Gualcamayo mining District, Argentina: An example of Carlin-like Au deposits

Abstract

In the mid-1990′s Minas Argentina S.A. (MASA), subsidiary of Viceroy Exploration, began a reevaulation of the Gualcamayo district that included a drilling program. Low-grade gold was intercepted in breccias and in the San Juan Formation limestones. MASA estimated an inferred resource of 37.7 million tonnes @1.16 g/t gold in 2000, and Mineral Resources Development, Inc (MRDI) estimated resources were 37.2 million tonnes grading 1.13 g/t gold. The mineralized zones were superimposed on skarn, marble, breccias, and veins that appear related to a dacitic intrusive complex. Viceroy was subsequently purchased by Yamana Gold in 2003 who continued to explore the project area. Their 2006 National Instrument (NI) 43–101 report for the QDD portion of the Gualcamayo system estimates a measured and indicated resource of 74 million tones (Mt) averaging 0.88 g/t Au (cut-off of 0.3 g/t Au) with an additional 13.9 Mt grading 1.17 g/t Au inferred, yielding 2.6 million ounces of Au. Annual production of gold by Yamana Gold has ranged from 180,412 (2014) to 180,674 (2015) to 154,052 (2017) ounces of gold.This study was initiated to determine the origin of the gold deposits. Field and laboratory studies were conducted by the senior author as part of a doctoral research program comparing ores from Gualcamayo with those of the Great Basin, Nevada, including polished and thin section studies of ore and gangue minerals, followed by fluid inclusion, SEM, stable isotopic, and 40Ar/39Ar analyses to support the interpretations on ore genesis. The area under study corresponds to the Quebrada Del Diablo (QDD) target, which is located in the eastern part of the property.The disseminated gold mineralization in Gualcamayo is localized mainly within collapse breccias with minor values in decarbonatized limestone; all ore intervals are closely associated with elevated arsenic intervals. The San Juan Formation, principal ore host, with 50 m of dolomitic öoid packstone represents the stratigraphic unit that reacted with hydrothermal fluids. Intensely decarbonatized zones have collapsed, forming extensive zones of breccia. Hydrothermal fluid percolated through E-, W- and ENE-trending fault zones and along some bedding planes. The gold grades are maximized where those faults cut an earlier-formed NNW-trending antiform.Alteration of wallrocks in Gualcamayo (decarbonatization, carbon enrichment, local silicification and argillization) associated with disseminated gold includes many of the assemblages associated with Carlin-type (CTD) and distal-disseminated gold (DDD) deposits (Theodore et al., 2002). Additionally, the associated realgar, orpiment, cinnabar, pyrite, arsenian pyrite, marcasite, and minor base metal sulfides are common to CTD and DDD deposits.The Gualcamayo Igneous Complex evolved from an early tholeiitic to calc-alkaline series of magmas. The first stage of igneous activity occurred near 13 ± 2 Ma, and is represented by the Filon Capa Las Vacas dacite. At 8.99 ± 0.01 Ma the Varela Dacite Porphyry was emplaced. The last episode of magmatism corresponds to intrusion of El Rodado Dacite Porphyry at 5.6 ± 0.2 Ma. Field and petrographic relationships observed, together with the δ34S 0 permil values from disseminated sulfide minerals and skarn, suggest that gold mineralization stages were related to the emplacement of the Varela Dacite Porphyry. The inferred age for the disseminated-gold mineralization is <8.99 ± 0.01 Ma.The stable isotope values obtained for the host San Juan Formation limestone (δ18O values: 21.7 to 24‰ and δ13C values: −1.9 to 1.7‰), dolostone (δ18O values: 20.7–22.2‰ and δ13C values: −0.1 to 1.5‰), marbles and breccias at Gualcamayo are interpreted to have been developed as a result of exchange with hydrothermal fluids. The more altered and mineralized rocks exhibit a greater decrease in δ18O values. Some samples show an increase in their δ13C due to the presence of organic carbon in the system.Many characteristics common to Carlin-type (CTD) and Distal-disseminated (DDD) deposits are found in the Gualcamayo disseminated gold deposits. The similarities include the carbonate host rock, the disseminated minerals and textures, sulfide and gangue paragenesis, pyrite with arsenian pyrite overgrowths and associated gold, the alteration types, and comparable isotopic ratios. These similarities allow classifying the Gualcamayo deposits as Carlin-like deposits. Based on the relationship between the mineralization and the igneous activity in the area, the deposit could be interpreted as an intrusion-centered mineral deposit (Sillitoe and Bonham, 1990), characterized by a porphyry complex in the center, which grades outward through Fe-(Cu) skarn with minor Mo, Au, Ag, Bi and W to Pb-Zn-Ag vein, and to the disseminated carbonate- and breccia-hosted Au deposits similar to the McCoy-Cove system along the Battle Mountain-Eureka trend of Nevada.