Geochemical indicators of metalliferous fertility in the Carboniferous San Blas pluton, Sierra de Velasco, Argentina

ROSSI, J.N.; TOSELLI, A.J.; BASEI, M.A.; SIAL, A.N.; BÁEZ, M.

Granite-Related Ore Deposits

The Geological Society of London

Lugar: London; Año: 2011; p. 175 - 186

Abstract: In the Sierra de Velasco, northwestern Argentina, undeformed Lower Carboniferousgranitoids (350–334 Ma) intrude deformed Lower Ordovician granites and have been emplacedby passive mechanisms, typical of tensional environments. The semi-elliptic, about 300 km2shallow-emplaced San Blas pluton is 340–330 Ma old, with 1Ndt between 21.3 and 21.8which indicates that, different from the nearby Famatinian–Ordovician granitoids, the San Blaspluton had a relatively brief crustal residence, with an interaction between asthenospheric materialand greywackes. The cupola of the pluton was almost totally eroded down during the UpperCarboniferous.The San Blas pluton is a porphyritic granite composed mainly of monzogranite to syenograniteand shows graphic intergrowth and miarolitic cavities up to 5 cm in diameter, filled with quartz.Two different textures are recognized: perthitic microcline megacrysts (30–45 vol%) set in amedium- to coarse-grained groundmass of quartz, microcline and oligoclase, with sericitic alteration.Biotite, muscovite, apatite, zircon, fluorite and opaque minerals are the accessory phases. Theother textural variation consists in microcline megacrysts (10%–15 vol%) and a fine-grainedgroundmass, of quartz, microcline and oligoclase, biotite, apatite, muscovite, zircon and magnetite.The average SiO2 content in this pluton is 74.94%, the ASI ¼ 1.1, CaO and MgO are less than1%, total Fe2O3 and P2O5 contents are low, and K2O . Na2O. Low Ba, Sr and high Rb contents,coupled with Sn contents (c. 15 ppm),W(c. 380 ppm), Nb, Y, Ta, Th and U confirm this is a specialgranite. The K/Rb ratio (c. 75) indicates that Rb has been fractionated to the residual melt whereasthe Zr/Hf (c. 25) demonstrates that hydrothermal alteration occurred. The Sr/Eu ratio of c. 75along other geochemical features characterize this pluton as a fertile evolved granite.The chondrite-normalized rare earth element (REE) diagram shows the tetrad effect that allowsthe subdivision of the lanthanides into four groups.In general, the tetrad effect is recognized in evolved granites and products of hydrothermalalteration such as greisens. The above-mentioned features show that the San Blas granite isfertile, and the absence of ore deposits has been probably caused by erosion of a mineralizedcupola during Carboniferous and Cenozoic exhumation. The finding of alluvial cassiterite andwolframite in drainage systems is the first evidence of the fertile character of this granite.