Organic Alteration in Hydrothermal Sulfide Ore Deposits
Andrew P. Gize†
Department of Earth Sciences, University of Manchester, Manchester M13 9PL, United Kingdom
Abstract
Many
ore deposits are enriched in transition metals, especially iron, whereas
organic carbon is a minor component. Based on currently available analyses,
variations in bulk chemistry of organic matter in ore deposits correlates
with differences in phase equilibria in the system Fe-S-H2O.
In the pyrrhotite and magnetite stability fields, extractable organic compositions
are restricted to molecules with relatively few carbons. Organic matter
in pyrite-rich deposits reflects biochemical precursors, pyrite also being
the stable Fe-S phase in fossil fuels. Oxidation of relatively iron-poor
deposits results in microbial oxidation of the organic matter. In iron-copper
deposits, oxidation results in aromatization and the generation of organosulfurs.
In H2O-dominated systems, the fluid is a hydrogen source to
the organic matter, whereas organic products in CO2-dominated
fluids are hydrogen depleted. The organic matter in ore deposits, therefore,
is often considered to be an alteration product, reflecting the dominant
iron phase, temperature, and the alteration fluid composition.
Organic Matter in the Upper Silesian (Mississippi Valley-Type) Zn-Pb Deposits, Poland
Maria
Sass-Gustkiewiczand Barbara KwieciÑska
University of Mining and Metallurgy, Faculty of Geology, Geophysics, and Environmental Sciences, al. Mickiewicza 30, 30 059 Kraków, Poland
Abstract
Organic matter contained in large amounts in the Upper Silesian Zn-Pb ore deposits has been identified as dopplerite-calcium humate. This humic, amorphous substance was precipitated from humic acids by calcium ion loading. The precipitation of dopplerite was contemporaneous with sulfide deposition at the time of initial and mature karst processes. The hydrothermal karst dissolution supplied calcium ions but the source of humic acids is still conjectural. The geochemical data indicate that the most probable source for the organic matter deposited in the Zn-Pb ores are overlying Triassic Keuper shales containing dispersed humic organic substance.
Precipitation Kinetics of Uranium by Sedimentary Organic Matter under Diagenetic and Hydrothermal Conditions
Satoru
Nakashima,†,*
Centre de Recherches sur la Synthèse et la Chimie des Minéraux, G.I.S., C.N.R.S.-B.R.G.M., 1A, rue de la Férrolerie, 45071 Orléans Cedex 02, France and U.A. 724 du C.N.R.S. (Université d’Orléans)
Jean-Robert
Disnar,
Départment Gîtes Minéraux, B.R.G.M., B.P. 6009, 45060 Orléans Cedex, France, and U.A. 724 du C.N.R.S. (Université d’Orléans)
and
Alain Perruchot**
Centre de Recherches sur la Synthèse et la Chimie des Minéraux, C.N.R.S., 1A, rue de la Férrolerie, 45071 Orléans Cedex 02, France
Abstract
The
reduction kinetics of uranyl cations to uraninite in aqueous solution by
two lignites with different maturities were experimentally determined under
diagenetic or hydrothermal conditions (180°–200°C). Both the uranyl
reduction and lignite dehydrogenation reactions observed confirm previous
mechanistic hypotheses (Nakashima et al., 1984, 1987; Nakashima, 1992a,
b). Reduction is a first-order reaction with respect to uranyl cation and
organic substrate. For both of the reactions studied, reduction and dehydrogenation,
the activation energies measured in experiments using two different lignites
were within experimental error. These values are 115 ± 15 kJ/mol
for uranyl reduction, and 59 ± 13 kJ/mol for lignite dehydrogenation.
Estimates of the half-lives of U in aqueous solutions have been completed
based on the kinetic parameters by assuming that the rate-determining step
is uranyl reduction to uraninite. The results suggest a rough time-scale
range of U deposition rates with large uncertainties on the pre-exponential
factor. The half-life of U precipitation is estimated to be on the order
of 3 h to 1 yr under modest thermal conditions (200°–100°C), 340
yr for radioactive waste repositories (50°C), and 104 to 105 yr at
the Earth’s surface (25°–4°C).
Origin of Black Shales and the Serpentinite-Associated Cu-Zn-Co Ores at Outokumpu, Finland
Kirsti
Loukola-Ruskeeniemi†
Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland
Abstract
The
Outokumpu Cu-Zn-Co deposits are associated with serpentinites, interpreted
as ophiolitic. The serpentinites are surrounded by dolomite-rich rocks,
calc-silicate rocks, fine-grained quartz rocks, metamorphosed black shales
(black schists), and mica schists. The ore is hosted mainly by quartz rocks,
but also by calc-silicate rocks and black schists. Calc-silicate and quartz
rocks are anomalously enriched with respect to Ni and Cr. The original
reserves in the Outokumpu area were 50 million tonnes (Mt) sulfide ore
grading 1.2 to 3.8 percent Cu, 0.6 to 1 percent Zn, 0.1 to 0.2 percent
Co, 0.1 percent Ni, 0.1 to 0.8 g/t Au, and 1 to 9 g/t Ag.
Black
schists in the serpentinite-associated prospects and mines in the Kainuu-Outokumpu
area were compared by studying more than 100 drill cores and over 800 samples.
The black schists over the entire area exhibit many features in common:
an average of 7 percent S and graphitic C, formation thicknesses commonly
exceeding 50 m, and textures such as fine-grained laminae rich in spheroidal
pyrite and locally veined by quartz and sulfides. The uniform distribution
of S and heavy metals in the thick and widespread black shale formations
provides evidence for influx of hydrothermal fluids to seawater during
the sedimentation of the organic-rich mud.
Carbon
isotope d13C
values in the Kainuu-Outokumpu black schists grade from –30 to –19 per
mil. Values of about –20 per mil are probably the result of isotope exchange
reactions between organic and carbonate C during metamorphic processes.
Fine-grained (<0.01 mm) spheroidal pyrite exhibits d34S
values from –12.7 to –6.4 per mil, with those of coarse-grained pyrite
ranging from –5.9 to +1.4 per mil in the Kainuu-Outokumpu black schists.d34S
values in the Outokumpu Cu-Zn-Co ores are comparable to those in the black
schists. These d34S
values suggest bacterial reduction of seawater sulfate, with addition of
hydrothermal S. Nickel concentrations in pyrite, as high as 0.7 percent,
occur in black schists from the Talvivaara black schist-hosted occurrence
(300 Mt, grading 0.26% Ni, 0.14% Cu, and 0.53% Zn). Cobalt concentrations
in pyrite in black schists (0.3% maximum) and the Outokumpu ores (average
1.1%) differ. Cobalt concentrations also vary widely in the Outokumpu
ore pyrites. This suggests that the composition of the ore-forming fluids
was not constant; rather, the Co, Cu, and Zn concentrations differed in
the successive fluids.
The
Kainuu-Outokumpu thick (>20 m) black shale formations were deposited in
basins where the bottom waters were enriched by hydrothermal fluids. According
to the genetic model now presented, the black shale formed a cap, enabling
pulses of Cu-Zn-Co-rich fluids to precipitate metals beneath the cap. Subsequently,
tectonic metaperidotites intruded the black shale–ore association. Alteration
of the serpentinite released Ca- and Si-rich material that precipitated
between the black shale and the serpentinite. During tectonic and metamorphic
processes, continued concentration of Cu, Co, and Zn in the silica-rich
host rocks resulted in massive orebodies. Serpentinite-associated Cu-Zn-Co
ores are rare because the close association of the Cu-Zn-Co ores with the
serpentinites is coincidental.
The
feasibility of using characteristics of black shales as an exploration
tool was tested in the Hammaslahti Cu-Zn-Au and the Vihanti Zn-Cu-Pb ore
provinces in Finland, as well as in the Kainuu-Outokumpu area. At both
Hammaslahti and Vihanti, black shale could have served as a cap rock under
which the ore precipitated.
The Oil-Bearing, Carlin-Type Gold Deposits of Yankee Basin, Alligator Ridge District, Nevada
Jeffrey
B. Hulen† and James W. Collister
Energy & Geoscience Institute, Department of Civil and Environmental Engineering, University of Utah, 423 Wakara Way, Salt Lake City, Utah 84108
Abstract
The
Carlin-type gold orebodies of Yankee basin, in the Alligator Ridge district
of northeastern Nevada, are unique for this deposit type in that they contain
abundant oil. This liquid hydrocarbon occurs as (1) primary and secondary
fluid inclusions in calcite ± realgar vein networks encircling the
orebodies in variously fresh-appearing to strongly decalcified, silicified,
and collapse-brecciated Mississippian to Devonian Pilot Shale; and (2)
vug- and fracture-filling free oil proximal to the orebodies in scattered,
residual pods and lenses of unoxidized, basal Pilot limestone. The fluid-inclusion
and free oils are geochemically equivalent and have similar thermal maturities
(early to peak oil-generation stage). Timing of entrapment of the fluid-inclusion
oils is paragenetically constrained as dominantly premineral and synmineral.
Associated free oil could have arrived at any time prior to, during, or
after mineralization but before late, oil-barren, spelean calcite vein-mineralization
and subsequent supergene oxidation. Biomarker fingerprints and carbon isotope
signatures indicate that the oils were self-sourced from the Pilot Shale;
their concentration in the organically lean basal limestone suggests derivation
from carbonaceous siltstones higher in the formation, but in structurally
lower configurations.
The
Yankee fluid-inclusion oils were clearly involved in the gold-mineralizing
hydrothermal system but were not thermally degraded to pyrobitumen, the
analogous solid hydrocarbon characteristic of Carlin-type gold deposits.
This relationship suggests that the Yankee system was cooler than the 175°
to 250°C widely cited as typical for such mineralization, a contention
supported by independent geothermometers. The oil-bearing fluid inclusions
all have homogenization temperatures lower than 150°C, with most less
than 120°C. Temperature-sensitive biomarker transformation ratios of
the oils, expressed as equivalent vitrinite reflectance (Ro;
0.75–0.95%) suggest peak paleotemperatures no higher than about 145°C.
These implied low system temperatures are consistent with the lack of evidence
for a contemporaneous igneous heat source.
The
fossil Yankee basin hydrothermal system is similar in many ways, such as
paleotemperature, host rocks, hydrocarbons, hydrothermal alteration, and
geochemistry, to nearby, active, moderate-temperature (120º– 130°C)
but gold-poor systems which encompass producing oil reservoirs. Numerous
such warm systems have likely existed in the past in this region. We suggest
that many of them formed small Carlin-type gold deposits and/or oil reservoirs
that still await discovery.
Sedimentary Exhalative Nickel-Molybdenum Ores in South China
D.
A. Lott, R. M. Coveney, Jr.,† J. B. Murowchick,
Department of Geosciences, University of Missouri, 5110 Rockhill Rd., Kansas City, Missouri 64110
and
R. I. Grauch
U.S. Geological Survey, Mail Stop 973, Denver, Colorado 80225
Abstract
Unique bedded Ni-Mo ores hosted by black shales were discovered in localized paleobasins along the Yangzte platform of southern China in 1971. Textural evidence and radiometric dates imply ore formation during sedimentation of black shales that grade into readily combustible beds, termed stone coals, which contain 10 to 15 percent organic carbon. Studies of 427 fluid inclusions indicate extreme variation in hydrothermal brine salinities that were contained by Proterozoic dolostones underlying the ore zone in Hunan and Guizhou. Variations of fluid inclusion salinities, which range from 0.1 to 21.6 wt percent NaCl equiv, are attributed to differences in the compositions of brines in strata underlying the ore bed, complicated by the presence of seawater and dilute fluids that represent condensates of vapors generated by boiling of mineralizing fluids or Cambrian meteoric water. The complex processes of ore deposition led to scattered homogenization temperatures ranging from 100° to 187°C within the Hunan ore zone and from 65° to 183°C within the Guizhou ore zone. While living organisms probably did not directly accumulate metals in situ in sufficient amounts to explain the unusually high grades of the deposits, sulfur isotope ratios indicate that bacteria, now preserved as abundant microfossils, provided sufficient sulfide for the ores by reduction of seawater sulfate. Such microbiota may have depended on vent fluids and transported organic matter for key nutrients and are consistent with a sedex origin for the ores. Vent fluids interacted with organic remains, including rounded fragments of microbial mats that were likely transported to the site of ore deposition by the action of waves and bottom currents prior to replacement by ore minerals.
An Evaluation of the Inorganic and Organic Geochemistry of the San Vicente Mississippi Valley-Type Zinc-Lead District, Central Peru: Implications for Ore Fluid Composition, Mixing Processes, and Sulfate Reduction
Jorge
E. Spangenberg,†
Institut de Minéralogie et Pétrographie, Université de Lausanne, BFSH-2, CH-1015 Lausanne, Switzerland
Lluís
Fontboté,
Département de Minéralogie, Université de Genève, 13, rue des Maraîchers, CH-1211 Genève 4, Switzerland
and
Stephen A. Macko
Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22903
Abstract
Mississippi
Valley-type zinc-lead deposits and ore occurrences in the San Vicente belt
are hosted in dolostones of the eastern Upper Triassic to Lower Jurassic
Pucará basin, central Peru. Combined inorganic and organic geochemical
data from 22 sites, including the main San Vicente deposit, minor ore occurrences,
and barren localities, provide better understanding of fluid pathways and
composition, ore precipitation mechanisms, Eh-pH changes during mineralization,
and relationships between organic matter and ore formation. Ore-stage dark
replacement dolomite and white sparry dolomite are Fe and rare earth element
(REE) depleted, and Mn enriched, compared to the host dolomite. In the
main deposit, they display significant negative Ce and probably Eu anomalies.
Mixing of an incoming hot, slightly oxidizing, acidic brine (H2CO3
being the dominant dissolved carbon species), probably poor in REE and
Fe, with local intraformational, alkaline, reducing waters explains the
overall carbon and oxygen isotope variation and the distributions of REE
and other trace elements in the different hydrothermal carbonate generations.
The incoming ore fluid flowed through major aquifers, probably basal basin
detrital units, with limited interaction with the carbonate host rocks.
The hydrothermal carbonates show a strong regional chemical homogeneity,
indicating access of the ore fluids by interconnected channelways near
the ore occurrences. Negative Ce anomalies in the main deposit, that are
absent at the district scale, indicate local ore-fluid chemical differences.
Oxidation of both migrated and indigenous hydrocarbons by the incoming
fluid provided the local reducing conditions necessary for sulfate reduction
to H2S, pyrobitumen precipitation, and reduction of Eu3+
to Eu2+. Fe-Mn covariations, combined with the REE contents
of the hydrothermal carbonates, are consistent with the mineralizing system
shifting from reducing/rock-dominated to oxidizing/fluid-dominated conditions
following ore deposition. Sulfate and sulfide sulfur isotopes support sulfide
origin from evaporite-derived sulfate by thermochemical organic reduction;
further evidence includes the presence of 13C-depleted calcite
cements (~–12‰ d13C)
as sulfate pseudomorphs, elemental sulfur, altered organic matter in the
host dolomite, and isotopically heavier, late, solid bitumen. Significant
alteration of the indigenous and extrinsic hydrocarbons, with absent bacterial
membrane biomarkers (hopanes) is observed. The lightd34S
of sulfides from small mines and occurrences compared to the main deposit
reflect a local contribution of isotopically light sulfur, evidence of
local differences in the ore-fluid chemistry.
Bitumens in the Late Variscan Hydrothermal Vein-Type Uranium Deposit of P?ríbram, Czech Republic: Sources, Radiation-Induced Alteration, and Relation to Mineralization
Bohdan
Kríbek,† Karel Zák,
Czech Geological Survey, Klárov 131/2, 118 21 Prague 1, Czech Republic
Jorge
Spangenberg,
Institute of Mineralogy and Petrology, University of Lausanne, CH 1015 Lausanne, Switzerland
Jan
Jehlicka,
Institute of Mineralogy, Geochemistry and Raw Materials, Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic
Stanislav
Prokes, and Jirí Komínek
Diamo State Enterprise, 471 28 Stráz p. Ralskem, Czech Republic
Abstract
The
late Variscan (275–278 Ma) P?ríbram uranium deposit is one of the
largest known accumulations of uraniferous bitumens in hydrothermal veins.
The deposit extends along the northwestern boundary of the Central Bohemian
pluton (345–335 Ma) with low-grade metamorphosed Late Proterozoic and unmetamorphosed
Cambrian rocks. From a net uranium production of 41,742 metric tons (t),
more than 6,000 t were extracted from bitumen-uraninite ores during 43
years of exploration and mining. Three morphological varieties of solid
bitumen are recognized: globular, asphaltlike, and cokelike. While the
globular bitumen is uranium free, the other two types are uraniferous.
The amount of bitumen in ore veins gradually decreases toward the contact
with the plutonic body and increases with depth.
Two
types of bitumen microtextures are recognized using high-resolution transmission
electron microscopy: amorphous and microporous, the former being less common
in uraniferous samples. A lower Raman peak area ratio (1,360/1,575 cm–1)
in mineralized bitumens (0.9) compared with uranium-free samples (2.0)
indicates a lower degree of microtextural organization in the latter. The
H/C and O/C atomic ratios in uranium-free bitumens (0.9–1.1 and 0.09, respectively)
are higher than those in mineralized samples (H/C = 0.3–0.8, O/C = 0.03–0.09).
The chloroform extractable matter yield is very low in uranium-free bitumens
(0.30–0.35% of the total organic carbon,TOC) and decreases with uranium
content increase. The extracted solid uraniferous bitumen infrared spectra
show depletion in aliphatic CH2 and CH3 groups compared
to uranium-free samples. The concentration of oxygen-bearing functional
groups relative to aromatic bonds in the IR spectra of uranium-free and
mineralized bitumen, however, do not differ significantly. 13C
NMR confirmed than the aromaticity of a uraniferous sample is higher (Far
= 0.61) than in the uranium-free bitumen (Far = 0.51). Pyrolysates
from uraniferous and nonuraniferous bitumens do not differ significantly,
being predominantly cresol, alkylphenols, alkylbenzenes, and alkylnaphthalenes.
The liquid pyrolysate yield decreases significantly with increasing uranium
content. The d13C
values of bulk uranium-free bitumens and low-grade uraniferous, asphaltlike
bitumens range from –43.6 to –52.3 per mil. High-grade, cokelike, uraniferous
bitumens are more 13C depleted (–54.5 to –58.4‰). In contrast
to the very light isotopic ratios of the high-grade uraniferous cokelike
bitumen bulk carbon, the individual n-alkanes
and isoprenoids (pristane and phytane) extracted from the same sample are
significantly 13C enriched. The isotopic composition of the
C13-24n-alkanes
extracted from the high-grade uraniferous sample (d13C
= –28.0 to –32.6‰) are heavier compared with the same compounds in a uranium-free
sample (d13C
= –31.9 to –33.8‰).
It
is proposed that the bitumen source was the isotopically light (d13C
= –35.8 to –30.2‰) organic matter of the Upper Proterozoic host rocks that
were pyrolyzed during intrusion of the Central Bohemian pluton. The 13C-depleted
pyrolysates were mobilized from the innermost part of the contact-metamorphic
aureole, accumulated in structural traps in less thermally influenced parts
of the sedimentary complex and were later extracted by hydrothermal fluids.
Bitumens
at the P?ríbram deposit are younger than the main part of the uranium
mineralization and were formed through water-washing and radiation-induced
polymerization of both the gaseous and liquid pyrolysates. Direct evidence
for pyrolysate reduction of uranium in the hydrothermal system is difficult
to obtain as the chemical composition of the original organic fluid phase
was modified during water-washing and radiolytic alteration. However, indirect
evidence—e.g., higher O/C atomic ratios in uranium-free bitumens (0.1)
relative to the Upper Proterozoic source rocks (0.02–0.05), isotopically
very light carbon in associated whewellite (d13C
= –31.7 to –28.4‰), and the striking absence of bitumens in the pre-uranium,
hematite stage of the mineralization—indicates that oxidation of organic
fluids may have contributed to lowering of aO2
and uraninite precipitation.
The Role of Organic Matter in the Genesis of the El Soldado Volcanic-Hosted Manto-Type Cu Deposit, Chile
Nicholas
S. F. Wilson†,* and Marcos Zentilli
Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5
Abstract
Solid
bitumen (residual petroleum) occurs intimately associated with sulfides
in several manto-type Cu deposits of central Chile. El Soldado (32º38'
S; 71º04' W) is one of the largest manto-type deposits (>130 million metric
tons @ 1.5% Cu) and is hosted by rhyolites and andesites of the Lower Cretaceous
Lo Prado Formation.
As
a result of basinal burial in the Early Cretaceous back-arc basin, petroleum
was generated from organic-rich shales in the underlying lower Lo Prado
Formation and migrated into primary and structural porosity in the upper
Lo Prado Formation. Migration was controlled by extensional basinal faults.
Low-temperature (<90ºC) biodegradation of petroleum developed an
early stage I assemblage of (mostly framboidal) pyrite (± sphalerite
± chalcopyrite). Geopetal structures related to pressure degassing
of semi-solid petroleum suggest that migration occurred while the strata
were horizontal, at ca. 130 Ma. Continued basinal burial led to thermal
degassing, increased maturation, and solidification of the petroleum.
Low-grade
regional (pumpellyite-prehnite) metamorphism of the sequence was followed,
at ca. 110 to 103 Ma, by the influx of high-temperature (ca. 300ºC),
Cu-rich fluids concurrently with emplacement of the Cretaceous batholith
into the basin. Basinal fluids were focused into a now tilted sequence
by a brittle shear (north-south to north-northwest) fault system. Copper
sulfides (stage II assemblage) replaced preexisting pyrite and bitumen,
forming zoned orebodies with a central chalcocite-hematite zone, followed
outward by bornite-chalcocite, chalcopyrite-bornite, and pyrite-chalcopyrite
zones. The mineralizing fluids altered the bitumen thermally and chemically
as follows: (1) reflectance (Ro) increased up to ca. 5 percent
in the core of the orebodies against a background of ca. 3 percent; (2)
anisotropy (nongranular, microscale, and domain) developed and the bitumen
was locally graphitized; (3) chlorine (± Cu ± Fe) was incorporated
into the bitumen affecting its reflectance. The source of Cu was most likely
the overlying oxidized volcanic pile; the Cu was transported as chloride
(CuCl) complexes, precipitating where the CuCl-rich solutions reacted with
bitumen (adsorption/reduction) and pyrite from the stage I assemblage.
The
study demonstrates that in the Lower Cretaceous basin of Chile, degraded
petroleum reservoirs can be important controls for metallic mineralization
derived from hydrothermal solutions of different sources, particularly
if biodegradation generated pyrite.
Noble Metals in Organic Matter and Clay-Organic Matrices, Kupferschiefer, Poland
H.
Kucha†,*
Institute of Geology and Mineral Deposits, 30-059 Kraków, Ave Mickiewicza 30, Poland
and
W. Przybyl-owicz*
University of the Witwatersrand, Schonland Research Centre, Private Bag 3, WITS 2050, South Africa
Abstract
In
the Kupferschiefer deposits of Poland, Cu, Ag, Pb, Zn, Fe, noble metals,
and other transition elements were concentrated by a process of catalytic
oxidation and desulfurization of organic matter. They occur as several
well-defined horizons that are zoned from the bottom upward: noble metals
(southwest perimeter of orebodies), Cu, Pb, Zn, and pyrite.
The
noble metal content in black shale clay-organic matrices reaches 1,900
ppm Au, 1,900 ppm Pd, and 600 ppm Pt. Gold valence determinations suggest
that Au occurs in two forms: native microinclusions with 0 valence containing
silver, and Ag-free microinclusions with gold valence +1 probably bound
to bitumens. The noble metals content in the organic matrix of thucholite
reaches 2,528 ppm Au, 5,000 ppm Pd, and 1,770 ppm Pt. Pt is probably bound
to tetrapyrrole rings, with Ni and V as vanadyl.
The
vertical distribution of metals is controlled by redox interfaces. On the
reduced side of the interface, up to 0.5 m thick, the following minerals
occur: a continuous Ag-Au series, several Pd arsenides and sulfide-arsenides,
Bi sulfides, tennantite, Co-Ni arsenides, castaingite CuMo2S5,
thucholite, covellite, and digenite. On the oxidized side of the interface,
up to 1 m thick, the following minerals occur: gold of high fineness, hematite,
PdBi sobolevskite, clausthalite, native Pb, plumbian gold, native palladium,
graphite, and relicts of organic matter. The oxidized portion of the redox
interface, containing lower noble metal contents, occupies a much larger
area than the reduced counterpart with its higher pay metals values.
SCIENTIFIC COMMUNICATIONS
Discovery
of a Palladium-Platinum-Gold-Mercury Bitumen in the Boss Mine, Clark County,
Nevada
Jacques
Jedwab,†
Laboratoire de Géochimie et de Minéralogie, CP 160/02, Université Libre de Bruxelles, 50, Avenue Roosevelt, B-1050-Brussels, Belgium
Denise
Badaut, and
Laboratoire de Géologie, Muséum National d’ Histoire Naturelle, 43, Rue Buffon, F-75005-Paris, France
Patricia
Beaunier
Service Microscopie Electronique, Université Paris VI, 8, Rue Cuvier, F-75252 Paris CEDEX 05, France
Abstract
A bitumen containing high concentrations of sulfur, palladium, platinum, gold, and mercury has been found in the Boss mine, Clark County, Nevada, where rich ores of “invisible” palladium and platinum were mined from 1916 to 1919. The bitumen, analyzed with X-ray energy dispersive spectrometry on areas devoid of discrete particles down to a 40 Å limit, contains 2.74 wt percent Pd + Pt + Au (Pd = 1.85%) and 5.83 wt percent Hg. It is associated with particulate potarite (Pd, Au, Pt)Hg, native gold, plumbojarosite, hydronium jarosite, argentojarosite, florencite, and bismuth oxides. The first four mineral species are also found included in the bitumen. The paragenesis was derivation and formation of bitumen from the local limestone, followed by interaction with the hydrothermal potarite and the supergene jarosite. This type of organic material, highly enriched in platinum group elements, previously had been reported only in Polish Kupferschiefer thucholite.
H.
Kucha†
Institute of Geology & Mineral Deposits, 30-059 Krakow, av. Mickiewicza 30, Poland
and
I. R. Plimer
School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052, Australia
Abstract
Submicroscopic
mixtures of metallic gold and solid hydrocarbons form aggregates of up
to 100 em
in fractures and vugs in loellingite from gold-rich samples from Maldon,
Australia. Electron microprobe analyses show that solid hydrocarbons contain
about 1 wt percent S and Fe, up to 3 wt percent O, and several wt percent
Au. Infrared microscopy indicates that the solid hydrocarbons comprise
mainly conjugated benzene rings with short-chain functional groups composed
of carbonyl groups (C=O), CH3, CH2, and OH. Metals
(Fe and Au) are suggested to be bound to aryl ketones, or may be bound
to hydrocarbons by S-functional groups. It is proposed that metal-bearing
solid hydrocarbons are replaced by siderite, producing spectacular nebular
inclusions of minute gold grains in replacive carbonate forming veinlets
in the host quartz.