Summary:
Cerro Moro is located within the Deseado Massif, a tectonic block in the central portion of the Santa Cruz Province that covers an area of approximately 60,000 square kilometres. The Deseado Massif is host to several producing and past-producing gold and silver mines, all of the low-sulfidation gold-silver quartz vein deposit type. This deposit type is characterized by quartz veins, stockworks, and breccias that contain gold, silver, electrum, argentite, and pyrite with lesser and variable amounts of sphalerite, chalcopyrite, galena, rare tetrahedrite and sulphosalt minerals that form in high-level (epizonal) to near-surface environments.
At Cerro Moro, gold-silver mineralization is associated with low-sulphidation epithermal veins, stockwork, and breccia units. Phreatic breccias (unit Hbx) are commonly spatially associated with low-sulphidation veins consisting of quartz, some adularia, and variable concentrations of pyrite, acanthite, sphalerite, and other very minor base metal sulphides.
The Cerro Moro property is underlain by Tertiary marine sediments, Quaternary gravels and volcanic rocks of Jurassic age assigned to the Bahía Laura Group by Panza et al. (1994).
The current distribution of rock units is strongly controlled by faulting. Stratified rocks generally dip gently to the south but are displaced along numerous faults. Actual displacement vectors on faults are poorly defined and structural observations of veins and fault surfaces show a complex history, with reactivation of fault surfaces showing different displacement vectors during different periods of deformation and resultant mineralization.
Gold-silver mineralization at Cerro Moro is associated with epithermal veins. Geological mapping and Ar-Ar age dating on vein adularia have defined at least three episodes of veining, spread over 9 million years from 180 to 171 Ma. The different ages of veining tend to have different orientations and structural controls on high-grade shoots. The earlier pulses of veining (Michelle vein at 180 Ma, Esperanza at 175 Ma, and Gabriela at 178 Ma) are characterized by banded crystalline quartz veins with local adularia and low sulphide content. These veins are generally poorly mineralized although they locally contain significant ore shoots. Grades are lower than in the younger pulse of mineralization and ore shoots terminate at shallow depths, suggesting significant erosion of the vein systems has taken place.
A second later pulse (171 Ma) consisting of black silica, is rich in base metal and silver sulphides and hosts high-grade gold mineralization, mainly in the Escondida-Zoe vein system. These high-grade veins and stockworks consist of banded veins with white quartz, fine-grained black silica, and coarse sulphides including pyrite, palecoloured sphalerite, galena, and acanthite as well as local electrum. The black silica is also characterized by anomalously high molybdenum content.
Veining at Cerro Moro is complex and widespread. Veining varies from simple single veins to complex vein systems. Veins are typically steeply dipping to sub-vertical. Outcropping veins locally reach widths up to 4 m, whilst associated zones of quartz stringers and stockwork may reach widths in the order of 10 to 15 m. The strike length of individual veins is variable and ranges generally between 200 metres and 1 kilometre. Alteration has been identified by Terraspec using spectrometry and is typical of the low-sulphidation model, with broad haloes of white mica and less common kaolinite alteration around the mineralized veins.
Structural controls on veining at Cerro Moro vary with the age of the veins. The oldest veins at Cerro Moro trend north to northeast and mineralization is preferentially hosted in northeast-trending segments, especially in areas close to intersections with northwest or east-west structures, suggesting possible reactivation with emplacement of younger mineralization. A second episode of white quartz-adularia veining was emplaced along northwest-trending structures. These veins are widespread in the main mine area and host lower-grade but significant mineralization in the Gabriela and Esperanza-Nini areas. The mineralization in these veins extends to relatively shallow depths below the current surface and probably represent the roots of deeply eroded veins. The third high-grade episode of sulphide-rich mineralization is also hosted along northwest trending faults. The main Escondida fault is a large displacement south side-down fault. Mineralization is localized around east-west trending segments as well as in small east-west splays off the main structure. These observations, along with the stratigraphic displacement observed above, suggest a strong sinistral-normal oblique movement vector that controls this part of the mineralization.