Disseminated Gold Deposits Research Articles

Ion microprobe determination of sulfur isotope variations in iron sulfides from the Post/ Betze sediment-hosted disseminated gold deposit, Nevada, USA

Secondary ion mass spectrometric analyses of ore samples from the Post/Betze sedimenthosted disseminated gold deposit were utilized to constrain elemental distribution of Au and As in iron sulfide phases. Most of the Au was deposited very early in the paragenetic sequence. Although Au and As are covariant in arsenian pyrite, Au apparently was depleted much more rapidly from the hydrothermal solutions than was As. Sensitive high-resolution ion microprobe (SHRIMP) sulfur isotope analyses of iron sulfides from the Post/Betze deposit vary widely from δ 34S = −29 to 23‰ and provide important information on the origin of sulfur and constraints on depositional mechanisms. Ore solutions had high δ 34S values and were most likely derived, at least in part, from thermochemical reduction of lower Paleozoic seawater-derived sulfate, possibly bedded barite. Late-stage ore fluids ( δ 34S sulfide = −12 to −29‰ ) are extremely depleted in 34S relative to main-stage ore fluids ( δ 34S sulfide = 16–23‰ ). Although such low δ 34S values can be generated hypothetically from the original ore fluids by oxidation (possibly boiling), the stability of pyrite is compromised. Introduction of and mixing with Fe-rich fluids is necessary to deposit pyrite having low isotopic values.

Gold and arsenic in iron sulfides from sediment-hosted disseminated gold deposits; implications for depositional processes

Secondary ion microscopic elemental distribution maps and quantitative measurements of Au and As concentrations in samples from five sediment-hosted disseminated gold deposits in Nevada present a consistent picture of the location and nature of gold in these samples. Gold is present in arsenian pyrite that forms overgrowth rims on, and narrow veinlets in, gold-free pyrite (and possibly other minerals, but only to a minor extent). Auriferous overgrowth rims that may exhibit compositional zoning, either simple or oscillatory, range from approximately 25 mu m in thickness to the limits of microscopic resolution. Because all of the observed gold-bearing pyrite contains sufficient As to be metastable (i.e., in excess of approximately 0.5%), it is suggested that small native gold grains occasionally observed in unweathered sedimenthosted disseminated gold ores resulted from exsolution of gold from the arsenian pyrite lattice.Available evidence suggests that gold in arsenian pyrite is most likely present as a charged species (Au (super +3) ) and that gold probably was deposited with arsenic as coupled solid solution substitutions in pyrite. The codeposition of gold and arsenic in pyrite in these and many other deposits may be a function of a redox reaction involving oxidation of gold and concurrent reduction of arsenic.Commonly proposed reactions involving gold deposition in sediment-hosted disseminated gold deposits usually involve gold precipitation as a neutral species from a bisulfide complex. It is proposed that such reactions are inappropriate for gold deposition in arsenian pyrite but that reactions involving oxidation of gold and concurrent reduction of arsenic are probably more appropriate. Mechanisms effecting gold deposition as native metal also will effect deposition as Au (super +3) if it is deposited along with arsenic (in either arsenopyrite or arsenian pyrite).

The Association of Gold and Hydrocarbons in Sediment-Hosted Disseminated Gold Deposits

The Association of Gold and Hydrocarbons in Sediment-Hosted Disseminated Gold Deposits

Geology and geochemistry of wall-rock alteration at the Carlin gold deposit, Nevada

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and 50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a CO 2 -rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, aud CO 2 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolinite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.

Application of mineral deposit models in exploration: a case study of sediment-hosted gold deposits, Great Basin, Western United States

Abstract This paper emphasizes the development of mineral deposit models for exploration using as a case study the sediment-hosted, structure-controlled, disseminated gold deposits in the Great Basin of the Western United States. More than 60 deposits have been found in this area since the discovery of the Carlin deposit in 1962. Most deposits were discovered by direct prospecting and sampling of outcropping silicified zones (jasperoids). Similar poorly exposed and unexposed deposits remain to be discovered, but future exploration will require improved models that indirectly predict locations of these deposits. Many of these sediment-hosted gold deposits occur within silicified and structurally complex zones in calcareous rocks below regional east-directed thrusts. Geological characteristics that have been successfully employed in exploration include: erosional windows (fensters) through the thrusts; silty to arenaceous calcareous host rocks below the thrusts; silicified zones; anticlines and domes of the lower plate calcareous sediments; alignment of deposits along linear trends; and geochemical anomalies (Au, Ag, As, Sb, Tl, etc.). Although the use of these criteria will discover new deposits, exploration now requires additional criteria that are reliably, but not so directly, associated with the deposits. Identification of these criteria will require the development of genetic models that tie more regional geological characteristics to ore formation and localization. Development of successful mineral deposit models first requires a working definition of the sediment-hosted gold deposit class and its geological characteristics. The characteristics of each deposit included in this class are then documented and validated. Next, the geological processes most likely responsible for the shared characteristics are identified. Finally, the most reliable, informative and accessible criteria, reflective of each ore-forming process, are selected. From these criteria are chosen the most efficient and effective geological features for continued exploration. Processes currently suggested to be important in the formation of sediment-hosted gold deposits include: regional wrench fault tectonics; plutonism; hydrothermal fluid flow driven by intrusions and/or thrusting; fluid flow along regional unconformities; gold precipitation following carbonate dissolution, or fluid mixing; and dissolution of synsedimentary gold from the allochthonous western assemblage sediments and reprecipitation of that gold in the deposits. A general approach for identifying additional possible processes, testing alternative processes and finally selecting exploration criteria is presented.

Fluid inclusion gas chemistry as a potential minerals exploration tool: Case studies from Creede, CO, Jerritt Canyon, NV, Coeur d'Alene district, ID and MT, southern Alaska mesothermal veins, and mid-continent MVT's

Recent advances in instrumentation now permit quantitative analysis of gas species from individual fluid inclusions. Fluid inclusion gas data can be applied to minerals exploration empirically to establish chemical (gas composition) signatures of the ore fluids, and conceptually through the development of genetic models of ore formation from a framework of integrated geologic, geochemical, and isotopic investigations. Case studies of fluid inclusion gas chemistry from ore deposits representing a spectrum of ore-forming processes and environments are presented to illustrate both the empirical and conceptual approaches. We consider epithermal silver-gold deposits of Creede, Colorado, Carlin-type sediment-hosted disseminated gold deposits of Jerritt Canyon, Nevada, metamorphic silver-base-metal veins of the Coeur d'Alene district, Idaho and Montana, gold-quartz veins in accreted terranes of southern Alaska, and the mid-continent base-metal sulfide deposits of Mississippi Valley-Type (MVT's). Variations in gas chemistry determine the redox state of the ore fluids, provide compositional input for gas geothermometers, characterize ore fluid chemistry (e.g., CH 4CO 2, H 2SSO 2, CO 2/H 2S, organic-rich fluids, gas-rich and gas-poor fluids), identify magmatic, meteoric, metamorphic, shallow and deep basin fluids in ore systems, locate upwelling plumes of magmatic-derived volatiles, zones of boiling and volatile separation, interfaces between contrasting fluids, and important zones of fluid mixing. Present techniques are immediately applicable to exploration programsas empirical studies that monitor fluid inclusion gas threshold concentration levels, presence or absence of certain gases, or changes in gas ratios. We suggest that the greater contribution of fluid inclusion gas analysis is in the integrated and comprehensive chemical dimension that gas data impart to genetic models, and in the exploration concepts based on processes and environments of ore formation derived from these genetic models.

A microbial method of mineral exploration: a case history at the Mesquite deposit

A unique method of gold exploration using a particular soil bacterium, Bacillus cereus, has been successfully tested over a variety of mineral deposits in terrains ranging from desert shrub vegetation to boreal forest. Results of an orientation survey using soil Bacillus cereus, soil Au geochemistry and creosote Au geochemistry over the Mesquite disseminated gold deposit, southern California, are presented in this report. Mineralized bedrock is exposed at the surface and extends to 100 meters beneath alluvium. Samples of soil and creosote vegetation were analyzed for Au to determine if Au concentrations could be used as an exploration guide. Results show increases in Bacillus cereus spore populations coincident with the known location of buried disseminated ore. Soil Au concentrations delineated the trend of the orebody at the surface and at depth provided the soils were collected at the base of creosote vegetation. Gold concentrations in creosote vegetation define only surface outcrops of mineralized bedrock. An effective exploration program for buried ore deposits in extremely arid terrains of this region would involve sampling soils beneath vegetation and analysis of soils for Au and Bacillus cereus concentrations.

Geology and geochemistry of three sedimentary-rock-hosted disseminated gold deposits in Guizhou Province, People's Republic of China

Five sedimentary-rock-hosted disseminated gold deposits have been discovered since 1980 in southwestern Guizhou Province (PRC). Submicron-sized gold is disseminated in silty carbonate and carbonaceous shale host rocks of Permian and Triassic age. Arsenic, antimony, mercury, and thallium accompany the gold. Associated hydrothermal alteration resulted in decarbonatization of limestone, silicification, and argillization, and depletion of base metals, barium, and many other elements. Organic material occurs in most host rocks and ores. It was apparently devolatilized during a regional heating event that preceded hydrothermal activity, and thus was not mobilized during mineralization, and did not affect gold deposition. The geologic setting of the Guizhou deposits includes many features that are similar to those of sedimentary-rock-hosted deposits of the Great Basin, western United States. The heavy-element suite that accompanies gold is the same, but base metals are even scarcer in the Guizhou deposits than they are in U.S. deposits. The Guizhou deposits discovered to date are smaller than most U.S. deposits and have no known spatially associated igneous rocks.

XRD mineralogic logging of drill samples from gold and copper mining operations

Traditionally, the logging of drill core and cuttings has been left largely to the visual acuity and interpretive acumen of the geologist. Supplementary X-ray diffraction-X-ray fluorescence (XRD-XRF) methods have been developed in recent years to provide rapid quantitative mineralogic and geochemical data on drill cuttings and core samples that can be plotted and contoured in vertical or plan sections. Such sections serve to outline alteration and geochemical zoning around various mineralized prospects and ore bodies, including porphyry deposits as well as disseminated gold deposits. Metallurgical ore types as well as lithologic rock types are classified routinely by computer processing of XRD-XRF data, which in turn are compiled into data base to facilitate metallurgical composting and rock type logging.

Geology of the Gold Quarry mine

The Gold Quarry mine has developed a large, epithermal, disseminated gold deposit hosted by Paleozoic siliciclastic sediments. The deposit consists of two, easterly tilted, stacked orebodies; an upper structural stockwork and a lower stratabound replacement. Anomalous crustal heat flow associated with Oligocene volcanic rocks appears responsible for the formation of a large hydrothermal system at Gold Quarry. Wallrock alteration and mineral concentration are spatially and temporally associated with this event. Rock alteration types include decalcification, silicification, alunitization, argillization and oxidation. Complex and repeated compressional and extensional tectonic events combined with litho-chemical characteristics of the host sediments to form sites favorable to mineral deposition. A dominant elemental association of AuAsSbHgTl is directly associated with mainstage Oligocene mineralization. A subordinate association of AgPbZnCuNi appears to represent earlier mineralizing events. A continuing deep exploration program has shown that mineralization remains open to the south.

Identification of illite polytype zoning in disseminated gold deposits using reflectance spectroscopy and X-ray diffraction-potential for mapping with imaging spectrometers

X-ray diffraction studies of clay minerals from disseminated gold deposits in Nevada demonstrate that illite polytypes are often laterally and vertically zoned around ore bodies. Polytypes act as geothermometers indicating temperatures of ore deposition and thus proximity to the hydrothermal fluids during deposition. Visible/infrared reflectance spectroscopy for field identification and mapping of the illite polytypes is evaluated. The results demonstrate that reflectance spectroscopy can be used to assist gold exploration efforts by providing detailed mineralogical information in real time at the field location. As the new generation of imaging spectrometers is developed, it is likely that subtle spectral differences such as those between the illite polytypes will become useful for remote exploration for gold deposits.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Genesis of sediment-hosted disseminated-gold deposits by fluid mixing and sulfidization: Chemical-reaction-path modeling of ore-depositional processes documented in the Jerritt Canyon district, Nevada

Integrated geologic, geochemical, fluid-inclusion, and stableisotope studies of the gold deposits in the Jerritt Canyon district, Nevada, provide evidence that gold deposition was a consequence of both fluid mixing and sulfidization of host-rock iron. Chemical-reaction-path models of these ore-depositional processes confirm that the combination of fluid mixing, including simultaneous cooling, dilution, and oxidation of the ore fluid, and wall-rock reaction, with sulfidization of reactive iron in the host rock, explains the disseminated nature and small size of the gold and the alteration zonation, mineralogy, and geochemistry observed at Jerritt Canyon and at many other sediment-hosted disseminated gold deposits.

Geology and geochemistry of the Vantage gold deposits, Alligator Ridge-Bald Mountain mining district, Nevada

Geologic relationships, major element data, and isotopic geochemistry of a group of Carlintype Au deposits in the Alligator Ridge-Bald Mountain district of east-central Nevada were investigated to help constrain the origin and relative timing of Au mineralization and associated alteration. The Vantage gold deposits were the largest of 18 known sediment-hosted, disseminated gold deposits and prospects that are distributed over a strike length of 40 km. The district consists predominantly of Paleozoic carbonate and siliciclastic sedimentary rocks. Minor amounts of Tertiary volcanic and volcanic-related sedimentary rocks and a small granitic stock in the northern end of the district are also present. The intrusion and its surrounding aureole also host gold mineralization, but most gold deposits in the district are not spatially associated with intrusive rocks.The Vantage Au deposits formed at the contact between the Devonian Devils Gate Limestone and the Devonian-Mississippian Pilot Shale adjacent to a major fault zone, herein referred to as the Vantage fault. Gold mineralization was accompanied by silver, antimony, and arsenic and showed a strong preference for the Pilot Shale. Alteration related in time and space to Au mineralization was the replacement of host-rock carbonates by quartz. Replacement in the Pilot Shale closely followed the ore-waste boundary, whereas replacement in the Devils Gate Limestone occurred as a stratiform jasperold not confined to the ore zones. Quartz + kaolinite + stibnite veins were common within the central, high-grade portion of each orebody. Peripheral to silicification, diagenetic dolomite in the Pilot Shale was replaced by hydrothermal calcite, and calcite veins constituted as much as 10 percent of the rocks. Two oxidation events that destroyed organic matter and sulfides occurred after Au deposition and silicification. The early oxidation event was accompanied by the destruction of detrital illite in the Pilot Shale and deposition of alunite + or - barite veins. This event was barren of Au but did redistribute metals introduced during silicification. The spatial distribution of this alteration was similar to that of the silicification. K-Ar dating of this alunite indicates an age of 11 Ma for its formation. Following a period of erosion, weathering-related oxidation overprinted much of the earlier alteration and deposited jarosite and goethite, but it did not affect metal distribution patterns or the matrix of the Pilot Shale.Whole-rock chemical analyses indicate that silica and sulfur were the only major components introduced during silicification and that very little material other than iron and organic matter was removed during the intense oxidation. Oxygen isotope compositions determined for jasperoid and vein quartz varied from 15.9 to 18.2 per mil, and from 16.1 to 20.1 per mil, respectively. Carbonate from whole-rock samples peripheral to silicification showed an increase in their carbon isotope values of up to 3 per mil (to 1.4ppm) and a decrease in their oxygen isotope values of up to 12 per mil (to 12.7ppm) in comparison with unaltered samples. Vein calcite had even more extreme values of up to 3.1 per mil for delta 13 C and 3.8 per mil for delta 18 O. Sulfur isotope measurements on stibnite and orpiment varied from 5.5 to 10 per mil and from 2.5 to 12.3 per mil, respectively. Vein alunite and barite associated with intense oxidation had sulfur isotope compositions that generally varied from 3.8 to 13.4 per mil, and from 21.0 to 29.2 per mil, respectively. Barite that appeared to be sedimentary in origin yielded values of 25.9 to 47.1 per mil.Hydrothermal fluids that formed the deposits ascended from depth along the Vantage fault. Near the level of the deposits, fluids were displaced into minor faults in the footwall limestone. At the contact between the Devils Gate Limestone and the Pilot Shale, fluids spread laterally and deposited Au, Ag, Sb, and As and quartz. Following Au deposition, hypogene oxidizing fluids using the same conduits destroyed reduced components in the Pilot Shale and produced major textural modifications. The distribution of the elements within the deposits appears to be related to (1) proximity to hydrothermal conduits, (2) composition of the host lithologies, and (3) extent of postmineralization oxidation. Oxygen isotope data of jasperoid and vein quartz indicate that the ore fluids were evolved meteoric waters. Isotopic data for epigenetic sulfides indicate that sulfur was derived from sedimentary sulfides; local stratigraphic considerations suggest that Cambrian siliciclastic rocks were the source. The deposits were geochemically similar to other sediment-hosted gold deposits in the Great Basin (e.g., Carlin), but they were uncomplicated by extraneous geologic features such as Paleozoic thrusts or premineralization intrusions. Thus, the Vantage deposits may be more easily understood examples of this class of ore deposits.

Seasonal variation of gold and arsenic in biogeochemical samples from a disseminated gold deposit in the Northern Cordillera

An orientation survey was conducted over the Quartz Mountain, Oregon, hot-spring type disseminated gold deposit to address three questions critical to successful exploration of the area: What is the relationship between bedrock geology and the trace-element content of trees and soils; do these relationships change seasonally; are these relationships similar in the two tree species which discontinuously blanket the area? Twig, needle and wood samples were collected four times during the year from both ponderosa pine ( Pinus ponderosa ) and white fir ( Abies concolor ). Soils were collected once. All samples were analyzed for Au and As and the wood samples were analyzed also for Sb. Arsenic was the best pathfinder element, having little analytical, spatial, or seasonal variation in the twig and needle samples. Anomalous levels were 150 μg/g (ppm) As in pine and 30 μg/g in fir. Gold showed analytical variation of 20–30%, sample duplicate variation up to 90%, spatial changes, and seasonal variation ranging from winter-summer background [10–20 ng/g (ppb) Au] to anomalies of 300–400 ng/g (ppb) Au in pine and 100 μg/g (ppb) Au in fir in the spring and fall. Antimony could not be completely evaluated as a pathfinder element because it was only determined in wood samples. Needles and twigs from both species proved to be viable sample media for exploration of the area. Wood was not a suitable medium because of low, erratic values, perhaps due in part to analytical technique. Needles had 2 to 20 times higher As concentrations than did twigs. Twigs had a slightly higher Au content [25 ng/g (ppb)] than did needles. The pine samples were higher in both elements than were the fir samples. The data, normalized by species, could be treated as one homogeneous data base. The soil Au and As data outlined the mineralization clearly with a central Au anomaly [100 ng/g (ppb) and greater] surrounded by As anomalies [100 μg/g (ppm) and greater] over a distance of 594 m. Neither spring nor fall tree Au anomalies were coincident with the soil Au anomaly. Consistent throughout the year, tree As anomalies coincided with the soil As anomalies, but covered a smaller area. Both the Au and As anomalies in trees appeared to be related to bedrock contact zones rather than to the soil the trees were growing in.

Newly discovered sedimentary rock-hosted disseminated gold deposits in the People's Republic of China

Newly discovered sedimentary rock-hosted disseminated gold deposits in the People's Republic of China

Applications of the laser Raman microprobe RAMANOR U-1000 to hydrothermal ore deposits; Carlin as an example

The laser Raman microprobe has been used effectively in the identification of polyatomic gases, complex ions, and daughter crystals in fluid inclusions in a variety of host rocks (e.g., Dhamelincourt et al., 1979a; Rosasco and Roedder, 1979). This is a generally nondestructive, in situ, semiquantitative analytical technique in which a monochromatic laser beam is focused into a sample. The spectrum resulting from the inelastically scattered radiation indicates the chemical speciation of the irradiated sample. Although laser Raman microprobe analysis is not very effective for brines composed of simple salts, the nonelectrolyte components of hydrothermal fluid inclusions can be readily analyzed. Raman spectroscopy also provides important information, on a microscale, for solid phases in the assemblage. Fluid inclusions in quartz and calcite from several types of samples from the disseminated gold deposit at Carlin, Nevada, were studied petrographically and analyzed by microthermometry and Raman spectroscopy. The laser Raman microprobe was used to scan for CO 2 , CO, N 2 , H 2 S, and CH 4 + or - H 2 O. The semiquantitative microprobe analyses confirmed the presence of three-phase inclusions containing water and different amounts of CO 2 , one-phase inclusions of high-density CH 4 -dominated fluid, and very weak aqueous brines. Very minor H 2 S was detected in a CO 2 -H 2 O inclusion and small amounts of CO 2 were detected in some of the CH 4 -dominated inclusions. These gases may have affected ore deposition. In any event, their identification is necessary for correct estimations of pressure to be derived from homogenization temperatures of the fluid inclusions. The laser Raman microprobe provides more than compositional data. For instance, the exact spectral position and shape of the CH 4 peak indicate the internal pressure, even in inclusions too small ( 2 polymorphs anatase and brookite (both present) in hydrothermal quartz deposits at Magnet Cove, Arkansas. The ability of the laser Raman microprobe to provide pinpoint analyses has many advantages over bulk extraction methods, which mix the volatiles from separate generations of fluids and may also produce chemical artifacts.

Primary Gas Dispersion Halos in Disseminated Gold Deposits: Examples from Nevada and California

Primary Gas Dispersion Halos in Disseminated Gold Deposits: Examples from Nevada and California

Relationship of the Cortez Caldera to the Cortez Disseminated Gold Deposit, Nevada

Relationship of the Cortez Caldera to the Cortez Disseminated Gold Deposit, Nevada

Hydrothermal maturation of indigenous organic matter at the Alligator Ridge gold deposits, Nevada

The organic matter of the Devonian-Mississippian Pilot Shale, which hosts the disseminated gold mineralization at Alligator Ridge, Nevada, has been investigated in an attempt to determine its source and maturation processes and to find an alteration parameter which reflects precious metal distribution more accurately than mineralogical alteration patterns. The Alligator Ridge deposits are similar to other disseminated gold deposits in the Great Basin of the western United States. Visible alteration features, consisting of quartz replacement of carbonates and oxidation of sulfides and organic matter, do not provide significantly useful guides for exploration.In this study, the type, abundance, and maturity of organic matter in the Pilot Shale were determined using techniques standard to petroleum exploration. The type and abundance of organic matter in background and mineralized samples are similar. Transmitted light examination of kerogen concentrates shows that most of the organic matter consists of fine amorphous (algal) aggregates with a minor amount of vitrinite. This corresponds with type I or II organic matter of Tissot and Welte (1978) which is derived from a marine source. Total organic carbon makes up 1 to 3 percent of most carbonaceous rocks at Alligator Ridge. No evidence was found for hydrothermal introduction or remobilization of organic matter.The maturity of organic matter, as determined by pyrolysis and vitrinite reflectance, is considerably higher in mineralized samples than in samples unaffected by the processes of mineralization. Hydrogen index values for mineralized samples are generally <40 (H/C approximately 0.5) compared with a background of 230 to 315 (H/C approximately 1). Changes in vitrinite reflectance show a similar but less dramatic change. Mineralized samples have reflectance values of 1 to 1.3 percent; background reflectance is 0.4 to 0.5 percent. All reflectance values, however, are anomalously low due to the extremely small grain size of the vitrinite and therefore cannot be used to determine paleotemperatures.Spatially, hydrogen index contours increase concentrically away from gold mineralization, except near the lower Pilot Shale contact. Here hydrogen index isopleths are nearly concordant and parallel other alteration features. The spatial pattern indicates that hydrothermal flow was dominantly along this contact and away from mineralization.This study shows that the organic matter in the Pilot Shale is of a marine sedimentary origin. The total organic carbon of mineralized and background material is similar, indicating that the organic matter was not added to the Pilot Shale hydrothermally. However, hydrothermal activity has increased the organic matter maturity, suggesting that maturation studies can be useful to exploration programs.

A comparison of iron oxide-rich joint coatings and rock chips as geochemical sampling media in exploration for disseminated gold deposits

We evaluated the effectiveness of iron oxide-rich fracture coatings as a geochemical sampling medium for disseminated gold deposits, as compared with conventional lithogeochemical methods, for samples from the Pinson mine and Preble prospect in southeastern Humboldt County, Nevada. That disseminated gold mineralization is associated with Hg, As, and Sb is clearly demonstrated in these deposits for both fracture coatings and rock chip samples. However, the relationship is more pronounced for fracture coatings. Fracture coatings at Pinson contain an average of 3.61, 5.13, 14.37, and 3.42 times more Au, As, Sb and Hg, respectively, than adjacent rock samples. At Preble, fracture coatings contain 3.13, 9.72, 9.18, and 1.85 times more Au, As, Sb and Hg, respectively, than do adjacent rock samples. Geochemical anomalies determined from fracture coatings are thus typically more intense than those determined from rock samples for these elements. The sizes of anomalies indicated by fracture coatings are also somewhat larger, but this is less obvious. In both areas, Sb anomalies are more extensive in fracture coatings. At Preble, some Hg and Au anomalies are also more extensive in fracture coatings. In addition to halos formed by the Hg, As and Sb, high values for Au/Ag and Zn/(Fe + Mn) are closely associated with gold mineralization at the Pinson mine. The large enhancement in geochemical response afforded by fracture coatings indicates a definite potential in the search for buried disseminated gold deposits.