Constraint on the age and genesis of the Sierra de Valle Fértil pegmatites (Western Sierras Pampeanas, Argentina)

GALINDO, C.; PANKHURST, R.J; CASQUET, C.; BALDO, E.; RAPELA, C.W.; SAAVEDRA, J.

Buenos Aires

Simposio; Simposio Internacional “Proto-Andean Margin of Gondwana”, XIII Congreso Geológico Argentino; 1996

Asociación Geológica Argentina

Neodymium and strontium isotope analyses have been used to constrain the age and source of the ore-forming fluids of fluorite lodes from Cabalango (Achala batholith - Sierra Grande) and Alpa Corral (Cerro Aspero batholith - Sierra de Comechingones) near Córdoba in the Sierras Pampeanas, western Argentina. Fluorite occurs as veins hosted by late Famatinian granites belonging to the Achala and Cerro Áspero batholiths, close to the contact with the metamorphic basement. By applying the Mind method to the fluorites, an isochron age of 131±22 Ma (MSWD 1.4) has been found at the La Nueva mine, Cabalango (64°36’ W, 3 1°27’ S), whilst those from the Bubu mine, Alpa Corral (64°5Y W, 32°34’ S) have provided an age of 1 17±26 Ma (MSWD = 2.8). Thus these veins must be related to the Lower Cretaceous late-Gondwanian extensional event that affected 1he Sierras Pampeanas basement of Argentina, with which alkali basalt magmatism is also associated. Initial 87Sr/86Sr ratios of the fluorites from La Nueva fail within the range 0.7223 to O.7268. The corresponding neodymium composition is εNd(131) = -7.4 to -8.3, which is similar to that of the Bubu fluorites: εNd(117)= -7.4 to -8.3. These values are more negative than those of the Achala granites in mid-Cretaceous times (-6.5 to -6.9), implying that the fluids had a Nd component derived from the older metamorphic envelope (εNd(131) = -11). However, the Cretaceous 87Sr/86Sr values of the fluorites at Mina La Nueva are lower than those of both the Achala granites and the metamorphic rocks, which suggests that a relatively unradiogenic Sr source was also involved. We suggest that the hydrothermal fluids responsible for the formation of  fluorite veins were derived by mixing of two fluids, an ascending one with low 6Nd and high R7SÍII6Sr values, that probably flowed through the basement metamorphics prior to penetrating the Achala batholith, and a “descending” one with low 87Sr/86Sr values, of probable meteoric origin. These fluorite veins are of the unconformity-related type, formed at shallow depth and low to moderate temperatures. Fluid convection was triggered by a regional thermal anomaly, probably related to the extensional tectonics.