New Mexico Mineral Symposium — Abstracts

Native copper at Santa Rita resulted in some of the earliest mining in the Southwest. Native copper artifacts from a Georgia archaeological site dated at A.D. 880 have been identified by trace element chemistry as being from Chino, and a copper bell dated ca A.D. 1150 has been excavated from a Mogollon site. Spanish explorers came north from Mexico to explore the area beginning with Don Juan de Otiate in 1598. The exact time when the Spanish gained knowledge of the deposit is unknown, but ca 1795 Captain Francisco Martinez stationed at El Presidio de Carizal mentions "El Cobre" near Santa Lucia Springs as a "criadero." A criadero or nursery (where minerals "grow") ~vas at the time considered a natural wonder, with such occurrences of metals reserved to the Spanish Crown. Consequently, there was little incentive to develop such deposits of native metal "growing" from the soil until Jose Manuel Carrasco, a soldier stationed about 150 mi south of Santa Rita at El Presidio de Janos, took the initiative to develop the deposit in the early 19* century. By about 1801 Carrasco had interested his friend Don Francisco Manuel Elguea a wealthy and influential merchant from Chihuahua in Santa Rita copper, which by 1804 resulted in contracts to supply copper for Mexican coinage.

Native copper is common in the oxidation zone of porphyry copper deposits, typically forming near the top of chalcocite enrichment. Here, copper in chalcocite is reduced to the native state with accompanying oxidation of sulfur to sulfate, as suggested by Lindgren from observation of Morenci ore and by the reaction:

Cu⁺¹₂S + 3Fe⁺³₂(SO₄)₃+ 4H₂0 = Cu⁰+ 6Fe⁺²SO₄ + 4H₂SO₄

Even though ferric iron is responsible for the oxidation of chalcocite in this geologic setting, little hematite associated with native copper is present in recent finds at Chino perhaps because of the relatively low pyrite content in this area of the pit. Much of the native copper at Chino is in the Santa Rita granodiorite stock to the west and beneath retrograde skarn mineralization in a "roof pendant" in the East pit area of the mine. Shiny, flattened specimens were collected in 1998 from fractures in skarn hosted by the Syrena Formation, which suggested a change in pH, rather than Eh, was responsible for the deposition of native copper from solution.

The best native copper specimens are found in the stock and in the most recent finds intimately associated with alunite. Additional associated minerals at Chino include quartz, sericite, gypsum, and minor orthoclase. Pyrite and hematite are present but in relatively small amounts. Cuprite is often found as an oxidation product on the native copper as coatings and in crystalline form as cubes, octahedrons, and dodecahedrons. Native copper has also been found recently in the South pit area of the Chino mine as finely crystalline masses in fractures associated with chrysocolla and the chalcotrichite variety of cuprite.

The main body of native copper mineralization in the East pit was first encountered at about 700 ft below the original surface at an elevation of 5,650 ft. The best specimens have been found sporadically between an elevation of about 5,500 ft and the current pit bottom on the 5,150-ft bench. Mining was stopped in 2001 about 50 ft above the bottom of the native zone that produced the most recent nicely crystallized specimens. Copper mineralization extends in an adjacent zone to about the 4,750-ft elevation, a total vertical extent of 900 ft, but current plans include mining only to the 5,000-ft elevation.

The New Mexico Bureau of Geology and Mineral Resources Mineral Museum contains specimens from at least as far back as the early part of this century (C.T. Brown Collection). Additions to the museum collections over time provide us a potential chronology for the production of fine specimens from the mine. The earliest documented specimens are typically massive vein fillings of native copper. A few early examples consist of arborescent growths of crystals. Later specimens often dominated by spinel and polycyclic twinning were produced from the 1970s to the early 1980s. A discovery in late 1993 had some nicely crystallized copper and cuprite and included some unusual forms. More recently a single specimen mined in March 2001 was recovered from an inactive concentrator stockpile that yielded some spectacular crystalline specimens sold in early 2002. Crystal forms include spinel twins, dodecahedrons, and modified cubes. Often single specimens show differing crystal habits. Some finely crystalline copper "wool" is present on some specimens as a secondary crystallization. Additionally, many examples of good quality crystalline copper, reasonably priced, were found in the area. Additional discoveries are anticipated when mining resumes.