New Mexico Mineral Symposium — Abstracts

The rare mineral betekhtinite, a sulfide of copper, lead, and iron, occurs in significant amounts in the St. Cloud copper-silver deposit located near Winston in the Chloride mining district, Sierra County, New Mexico. The mine, situated on the eastern slopes of the Black Range approximately 40 mi west of Truth or Consequences, is owned and operated by the St. Cloud Mining Co.

The St. Cloud and adjoining Atlanta and Mayflower lode mining claims were located in 1884. A few tons of hand-sorted oxidized copper-silver ore were shipped from near-surface workings on the Atlanta-St. Cloud claims in the 1880's and early 1890s. Drilling by the Goldfield Corp. in 1968 on the St. Cloud-U.S. Treasury vein intercepted ore-grade copper-silver mineralization at a depth of 450 ft. Subsequent drilling established the presence of two ore deposits, the St. Cloud and the U.S. Treasury.

The St. Cloud ore deposit is a mesothermal vein-type deposit occurring in a strong northwesterly striking fault structure. Host rocks are the Madera Limestone (Pennsylvanian) and andesitic flows and sills (Tertiary). The fault zone and vein structures are 20-60 ft wide. Ore-grade mineralization is typically 10-20 ft wide and occurs as ore shoots within the structure. Minerali¬zation appears to favor those areas where the vein structure has limestone on one wall and andesite on the other.

Mineralization, which is often massive, contains (in decreasing order of abundance) sphalerite, bornite, betekhtinite, galena, chalcocite, and stromeyerite in a siliceous breccia. The outer portions of the orebodies contain some chalcopyrite. Pyrite, although common in the wall rocks, is noticeably less abundant in the ore. Late-stage calcite is common. Oxidation in the upper parts of the deposit created very minor amounts of native copper, curite, and copper carbonates.

Betekhtinite was first described in 1955 as a Cu-Pb-Fe sufide from veins in the Mansfeld "Kupferschiefer" (copper shales) of East Germany. Since then it has been found in Europe, Asia, Africa, South America, and Australia. In 1982 betekhtinite was identified in concentrates from the St. Cloud mine. We believe that this is the first reported occurrence of the mineral in North America.

Chemically, the New Mexico betekhtinite is a Cu-Pb-Fe sufide with minor amounts of silver, but microprobe analyses indicate that the published formula—Cu₁₀(Pb,Fe)S₆--is in error. In the literature on this mineral, the formula has undergone a number of changes, partly because the silver and iron may be assigned to either the Cu site or the Pb site. Analyses from the present study and from many literature sources seem to best fit a formula of Cu₁₃(Pb,Fe,Ag)₂S₉.

*A portion of this work was performed at Sandia National Laboratories and was supported by the U.S, Department of Energy under Contract #DE-AC04-76DP00789. The authors would like to acknowledge the cooperation and assistance of the St. Cloud Mining Co.