Gold mineralization in Gualcamayo Mining District. Central Precordillera, San Juan Province, Argentina

MARÍA CELESTE D´ANNUNZIO

Lima

Congreso; XVI Peruvian Geological Congres and SEG 2012 Conference, Lima, Peru; 2012

SEG, Sociedad Geológica del Perú

The Precordillera of Argentina is a fold-and-thrust belt predominantly composed of Lower Palaeozoic marine to non-marine sedimentary rocks, with minor volcanic and intrusive rocks towards the west, along with later Middle to Upper Carboniferous continental to shallow marine clastic rocks and rare Permian conglomerates and Triassic red beds. Cenozoic continental sedimentary rocks generated in terrestrial environments related to the uplift of the Andes occur in some major and minor roughly N‐S trending basins. The 13 - 5.6 Ma dacitic to tonalitic porphyries intrude the Paleozoic sequence. The Gualcamayo Mining District (68°38´26"W; 29°43´10"S) includes three ore deposits under exploration (QDD Main –formerly called “Quebrada del Diablo”-, Amelia Ines-Magdalena and QDD Lower West). QDD Main is a Carlin-type gold deposit that is currently being mined. Amelia Ines-Magdalena are gold-bearing skarn deposits. QDD Lower West is a blind ore deposit hosted by limestone and calcareous breccias at about 500-600 meters of depth. The host rock of Gualcamayo Mining District is a thick Cambrian/Ordovician carbonate platform sequence highly deformed by the Andean tectonic. This sequence is intruded by dacitic to tonalitic porphyries, leading to a favourable scenario for distal epithermal disseminated deposits and particularly Carlin-type deposits. QDD Main is the first Carlin type deposit recognized in the Lower Paleozoic carbonates of Precordillera as well as the first Carlin type deposit described in Argentina. Although initially QDD Lower West was considered as the deeper part of QDD Main, preliminary results of this research have shown several differences between both deposits. Breccias that host the mineralization are mostly calcareous. Two types of breccias have been recognized based on clasts / matrix-cement ratio. Type 1 has a matrix-supported texture and type 2 has a clast -supported texture. Then, based on the composition of matrix and clasts, each type was divided into subtypes. Type 1 (Picture 1A) includes: 1a) breccias with quartz clasts cemented by calcite and sulphides; 1b) breccias with gray limestone clasts and a calcareous matrix; 1c) breccias composed of porphyry, quartz and imestone clasts in a calcareous matrix; 1d) breccias with white marble clasts and a alcareous matrix; 1e) breccias with quartz and limestone clasts and a calcareous matrix; 1f) breccias with quartz and chert clasts and a calcareous matrix. Type 2 (Picture 1B) includes: 2a) breccias whit white and gray marble, quartz and sandstone clasts and a calcareous matrix; 2b) breccias with gray limestone and white marble clasts in a quartz-bearing matrix and calcareous- sulphide cement. The ore consists of pyrite, sphalerite and arsenian pyrite in very small crystals with traces of Cu, Ag, Zn,Hg, Bi or As. Au appears as free gold or as inclusion within the sulphides. Sulphides can occur as breccia cement, disseminated within calcareous cement, or in calcite-bearing veinlets.