New Mexico Mineral Symposium — Abstracts

Granite Gap is in the central Peloncillo Mountains near the "boot heel" of southwestern New Mexico and was one of the major producers of the San Simon mining district. It can be reached by traveling west on I-10 for 16 mi past Lordsburg to Road Forks then south on U.S. 80 for approximately 11 mi. The mining area is on a hill southeast of the highway and is about one quarter of a mile from the highway. Most of the area is in private ownership as patented claims.

Mining exploration at Granite Gap began about 1880, and there was a great deal of activity at the site in 1904 when it was visited by Lindgren, Graton, and Gordon (1910, p. 330). Extensive production stopped around 1915 although small amounts of ore were shipped until at least 1926 and probably much later as reported by Gillerman (1958, p. 101). The mines were operated for lead and silver with some minor amounts of gold. The ore occurs in hydrothermal veins in Mississippian limestones near or adjacent to Tertiary dikes of a light-colored, fine-grained, quartz monzonite porphyry (Williams, 1978). No contact metamorphic effects were observed in the limestones adjacent to the dikes (Gillerman, 1958; Williams, 1978) although an extensive contact metamorphic aureole does occur around the later, genetically related, main quartz monzonite intrusive (Williams, 1978).

The main workings consist of several shafts and a number of adits, all of which were apparently interconnected at one time. Workings that are currently accessible form a Byzantine series of drifts, declines, stopes, and "dog holes." Mining by "following the ore" is probably responsible for this extreme irregularity of openings and suggests that the ore shoots themselves were highly irregular although they were always near the intrusive dikes.

Arsenic was detected early and was attributed to oxidized tetrahedrite-group minerals [(Cu, Fe)₁₂(Sb,As)₄S₁₄ (Lindgren et al., 1910) although no relict tetrahedrite has been found. Williams (1978) reported a number of arsenate minerals but did not provide specifics of occurrence, associations, and physical descriptions for most of them. He did, however, note the ubiquitous presence of arsenopyrite in unctuous phlogopite surrounding the other sulfides. This material was not detected during this investigation. Williams (1978) determined that the silver in the ore was originally derived from matildite [AgBiS₂] embedded in the galena.

Granite Gap is relatively unknown to mineral collectors with the exception of a small cadre of micromineral enthusiasts. This is probably due to the remoteness of the area plus the paucity of hand-sized specimens. However, it is now revealed to have a significant suite of attractive secondary arsenate minerals plus a number of additional species of interest to mineral collectors. Arsenate species found during this study, in order of abundance, are mimetite ([Pb₅(AsO₄)₃Cl], conichalcite [CaCu(AsO₄)(OH)], beudantite [PbFe₃³⁺(AsO₄)(SO₄)(OH)₆], agardite-(Y)[Y,Ca)Cu₆(AsO₄)₃(OH)₆ • 3H₂O] (Williams, 1978, identified the related species mixite), adamite (cuprian) [(Zn,Cu)₂(AsO₄)(OH)], olivenite [Cu₂(AsO₄)(OH)], and carminite (PbFe₂³⁺(AsO₄)₂(OH)₂]. Species identification was accomplished by physical characteristics and electron microprobe analysis. Mimetite color and habit are quite variable. Crystals may be bright canary yellow or completely white, and the habit varies from 1 mm spheres to hexagonal prisms that may terminate with the pinacoid or branch into "wheat sheaves." Conichalcite is typically grass green and may be individual spheres or mats and crusts of intergrown sheaves. Coverage of some specimens exceeds several square centimeters. Beudantite is found in very small (less than 0.1 mm) crystals that are yellow green to brown and sub-translucent. Agardite-(Y) occurs in greenish-blue, acicular sprays with individual crystals to 0.5 mm. Some crystals are extremely fine whereas others are sufficiently coarse to recognize the hexagonal symmetry. It is found with conichalcite and adamite but is difficult to distinguish from the ubiquitous aurichalcite [(Zn,Cu)₅(CO₃)₂(OH)₆]. Adamite was determined to be cuprian by electron microprobe analysis. Individual, bluish-green, equant crystals about 0.2 mm across are translucent with white patches. Olivenite was determined to be zincian, and the crystals are 0.2 to 0.3 mm and equant. They are lustrous, transparent, and sea-blue green. Carminite occurs as bright, carmine-red sprays less than 0.2 mm across and as drusy crusts with very finely crystalline "blebs" of yellow-green beudantite. A copper arsenate, as yet to be identified, was found as 0.1 mm purple-red octahedra and remains under study.

Bromargyrite [AgBr] is usually found as individual, 0.5 mm, olive-green crystals perched on crusts of conichalcite. Aurichalcite and rosasite [(Cu,Zn)₂(CO₃)(OH)₂] are both found in attractive specimens in a number of areas throughout the mines, always in association with colorless, bladed, hemimorphite [Zn₄Si₂O₇(OH)₂ • H₂O] crystals. Carminite was found in a single specimen on the dump of an adit high on the northwest side of the Granite Gap hill. This adit and dump were the source of the greatest volume and variety of arsenates. Pyrite [FeS₂] oxidized to goethite [FeO(OH)] is common on this dump, and blue botryoidal hemimorphite can be found as well. Plattnerite [PbO₂] associated with white mimetite and brown beudantite is found in this adit in a small area with difficult access. Willemite in colorless, hexagonal prisms and botryoidal, gray smithsonite [Zn(CO₃)] along with two single crystals of azurite [Cu₃(CO₃)₂(OH)₂] were found on the dump of the highest adit on the southwest side of the hill.

As the only known New Mexico location for carminite and one of a very few arsenate locations in New Mexico, Granite Gap has added one more interesting chapter to the story of New Mexico minerals although the story is by no means complete.

References:

1. Gillerman, Elliot, 1958, Geology of the central Peloncillo Mountains, Hidalgo County, New Mexico, and Cochise County, Arizona: New Mexico Bureau of Mines and Mineral Resources, Bulletin 57, 152 p.
2. Lindgren, Waldemar, Graton, Louis C., and Gordon, Charles II, 1910, The ore deposits of New Mexico: U.S. Geological Survey, Professional Paper 68.
3. Williams, S. A., 1978, Mineralization at Granite Gap, Hidalgo County, New Mexico: New Mexico Geological Society, Guidebook 29, pp. 329-330.