New Mexico Mineral Symposium — Abstracts

History—The Linchburg mine is in Socorro County, New Mexico, approximately 9 km south of the village of Magdalena. It was first operated by the American Zinc, Lead, and Smelting Co. in 1910. Early development consisted of driving a 500-m-long adit to explore the sulfide ore deposits adjacent to the Linchburg fault. In 1912, C. T. Brown purchased the property and then sold it to the Empire Zinc Co. that same year. Empire developed the deposit by stoping off the main adit, primarily to the north, in 1915 and 1916. Minor production continued to the mid-1920s. The mine was then idle until purchased by the New Jersey Zinc Co. in 1942 when the high demand for strategic metals during World War II resulted in renewed activity. New Jersey Zinc leased the property to various operators after the war. Operations ceased during the mid-1960s, primarily because of the distance required to ship concentrates. Cobb Resources, Inc. leased the property in 1972 and performed additional development work until 1982. Hydro Nuclear Corp. purchased the property from New Jersey Zinc in 1989, but there has been no production in recent years.

In 1990, the property was leased by the U.S. Army Corps of Engineers to conduct experiments as part of an underground munitions storage facility research project. This project is a joint venture between the United States and the Republic of Korea designed to optimize safety for underground munitions storage facilities. Two drifts will extend from the existing adit to provide access to seven chambers that are 1/3-scale models of the final design. Each chamber incorporates different features designed to attenuate gas and blast pressures resulting from the detonation of the explosives contained inside. A series of instruments located throughout the project and adit will measure the effects of the blasts.

Geology—The Linchburg mine is on the north end of Magdalena uplift, a horst block homocline feature in the Rio Grande rift. Sedimentary rocks in the homocline generally strike north-northwestward and dip W20°-40°SW. Most mine workings are in the Mississippian Kelly Limestone where large replacement bodies of zinc, lead, and minor copper sulfide ore have developed adjacent to major faults. In addition to the Kelly Limestone, there are exposures of Pennsylvanian shale and limestone of the Madera and Sandia Formations, Precambrian greenstone and granite, Tertiary monzonite porphyry, and Tertiary andesite and rhyolite. The munitions project is in an interval of the upper Kelly Limestone exposed in the adit between 215 and 275 m from the portal. The intent was to locate the project in a homogeneous body of relatively hard and dense material barren of ore.

Mineralogy—Two mineral species not previously reported/confirmed from New Mexico were found by the authors during Spring 1993. Both minerals, ktenasite (Cu⁺²,Zn)₅(SO₄)2(OH)₆ • 6H₂O and serpierite Ca(Cu⁺²,Zn)₄(SO₄)₂(OH)₆ • 3H₂O, were found on fault/fracture surfaces and in gouge and breccia associated with the Linchburg fault where the hanging-wall replacement ore contains zinc and copper. Observations and production records indicate that significant copper-bearing ore was located only to the north of the main adit, which is also where the ktenasite and serpierite were found. Ktenasite was confirmed by microprobe analysis (Hlava, personal communication) and by x-ray diffraction (McKee, personal communication). Serpierite identification was established by microprobe analysis (Hlava, personal communication). Ktenasite is found as crusts and druzes of transparent, very thin, tabular crystals usually 0.1 mm or less. Some crystals are more equant. The color ranges from emerald green to blue-green with a vitreous luster. Ktenasite is often found overgrown on serpierite, which makes it appear blue. Serpierite is found as spherical aggregates of robin's egg blue crystals with a pearly luster.The individual sword-shaped crystals are considerably less than 0.1 mm in length. Much of the serpierite is covered by ktenasite. An unknown zinc carbonate-sulfate was also found in association with the ktenasite and serpierite. It occurs as crusts of thin, hexagonal platelets that are transparent and colorless with a vitreous luster. Crystals are 0.1 mm or smaller. Identification is pending based on further study (Modreski and Hlava, personal communi¬cation).

To our knowledge, ktenasite has been reported in the United States only from the Commodore mine in Colorado and the 79 mine in Arizona. The type location is at Laurium, Greece, and the only other known world occurrence is in Modum, Norway. The Blanchard mine is the only previously reported serpierite location in New Mexico, but analyses have failed to confirm these earlier reports.

Minerals from the Linchburg mine

†† New to New Mexico
†Reported but not observed
* Many collectors have specimens from the Linchburg mine that are labelled siderite. Numerous analyses of the tan to dark-brown siderite-looking mineral have all indicated that it is ferruginous calcite. In addition, ankerite has not been observed, indicating that siderite is not likely to occur.