Rogelio Daniel Acevedo

Polymetallic VMS deposits of the Andes Fueguinos (southernmost Argentina): Preliminary report

The polymetallic volcanic-hosted massive sulphide deposits of the Andes Fueguios occur in a highly deformed but coherent stratigraphic succession of Late Jurassic metavolcanic and metasedimentary rocks. Local stratigraphy consists of porphyries along with rhyolitic lava, acidic tuff, breccias, conglomerates, radiolarian lutites and basaltic rocks: Mineralization occurs as massive stratiform, massive replacement and sulphide stringer veins. Sulphides are typically massive, fine-grained, layered and locally brecciated andconsists of pyrite and sphalerite, with lesser pyrrhotite, chalcopyrite, galena, tetrahedrite-freibergite, bourmonite, cobaltite and arsenopyrite. Styles of hydrothermal alteration identified in the host rocks include proximal silicification and more distal chloritization, sericitization and, in places, carbonatization. Future research will be focused on identifying the salient physico-chemical controls on the mineralization process and their implications for volcanic-hosted massive sulphide exploration in the district.

Nature and Synthesis of the Results Obtained

The preceding chapters have tried to present the author’s direct observation data, locality by locality, about the set of eruptive, and metamorphic rocks in the Fuegian Andes. This has been done in this way based upon the idea that an integration of the data as well as interpretations and emerging ideas can be, to an acceptable extent, separated from the data themselves.

The Cretaceous Layers Later than the Early Cretaceous

The marine continuity after the Early Cretaceous implies an essential difference with the condition in the orogenic belt with a N-S trend of the Patagonian Cordillera, and it could be said, in a figurative sense, that the continent has kept its feet in the ocean.

The Magmatic Rocks Probably Corresponding to the Andean Batholith and the Associated Metamorphic Contact Phenomena

The knowledge about the presence of outcropping plutonic rocks in the Argentine side of the Fuegian Andes is limited exclusively to the references about the dioritic bodies, in a wider sense, of Mount Jeu-Jepen (immediately to the ESE of Lake Fagnano), Spion Kop (in the Lucas Bridges Range, on Paso Harberton) hills, and Estancia Tunel, to the E of Ushuaia.

The Synthesis About the Formational Matter-Lithostratigraphic Units

The high nomenclatural complexity of the present analysis is shown by the fact that, before the Early Cretaceous, the geological literature records a lot of formational names, fruit of the contributions of many different authors.

The Deformed Complex and the Formational Units

The formational units that appear in the available geological literature, and that are found within the Deformed Complex of the Fuegian Andes, are the “Metamorfita Lapataia” (i.e., Lapataia Metamorphics), and the Alvear, Lemaire, Yahgan, and Beauvoir formations, “Estratos (strata) de Policarpo” and “Estratos de Bahia Tethis.”

The Basic Eruptive Rocks of Alvear and Sorondo Ranges (Mount Olivia)

Another matter of interest is the one concerning the basalts–andesites and diabase–microgabbros of the Alvear and Sorondo ranges. They have been described as vein-layer interbedding in the sedimentary frame of the Yahgan Fm. and were joined later to the Lemaire Fm. as an expression of bimodal volcanism to explain the mineralization model of the volcanogenic massive sulfides.

Astroblemes-Wrong (Structures that Resemble Astroblemes but They Are Not)

A bowl-shaped depression has been dismissed as of impact origin because it is undoubtedly a glacial cirque. Its diameter is 1 km and its depth, 200 m. The geology is composed of rhyolites and rhyodacites of the Lemaire Formation (Jurassic).

Unusual PGE concentration in early disulfides of a low-temperature hydrothermal Cu-Ni-Co-Au deposit at Villamanin (Leon, northern Spain)

The early mineral assemblage of the Providencia mine in northern Spain includes metastable Cu-Ni pyrite-type disulfides with economic concentrations of Au and subeconomic concentrations of PGE in solid solution. High chondrite-normalized relations of Ir, Rh and Pt versus Pd, and the lack of Ru also characterize the ore. Our studies show that the ore-forming fluids were quasi-hydrostatic, low-temperature hydrothermal, and had a high salinity, and a high sulfur fugacity. The source of PGE remains unknown.