STRUCTURAL CONTROL OF VEIN FORMATION AND MINERALIZATION AT THE EPITHERMAL HUEVOS VERDES AG-AU VEIN SYSTEM, SAN JOSE DISTRICT, DESEADO MASSIF, PATAGONIA, ARGENTINA

The Deseado massif is an exposed block of rift-related, bimodal Jurassic volcanic rocks that is built up by the mid- to upper Jurassic andesitic Bajo Pobre formation and the rhyodacitic to rhyolitic Chon Aike formations. Volcanism was followed closely in time and space by hydrothermal activity and vein formation. The Deseado massif hosts a great number of low sulfidation, epithermal precious metal quartz vein districts such as Cerro Vanguardia, Martha, Manantial Espejo, and San José.

The San Jose district is located in the northwest portion of the Deseado massif. A number of vein sets are hosted by andesitic volcanoclastic rocks and lava flows of the Bajo Pobre formation that is locally overlain by rhyodacitic tuffs and ash flow tuffs of the Chon Aike and La Matilde formations. Cretaceous sedimentary rocks are present in small fault-controlled basins. Vein mineralization and its Jurassic host rock are exposed in isolated windows within Tertiary flood basalts that cover vast parts of the district.

The Huevos Verdes system is a set of low-sulfidation style epithermal Ag-Au-bearing quartz veins, and has a length of some 2000 m, with several ore shoots along strike. Vein formation at Huevos Verdes is related to a NNW-striking sinistral strike-slip fault system with an average orientation of 325°/65° NE. Variations of vein orientation in bends and jogs along, and in between, sub-parallel sinistral faults control vein width and mineralization styles.

The vein system at Huevos Verdes, as well as other NNW-trending veins of the district, is suggested to have developed between E-striking dextral faults and conjugate NNW-striking sinistral faults, under reactivation of pre-existing NNW-striking Triassic rifts that formed during early extension and rifting related to the break up of Gondwana. The district is segmented by NNE-trending lineaments and fault systems such as the Rio Pinturas lineament into individual blocks. Sinistral shearing along these NNE-trending structures may have resulted in overall extension within enclosed blocks that facilitated formation of structural openings along re-activated faults of favorable NNW-strike.