©2013 Society of Economic Geologists, Inc.Economic Geology, v. 108, pp. 1577–1589

Insights from Pb Isotopes for Native Gold Formation During Hypogene and Supergene Processes at Rich Hill, Arizona

GeorGe D. Kamenov,1,† eriK B. melchiorre,2 Faye n. ricKer,1 anD eD DeWitt3,*1 Department of Geological Sciences, University of Florida, Gainesville, Florida 32611 USA

2 Department of Geological Sciences, California State University San Bernardino, San Bernardino, California 92407 USA3 U.S. Geological Survey, Central Mineral and Environmental Resources Science Center, Box 25046, MS-973,

Denver Federal Center, Denver, Colorado 80225 USA

AbstractThe origin of gold nuggets in placers is enigmatic as they can be mechanically separated from the primary

vein deposit or can be of secondary origin, formed in the placer environment. Pb isotopes can be a valuable tracer as a match in the isotopic ratios between vein and placer gold will indicate that the placer nuggets are simply mechanically separated from the hydrothermal vein and unmodified by weathering.

Reconnaissance elemental analyses revealed that vein and placer gold from the Rich Hill district, Arizona, contain up to thousands of ppm Pb and therefore both are suitable for Pb isotope analyses. We measured Pb iso-topes in placer gold nuggets and compare the results to primary vein gold and sulfides and also gangue minerals and host rocks in the Rich Hill area. Primary vein gold and galena show similar Pb isotope ratios with 206Pb/204Pb ranging from 18.364 to 18.486, 207Pb/204Pb from 15.574 to 15.593, and 208Pb/204Pb from 37.150 to 37.359. This indicates that the ore metals were derived from a similar source during a hypogene hydrothermal event in the geologic past. Present-day Pb isotope compositions of local rocks in the area show distinct values compared to the primary vein mineralization. Age-corrected and modeled Pb isotope compositions for local magmatic rocks do not provide a clear picture for a possible hydrothermal ore metal source due to possible alteration processes and the long time elapsed since the mineralization event. The distinct present-day Pb isotopes in local lithologi-cal units, however, provide an excellent tracer for deciphering the origin of the placer gold.

All the placer gold nuggets show very distinct Pb isotope compositions when compared to their respective hydrothermal vein source represented by the vein gold and galena. The placer gold Pb isotopes show very wide variations, with 206Pb/204Pb ranging from 17.985 to 20.057, 207Pb/204Pb from 15.494 to 15.749, and 208Pb/204Pb from 36.863 to 38.606.The placer nuggets follow the present-day Pb isotope trend observed in rocks and sedi-ments in the area. In addition, the Pb isotopes in majority of the placer gold nuggets are far more radiogenic compared to the vein gold, sulfides, and even gangue minerals. This is indicating that a significant part, if not all, of the Pb in the placer gold was relatively recently acquired from the placer sediments. The observed distinct Pb isotope compositions between primary and placer samples suggest that gold undergoes significant chemical changes during the supergene stage. Considering a number of possible scenarios, the most plausible explana-tion for the observed present-day Pb isotope similarity between placer gold and local rocks is formation of a significant part of the total mass of these nuggets within the placer environment.

IntroductionGolD occurs in placer and hydrothermal deposits primarily in native form. The occurrence of native gold and gold-silver alloy (electrum) within hydrothermal veins of epithermal deposits is interpreted as hypogene Au precipitated during the hydrother-mal stage (e.g., Saunders et al., 2008, and references therein). On the other hand, the origin of native placer gold nuggets (defined as masses weighing >1 g or measuring >4 mm) and smaller placer gold is not well understood as they may have pri-mary, secondary, or a combination of primary and secondary formation (e.g., Hough et al., 2009). In the classic interpreta-tion, it is assumed that a gold mass forms during a hypogene hydrothermal event and survives the weathering stage intact due to the low reactivity and malleability of gold to ultimately become part of a placer deposit (e.g., Lindgren, 1911; Chapman et al., 2010, 2011). However, several features observed in nug-gets, including rounded forms, overgrowths, bacterioform gold, and nanoparticle biofilms suggest a secondary origin (Watterson,

1992; Mossman et al., 1999; Reith et al., 2007, 2010; Hough et al., 2009; Southam et al., 2009). However, other authors have rejected the notion of biological or low-temperature geochem-ical nugget growth in favor of an abiological, high-temperature origin within lode deposits (e.g., Hough et al., 2007). The recent consensus is that the processes of both models likely play a role in nugget formation (e.g., Reith et al., 2010).

In nature, gold is usually alloyed to some extent with Ag and contains trace levels of other elements, including Pb (Pettke and Frei, 1996; Kamenov et al., 2007). It is therefore possible to measure the isotopic composition of Pb alloyed in native gold to provide important clues regarding the potential source(s) and process(s) that contributed to the formation of placer and lode gold deposits. In the case of placer gold nuggets, the Pb isotope information can show if the nuggets are “primary,” i.e., have the same isotopic signature as the primary hydrothermal vein gold, or are “secondary” and have an isotopic signature distinct from their primary gold source. In this study, nine placer gold nuggets from the Rich Hill district, Arizona, were analyzed, and their Pb isotope composition compared to data from primary vein gold, sulfide, and gangue minerals, and host-rock samples in order to examine their possible origin.

0361-0128/13/4153/1577-13 1577Submitted: March 2, 2012Accepted: March 14, 2013

†Corresponding author: e-mail, kamenov@ufl.edu*Deceased August 7, 2013. Please see the obituary for Dr. DeWitt in the

October SEG Newsletter.

1578 KAMENOV ET AL.

Geologic Settings

Regional geology

The Rich Hill district of Arizona (also known as the Weaver II district) is located in central Arizona, USA (Fig. 1). The bedrock geology of the region consists of metasedimentary and metavolcanic rocks of the 1.8 to 1.74 Ga Yavapai Super-group (e.g., Anderson, 1989; Karlstrom et al., 2001) and the “granite of Rich Hill” (DeWitt, 1989). The Rich Hill granite is a coarse-grained muscovite-biotite–bearing, light gray rock with rusty spots from the weathering of magnetite crystals (up to 1% of the rock). Similar granitic rocks from north-central Arizona usually date to 1.7 to 1.75 Ga (DeWitt, 1989). The

metamorphic and intrusive units are cut by diabase dikes composed of fine equigranular plagioclase, hornblende, bio-tite, and clinopyroxene. These dikes are mineralogically and chemically similar to the 1.1 Ga diabase veins found in the Sierra Ancha Mountains and other locations throughout cen-tral Arizona (Wrucke, 1989). The diabase dikes are closely associated with elevated gold where they border lode veins.

Although tentatively classified as Tertiary in age by Keith et al. (1983), vein deposits in the Rich Hill and nearby Martinez metallic mineral districts are possibly Middle Proterozoic in age and related to emplacement of mafic dikes that are chemi-cally and mineralogically identical to ~1.1 Ga Sierra Ancha diabase. Gold-rich veins at the Congress and Alvarado mines,

FiG. 1. Map of the Rich Hill area, insert showing the location in Arizona. Letters represent lode sample localities and num-bers represent placer sample localities of samples analyzed for Pb isotopes. The numbered “x” markers denote the location of samples examined only for silver content and transport shown on Figure 12 (samples 5 and 6 are located many kilometers south of this map, and are not shown). The solid red line is the surface trace of the Octave-Beehive vein.

INSIGHTS FROM Pb ISOTOPES FOR NATIVE GOLD FORMATION AT RICH HILL, ARIZONA 1579

located to the northwest of Octave, are parallel to mafic dikes that strike northwest and dip shallowly northeast. Hydrother-mal alteration related to vein formation has turned the adja-cent mafic dikes into mixtures of chlorite, biotite, and calcite. The mafic dikes crop out extensively to the west and north-west, throughout the Weaver Mountains and to the Santa Maria River. In the late 1980s, Echo Bay Exploration Inc. began producing gold ore from a previous unmined part of the Niagara vein structure of the Congress mine. During their mapping and production, the company noted two postmineral dikes, one mafic and one felsic that cut perpendicularly across the gold-bearing vein on the second development level drift, west of the main haulage decline. The 2-ft-thick, fine-grained, equigranular mafic dike was oriented N 43° E and dipped 45° SE. In thin section the dike was composed of intermediate plagioclase, hornblende, clinopyroxene, and minor biotite and calcite. Importantly, this dike was not the “diabasic” main mafic dike at the Congress mine that paralleled and was exten-sively altered by the vein system, but was a younger dike, ori-ented perpendicular to the vein system. Hornblende was separated from the dike and analyzed by the 40Ar/39Ar step heating technique. A plateau date of 103.7 ± 2.1 Ma was determined for 88.6% of the gas from the hornblende (DeWitt, unpub. data, 1989). This mafic dike, which clearly crosscuts the mineralized Niagara vein in the Congress mine area, is Early Cretaceous and indicates that gold mineraliza-tion must predate 103.7 Ma. The gold-bearing veins at the Congress and Alvarado mines cut the granodiorite at Yarnell (DeWitt, 1989), which is a 1.4 Ga pluton (J.L. Wooden, unpub. U-Pb zircon data, 1990). Therefore, mineralization in the area must be younger than 1.4 Ga and older than 103.7 Ma.

Rich Hill lode and placer deposits

Quartz veins hosting the gold mineralization in the Mar-tinez and Rich Hill districts are very similar. Rich Hill veins range from a few centimeters to 2 m in thickness (Fig. 2), and are composed of quartz, base metal sulfides (pyrite, galena, minor chalcopyrite), and native gold or electrum. The veins strike northeast and dip about 30° to the north. These lode veins cut older gold-barren quartz-mica pegmatite veins and

aplite dikes. Most of the lode veins are paralleled by diabase dikes and/or Octave diorite. The Octave diorite is a small intrusive body which forms a localized dike that borders the Octave vein and diabase dike (Melchiorre, 2009). The diabase and diorite are believed to be contemporaneous, as they do not cut each other and have the same crosscutting relation-ships with older and younger rocks. The age of the diorite is therefore tentatively assigned as 1.1 Ga. The main lode is the Octave-Beehive vein (Fig. 1). The vein typically consists of distinctive white and blue quartz (Fig. 2). The blue quartz is usually only weakly mineralized, forms the footwall of the vein, and was used by miners to delineate the bottom of the ore zone at Octave. The white quartz is always present in the vein and is the source of virtually all of the ore from the vein. The white quartz is weakly to strongly banded in most places, and contains irregularly distributed masses and dissemina-tions of native gold-electrum and gold- and silver-bearing galena, pyrite, chalcopyrite, sphalerite, and bornite.

Placer gold occurs in small quantities within modern stream sediments and debris flow deposits in an approximately 16 km2 area centered on these vein deposits (Fig. 1). Suban-gular to subrounded gold particles ranging from 0.1 mm with a mass of a few mg, to fist-sized nuggets with a mass measured in kilograms, occur throughout the placers (Figs. 3, 4A, B,

FiG. 2. Photograph of the Octave vein in outcrop, showing white quartz with boxwork cavities produced by the weathering of sulfides, above the “blue” quartz (gray portion of vein) that marks the footwall of the vein.

FiG. 3. Nearly 40 troy oz of gold nuggets from the Devils Nest placer of the Rich Hill area, mined in 2008 to 2010 by Jeff Yahn. These nuggets come from a variety of placer deposits (e.g., red, white, and black), but are domi-nantly from the base of the red placers.

1580 KAMENOV ET AL.

C). While placer gold does occur within the sediments of the modern, active channel of Weaver Creek, most of the placer gold occurs within “red placers” (Crombie et al., 2002). These older alluvial deposits are both colored and well cemented by the dried, iron-rich red smectite clay that makes up a large percentage of the matrix of the unit (Fig. 4D). Sedimentary clasts are well rounded to subangular, range in size from sand to boulders over 1 m in diameter, and are local granite and metamorphic rocks. The estimation of placer gold grade in this district is exceptionally difficult due to irregular gold dis-tribution and the nugget effect. Average grade of the red plac-ers is 1 to 2 g gold/m3, with gold more concentrated in the bottom 10 cm of the deposit, directly above the bedrock, and especially within crevices and joints in bedrock. Gold from the richest and thickest of these placers, the so-called “Dev-ils Nest” claims, occurs within red placers, but also as other rich placer types. Occasionally, exceptionally rich bonanzas are found in bedrock channels under the red placers, within the “white caliche” unit (Fig. 4E, F). These deposits aver-age 5 to 10 g gold/m3, and consist of larger and much more rounded and often flattened gold particles than those found in the overlying red placers (Fig. 4D). Though this unit is called white caliche, it often contains little calcium carbonate. The white appearance is due to bleached cobbles and sediments that are cemented by sericite and highly silicified material. In the deepest bedrock depressions, beneath the white caliche, are gold-bearing gravels cemented by a black manganese-iron oxide and barium manganese oxide (hollandite) precipitate. These “black gravel” deposits average 1.5 to 4 g gold/m3, and

consist of two-dimensional, well-rounded, flattened gold par-ticles that are nearly completely encapsulated by black crust (Fig. 4C). These black gravels typically occur as thin seams near bedrock, and as fill material for the locally deepest pot-holes in bedrock.

Total placer gold production, from the discovery in 1863 to present, is officially 100,000 troy oz. However, the real total is undoubtedly much higher as most gold went unreported by individual miners (Crombie et al., 2002). Lode gold at Rich Hill was dominated by production of the Octave mine (Figs. 1, 2). This mine was most active from 1895 to 1942 and produced at least 80,000 troy oz of gold. The Beehive, Mey-ers, and Johnson mines have produced considerably lesser amounts of gold. Recent geochemical and mineralogical work on the Rich Hill deposits, published as general interest books, consists of electron probe, scanning electron microscope (SEM), and X-ray energy dispersive spectra characterization of the mineralogy of the lode (Melchiorre 2008, 2009). As these publications are not widely available, relevant data are represented in this paper.

Materials and MethodsVein gold, sulfides, and gangue minerals were recovered

from Octave, Johnson, Meyer, Joker, and Beehive sites (Fig. 1). Representative mineral samples were carefully separated from the bulk vein with stainless steel tools. Gold nuggets were collected from the main placer deposits located near by the main hydrothermal veins (Fig. 1). Gold was recovered from the placer sediments by utilizing a Keene Engineering

FiG. 4. Samples of placer gold from the (A) red placer (B) white caliche (C) and black gravel placers, showing relative physical form. The placers that produced this gold are also unique in appearance due to (D) iron staining and smectite clay in red placers, (E) bleached cobbles and boulders of bedrock in white caliche placers (dashed line marks boundary between upper modern overburden and lower placer material), (F) and thin black manganese-rich zones above bedrock within local bedrock lows.

INSIGHTS FROM Pb ISOTOPES FOR NATIVE GOLD FORMATION AT RICH HILL, ARIZONA 1581

vibrostatic dry washer for smaller pieces (0.2–0.5 g), while larger gold (>0.75 g) was recovered by use of a MINELAB GP-Extreme metal detector. It was not practical to obtain additional samples owing to the elevated price of gold, dif-ficulty in maintaining access to claims, and time required for obtaining samples (up to 6 hours per sample). These placer samples represent individual gold grains as recovered by, or in the presence of, one of the authors (E. Melchiorre) and are not bulk bullion samples or amalgamated placer gold sam-ples. Representative sediment samples were collected from the placers and modern streams in the area. Magmatic and metamorphic rocks were collected from several sites nearby Octave-Beehive vein and from the Rich Hill granite.

SEM imaging and in situ elemental analyses of represen-tative gold grains were generated using a JEOL JXA–8500F using standard off-peak interference and matrix corrections (Armstrong, 1988; Donovan et al., 1993). The error associated with the in situ trace element microprobe analyses is less than ±10%. Selected single gold grains with a mass of 0.25 g per sample were dissolved in a closed vessel aqua regia (hydro-chloric and nitric acid) digestion. Twenty-four hours of cold digestion were followed by 1 hour of digestion at 100°C. No insoluble residue was observed, and mass loss through insol-uble residue was below detection limits on a 0.1 mg balance. The resultant solution was analyzed using an Element 2 HR-ICP-MS at the University of Arizona. The error on the gold grains trace element concentrations is less than ±10%. U, Th, and Pb concentration analyses on whole-rock samples were performed on an Element 2 HR-ICP-MS at the University of Florida on the same powders used for Pb isotope analyses following Kamenov et al. (2008) methods. The error on the whole-rock U, Th, Pb concentrations is less than ±5%.

Sample preparation for isotope analyses was performed within a clean laboratory (class 500) equipped with class 10 laminar flow hoods, using Optima-grade reagents. Whole rocks and silicate gangue minerals were digested in pre-cleaned Teflon vials with HF-HNO3 and then evaporated to dryness. The residue was dissolved in 1N HBr and loaded on columns packed with Dowex 1X-8 resin, preliminary cleaned with 2 ml 6N HCl. The sample was washed three times with 1 ml 1N HBr and the Pb fraction was eluted in 1 ml 3N HNO3. Sulfide vein minerals were digested by adding 1 to 2N HNO3 and evaporating the solution to dryness. The dry residues from the decomposed sulfides were processed for Pb isotopes similar to the whole-rock samples. Galena was also dissolved in HNO3 and the resultant solution was directly analyzed for Pb isotopes without column purification.

As is generally known, modern soils and sediments can be contaminated with anthropogenic Pb released during the era of leaded gasoline usage; therefore, the sediments were sub-jected to a three-step leaching protocol, following Kamenov (2008), to remove any anthropogenic Pb present in the sam-ple. This anthropogenic Pb is typically present as labile Pb and is expected to be found mainly in the first 0.1N HCl leachate, and to some extent in the second 2N HCl leachate (Kamenov, 2008). The residue after the two leaching steps was digested with HF-HNO3, and Pb from all fractions was separated fol-lowing a similar protocol as the whole-rock samples.

Representative clean vein and placer gold grains without any surficial coatings and/or inclusions were subjected to Pb

isotope analyses. In order to avoid any possibility of contami-nation, the grains selected for Pb isotope analyses were not subjected to SEM imaging but were carefully examined under a binocular microscope. Larger grains were cut with stainless steel tools and small, clean portions (about 0.02 g–0.05 g) of the selected gold grains were cleaned overnight in hot (~100°C) 3.5N HNO3, rinsed several times with 4× distilled H2O, and then dissolved with aqua regia. The nitric acid cleaning step should remove any invisible by binocular microscope sulfides, carbonates, and oxide coatings that may be present in discrete cracks or imperfections on the outside of the gold grains. Even if present inside the gold, micron-size silicate inclusions (e.g., quartz, zircons) will not dissolve during the aqua regia gold dissolution step. Lead was extracted from the gold sam-ples following a protocol modified after Bendall et al. (2009). The resultant aqua regia solution was evaporated and the gold residue was dissolved in 2 ml 6N HCl. The 6N HCl solution was loaded on columns packed with Dowex 1X-8 resin and the elutant was collected in a clean Teflon vial. The Dowex resin attracts the AuCl– ions, while Pb passes through in the 6N HCl elutant (Bendall et al., 2009). The gold-free solution was then evaporated to dryness and Pb was further purified with 1N HBr following the protocol described above for the whole-rock and gangue mineral samples.

Pb isotope compositions for all samples were determined on a “Nu-Plasma” MC-ICP-MS with Tl normalization fol-lowing a method described in Kamenov et al. (2004). Long-term analysis of the NBS 981 Pb isotope standard yielded the following average values: 206Pb/204Pb = 16.937 (±0.004, 2σ), 207Pb/204Pb = 15.490 (±0.003, 2σ), and 208Pb/204Pb = 36.695 (±0.009, 2σ).

ResultsThe majority of the gold in the Rich Hill hydrothermal

quartz veins occurs as micron-sized inclusions within the quartz and/or within or adjacent to sulfide minerals. Vis-ible native gold in the veins is rare. On Figure 5 is shown a

FiG. 5. Backscatter electron image of a typical lode gold sample from the Octave gold mine. Minerals are quartz (qtz), galena (ga), and acanthite (ace). Microprobe analysis of the fine-grained native gold indicates an average com-position of 83.9 wt % Au, 15.3 wt % Ag, 0.6 wt % Pb, 0.2 wt % Cu.

1582 KAMENOV ET AL.

backscatter electron image of a typical run-of-mine ore with the occurrence of native gold as micron-sized grains in thin quartz veinlets between galena and acanthite. Microprobe analysis of this hypogene native gold indicates a composition rich in silver (15.3 wt %), lead (0.6 wt %), and copper (0.2 wt %) (Fig. 5; data presented in the figure caption).

The majority of the gold grains recovered from the red placers and white caliche placers are clean, mm- to cm-size nuggets (Fig. 3). Although the gold from the red placers is largely devoid of coatings, some relict crustlike material can be observed in pits and cracks (Fig. 6A, B, C). In contrast, the black gravels gold grains are often nearly completely encap-sulated by a crust up to 80 µm thick (Fig. 4C). Microprobe investigation revealed that the crust is composed of Mn, C, and clays (Fig. 6D). Backscatter imaging data reveal that placer gold grains from Rich Hill rarely contain mineral inclu-sions. Where present, inclusions include quartz, annite mica, and caliche breccias (Fig. 7A, B). No sulfide inclusions were observed in this study. Where present, inclusions are mainly restricted to the outer rims of the gold grain and are obvious to external visual examination under a binocular microscope. The vast majority of gold grains from the Rich Hill placers are devoid of obvious mineral inclusions (Fig. 8). The gold grains typically have a silver-poor, gold-rich rim, which shows well under backscatter electrons (Figs. 7A, B, 8). Reconnaissance

ICP-MS analyses of bulk gold grains from the three types of placers show variable Pb concentrations ranging between 1,000 and 10,000 ppm (Table 1). Microprobe investigation confirmed that Pb is in the gold matrix with up to thousands of ppm in within the rims and the cores of the placer gold grains (Table 2; Figs. 7A, B, 8).

Whole-rock U, Th, and Pb concentrations and resultant U/Pb and Th/Pb ratios are presented in Table 3. Pb isotope data for native gold, sulfides, gangue vein minerals, sediment leachates and residues, and whole rocks from the Rich Hill district are shown in Table 4. Vein gold found in situ within the vein shows relatively uniform Pb isotope ratios with 206Pb/204Pb ranging from 18.369 to 18.486, 207Pb/204Pb from 15.574 to 15.593, and 208Pb/204Pb from 37.150 to 37.359. Vein sulfides overall show very similar Pb isotope values to the gold ranging from 18.364 to 18.488 for 206Pb/204Pb, 15.581

FiG. 6. Scanning electron microscope image of the outer surface of gold (A) from red placer, showing the entire gold grain and dark biofilmlike forms in pits and crevices. Boxed and lettered areas show the detail of these biofilmlike forms at (B) and (C). The gold from the black gravel placers (D) is nearly entirely enclosed by a black coating of biofilm.

taBle 1. ICP-MS Trace Element Concentrations (ppm) Measured in Three Placer Gold Samples

Hg Bi Cu Sb Te Pb

Red placer gold 2,400 270 5,200 260 150 10,300 Black gravel gold 490 30 5,720 380 26 5,900 White caliche gold 1,100 14 4,600 40 7 980

INSIGHTS FROM Pb ISOTOPES FOR NATIVE GOLD FORMATION AT RICH HILL, ARIZONA 1583

to 15.591 for 207Pb/204Pb and 37.249 to 37.345. Two sulfide samples, pyrites from Meyer and Johnson mines, show more radiogenic Pb isotope values compared to the gold and galena (Table 4). The Pb isotopes for these two pyrites fall within the range observed for the gangue minerals (Table 4). In contrast, placer gold, sediments (including leachates and residues), and whole rocks show very large variations in their Pb iso-tope compositions with 206Pb/204Pb ranging from 17.088 to 23.581, 207Pb/204Pb from 15.429 to 15.850, and 208Pb/204Pb from 35.852 to 40.876. All of the samples form a linear trend in 206Pb/204Pb-207Pb/204Pb space with slope corresponding to 1,580 (± 60) Ma (Fig. 9), as calculated with Isoplot (Ludwig, 1999).

Discussion

Pb isotopes in hypogene native gold

Pb is a common ore metal and the isotopic composition of galena and vein gold, therefore, can be used to estimate

FiG. 7. Images of a red placer gold grain that displayed obvious inclusions on its outer surface when examined under a binocular microscope. (A) Back-scatter electron image, showing locations of microprobe analyses presented in Table 2. Contrast has been set to highlight the distinct and sharp, yet irreg-ularly distributed zones of gold enrichment-silver depletion. (B) Backscatter electron image of the same nugget, with contrast set to highlight mineral inclusions of quartz (qtz), annite mica (ann), and caliche that consists of cal-cite (cal) matrix and quartz breccia.

FiG. 8. Backscatter electron image of a typical inclusion-free red placer gold grain, showing locations of microprobe analyses presented in Table 2. Contrast has been set to highlight the distinct and sharp yet irregularly dis-tributed zones of gold enrichment-silver depletion.

taBle 2. Microprobe Analyses of Two Nuggets, Spot Positions Shown on Figures 7 and 8

Sample point Position Fe (wt %) Cu (wt %) Zn (w t%) Ag (wt %) Sb (wt %) Te (wt %) Ba (wt %) W (wt %) Au (wt %) Pb (wt %) Total

Figure 7 1 Core 0.052 0.053 b.d.l. 12.2 0.023 0.030 b.d.l. 0.020 88.5 0.045 100.9 2 Core b.d.l. 0.048 b.d.l. 11.6 0.005 0.017 0.003 b.d.l. 89.0 b.d.l. 100.8 3 Rim b.d.l. b.d.l. b.d.l. 1.3 0.030 0.020 0.016 b.d.l. 97.9 b.d.l. 99.2 4 Core 0.022 0.053 b.d.l. 10.9 b.d.l. b.d.l. 0.012 0.053 89.3 0.048 100.5

Figure 8 1 Rim 0.037 b.d.l. b.d.l. 0.5 b.d.l. b.d.l. 0.008 b.d.l. 99.6 0.244 100.4 2 Rim 0.017 0.010 b.d.l. 10.3 0.016 0.015 0.000 0.004 90.3 0.047 100.8 3 Core 0.052 0.033 0.019 10.4 0.025 0.007 0.000 b.d.l. 89.8 b.d.l. 100.4 4 Core 0.011 0.043 0.007 10.2 0.019 0.023 0.012 b.d.l. 90.0 0.278 100.5 5 Core 0.004 0.007 0.013 10.3 0.017 0.009 0.011 0.009 89.8 0.672 100.9

b.d.l. = below detection level

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the source of gold and other ore metals in the vein. Galena contains 86.6% Pb by weight and has extremely low concen-trations of U and Th, therefore its present-day Pb isotope composition is representative of the original Pb present in the hydrothermal fluids at the time of the formation of the vein. As can be seen in Table 4 and Figure 10, galena and vein gold show very similar Pb isotope compositions. The similarity in the Pb isotope compositions between galena and vein gold indicates that both are primary minerals formed in situ in the vein during the hypogene, hydrothermal stage. The similarity of the Pb isotope compositions in galena concentrates from Johnson and Octave-Beehive (Table 4) shows that both areas of mineralization are related to the same hydrothermal event and share the same source of ore metals. Similarly, analyzed pyrites from Octave also show similar Pb isotope composition

taBle 3. Pb, Th, and U Concentrations (ppm) and Resultant 238U/204Pb and 232Th/204Pb Ratios for Whole Rocks from the Rich Hill Area

Sample Pb Th U 238U/204Pb 232Th/204Pb

Diabase dike, Octave 10.9 3.9 2.0 13.3 26Pegmatite, Octave 0.3 0.0 0.0 1.8 3.9Diorite, Octave 11.2 0.9 0.5 3.5 5.5Greenstone, Octave 2.3 2.2 1.0 31.6 71Greenstone, Stanton 7.0 0.4 0.2 2.0 3.8Diorite, Joker 2.6 3.7 1.0 28.8 105Rich Hill granite 30.5 11.6 3.5 8.3 28

taBle 4. Pb Isotope Compositions of Rich Hill Samples

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Placer native goldPlacer gold, Potato Patch placer (1)1 19.243 15.662 37.746Placer gold, Lower Weaver Creek (2) 19.389 15.682 37.049Placer gold, Meyer Creek (3) 19.093 15.668 38.325Placer gold, Bottom main (4) 19.006 15.662 37.562Placer gold, black placer (5) 18.956 15.635 37.523Placer gold, black gravel (5) 20.057 15.749 38.606Placer gold, white caliche (6) 17.985 15.494 36.863Placer gold, Beehive (7) 18.971 15.627 37.584Secondary gold in schist joint, 18.784 15.612 37.430 Beehive (B)

Vein native goldVein gold, Octave 1 (O) 18.369 15.574 37.162Vein gold, Octave 2 (O) 18.409 15.576 37.150Vein gold, Octave 3 (O) 18.486 15.593 37.359

SulfidesPyrite, Meyer mine (M) 18.678 15.608 37.327Pyrite, Johnson mine (JM) 18.583 15.602 37.363Pyrite, Octave mine (O) 18.488 15.590 37.310Pyrite, Octave 2 (O) 18.446 15.584 37.332Pyrite Octave 3 (O) 18.450 15.590 37.345Galena, Octave (O) 18.364 15.581 37.310Galena, Joker (J) 18.457 15.591 37.285Galena, Beehive (B) 18.442 15.586 37.249Galena, Johnson mine (JM) 18.443 15.589 37.249

Gangue vein mineralsQuartz, gold vein Octave (O) 18.750 15.618 37.677White quartz, Octave (O) 18.558 15.603 37.355Blue quartz, Octave (O) 18.710 15.613 37.501Malachite, Beehive Twin Tunnel 18.608 15.601 37.303 area (B)Quartz, Beehive Twin Tunnel area (B) 18.615 15.599 37.312Quartz, Johnson mine (JM) 18.565 15.602 37.373

Modern sediments leachates and residues0.1N HCl leach, Weaver sediment 18.795 15.608 37.9292N HCl leach, Weaver sediment 18.794 15.609 37.899Residue, Weaver sediment 18.969 15.623 37.9042N HCL leach, heavy fraction 19.507 15.676 38.394 Weaver sedimentResidue, heavy fraction Weaver 19.040 15.613 37.683 sediment0.1N HCl leach, black placer sediment 22.431 15.972 39.2442N HCl leach, black placer sediment 21.544 15.878 38.743Residue, black placer sediment 21.103 15.833 38.1510.1N HCl leach, Bottom main sediment 22.868 16.006 39.3822N HCl leach, Bottom main sediment 21.886 15.911 38.890Residue, Bottom main sediment 20.450 15.764 37.8302N HCl leach, white caliche sediment 21.685 15.889 38.811Residue, white caliche sediment 19.606 15.688 37.3510.1N HCl leach, black gravel sediment 21.638 15.883 39.0032N HCl leach, black gravel sediment 21.690 15.889 39.023Residue, black gravel sediment 20.658 15.790 37.944

Magmatic and metamorphic rocks:Diabase dike, Octave 19.494 15.673 37.679Pegmatite, Octave 18.236 15.550 36.634Diorite, Octave 18.591 15.589 37.603Greenstone, Octave 23.581 16.062 40.876Greenstone, Stanton 17.088 15.429 35.852Diorite, Joker 20.962 15.839 38.121Rich Hill granite 21.280 15.850 37.540Beehive schist, 18.793 15.620 37.456 Nugget pit silicate fraction

1 Sample locations on Figure 1

15.4

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16

17 18 19 20 21 22 23 24

sedimentsrocksplacer goldvein gold

36

37

38

39

40

41

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207 P

b/20

4 Pb

208 P

b/20

4 Pb

Age=1580 +/-60 Myr; MSWD=49;

FiG. 9. Present-day Pb isotope compositions of all of the samples from the Rich Hill area. Note that the samples follow a linear trend corresponding to 1580 (±60) Ma isochron. Isochron fitted with Isoplot (Ludwig, 1999).

INSIGHTS FROM Pb ISOTOPES FOR NATIVE GOLD FORMATION AT RICH HILL, ARIZONA 1585

to the galena and vein gold (Table 4; Fig. 10). Only two pyrites from Meyer and Johnson mines show distinct Pb isotope compositions compared to the primary gold and galena and overlap with the gangue minerals in 206Pb/204Pb vs. 207Pb/204Pb space (Fig. 10). Note that these two pyrites have the same 208Pb/204Pb as the rest of the sulfides (Fig. 4), only more radiogenic 206Pb/204Pb and 207Pb/204Pb ratios. Most likely these two pyrites contained significant amount of U (but no significant Th) so their present-day Pb isotope compositions are affected by U decay. The gangue vein minerals have Pb isotope composition that is distinct from the vein gold and primary sulfides (Fig. 10). An offset between gold and gangue vein minerals Pb isotopes was observed in Miocene Nevada epithermal gold deposits (Kamenov et al., 2007). The offset in Nevada deposits was interpreted to be a result of different sources contributing Pb to the gold (magmatic) and gangue vein minerals (local host rocks). Therefore, the observed dif-ference between gangue and gold may also be attributed to different sources in the Rich Hill deposit. However, Arizona mineralization is most likely Proterozoic and therefore much older compared to Nevada Miocene deposits. As can be seen in Table 4 and Figure 10, the gangue Pb isotope compositions are all more radiogenic than the primary sulfides and gold; therefore, the observed differences must also reflect in situ radiogenic Pb growth. The latter is also confirmed by the fact that the gangue minerals lie on the overall trend formed by

all of the samples in Pb-Pb isotope space (Fig. 9). Due to the very long time elapsed since the formation of the mineral-ization, the gangue minerals and also any potential lithogenic source will likely have much more radiogenic Pb at present compared to the galena and the vein gold. As can be seen on Figure 9, there is no direct match between the present-day values of any of the analyzed whole rocks and the vein gold and sulfides, which is expected as the whole-rock Pb isotope compositions evolved through time due to U and Th decay.

Previous works (Page et al., 1994; Melchiore, 2008, 2009) proposed that the gold mineralization is associated with the 1.1 Ga magmatic event represented by the diabase dikes in the area. The analyzed diabase dike shows much more radio-genic present-day Pb with 206Pb/204Pb = 19.494 (Table 4) compared to the primary vein gold and sulfides. However, the present-day more radiogenic Pb isotope composition of the dike is affected by at least 1.1 Ga of U and Th decay. Based on geologic considerations, discussed above in the Geologic Settings section, the gold mineralization can be anywhere from about 100 Ma to 1.4 Ga, but is possibly associated with the 1.1 Ga dike event. On Figure 11 we plotted vein gold data in comparison to whole-rock data corrected for U and Th decay for three different time intervals, 100 Ma, 500 Ma,

15.45

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17.5 18 18.5 19 19.5 20 20.5

sulfidesgangueplacer goldvein gold

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39

206Pb/204Pb

207 P

b/20

4 Pb

208 P

b/20

4 Pb

Meyer pyrite

Meyer pyrite

Johnson pyrite

Johnson pyrite

FiG. 10. Present-day Pb isotope compositions for sulfides, gangue miner-als, and vein and placer gold. Note the overall more radiogenic nature of the placer gold samples, even compared to gangue vein minerals, for more discussion, see the text.

15.3

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14 16 18 20 22 24

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vein goldrocks, present dayrocks, 100 Myrocks, 500 Myrocks, 1100 My

208 P

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Octave

diab

ase d

ike

Octav

e diorit

e

Octave diabase dike

Octave diorite

FiG. 11. Pb isotope data for whole rocks corrected for U-Th decay for 100, 500, and 1100 Ma compared to vein gold. U, Th, Pb concentrations used for age corrections are listed in Table 3.

1586 KAMENOV ET AL.

and 1.1 Ga, based on the measured U, Pb, and Th concen-trations (Table 3). Octave diorite shows relatively close Pb isotope compositions to the vein gold and sulfides at 100 Ma (Fig. 11), suggesting that it can be a possible source for the ore metals. However, this will indicate younger age of the hydrothermal veins and no relations to the 1.1 b.y. old dia-base dikes. In addition, field relationships discussed the above indicate that Octave diorite and the diabase dikes are most likely contemporaneous. Octave diabase dike and Octave dio-rite are relatively close to the vein gold and sulfides Pb iso-tope compositions at 500 Ma (Fig. 11); however, it is unclear if 500 Ma is a plausible age, based on the available data for the regional geology. None of the whole rock corrected for 1.1 Ga decay is equivalent to the vein gold and sulfides (Fig. 11). For example, the calculated initial Pb isotopes at 1.1 Ga for the diabase dike are 206Pb/204Pb = 17.020, 207Pb/204Pb = 15.484, and 208Pb/204Pb = 36.218, very distinct from the vein gold and galena. This can be interpreted in several ways, including that none of the analyzed rocks was a source for the gold or that the vein was not formed at 1.1 Ga or that the U-Th-Pb system-atics of the rocks were affected by secondary processes. If the measured present-day U-Pb-Th concentrations in some of the rocks—for example, the diabase dike—were affected by sec-ondary alteration process, then this can lead to U-Pb-Th gain/loss that will strongly influence our initial Pb calculations. As noted above, the dikes are often hydrothermally altered and may contain secondary minerals such as chlorite, biotite, and calcite. If we assume lower U/Pb and Th/Pb ratios in the dike we can model Pb isotope compositions very close to the pri-mary mineralization. Assuming a model with 238U/204Pb = 5.5, we get 206Pb/204Pb = 18.471 and 207Pb/204Pb = 15.596; simi-larly, assuming 232Th/204Pb = 6.1, we get 208Pb/204Pb = 37.287 at 1.1 Ga. Although lower than measured, such U/Pb and Th/Pb ratios are observed in rocks in the area (Table 3). The model-corrected dike Pb isotope ratios are very close to the ratios observed in the primary vein gold and sulfides, suggest-ing the possibility of magma that fed the dikes was the source of gold and other ore metals. However, this is highly specula-tive as the modeled U/Pb and Th/Pb ratios are very different from the measured ratios for the dike (Table 3). Based on the available data, we cannot really resolve if the dike or diorite magmas were the true metal source for the deposits due to the long time that passed since their emplacement and the possibility of secondary alteration.

Pb isotopes in placer gold nuggets

In contrast to the vein gold, the placer gold shows highly variable Pb isotope compositions (Figs. 9, 10). All the placer gold nuggets, with exception of the caliche gold, show much more radiogenic Pb isotope compositions compared to the primary vein gold and sulfides (Fig. 10; Table 4). Further-more, the placer gold is generally much more radiogenic than the gangue minerals from the veins (Fig. 10) and overlap with the local sediments and rocks (Fig. 9). The observed Pb iso-tope compositions in the placer gold point to a distinct Pb source as compared to the primary vein gold and sulfides. As can be seen in Figure 9, the placer gold follows the present-day Pb isotope trend observed in rocks and sediments in the area. This indicates that a significant amount of Pb in the placer nuggets is derived recently from the local rocks and

sediments, rather than the ancient hydrothermal source as discussed above for the native gold in the lode deposits. In addition, the fact that the placer gold Pb isotopes are gen-erally far more radiogenic compared to vein gold and even gangue minerals clearly indicates that most of the lead in the nuggets was acquired relatively recently from the local sedi-ments and/or rocks.

White caliche gold shows less-radiogenic Pb isotopes com-pared to the rest of the placer nuggets and even primary vein gold and sulfides (Fig. 10; Table 4). Apparently, Pb derived from local units containing the least radiogenic Pb at present, such as the Stanton Greenstone metamorphic rocks (Table 4), became incorporated in the caliche gold. In addition, gold in the schist joint in Beehive also shows distinct Pb isotopes from the vein gold and galena (Table 4), suggesting that Pb in this gold is also derived from the local host rocks relatively recently. Regardless of the caliche gold having a less-radio-genic character, and Beehive schist gold’s secondary nature, both have isotopic compositions distinct from the vein gold but similar to present-day rocks and sediments from the study area (Fig. 9), also indicating recent Pb derivation from the latter.

A possibility that must be addressed is that the Pb isotope signal is related to discrete placer inclusions in the nuggets. Chapman et al. (2010, 2011) conducted an extensive study on placer gold in the Yukon and utilized elemental compositions and inclusions (assumed to be primary from the veins) to clas-sify the placer nuggets. In their studies, they conclude that inclusions are often obliterated as a consequence of chemi-cal and physical erosion and/or deformation and not likely to be mechanically trapped within the placer environment. Gold in the Rich Hill area contains significant amount of Pb (Tables 1, 2) and the elevated concentrations of lead are not related to inclusions within the gold grains (Figs, 7A, B, 8). Where present, inclusions include annite mica, quartz, and caliche breccias, none of which are expected to have high Pb concentrations. Simple mass-balance considerations show that even if micron-sized discrete inclusions are present, they must contain extremely high levels of Pb to shift the isotope composition of the gold nugget towards the placer sediments, however, no sulfide inclusions were observed in this study. As we explain above, samples selected for lead isotope analy-ses were examined to eliminate any gold grains with obvious inclusions; therefore, any Pb from such inclusions or in situ radiogenic Pb growth from U-decay related to included min-erals (e.g., zircon) is believed to be insignificant. Furthermore, even if discrete silicate minerals were present, they will not dissolve during the aqua regia dissolution and so are not likely to contribute to the Pb isotope composition measured in the nuggets. In addition, native gold and electrum contain insig-nificant amount of U (Pettke and Frei, 1996). This is clearly evident also in the preservation of the initial Pb isotope com-positions in the vein gold as described in the previous section.

Regarding the isotopic composition of the nuggets, the pos-sibility of lead diffusion in native gold should be examined. Although in experiments with pure metals gold diffuses rela-tively rapidly in lead, presence of impurities in the metallic lead strongly reduces the gold diffusion (Decker et al., 1979). It is unclear if the process can proceed in reverse, i.e., lead to diffuse in gold nuggets, especially in natural environment,

INSIGHTS FROM Pb ISOTOPES FOR NATIVE GOLD FORMATION AT RICH HILL, ARIZONA 1587

where gold is in metallic form but lead is usually not. In addi-tion, nuggets contain much higher Pb concentrations com-pared to local rocks (Tables 1, 2, 3). The observed Pb isotope disequilibrium between gangue host minerals and vein gold indicates that diffusion of Pb in native gold is not likely to be an important process (Fig. 10). In addition, similar Pb isotopic disequilibrium between gangue vein minerals and native gold was observed in Miocene epithermal deposits (Kamenov et al., 2007). The fact that gold did not equilibrate isotopically with the gangue vein minerals in these two cases suggests that Pb diffusion is not likely to be an important factor controlling the Pb isotope composition of the native gold.

Microprobe work shows that Pb in the placer nuggets is occurring within the gold matrix (Figs. 7A,B, 8) and the Pb isotopes show that this Pb is derived from the local placer sed-iments and/or rocks recently. The observed large range in Pb isotope variability in the local rocks and sediments shows that the latter are comprised of different components with highly variable time-integrated U/Pb and Th/Pb ratios (Figs. 9, 11). It is also possible that some of the most labile Pb fractions in close-to-the-surface sediments/soils were affected in modern times by anthropogenic Pb derived from fossil fuel combus-tion and/or mining and smelting operations. Anthropogenic Pb is typically associated with labile sediment fractions (e.g., Kamenov, 2008). However, we do not observe any particular trend in the Pb isotope relationships between sediment leach-ates and residues, indicating that anthropogenic Pb, if any, did not play a major role in the Pb isotope compositions of the leachate placer samples. As can be seen in Table 4, Weaver leachate is less radiogenic than the corresponding residue, but the rest of the leachates are more radiogenic than the corre-sponding residues.

Sediment leachates and residues, even from a single local-ity, show highly variable Pb isotope compositions (Table 4). For example, Weaver sediments show 206Pb/204Pb from 18.79 for bulk sediment leachates to 19.51 for 2N HCl leachate of the separated heavy fraction. The placer gold from Weaver falls within this range for 206Pb/204Pb and 207Pb/204Pb but shows somewhat lower 208Pb/204Pb. Black gravel placer gold shows the highest 206Pb/204Pb from all placer nuggets, which is similar to the black gravel sediments. On the other hand, black placer gold has Pb isotope compositions somewhat lower than the analyzed different fractions from the black placer sediment sample (Table 4) but falls within the overall present-day variability observed in the rocks in the area (Fig. 9). Overall, although there is similarity, there is no exact Pb isotope match between placer gold and the analyzed placer host sediments. This, however, is not surprising because the host placer sediments are highly heterogeneous, from boul-ders to clay-size fraction (Fig. 4 D, E, F). Although we per-formed leaching experiments, it is difficult to determine the exact sediment fraction that contributed Pb to the gold. Fur-thermore, rounded nuggets (Figs. 3, 4A, B) indicate that gold and sediments were transported for some distance from their respective sources but likely in highly differentiated rates due to density contrast. Therefore, it is possible that the present-day host sediments are not the exact source from which the placer gold acquired Pb. In addition, host sediments continu-ally evolve isotopically due to U and Th decay. Therefore, the time elapsed since the gold nugget formation and the present

day may also play a role in the observed differences between host sediment and nuggets.

Gold enrichment in placer nuggets rims and Pb isotope data

The Rich Hill placer gold grains show a silver-poor, gold-rich rim (Figs. 7A, B, 8), a very typical feature for placer gold (e.g., Knight et al., 1999). Although beyond the immediate scope of the Pb isotope work that is the focus of this paper, we reproduce data after Melchiorre (2009) on silver rims of placer gold from modern gravel banks within the active chan-nel of Weaver Creek (Fig. 1), and their relationship to distance from Johnson mine (Fig. 12). Backscatter electron images of the gold show zones of gold-enrichment/silver-depletion (Fig. 12). Average thickness of these rims for the four grains extracted from modern Weaver Creek in the vicinity of the Devils Nest is 10 µm, yet rims in placer gold from the older white caliche and red placers in the Devils Nest are gener-ally thicker (Figs. 7A, B, 8, 12). Knight et al. (1999) noted similar rims in placer gold from Yukon, and interpreted these rims to be the result of silver leaching, and not gold addition, based upon grain morphological characteristics measured for thousands of individual gold grains. The limited data set presented in Table 2 shows that Pb in the rims is somewhat lower compared to the cores, suggesting that Pb is likely fol-lowing Ag and is leached out during the rim development. Although it is unclear when such rims develop, a recent work on Celtic gold coins by Bendall et al. (2009) found that Ag and Cu are depleted within 50 to 200 µm from the outer rims of the coins after only 2,000 years of burial. This is suggesting that the observed Ag-depletion rims in nuggets may be very rapid, possibly a century-time scale process.

The observed secondary gold enrichment of the nugget rims is an evidence for a secondary chemical modification in the placer environment, but it is unlikely to have any substan-tial effect on the Pb isotope systematics of the gold grains. For example, the data for Weaver Creek placer gold show a cor-relation between the thickness of these gold-enrichment/sil-ver-depletion rims and the distance of the sample location from the Johnson mine (Fig. 12). The Johnson mine was

FiG. 12. Silver depletion in placer gold from modern gravel banks within the active channel of Weaver Creek; data and figure after Melchiorre (2009).

Weaver Creek Placer Gold

Distance from Johnson Mine (kilometers)

Gol

d-r

ich

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hick

ness

(mic

rons

)

1588 KAMENOV ET AL.

chosen as a reference point in the upper watershed of Weaver Creek, but was only a minor producer of about 1,000 oz of lode gold. We do not infer that the Johnson mine is the source of all placer gold within Weaver Creek but a possible comple-mentary source. As discussed above, the Johnson mine galena concentrate has the same Pb isotope composition to the major Octave-Joker-Beehive vein indicating that both are part of the same hydrothermal event (Table 4). Therefore, both possible sources will be releasing gold to the placer with similar Pb isotope compositions. However, none of the placer gold sam-ples are even remotely similar isotopically to the primary vein mineralization (Fig. 10). As can be seen in Table 4, Weaver Creek nugget 2 shows the second most radiogenic 206Pb/204Pb ratio, which is very distinct from the primary mineralization signal.

Insights from Pb isotopes for the origin of placer gold

As discussed above, in the Introduction to this paper, there are two competing hypotheses for the origin of placer gold nuggets (e.g., Hough et al., 2009). The first hypothesis is sim-ply unmodified derivation of the gold nugget directly from the vein deposit (i.e., mechanical weathering). Our Pb isotope data contradict placer gold formation and derivation from the lode solely by mechanical weathering as the nuggets show dis-tinct values when compared to the vein primary gold (Fig. 10). This clearly indicates that the placer gold is chemically modi-fied during weathering and/or within the placer environment. Therefore, our Pb isotope data are supporting the hypothesis for formation of the nuggets during the supergene stage. The placer gold shows distinct Pb isotope values compared to the primary vein gold and is, overall, similar to local sediments and rocks, strongly suggesting that the nuggets most likely formed or gained significant mass within the placer environment. In addition, presence of placer inclusions in some nuggets com-pletely embedded in the gold matrix, is also supportive of gold formation within the placer (Fig. 7A, B). The caliche inclusion shown on Figure 7 must be trapped within the placer environ-ment; it cannot be an inherited inclusion from the vein.

A number of experiments show that despite its low solubil-ity in water, gold is highly mobile in the near-surface environ-ment, especially in the presence of certain biological agents (e.g., Southam and Beveridge, 1994, 1996; Lengke et al., 2006a, b; Reith and McPhail, 2006; Lengke and Southam, 2007). Recent work by Reith et al. (2010) discovered living biofilms on gold grains and concluded that the gold grains grow by microbially assisted dissolution, precipitation, and aggregation. This model of nugget formation within placers is supported by our Pb isotope data. Furthermore, we observed possible coatings and microbial mats in placer gold from Rich Hill (Figs. 4C, 6A, B, C, D). The black gravel gold, which has the most radiogenic Pb of any placer gold at Rich Hill, is covered with a complex coating of Mn, C, silicate minerals, and clays (Figs. 4C, 6D), similar to the biofilms described by Reith et al. (2010). Surface textures and forms of this coating are also similar to reported bacterioform gold in a number of other works (Watterson, 1994; Mossman et al., 1999; Reith et al., 2007; Hough et al., 2009; Southam et al., 2009; Reith et al., 2010). The gold from the red placers is largely devoid of coatings with textures and compositions that are similar to reported biofilms, but some relict biofilmlike material is

preserved within deep cracks and pits on the gold (Fig. 6A, B, C). White caliche gold has no observed biofilmlike coatings, even within deep pits and cracks. These observations suggest that our set of samples may represent nuggets at different stages of evolution within the placer. Although speculative, it is possible that the gold with biofilm-like coatings in the black gravel may be an example of actively growing nuggets. In contrast, the red placer nuggets are likely indicative of higher energy and/or oxidation state environment so that the biofilms were mostly abraded away, preserved only in deep pits and cracks (Fig. 6A, B, C). However, regardless of the presence, partial preservation, or complete lack of biofilm coatings all of the studied placer gold nuggets show Pb isotope composi-tions consistent with formation during the supergene stage. Therefore, the biofilm preservation is likely dependent on the recent environment and energy of the placer so may or may not be present at the moment of the nugget collection. The Pb isotope signal, on the other hand, is apparently preserved and so provide evidence for the growth of the nugget within the placer environment.

ConclusionsHydrothermal vein gold and galena from the Rich Hill

area, Arizona, show similar Pb isotope signatures, indicating that both precious and base metals are derived from a similar source during a hydrothermal event in the geologic past. On the other hand, placer gold nuggets from the area show dis-tinct Pb isotope systematics when compared to the primary hydrothermal gold and sulfides. In addition, the placer gold Pb isotopes are overall more radiogenic even compared to the hydrothermal gangue minerals, indicating that the nuggets do not contain any significant amount of primary, hydrothermal vein Pb. The placer gold overall radiogenic character and the fact that it follows the modern Pb isotope trend formed by local sediments and rocks indicate that the nuggets acquired Pb relatively recently from the local sediments. The Pb iso-topes do not really provide information about the processes and/or fluids involved in the release and transport of gold from the vein to the placer but show that the starting and final product are markedly different, thus indicating signifi-cant evolution during the supergene stage. Most likely the primary gold from the hydrothermal vein was mobilized dur-ing weathering and the primary Pb isotope signal was lost and new Pb signal was acquired during the nugget aggregation in the placer. Observed biofilmlike features in some of the Rich Hill gold samples suggest that the nugget growth in the Ari-zona placer environment may also be driven or assisted by biochemical processes. The Rich Hill placer nuggets appear to be representative of different stages of the placer environ-ment. The presence of biofilmlike coatings on the black gravel gold suggests that these nuggets may be actively growing at present. The red placer gold apparently has experienced higher energy environment and so the nuggets are rounded and there are only relicts of biofilms preserved in recess pits and cracks. However, regardless of the presence, partial pres-ervation, or lack of biofilms, the Pb isotope systematics of the nuggets still record the placer growth stage. The biofilms are likely ephemeral features that may be lost from a given nug-get but the Pb isotopes apparently still keep a record of the growth within the placer environment.

INSIGHTS FROM Pb ISOTOPES FOR NATIVE GOLD FORMATION AT RICH HILL, ARIZONA 1589

AcknowledgmentsWe thank the many claim owners of the Rich Hill district

who made this study possible through the generous access to their properties. We especially thank Jeff Yahn and Rob Alli-son for access to the claims in the Devils Nest area. Many thanks to Jim Saunders for numerous discussions on gold formation during hypogene and supergene processes. We are indebted to Dante Lauretta and Alex Andronikov at the University of Arizona for the ICP-MS analyses. We thank Jim Mortensen, Robert Hough, and associate editor Sarah Glee-son for their constructive comments that greatly improved the manuscript. The authors thank the donors of the Ameri-can Chemical Society Petroleum Research Fund for partial support of the secondary mineralogy work, through a Type B research grant. Instrumentation was provided and supported by National Science Foundation EAR-0941106 (SEM/EDS upgrade).

REFERENCESAnderson, P., 1989, Stratigraphic framework, volcanic-plutonic evolution,

and vertical deformation of the Proterozoic volcanic belts of central Ari-zona, in Jenney, J.P., and Reynolds, S.J., eds., Geologic evolution of Arizona, Arizona Geological Society Digest, v. 17, p. 57–147.

Armstrong, J.T., 1988, Quantitative analysis of silicates and oxide minerals: Comparison of Monte-Carlo, ZAF and Phi-Rho-Z procedures, in Newbury, D.E., ed., San Francisco, San Francisco Press, Proceedings of the Micro-beam Analysis Society: p. 239–246.

Bendall, C., Wigg-Wolf, D., Lahaye, Y., Von Kaenel, H.M., and Brey, G.P., 2009, Detecting changes of Celtic gold sources through application of trace element and Pb isotope laser ablation analysis of Celtic gold coins: Archae-ometry, v. 51, p. 598–625.

Chapman, R.J., Mortensen, J.K., Crawford, E.C., and Lebarge, W.P., 2010, Microchemical studies of placer and lode gold in the Klondike district, Yukon, Canada: 2. Constraints on the nature and location of regional lode sources: economic GeoloGy, v. 105, p. 1393–1410.

——2011, Styles of lode gold mineralization contributing to the placers of the Indian River and Black Hills Creek, Yukon Territory, Canada as deduced from microchemical characterization of placer gold grains: Mineralium Deposita, v. 46, p. 881–903.

Crombie, M.K., Gholson, C.T., Lauretta, D.S., and Melchiorre, E.B., 2002, Rich Hill: The story of Arizona’s most amazing gold district: Tucson, Ari-zona, Golden Retriever Press, 262 p.

Decker, D.L., Melville, J.G., and Vanfllet, H.B., 1979, De-enhancement of gold diffusion in lead by impurities in the lead: Physics Review B, v. 20, p. 3036–3044.

DeWitt, E., 1989, Geochemistry and tectonic polarity of early Proterozoic (1700–1750 Ma) plutonic rocks, north-central Arizona, in Jenney, J.P. and Reynolds, S.J., eds., Geologic Evolution of Arizona, Arizona Geological Society Digest, v. 17, p. 149–163.

Donovan, J.J., Snyder, D.A., and Rivers, M.L., 1993, An improved interfer-ence correction for trace element analysis: Microbeam Analysis, v. 2, p. 23–28.

Hough, R.M., Butt, C.R.M., Reddy, S.M., and Verrall, M., 2007, Gold nug-gets: Supergene or hypogene?: Australian Journal of Earth Sciences, v. 54, p. 959–964.

Hough, R.M., Butt, C.R.M., and Fisher-Buhner, J., 2009, The crystallogra-phy, metallography and composition of gold: Elements, v. 5, p. 297–302.

Kamenov, G.D., 2008, High-precision Pb isotopic measurements of teeth and environmental samples from Sofia (Bulgaria): Insights for regional lead sources and possible pathways to the human body: Environmental Geology, v. 55, p. 669–680.

Kamenov, G.D., Mueller, P., and Perfit, M., 2004, Optimization of mixed Pb-Tl solutions for high precision isotopic analyses by MC-ICP-MS: Jour-nal of Analytical Atomic Spectrometry, v. 19, p. 1262–1267.

Kamenov, G.D., Saunders, J.A., and Hames, W.E., 2007, Mafic magmas as sources for gold in Middle-Miocene epithermal deposits of Northern Great Basin, USA: Evidence from Pb isotopic compositions of native gold: eco-nomic GeoloGy, v. 102, p. 1191–1195.

Kamenov, G.D., Perfit, M.R., Mueller, P.A., and Jonasson, I.R., 2008, Con-trols on magmatism in an island arc environment: Study of lavas and sub-arc xenoliths from the Tabar-Lihir-Tanga-Feni island chain, Papua New Guinea: Contributions to Mineralogy and Petrology, v. 155, p. 635–656.

Karlstrom, K., Ahall, K.I., Williams, M.L., McLelland, J., and Geissman, J.W., 2001, Long-lived (1.8 to 1.0 Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia: Precambrian Research, v. 111, p. 5–30.

Keith, S.B., Gest, D.E., DeWitt, E., Toll, N.W., and Everson, B A., 1983, Metallic mineral districts and production in Arizona: Arizona Bureau of Geology and Mineral Technology Bulletin, v. 194, 58 p.

Knight, J.B., Morison, S.R., and Mortensen, J.K., 1999, The relationship between placer gold particle shape, rimming and distance of fluvial trans-port; as exemplified by gold from the Klondike district, Yukon Territory, Canada: economic GeoloGy, v. 94, p. 635–648.

Lengke, M.F., and Southam, G., 2007, The deposition of elemental gold from gold (I)-thiosulfate complex mediated by sulfate-reducing bacterial condi-tions: economic GeoloGy, v. 102, p. 109–126.

Lengke, M.F., Ravel, B., Fleet, M.E., Wanger, G., Gordon, R.A., and Southam, G., 2006a, Mechanisms of gold bioaccumulation by filamentous cyanobacteria from gold (III)-chloride complex: Environmental Science and Technology, v. 40, p. 6304–6309.

Lengke, M.F., Fleet, M.E., and Southam, G., 2006b, Morphology of gold nanoparticles synthesized by filamentous cyanobacteria from gold (I)- thio-sulfate and gold (III)-chloride complexes: Langmuir, v. 22, p. 2780–2787.

Lindgren, W, 1911, Tertiary gravels of the Sierra Nevada of California: U.S. Geological Survey, Professional Paper 78, 226 p.

Ludwig, K.R., 1999, User’s Manual for Isoplot/Ex Version 2.02, a geochrono-logical Toolkit for Microsoft Excel: Berkeley, California, Berkeley Geochro-nology Center Special Publication 53 p.

Melchiorre, E., 2008, Octave gold mine: San Bernardino, California, Rock Doc Publications, 131 p.

——2009, Gold atlas of Rich Hill, Arizona: San Bernardino, California, Rock Doc Publications, 110 p.

Mossman, D.J., Reimer, T., and Durstling, H., 1999, Microbial processes in gold migration and deposition: Modern analogues to ancient deposits: Geo-science Canada, v. 26, p. 131–140.

Page, T.C., Miller, M.A., Sell, J.D., Gibson, P.C., 1994, Geology and geo-chemistry of stockwork gold mineralization at the Yarnell mine, Yavapai County, Arizona [abs.]: Transactions Society of Mining Engineers of AIME, v. 294, p. 1878.

Pettke, T., and Frei, R., 1996, Isotope systematics in vein gold from Brusson, Val d’Ayas (NW Italy), Pb/Pb evidence for a Piemonte metaophiolite Au source: Chemical Geology, v. 127, p. 111–124.

Reith, F., and McPhail, D.C., 2006, Effect of resident microbiota on the solu-bilization of gold in soils from the Tomakin Park gold mine, New South Wales, Australia: Geochimica et Cosmochimica Acta, v. 70, p. 1421–1438.

Reith, F., Lengke, M.F., Falconer, D., Craw, D., and Southam, G., 2007, Winogradski Review: The geomicrobiology of gold: International Society for Microbial Ecology, ISME Journal, v. 1, p. 567–584.

Reith, F., Fairbrother, L., Nolze, G., Wilhelmi, O., Clode, P.L., Gregg, A., Parsons, J.E., Wakelin, S.A., Pring, A., Hough, R., Southam, G., and Brug-ger, J., 2010, Nanoparticle factories: Biofilms hold the key to gold dispersion and nugget formation: Geology, v. 38, p. 843–846.

Saunders, J.A., Unger, D.L., Kamenov, G.D., Fayek, M., Hames W.E., and Utterback, W.C., 2008, Genesis of Middle Miocene Yellowstone-hotspot–related bonanza epithermal Au-Ag deposits, Northern Great Basin, USA: Mineralium Deposita, v. 43, p. 715–734.

Southam, G., and Beveridge, T.J., 1994, The in vitro formation of placer gold by bacteria: Geochimica et Cosmochimica Acta, v. 58, p. 4527–4530.

——1996, The occurance of sulfur and phosphorous within bacterially derived crystalline and pseudocrystalline gold formed in vitro: Geochimica et Cosmochimica Acta, v. 60, p. 4369–4376.

Southam, G., Lengke, M.F., Fairbrother, L., and Reith, F., 2009, The biogeo-chemistry of gold: Elements, v. 5, p. 303–307.

Watterson, J.R., 1994, Artifacts resembling budding bacteria produced in placer-gold amalgams by nitric acid leaching: Geology, v. 20, p. 1144–1146.

Wrucke, C.T., 1989, The middle Proterozoic Apache Group, Troy Quartzite, and associated diabase of Arizona: in Jenney, J.P., and Reynolds, S.J., eds., Geologic evolution of Arizona, Arizona Geological Society Digest, v. 17, p. 239–258.