Los skarns San Marcos Sierra y La Falda: Geología, mineralogía y composición de los fluidos hidrotermales. Comparación con otros skarns de las Sierras Pampeanas Orientales de Córdoba

MARTA FRANCHINI; RAÚL LIRA; JORGE SFRAGULLA

Revista de la Asociación Geológica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 1999 vol. 54 p. 420 - 435

0004-4822

The San Marcos Sierra and La Falda skarns occur north of the Achala granitic intrusive complex, the former west of the Sierra de Cuniputo and the latter west of the Sierra Chica range. They are developed in Paleozoic igneous rocks (mostly pegmatites) and in late Precambrian-Cambrian polymetamorphic basement rocks. The San Marcos Sierra skarn consists of massive wollastonite-rich exoskarn with garnet, vesuvianite spots, and pyroxene rich bands that replace impure marbles in the contact with granitic pegmatites and in the proximity to trondhjemite-granodiorite plutons. The La Falda skarn consists mainly of garnet-rich massive exoskarn with a clinozoisite-vesuvianite band, hosted by gneisses in the contact with granitic pegmatites and nearby a tonalite stock. In both skarns pyroxene is Mg rich (Hd22.3-32.3 Jo0.4-3); garnet is iron poor (Ad 7-25 Py0.3-1.7), contains 0.09 to 0.38 wt. % fluorine, and nF/ (n F+ nOH) is lower than 0.55. Vesuvianite is iron poor and contains between 0.99 and 1.47 wt. % fluorine. Fluid inclusion analyses of garnet, pyroxene, quartz, clinozoisite and vesuvianite suggest that most skarns formed between 210° and 400° C (Th; not pressure corrected); salinity in silicates from La Falda skarn ranges from 2.3 to 6.7 wt. % eq. NaCl. Overall, the two skarns described in this paper are similar to others found in the contact zones of the Achala batholith, such as Copina and Cañada del Puerto skarns, except for some differences in the zonation pattern, the abundance of quartz, fluorine and the presence of late minerals enriched in Fe and Mn that characterize the latter. These differences might be related to the composition of the metamorphic protholiths and the composition of hydrothermal fluids that reacted with them. Assessment of isobaric phase relations in the system CaO-Al2O3-SiO2-H2O-HF, allowed the estimation of fluid characteristics in equilibrium with fluorian grandite - bearing assemblages. Phase relations in Cañada del Puerto and Copina skarns show that log aH+aF? in the coexisting fluid phase was between -10. 32 and -10.16, at 350 °C and 1 kbar. For San Marcos Sierra and La Falda skarns, values of log aH+ aF? between -10. 5 and -10.7, at 1 kbar and 200 °C would indicate lower u HF in the coexisting fluid phase than fluids in skarns proximal to the Achala batholith.