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.
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