Epithermal precious and base metal mineralisation and related magmatism of the Northern Altiplano, Bolivian
The Bolivian Altiplano is part of the inner arc Polymetallic Belt of the Andes, and is a Cretaceous-Cenozoic intermontane basin located between the Andean arc of the Western Cordillera and the Paleozoic fold belt of the Eastern Cordillera. Reconnaissance geological mapping shows that epithermal mineralisation in the NE Altiplano is related to silicic magmatism located on NW-trending Altiplano growth faults and intersections with NE and E-W lineaments. Magmatism was episodic and occurred during the Miocene arc broadening episode, which correlates with increased plate convergence rates. Most magmatism is mid Miocene (19-10 Ma), and formed flow-dome-sill-stock complexes. The upper (9-7.5 Ma) and late (6.5-4 Ma) Miocene episodes, in contrast, generally formed ash-flow calderas and strato-volcanoes. The three episodes are mainly dacites and rhyolites of the high-K calc-alkaline suite, with some shoshonites, and can only be distinguished isotopically, with progressively stronger crustal contamination in the younger episodes. Sr-Nd-O isotopes and trace elements show that the magmas evolved by variable fractionation and assimilation from subduction-related, mantle-derived magmas which were isotopically enriched by bulk contamination with Precambrian gneisses. Mapping, petrography and XRD show that the epithermal deposits have large areas of pervasive phyllic alteration with a propylitic halo. Tourmaline alteration occurs in the cores of Sn-bearing deposits. Argillic and silicic alteration in some deposits are subsurface features of hot spring systems. Mineralisation (Au-Ag-Cu-Pb-Zn) is disseminated and in sheeted veins and veinlets which have a NE-trend, related to the regional tectonic stress. Dating and O-H isotopes show that the mineralisation is genetically related to the dacitic magmatism and formed from a dominantly magmatic fluid, with meteoric mixing in the upper levels. Differences between the Polymetallic Belt and the Copper Belt are mainly a function of erosion level. Polymetallic deposits of the Eastern Cordillera contain important Sn and form the main part of the Tin Belt. Minor Sn also occurs in Altiplano deposits hosted by Paleozoic marine sediments, but not in those in Tertiary red beds. Tin was probably derived from the Paleozoic sediments, and is not related to deep subduction.