New Mexico Mineral Symposium — Abstracts

Turquoise, Cu⁺²Al₆(PO₄)₄(OH)₈.4H₂O, occurs in two distinctly different geological environments in the Orogrande district, Otero County, New Mexico. The two deposits are hosted in different rock types. Associated vein mineral and gangue assemblages are also variable between the deposits. Turquoise also has distinctly different physical character at each deposit. The individual features at each deposit suggest a different model origin for the turquoise.

Providence (a.k.a. DeMueles) mine area—The Providence mine is near the terminus of an old railroad grade near the Cinco de Mayo and Iron Duke iron skarn deposits. The turquoise is hosted by altered quartz monzonite. The quartz monzonite represents the main-stage plutonism in the district. Alteration assemblages typical of porphyry copper systems are present. The dominant hydrothermal alteration stage is quartz-sericite with a superimposed weathering assemblage of kaolinite and limonite (jarosite). The turquoise occurs as vein fillings and nodules in veins. Most of the material on the surface of the dumps is faded. Freshly broken fractures containing turquoise and unfaded material on the dump are deep sky blue. The material tends to be thin but hard. Associated minerals are limited and consist mainly of kaolinite and mixed iron oxides (limonite) with minor gypsum. On the margins of mineralization the turquoise grades into jarosite veins. This feature is similar to the reported turquoise occurrences at Hatchita, New Mexico.

Iron Mask mine area—The Iron Mask mine approximately halfway between the Lucky and the Cinco de Mayo mines. Most of the mining in this area was for iron, hosted in skarn and as large magnetite blocks cemented by caliche. The turquoise deposits are immediately southwest of the Iron Mask workings. Turquoise occurs as vein fillings and nodules in a 7-meter-thick shale unit in the Laborcita Formation (upper Pennsylvanian to lower Permian). The most unusual feature of this deposit is the association of abundant gypsum and halite with the turquoise. Most of the turquoise is chalky and light colored. Occasional hard nuggets or veins are encountered but it is uncommon. The material tends to be lighter blue than that from the Providence mine. No turquoise was observed in rocks outside the shale formation. Jarosite and copiapite are found on the margins of turquoise mineralization as replacement masses.

Three models for the origin of turquoise have been presented by previous investigators. These models include (1) magmatic-related processes, e.g. hydrothermal alteration; (2) contact metasomatic processes; and (3) weathering-related (supergene) processes. The geologic relationships and mineral assemblages suggest a weathering mode of formation for the turquoise at both deposits at Orogrande.

At the Providence mine, weathering of disseminated pyrite and chalcopyrite in the monzonite porphyry led to the formation of acid waters and provided a source of copper. Percolating acid waters altered feldspar to kaolin and dissolved some aluminum. The same acid waters weathered disseminated apatite as a source for phosphorous. The solutions were concentrated along fractures where the turquoise precipitated. "Spent" solutions (lacking copper and phosphorus) precipitated jarosite and gypsum-goethite on the margins of the turquoise-mineralized area.

A different weathering model is required for the turquoise deposits at the Iron Mask locality. Pyrite replacement deposits occur above and in the upper portions of the host shale. The oxidation of these pyrite deposits (which show copper staining) led to the formation of acid sulfate waters. The acid sulfate waters percolated along fractures and through the shale and reacted with collophane-rich (probably carbonate-hydroxylapatite, Ca₅(PO₄,CO₃)₃(OH), because fluorite was not observed) zones to form the turquoise. The abundance of gypsum is an artifact of the acid sulfate alteraton. The presence of jarosite and copiapite on the margins of the shale are also reaction products of the pyrite oxidation.