New Mexico Geological Society Annual Spring Meeting — Abstracts

Turquoise, Cu²⁺AI₆(PO₄)₄(OH)₈, occurs in a shale unit of the Gobbler Fm. at the Iron Mask Mine in the Orogrande District, Otero County, New Mexico. The deposit lies on the margin of porphyry copper and skarn deposits. Veins of turquoise and gypsum, CaSO₄•2H₂O, up to 5 cm thick, occur throughout the mine. The largest veins are parallel to bedding. Alunite, KAl₃(SO₄)₂(OH)₆, and jarosite, KFe₃³⁺(SO₄)₂(OH)₆, also occur with turquoise typical of acid sulfate precipitation.

Pyrite, FeS₂, occurs as nodules in the shale and, with chalcopyrite, CuFeS₂, as replacement layers in rocks associated with the adjacent skarn. These sulfides oxidized to form acid sulfate solutions and mobilize copper. The presence of other secondary copper minerals (atacamite, chrysocolla, and malachite) with turquoise supports the inference of Cu²⁺ mobilization and introduction into the host shales. The ubiquitous presence of gypsum indicates that acid solutions were abundant. These solutions leached phosphate, and possibly aluminum from the shale. Two sampling patterns over an area of 14 cm by 140 cm indicate that the greatest amount of leaching occurred near veins. Shale altered by acid sulfate solutions contains as little as 0.04% ± 0.01% P₂O₅. Whereas unaltered shale contains up to 0.40% ± 0.01% P₂O₅.

The host-rock shale has alternating light and dark bands. The dark bands contain 0.40% ± 0.01 % P₂O₅, twice as much phosphate as lighter bands which only contain 0.20% ± 0.01 % P₂O₅,. The dark bands are rich in a species of apatite, Ca₅(PO₄,CO₃)₃(CI,F,OH), and xenotime, YPO₄. Dissolution textures exhibited by these minerals in altered samples suggest they are the source of phosphate for turquoise genesis. On one sample of the shale, turquoise precipitated only on the dark bands reflecting the source of phosphate that is abundant in the shale. The geologic setting and mineral assemblages at the Iron Mask Mine suggest a supergene origin of turquoise at this deposit.