Mineralization in the Blanca Creek, La Huerta range, San Juan Province, Argentina

MOGESSIE, ABERRA; SCHNEIDER, ISABELLA; GALLIEN, FLORIAN; CASTRO DE MACHUCA, BRÍGIDA; MEISSL, ESTELA; BJERG, ERNESTO; DELPINO, SERGIO; PREVILEY, LORENA; PONTORIERO, SANDRA; KOSTADINOFF, JOSÉ

Innsbrück

Conferencia; PANGEO Austria 2006; 2006

University of Innsbrück

The La Huerta range is part of the Western Sierras Pampeanas and consists of a crystalline basement and neopaleozoic  extrusives and breccias. Old papers classified the ore deposits  in three groups: 1) sulphide replacements along shear zones in non-calcareous metamorphic rocks, including auriferous pyrite deposits (Caledonia Mine in Blanca Creek) and galena-sphalerite deposits with carbonate gangue; 2) lenticular deposits of auriferous quartz (e.g. El Morado); and 3) fissure veins with argentiferous galena (e.g. Santo Domingo). The most important mine is the Caledonia mine situated 6 kms north of Marayes. It is a pyritic gold deposit, apparently formed at considerable depth and at medium to high temperatures. The greater part of the commercial-grade ore may have been formed by supergene enrichment in the zone of cementation, the gossan outcrops being poor in gold, and the primary rather lower grade. Field observations indicate that the mineralized zones are mostly associated with the occurrence of metacarbonate outcrops within the high grade basement. In some areas like the Caledonia mine massive rhyolites and rhyolite breccias may have played a role in the mineralization. Preliminary reflected light microscopic investigation and electron microprobe analyses of representative mineralized samples from the Caledonia mine show the presence of several ore minerals. Among them are pyrite (Fe0.33S0.67), bornite (Fe0.11Cu0.48S0.41), chalcopyrite (Cu0.25Fe0.25S0.50), galena (Pb0.40Cu 0.01Cd0.04As0.02S0.53), arsenopyrite (Fe0.35As0.29S0.36), tetrahedrite (Cu0.34Zn0.07Bi0.03Sb0.02As0.08S0.46), sphalerite (Zn0.49 Fe0.07 S0.50), greenockite (Cd0.47Ag0.06Zn0.06Cu0.05S0.50) and hematite. Greenockite is reported for the first time from this mine. The composition of the sphalerite is variable with Zn contents ranging from 48 to 64 wt.%. Where the Zn-content is low,  Fe, Cu and Sb are found to be high. On a thin section level the mineralization is generally zoned in three layers 1) rich in pyrite + arsenopyrite + galena + sphalerite¸ 2) mainly hematite rich with some galena and pyrite and 3) a mixture of 1) and 2.