New Mexico Mineral Symposium — Abstracts

The San Pedro mine is located approximately 2.1 air miles southeast of the town of Golden, in the western San Pedro Mountains, New Placers district, Santa Fe County, New Mexico. The mine was discovered in 1840 and was operated intermittently until 1982. Although total production is unknown, published lode production for the New Placers district from 1904 to 1938 was 203,965 short tons of ore yielding 11,402.57 troy ounces of gold (0.056 oz/ton), 242,791 troy ounces of silver (1.19 oz/ton), and 9,773,773 pounds of copper (2.4%). Most of this production came from the San Pedro mine.

The ores of the San Pedro mine are contact metasomatic (skarn, tactite) deposits in the upper limestone beds of the Pennsylvanian Madera group. The dip of the Madera beds in the vicinity of the mine is about 15° to the east. The tabular orebodies are localized beneath a rhyolite sill, locally known as the Puzzle sill. Heat and accompanying mineralizing solutions probably emanated as a late stage from the underlying monzonite porphyry laccolith that forms the core of the western part of the San Pedro Mountains.

At least three stages of skarn formation are recognized. First, limestone was metamorphosed to andradite garnet with minor epidote. Reduction in volume during the alteration of limestone to garnet resulted in a vuggy, porous garnet bed. These pores were partially filled by a second mineralizing event consisting of chalcopyrite, bornite, pyrite, pyrrhotite, calcite, specular hematite, quartz, and chlorite. The final mineralizing event filled the larger cavities with calcite, quartz, pyrite, chalcopyrite, scheelite, and adularia. Wire gold was deposited locally along the edges of these large vugs and has been recovered recently as beautifully contrasting specimens of gold in calcite.

Other minerals of interest to collectors include chalco-pyrite, Japan-law twin quartz crystals, twinned scalenohedrons of calcite (rarely sixlings), and rarely amethyst, scheelite, native copper with cuprite, and pyrite pseudomorphs after calcite. Large chalcopyrite crystals (up to 3 inches on an edge) were collected for many years from the district. They are commonly coated with a dusting of malachite. Recently, limonite-coated Japan-law twins were collected that exhibit unusual scepter terminations. Gold in calcite and occasionally on garnet and pyrite were collected in 1984 by Wayne Holland of Albuquerque. These specimens came from a restricted zone of abundant calcite (possible an extremely vuggy area) discovered some time earlier by Ira Young.