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THE NATURAL DEFENSES OF COPPER FLAT SIERRA COUNTY, NEW MEXICO

Copper Flat, located in southwestern New Mexico, approximately 23 miles southwest of Truth or Consequences, is a porphyry copper deposit with associated gold, silver, molybdenum, and sulfide minerals. The stock contains a 75 million-year-old quartz monzonite breccia pipe forming the center of an eroded andesite strato-volcano. Quintana Minerals Corporation mined the property for three months in 1982 producing 7.4 million pounds of copper, 2,306 ounces of gold, and 55,966 ounces of silver. Mining activities ceased because of low copper prices. The mining equipment was dismantled and sold. Since no mining activities have occurred since 1982, the site is an excellent field laboratory for studying the behavior of metals and sulfide minerals exposed with waste rock and tailings in the arid southwest. There is a 12.8-acre pit and pit lake on site that is located near the center of the breccia pipe. The entire study undertaken at Copper Flat focused on the potential impact of the pit lake, the waste rock piles, and the tailing impoundment on the local surface and groundwater quality; however, this paper focuses on the pit lake. The pit lake has been sampled at least 65 times between 1989 and 1997. The pH of the lake is typically neutral to alkaline, with exception occurring in 1992 and 1993, where the pH dropped as low as 4.4. At least one intermittent seep from the pit wall has been sampled and the results reported a pH of 2.64, a total dissolved solid concentration (TDS) of 12,770 milligrams per liter (mg/L), and a sulfate concentration of 790 mg/L. The andesitic host rocks surrounding the ore body and groundwater inflow have a high acid buffering capacity as shown by the partial dissolution of calcite and the precipitation of gypsum and goethite. The alkalinity of the groundwater and host rocks quickly neutralizes and dilutes acidic discharges into the pit lake. Groundwater samples collected from monitoring wells located down gradient from the pit lake indicate groundwater chemistry is similar before and after the excavation of the pit.