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GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 1:30 PM-5:30 PM

GEOCHEMICAL INVESTIGATION OF SKARN AND ORE-FORMING PROCESSES IN THE EL MOCHITO ZN-PB-AG CARBONATE REPLACEMENT DEPOSIT, HONDURAS


AULT, K. M., Department of Earth and Planetary Sciences, McGill University, 3450 University St, Montreal, QC H3A 2A7, Canada and WILLIAMS-JONES, A. E., Department of Earth and Planetary Sciences, McGill Univ, 3450 University St, Montreal, QC H3A 2A7, Canada, kault@eps.mcgill.ca

The El Mochito mine, west-central Honduras, is a distal chimney-manto Zn-Pb-Ag replacement deposit (skarn) hosted by Cretaceous limestones.

The deposit exhibits typical Zn-skarn mineralogy, consisting of pyroxene (Hd37-80Di10-30) and garnet (Ad55-77Gr14-35) with sphalerite, argentiferous galena, pyrrhotite, pyrite, chalcopyrite and arsenopyrite. Orebodies along the fault-controlled Salva Vida-Nacional trend grade outward and upward from Zn-Fe rich cores to Pb-rich and marginal Ag-rich zones. Garnet:pyroxene ratios decrease progressively away from fault zones toward unaltered limestone and along the trend of the orebodies (NE to SW). Changes in Zn/Pb, Zn/Cu, Pb/Cu and Cu/Ag ratios along trend appears to reflect the direction of hydrothermal fluid flow (NE to SW) and increased distance from a potential source region.

Fluid inclusion studies of sphalerite, garnet and pyroxene indicate that two fluids were involved in the formation of the deposit; a high temperature/low salinity fluid (480oC, 5 wt% NaCl eq.), and a low temperature/high salinity fluid (390oC, 17 wt% NaCl eq.).

Lead isotopic ratios of galena are comparable with those of the nearby Miocene Padre Miguel ignimbrite, suggesting that the ignimbrite or a related magma was the source of lead. The Pb ratios of altered and non-mineralised limestones also define the direction of fluid flow (NE to SW) and the limit of alteration along trend. Sulphur isotopes of sphalerite, galena and pyrrhotite give d34S values which range from –3.5 to +3.9‰. These values fall within the range expected for magmatic fluids, demonstrating that the source of sulphur at El Mochito is primarily igneous.

Zoning distribution of mineralogical, metal and isotopic ratios (S, Pb), and fluid inclusion temperatures indicate that the El Mochito Zn-Pb-Ag deposit is orthomagmatic in origin, and that the source of heat and metals lies to the NE, related to magmatism associated with the Padre Miguel ignimbrite.

Session No. 173--Booth# 20

Economic Geology (Posters)
Thursday, 8 November 2001: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 33, No. 6, October 2001, p.2483
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