New Mexico Mineral Symposium — Abstracts

The San Pedro mine has held a certain fascination for the authors since they were teenagers, first as a place to explore and then, as we began to notice the crystals, a source for a new hobby and later a profession. Thus inspired, we both went on to careers in geology, Bill as a professor at the University of Colorado and Sherm as a geologist for the U.S. Geological Survey. The mine is located approximately 40 mi south of Santa Fe, and 32 mi northeast of Albuquerque. The mine exploited a skarn deposit, producing copper, gold, and silver. Garnet associated with metal deposition replaced limestone beds around a mineralizing porphyry copper-molybdenum type intrusion.

History—Historical records are fragmentary. Fray Dominguez made a journey surveying the missions of New Mexico in 1776. His map of the missions shows a village of San Pedro at the location of the mine. It is most likely that it was a mining village at that time, in an area poor for agriculture, and far from protection from Indian raids.

Gold was discovered in the nearby Ortiz Mountains in 1833, followed by discovery of the New Placers in the San Pedro Mountains in 1839. Miscellaneous historical references mention activity at the San Pedro mine 1840–1846, which was owned by Mexican mining interests. The Mexican government awarded the Cañon del Agua land grant, just to the west of the San Pedro mine, to a rancher at some time before 1846, which was later recognized by the United States in 1875. In 1880 a criminal gang arranged to have the grant resurveyed to include San Pedro mine, then seized the mine. When the owner at that time, M. A. Otero, a former governor of New Mexico, sued to recover the mine, the gang fortified it. In a raid at night, Otero brought a dozen armed men who entered the mine by sliding down a rope in a shaft, forced 162 miners out of the mine and took it over. The owner eventually recovered the mine through legal proceedings.

Production—From 1904 to 1967, 273,129 tons of ore was produced yielding 16,549 oz gold, 304,625 oz silver, and 7,476 tons of copper. Average grade about 2.7% Cu, 0.05 oz/t gold, 0.8 oz/t Ag.

Geologic setting—The mine lies in a short chain of intrusions parallel with the Rio Grande rift, aproximately 10 mi to the west. The intrusions may represent an early stage of rifting. The rocks intruded include Precambrian granite and gneiss, Pennsylvanian limestone and shale, Permian siltstone and sandstone, and Triassic siltstone, shale, and sandstone.

Skarn formation—High temperature solutions from the mineralizing intrusion in San Lazarus gulch brought abundant trace elements through stockwork veinlets, producing mineralogical alteration of the surrounding rocks, and forming the mineral deposit. High-temperature solutions from the intrusion, some trapped as fluid inclusions in crystals, introduced SiO₂, Al, Fe, Mg, Na, K, Ba, Mn, Ti, Cu, W, Ag, Au, Mo, Cl, S, and F, among other elements, most of which were transported by chloride ions. On encountering the calcite of the limestone, calcium quickly robbed the chloride ions to deposit magnetite, garnet, pyroxene, wollastonite, quartz, chalcopyrite, pyrite, pyrrhotite, molybdenite, scheelite, fluorite, and gold in prograde early deposition. At high temperatures, sulfur is carried mostly as sulfur dioxide. During cooling of the system, sulfur dioxide reacted with water to produce sulfuric acid and hydrogen sulfide at temperatures of about 400° C. The then somewhat acid solution destroyed the garnet to produce quartz, chlorite, calcite, pyrite, and specular hematite partially filling cavities in the garnet. Chalcopyrite was remobilized to form very large crystals, up to 4 inches across.

Less common minerals include adularia, sphalerite, galena, idocrase, scapolite, epidote, sphene, anatase, allanite, laumontite, and siegenite. The nickel in the rare sulfide siegenite was shown by Lee (1987) to have come from the Pennsylvanian limestone and shale. Supergene minerals include bornite, covellite, malachite, azurite, chrysocolla, limonite, cuprite, and native copper. 

The skarn is zoned outward from the mineralizing intrusion from (1) the garnet zone, (2) the ore zone (also designated as the “marble line”), containing garnet, chalcopyrite, gold, scheelite, quartz, and calcite, (3) the marble zone to (4) unaltered limestone. Shales interbedded with limestone are altered to fine-grained hornfels, which shows complex zoning from the intrusion outward from a light-green zone characterized by diopside and actinolite-tremolite to a peripheral zone characterized by biotite to unaltered rocks consisting of clays. All zones contain significant amounts of calcium plagioclase or anorthite. Orbicules common in the hornfels have complex mineralogy differing somewhat from that of the main hornfels, with an inner zone adjacent to the intrusion characterized by orbicules containing garnet and wollastonite and an outer zone with orbicules containing epidote, diopside, actinolite-tremolite, and chlorite. Alteration of shales extends beyond the marble zone in the limestones, providing a guide for exploration.

Mineral specimens—Some of the largest chalcopyrite crystals in the world come from the San Pedro mine. One specimen collected in 1956 by the authors and Bill’s fiancée at the time, Carol Bambrook, measured 3.5 inches! This specimen was later donated to the University of New Mexico Geology Museum. Many of the chalcopyrite crystals enclose cubes and pyritohedrons of pyrite. In addition, many of the chalcopyrite specimens exhibit very unusual twinning. Many other good mineral specimens collected by the authors occurred principally in the marble line zone, where replacement of limestone reduced the volume of the rock, producing abundant cavities. In addition to the chalcopyrite, notable specimens include pyrite intergrown with calcite, quartz, and small rosettes of specular hematite on a base of garnet crystals. In certain areas of the mine all the quartz occurs as Japanese twins. Calcite shows a number of habits, including scalenohedrons, some twinned on the basal pinacoid, and rhombohedrons, some twinned on rhombohedral planes. The surfaces of chalcopyrite crystals are partially oxidized down to the water level, but a few excellent unoxidized specimens were recovered in early mining and by the authors. A very few specimens of pyrite pseudomorphous after calcite scalenohedrons were found at the water level. Some spectacular specimens of gold are exhibited in the museum of New Mexico Tech

References:

1. Atkinson, W. W., Jr., 1961, Geology of the San Pedro Mountains, Santa Fe County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 77, 50 pp.
2. Lee, C. G., 1987, Skarn and hornfels petrogenesis at the San Pedro mine, Santa Fe County, New Mexico: unpublished Ph.D. dissertation, University of Colorado, Boulder.
3. Sanders, M. R., 1992, The San Pedro mine, Santa Fe County: Rocks and Minerals, v. 67, pp. 390???395.