Genesis of the La Espingarda kaolin deposit in Patagonia

摘要

The La Espingarda kaolin deposit was formed by "in situ" alteration of sub-alkaline rhyolites belonging to thernJurassic Marifil Formation. Three altered volcanic lithofacies were identified: a porphyric biotitic ignimbritern(RPB), a coarse lithic ignimbrite (ILG), and a fluidal intrusive rhyolite (RFI).rnThe kaolinization covers an ellipsoidal surface area of ～20,000 m~2, with the alteration intensity decreasingrndownwards and disappearing at 8-12 m from the surface. In two mine sectors small stockworks of finernquartz veins appears (<3 m~2). The deepest alteration is related to two fault zones where the three volcanicrnunits are in contact. There is no lateral clay zoning at the faults. The mineralogical composition is kaolinite ±rnhalloysite ± illite + quartz + feldspars + Fe-hydr(oxides). At least three kaolinite generations were identified.rnThe first is pervasive; the second appears as a filling of vugs in the quartz veinlets that crosscut thernpervasively altered rocks; and the third occurs as pure kaolin veins without quartz vein cross cuts. During thernalteration processes almost total alkali cations were leached. The argillized lithofacies showed Ni enrichmentrnand Cu, Sr, and Ba depletions.rnThe main weathering genesis for La Espingarda is supported by the deposit morphology, its location inrntopographic lows, the paleoclimatic record, its simple mineralogical composition; vertical zonation, thernkaolinite veins isotopes (δ~(18)O %? 18.3; δD %, -59.0), and the trace element distribution.rnA steam heated water activity produced some kaolinite overprint according to one isotopic value and the S and Prncontents slightly higher in the kaolinized rocks. Neither Au, Ag, As, Sb, Hg, or Ba epithermal pathfinders' anomaliesrnnor drill data support the existence of any metallic mineralization at the kaolin blanket bottom.rnIn Patagonia hydrothermal kaolinite manifestations are located around and beneath silicified, erosion-resistantrnhills and include some of the following minerals: dickite, alunite, pyrophyllite, or pyrite and have As, S., Ba, and Agrntrace elements within the range of weak geochemical anomalies.