apparent diameter.

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Figure 16 – Nanoparticles in Other Phases

Figure 16) TEM micrographs of gold nanoparticles in other mineral phases. A) Gold nanoparticles in illite-sericite. Near-atomic structure of the illite-sericite is visible and lattice spacings were measured to be 10 Å. B) Aggregation of gold nanoparticles in a grain boundary between illite-sericite regions. C) Titanium oxide crystals with gold nanoparticles. D) Higher magnification image of a Ti oxide twin plane from image C with some near-atomic structure. Gold nanoparticles are clearly visible as inclusions in the crystals.

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Figure 17 – STEM Ag Nanoparticles Elemental Maps

Figure 16) TEM photomicrographs of nanoparticles. Accompanying STEM elemental maps indicate that these are Ag nanoparticles of varying sizes. Dark phases in the corners of the micrographs were identified as Au.

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Figure 18 – STEM Elemental Maps

Figure 18) STEM elemental maps showing the distribution of all elements detected in the region of the sample, which is characteristic for the majority of the elemental maps collected in this study. Gallium occurs fairly evenly distributed through the sample as a result of the FIB milling process. The holey carbon film is often visible underneath the TEM foil and is readily detected with EDS. Au and Cu correlate strongly, though Ag is depleted from the surface but concentrated at a depth of a few hundred nanometers. Ag also correlates with darker nanoparticles. Fe is also present in solid solution with the gold in this area. Si, O, Al, and K correlate strongly suggesting the presence of the potassium feldspar adularia. Between gold and adularia there appears to be a gap at the interface that has been filled in with Spur’s resin, the elemental components are light and not readily detected by EDS.

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Figure 19 – STEM Elemental Maps II

Figure 19) STEM elemental maps showing the distribution of elements at the interface. K, Si, and O indicate the presence of adularia. Mapping shows that Cu and Ag are in solid solution with Au and that there is a zonation between Au - Cu and Ag. A concentration of Fe is found near the surface of the adularia grain. There appears to be a gap at the interface, likely filled in with resin from the embedding process. Gallium is evenly distributed throughout this region of the sample as result of the FIB milling process.

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Figure 20 – STEM Elemental Maps - Au, Ag, and Cu

Figure 20) TEM micrographs with accompanying STEM elemental maps showing the distrubtion of metals (Ag, Au, and Cu) in gold from three of the Round Mountain samples. Au and Cu correlate strongly at the surface, while the surface seems to be depleted below detection levels in Ag. Ag seems to be concentrated at a depth below approximately 200 nm beneath this Au-Cu rich zone. In the bottom set of maps, Ag correlates with darker nanoparticles in the top right of the images. The presence of both Cu and Ag at the surface could have important implications for cyanide leaching.

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Figure 21 – Linear Thermal Expansion Graph

Figure 21) Graph showing the linear thermal expansion of phases present at the interface over the range of temperatures corresponding with low sulfidation epithermal deposits. An initial model length of 100 µm was used for these calculations. Linear coefficients of thermal expansion vary for some minerals based on temperature and crystallographic orientation/axis, so the maximum thermal expansion coefficient for this temperature range was used for the entire temperature range to simplify calculations.

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Figure 22 – Gallium STEM Maps

Figure 22) STEM elemental maps showing the distribution of gallium in several regions of Round Mountain samples. Gallium is commonly distributed fairly evenly throughout the sample in low concentrations as a result of the FIB milling process. Higher gallium concentrations are observed in areas directly irradiated by the Ga+ beam such as along the bottom and edges as evident in these maps where the beam is used to cut the foil free from the surrounding material for extraction.

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Figure 23 – Mineralization Sequence Interpretation

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9. Appendices

Appendix A: Supplementary SEM

Additional Mineral of Interest - Pyrite

FESEM photomicrographs of euhedral pyrite grains observed in Round Mountain samples. Pyrite occurs mainly as cubes but other morphologies have been observed. The bottom left image shows triangular etch pits.

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Preliminary SEM from new macrocrystalline samples

Preliminary SEM photomicrographs collected from new macrocrystalline samples donated by the staff at Round Mountain. Some areas of the sample display a similar step/terrace texture (top left) observed in the samples purchased for this study. Steps and terraces are still often polygonized and may show a triangular morphology (bottom right). Triangular pits interpreted to be the result of dissolution (bottom center) are also present.

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Appendix B: Supplementary TEM Micrographs

TEM micrographs collected at high magnifications of nanoparticles in Round Mountain gold samples. Diffraction patterns collected from these ares typically display polycrystalline rings that have spacings consistent with those expected for gold-silver. On average the apparent diameter of the nanoparticles ranges from 2-10 nanometers, but larger nanoparticles have been observed.

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A) TEM micrograph of abundant gold nanoparticles in illite/sericite. B) Higher magnification image of gold nanoparticles in illite/sericite. Linear features represent the 2:1 layer structure. C) Nanoparticles in illite/sericite. Some of the nanoparticles are elongated in the same orientation of the mica and may indicate a replacement texture. D) High magnification TEM micrograph of gold nanoparticles that are aggregated together. Darker bands in some of the nanoparticles represent twin planes.

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SAED Pattern Formation – secondary diffraction

Diagram depicting how the SAED pattern of rings around individual reflections is generated. The primary electron beam is diffracted by a single crystal, and this diffracted beam is in turn diffracted again by crystals stacked beneath it. In the case of Round Mountain samples, it is believed that a single crystal overlies thin films of gold nanoparticles that result in the observed diffraction patterns. This is supported by the consist 0.230 nm spacing of each of the rings around individual reflections. Diagram is courtesy of Dr. Richard Edelmann.

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