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Primary Dispersion of Trace Elements, Fluid Inclusion Zoning, and Alteration Zoning at the Pueblo Viejo Au-Ag Deposit, Dominican Republic
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PRIMARY DISPERSION Primary Gas Dispersion Halos in Disseminated Gold Deposits: Examples from Nevada and California CHARLES G. CLIFTON
Exploration Research Laboratories, 131 NW 4th Street, Box 200, Corvallis, OR 97330, U.S.A. All hydrothermal systems contain significant amounts of volatiles, either as dissolved aqueous species, as free gaseous species, or both. As hydrothermal solutions traverse or permeate a rock mass, gases are "deposited" in the rock in several ways, including: adsorption onto alteration phyllosilicates (clays and micas), entrapment in vugs, and encapsulation in fluid inclusions. The gases are easily liberated from wallrock material by heating and are identified by gas chromatography/mass spectrometry. Primary gas dispersion halos occur as anomalous concentrations of individual gases (H~S, SO2, CO2, etc.) or as groups of gases (total hydrocarbons, total sulfur gases, etc.). On a weight basis, the amount of gas introduced to the wall rocks often exceeds the total amount of trace elements. This, coupled with high mobility, makes primary gases in wall rocks valuable geochemical indicators, particularly for deeply buried deposits which are not associated with strong trace element signatures. Primary gas dispersion halos have been identified in every epithermal deposit studied to date by Exploration Research Laboratories. The halos are of two types: (a) broad halos of general gas enrichment which reflect the aerial extent of pervasive hydrothermal fluid flow, and (b) restricted halos which reflect local disequilibrium in the system and are associated with mineral deposition and/or development of alteration phases. Gases incorporated into wallrock material do not appear to directly reflect the abundance or speciation of gases originally present in the hydrothermal solutions. Rather, the distribution is controlled by the affinity of different clays and micas for polar or non-polar gases, and reactions of dissolved gases with primary minerals. Local variations in the primary gas dispersion halo may reflect subtle changes in alteration mineralogy and thus, indirectly, local variations in the chemistry of the hydrothermal system. The results of studies of primary gas dispersion in several disseminated gold deposits will be presented. These will include the Hor~e Canyon and Getchell "Carlin-type" deposits in Nevada and the Hayden Hill volcanic-hosted deposit in northeast California.
Primary Dispersion of Trace Elements, Fluid Inclusion Zoning, and Alteration Zoning at the Pueblo Viejo Au-Ag Deposit, Dominican Republic KARR McCURDY 1, NORMAN RUSSELL 1 and STEPHEN E. KESLER 2
Rosario Dominicana, S.A., Apartado 944, Santo Domingo, Dominican Republic ~Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A. Zoning of hydrothermal alteration minerals is the most useful indicator of Au-Ag mineralization at the Pueblo Viejo Au-Ag hot spring deposit. Mineral sequence is characterized by an alteration funnel with high-level massive jasperoid underlain successively by quartz-pyrophyllite and alunite and surrounded by regionally extensive calcite. The distribution and degree of enrichment of trace elements within and surrounding the Au-Ag mineralization reveals that Te, Au, and As are very abundant in the funnelshaped zone of hydrothermal alteration. Assay values from 343 non-oxidized rock samples for Au, Ag, Cu, Zn, Pb, As, Sb, Hg, Te, and Se show that Au is well dispersed in the upper portion of the mineralized system.
260
The degree of enrichment of trace elements assayed from 53 oxidized rock samples shows that Au, Ag, Pb, As, Sb, Hg, Te, and Se are more enriched in the oxidized zone than in the non-oxidized ore zone. However, these elements' distributions are much more irregular, denoting leaching and secondary enrichment of all elements with the exception of Au. Gold geochemistry surveys have delineated the Pueblo Viejo ore bodies, but other elements, especially Hg, Te, and As, could also be used as pathfinders to potential Pueblo Viejo Au-Ag mineralization. Fluid inclusions from 33 jasperoid samples taken from within and surrounding the Pueblo Viejo ore body have been analyzed for their H~O, Ar, CH4, CO, COs, H2, N2, and O2 gas contents. Fluid inclusion gases from within the ore zone are H20 rich and CO-CO2 poor, while inclusions surrounding the ore body are CO-CO2 rich.
Stream Sediment Geochemistry of the McLaughlin Gold Deposit and the Knoxville
District, Napa and Yolo Counties, California C. NEIL UPCHURCH
Consultant, 2980 Middlecoff Circle, Reno, N V 89502, U.S.A. A series of stream sediment geochemistry orientation studies were conducted in the Knoxville mercury mining district of lyzed for Au, Ag, As, Sb and Hg. Many samples were separated into eight size fractions. Some were also separated into light; nonmagnetic, heavy; and magnetic, heavy mineral fractions. All fractions were analyzed separately. Analyses were also performed on the entire --1~ inch size fraction of all samples. A study of sample fractions shows that Au, Sb and As tend to be more concentrated in the +40 mesh fractions and that Ag is more concentrated in the --40 mesh fractions. The Au/Ag ratio is generally greater than one in all +40 mesh fractions and less than one in the --40 mesh fractions. The only consistent habit of Hg is its increased concentration in the nonmagnetic fraction of heavy minerals. The sorting tendency of each metal suggests the form in which it is traveling.
PRIMARY DISPERSION Primary Gas Dispersion Halos in Disseminated Gold Deposits: Examples from Nevada and California CHARLES G. CLIFTON
Exploration Research Laboratories, 131 NW 4th Street, Box 200, Corvallis, OR 97330, U.S.A. All hydrothermal systems contain significant amounts of volatiles, either as dissolved aqueous species, as free gaseous species, or both. As hydrothermal solutions traverse or permeate a rock mass, gases are "deposited" in the rock in several ways, including: adsorption onto alteration phyllosilicates (clays and micas), entrapment in vugs, and encapsulation in fluid inclusions. The gases are easily liberated from wallrock material by heating and are identified by gas chromatography/mass spectrometry. Primary gas dispersion halos occur as anomalous concentrations of individual gases (H~S, SO2, CO2, etc.) or as groups of gases (total hydrocarbons, total sulfur gases, etc.). On a weight basis, the amount of gas introduced to the wall rocks often exceeds the total amount of trace elements. This, coupled with high mobility, makes primary gases in wall rocks valuable geochemical indicators, particularly for deeply buried deposits which are not associated with strong trace element signatures. Primary gas dispersion halos have been identified in every epithermal deposit studied to date by Exploration Research Laboratories. The halos are of two types: (a) broad halos of general gas enrichment which reflect the aerial extent of pervasive hydrothermal fluid flow, and (b) restricted halos which reflect local disequilibrium in the system and are associated with mineral deposition and/or development of alteration phases. Gases incorporated into wallrock material do not appear to directly reflect the abundance or speciation of gases originally present in the hydrothermal solutions. Rather, the distribution is controlled by the affinity of different clays and micas for polar or non-polar gases, and reactions of dissolved gases with primary minerals. Local variations in the primary gas dispersion halo may reflect subtle changes in alteration mineralogy and thus, indirectly, local variations in the chemistry of the hydrothermal system. The results of studies of primary gas dispersion in several disseminated gold deposits will be presented. These will include the Hor~e Canyon and Getchell "Carlin-type" deposits in Nevada and the Hayden Hill volcanic-hosted deposit in northeast California.
Primary Dispersion of Trace Elements, Fluid Inclusion Zoning, and Alteration Zoning at the Pueblo Viejo Au-Ag Deposit, Dominican Republic KARR McCURDY 1, NORMAN RUSSELL 1 and STEPHEN E. KESLER 2
Rosario Dominicana, S.A., Apartado 944, Santo Domingo, Dominican Republic ~Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A. Zoning of hydrothermal alteration minerals is the most useful indicator of Au-Ag mineralization at the Pueblo Viejo Au-Ag hot spring deposit. Mineral sequence is characterized by an alteration funnel with high-level massive jasperoid underlain successively by quartz-pyrophyllite and alunite and surrounded by regionally extensive calcite. The distribution and degree of enrichment of trace elements within and surrounding the Au-Ag mineralization reveals that Te, Au, and As are very abundant in the funnelshaped zone of hydrothermal alteration. Assay values from 343 non-oxidized rock samples for Au, Ag, Cu, Zn, Pb, As, Sb, Hg, Te, and Se show that Au is well dispersed in the upper portion of the mineralized system.
260
The degree of enrichment of trace elements assayed from 53 oxidized rock samples shows that Au, Ag, Pb, As, Sb, Hg, Te, and Se are more enriched in the oxidized zone than in the non-oxidized ore zone. However, these elements' distributions are much more irregular, denoting leaching and secondary enrichment of all elements with the exception of Au. Gold geochemistry surveys have delineated the Pueblo Viejo ore bodies, but other elements, especially Hg, Te, and As, could also be used as pathfinders to potential Pueblo Viejo Au-Ag mineralization. Fluid inclusions from 33 jasperoid samples taken from within and surrounding the Pueblo Viejo ore body have been analyzed for their H~O, Ar, CH4, CO, COs, H2, N2, and O2 gas contents. Fluid inclusion gases from within the ore zone are H20 rich and CO-CO2 poor, while inclusions surrounding the ore body are CO-CO2 rich.
Stream Sediment Geochemistry of the McLaughlin Gold Deposit and the Knoxville
District, Napa and Yolo Counties, California C. NEIL UPCHURCH
Consultant, 2980 Middlecoff Circle, Reno, N V 89502, U.S.A. A series of stream sediment geochemistry orientation studies were conducted in the Knoxville mercury mining district of lyzed for Au, Ag, As, Sb and Hg. Many samples were separated into eight size fractions. Some were also separated into light; nonmagnetic, heavy; and magnetic, heavy mineral fractions. All fractions were analyzed separately. Analyses were also performed on the entire --1~ inch size fraction of all samples. A study of sample fractions shows that Au, Sb and As tend to be more concentrated in the +40 mesh fractions and that Ag is more concentrated in the --40 mesh fractions. The Au/Ag ratio is generally greater than one in all +40 mesh fractions and less than one in the --40 mesh fractions. The only consistent habit of Hg is its increased concentration in the nonmagnetic fraction of heavy minerals. The sorting tendency of each metal suggests the form in which it is traveling.