Isidoro B. Schalamuk

Jurassic epithermal Au–Ag deposits of Patagonia, Argentina

Abstract Important precious metal deposits have been discovered during the last 10 years in the Deseado Massif region of Patagonia, Argentina. This region is a plateau consisting of Middle to Upper Jurassic volcanic rocks that host fracture-controlled epithermal Au–Ag mineralization. These mineral deposits represent low sulfidation type hydrothermal systems and formed following the main period of volcanism, probably during the Late Jurassic period. The presence of silica sinters and veins interpreted to be feeder structures indicates that the tops of some deposits have been preserved, with erosion of younger sedimentary and volcanic rocks now exposing these systems. These deposits represent diverse erosional levels ranging from sinter formed at the paleosurface, to intermediate Au–Ag-rich quartz veins, to base metal-bearing Au–Ag veins that represent deeper levels of the epithermal systems. The best known deposits are in the Cerro Vanguardia district where vein systems occur over a 350 km 2 area and veins can be traced for more than 14 km. Geological mapping of the district has resulted in the definition of over 140 km of veins. Current reserves are over 3.2 million ounces Au equivalent with grades of 10 g/t Au and 120 g/t Ag. The geologic characteristics of the region and the presence of major Au–Ag deposits provide targets for continuing exploration in the Deseado Massif.

La Marciana Jurassic sinter—implications for exploration for epithermal precious-metal deposits in Deseado Massif, southern Patagonia, Argentina

Abstract A well-preserved Jurassic siliceous sinter has been identified at La Marciana farm in southern Patagonia, Argentina. Fossil sinters are a rare occurrence and this example bridges the gap between Devonian/Carboniferous examples and more abundant Tertiary sinters. Its presence is significant in the context of exploration for epithermal precious-metal deposits in the Deseado Massif geological province of southern Patagonia. Field and textural characteristics of the La Marciana siliceous deposit provide unequivocal evidence of its origin. It is located in an appropriate volcanic and epithermal environment and it displays a strongly developed non-planar depositional lamination, elongate cavities parallel to this lamination, small-scale stromatolitic columnar structures, moulds of stems and roots of plants and desiccation cracks. Geochemical and oxygen isotope studies provide additional information compatible with a sinter origin. The content of precious-metal elements—apart from in anomalous hydrothermal breccia—is low, and the average Au, Ag, As and Sb concentrations are similar to those of most known fossil sinters. The 18O SMOW values obtained on La Marciana sinter range from 7.3 to 16‰ and could indicate a lower temperature of formation when compared with quartz-vein isotope data from the Cerro Vanguardia area—the best-known example of an epithermal precious-metal deposit in the Deseado Massif. Hydrothermal alteration of the volcanic rocks and the presence of quartz veinlets and hydrothermal breccias with Au and Ag anomalies in the La Marciana area are indicative of gold- and silver-rich hydrothermal activity. Taken in conjunction with the presence of the La Marciana sinter, which is further evidence of Jurassic hot-spring deposits in the western border of the Deseado Massif, this suggests that exploration in the area should focus on locating epithermal quartz vein systems that may be preserved at depth.

A new metallogenical association (Sn-Cd-In-Zn-Ag-Au) in the Deseado auroargentiferous province, Deseado Massif, Patagonia, Argentina

A new metallogenical association (Sn-Cd-In-Zn-Ag-Au) is reported for the low sulphidation epithermal Deseado Auroargentiferous Province, in Patagonia, Argentina. The anomalous presence of Sn, Cd and In, together with anomalies of Zn and Ag, and also high contents of Cu, Mn, Pb, W and Bi in a complex sulphur-rich mineralogy, represent a new metallogenical association. This assemblage could be related to a higher temperature mineralization or a different type into the epithermal range. The presence of this new metallogenical association increases the mining potential of the region, extending exploration targets in this metallogenical province and must be considered during future exploration duties in the region.

Indium distribution and correlations in polymetallic veins from Pingüino deposit, Deseado Massif, Patagonia, Argentina

ABSTRACT The Pingüino deposit is characterized by the presence of indium-rich polymetallic vein mineralization representing an atypical epithermal occurrence for the low sulphidation epithermal mineralization from the Deseado Massif, Patagonia, Argentina. Polymetallic veins display high In, Zn, Pb, Ag, Cd, Au, As, Cu, Sn, W and Bi values represented by complex sulphide mineralogy. Mineralization developed in two main stages: a Cu-Au-In-As-Sn-W-Bi stage, and a Zn-Pb-Ag-In-Cd-Sb stage. Correlation coefficients are used to estimate the degree of inter-relation between metals concentrations in each stage and specifically to determinate the behaviour of indium. Indium concentrations show a wide range (3.4–1184 ppm In) and, based on the correlation coefficients of ore geochemistry, in the first stage indium is associated mainly with Sn, present in ferrokesterite and cassiterite, while the highest indium values are related to the late mineralization stage, closely associated with Zn and Cd and present in the Fe-rich sphalerite, the most important In-bearing mineral in the deposit.

Late Cenozoic Mineral Resources of Argentine Patagonia

Publisher Summary The Late Cenozoic mineral resources of Argentine Patagonia represent a group of deposits of varied characteristics, intimately related to the geology of the existing morphostructural units and to the sequence of geological, geomorphological, and paleoclimatic processes that took place during the Late Cenozoic times. The prevailing climatic conditions during the last 10 Ma in this region have played a significant role in the genesis of some of the mineral concentrations discussed in this chapter. The mineral deposits have been subdivided into three main groups according to the geological environment in which they have been formed: (1) detritic, (2) evaporitic, and (3) volcanic. The Quaternary gold-bearing detritic deposits are found both along the Atlantic coast and in the fluvial drainage systems at the Andean foothills. Their importance is related to their probable economic use and as a prospective tool in the search for older primary mineralization areas. The detritic iron-titaniferous deposits in the southern coast of Buenos Aires Province are the sole titanium resources of Argentina. However, their actual economic importance is still unknown. Evaporitic deposits are the most important economic Holocene mineral resources of Argentine Patagonia and show a wide regional distribution.