New Mexico Mineral Symposium — Abstracts

Azurite and malachite are common in oxidized skarn deposits in Paleozoic carbonate rocks adjacent to Laramide porphyry intrusives in the Morenci district. Recently, some excellent specimen-and polishing-quality azurite and malachite have been collected from fault breccias cutting igneous rocks in the Northwest Extension area of the Morenci district. Malachite with minor azurite also commonly occurs as thin fracture coatings in the oxidized ores in the district, but specimen-quality material is restricted to the skarn and Northwest Extension occurrences.

The Paleozoic section at Morenci includes two carbonate units, the Ordovician Longfellow Limestone and the Mississippian Modoc Formation, both of which have some dolomitic beds. Calcsilicate skarn has been developed in both units on the southeast side of the Morenci mine where they have been intruded by monzonite porphyry dikes and sills and in the Metcalf mine where intruded by granite porphyry. The skarns contain diopside, epidote, garnet, and tremolite with some localized actinolite, chlorite, magnetite, idocrase, specular hematite, and talc. Original sulfide minerals occurred as late-stage pyrite-chalcopyrite ± bornite veins, which subsequently reacted with oxygenat¬ed water forming acidic solutions that were almost immediately neutralized by remaining calcite, and perhaps dolomite, resulting in the precipitation of azurite and malachite. Other minerals formed from the supergene oxidation of the skarn deposits include chrysocolla, tenorite, and occasionally native copper and cuprite. Azurite and malachite commonly occur as alternating layers, usually with chrysocolla, as coatings on bedding planes and fractures. Occasionally, azurite crystals to approximately 2 mm occur as complete drusy coatings of open spaces. Azurite and malachite stalactites are a minor occurrence. Well-formed, deep-blue, blocky to platy azurite crystals to 2 cm have been found in a vuggy hematite-goethite matrix along a fault cutting Longfellow Limestone and a monzonite porphyry dike in the southeastern portion of the Morenci mine. Pseudomorphs of malachite after azurite are common from this zone. Cuprite, native copper, and native silver also occur in the fault, with cuprite crystals to 1 cm commonly coated by a pale-green silky aggregate of malachite and sericite.

Azurite and malachite are also common in faults cutting Precambrian granite and Laramide granite porphyry in the Northwest Extension deposit in the Morenci district. The Northwest Extension deposit is an oxide-copper deposit with chrysocolla as the major copper mineral with lesser brochantite, azurite, and malachite. Azurite cements fault breccias in some of the larger faults, and small, blocky azurite crystals have been found lining vugs in the breccias and "floating" in hematite-stained clay gouge. Breccia clasts to several inches are occasionally coated by several generations of thin bands of fine-grained malachite, followed by the deposition of more coarse grained azurite. Azurite also occurs as rosettes on chatoyant malachite. Some very dark green pseudomorphs of malachite after azurite have also been collected from the Northwest Extension deposit.