New Mexico Geological Society Annual Spring Meeting — Abstracts

The Hillsboro mining district lies in the Animas Mountains, eastern Black Range and has been mined approximately for the last 120 years, mainly for gold, silver, and copper. Four types of deposits are found in the district, Laramide vein, placer gold, porphyry-copper, and carbonate-hosted replacement deposits. Underground mining was the main mode of extraction until 1982 when open pit mining for copper began at Copper Flat. The Copper Flat mine operated only from March to June 1982. During the period 1877-1982 an estimated 270,000 ounces of gold, from lode and placer gold, 78,000 ounces of silver, and 24 million pounds of copper were produced from the district.

Geology of the Hillsboro area is dominated by Cretaceous andesite flows, breccias, and volcanoclastic rocks. Quartz monzonite intrusions and associated quartz latite dikes intrude the andesite. The Copper Flat quartz monzonite, which intrudes the andesite, is 75.1 ± 2.5 Ma (Hedlund, 1985). Field relationships and geochemical data suggest there are two stages ofthe quartz monzonite intrusion (Copper Flat and Warm Springs quartz monzonites). The Copper Flat porphyry-copper deposit consists of copper, gold, molybdenum, and silver disseminated in a quartz monzonite stock and in quartz veins. Quartz latite dikes propagate radially from the porphyry and intrude the andesite and the quartz monzonite. The quartz latite dikes contain variable amounts of silver, gold, copper, lead, and zinc. Previous fluid inclusion studies from Norman et al. (1989) and Fowler (1982) indicate the fluids at Copper Flat breccia and the Wicks vein are of similar chemical composition. Mineralization occurred at temperatures of 320-3600 C from low to moderate salinity fluids (10-45 eq. wt.% NaCI).

A crude district zoning has been recognized by many workers in the district. The Copper Flat porphyry-copper deposit forms the center. Propagating outward radially from the Copper Flat quartz monzonite are epigenetic Au-Ag-Cu veins hosted by many of the quartz latite dikes. Small carbonate-hosted replacement deposits (possible skarns) are found in the southern part of the district. Interpretation of published and unpublished geochemical data of veins in the district have shown a refinement ofearlier interpretations of district zoning. Au, Ag, and Cu decrease in concentrations along the Empire vein away from the Copper Flat quartz monzonite. Au, Ag, and Cu decrease in concentration away from the Warm Springs quartz monzonite along the Sherman vein. This suggests that the Warm Springs quartz monzonite may be a second source for mineralizing fluids. Additional work to confirm this possibility is in progress.