Helvine Mineralization in Devonian-Carboniferous Skarns of the Oriental Pampean Ranges, Córdoba, Argentina: Mineralogy and Nature of Ore-Forming Fluids

Abstract

Helvine [Be3Mn2+4(SiO4)3S] occurs in less than one meter-sized spotty concentrations in some calcic skarns of Córdoba province, Argentina. The local geology, mineral chemistry, paragenetic mineral associations, and the evolution of helvine mineralizing fluids were studied from two selected localities (i.e., the Chingolo scheelite mine and Casa la Plata). Helvine from the Chingolo scheelite mine occurs in idiomorphic crystals up to 15 cm long, partially intergrown with spessartine-rich garnet, and in part or totally included in vug-filling spar calcite in the prograde garnet-vesuvianite skarn zone developed between replaced aplite-pegmatite dikes and calcic phlogopite-bearing marbles. Helvine from Casa la Plata occurs abundantly in vein-like, fluorite-rich garnet-vesuvianite skarn associations, where tetrahedrons up to 2 cm long occur preferentially included in fluorite in an illite-sericite-chlorite strongly replaced schist. The composition of helvine from both localities does not differ from other worldwide known compositions. Associated garnet from both localities is enriched in molar subcalcic garnet, largely as spessartine. Clinochlore is a conspicuous phase that occurs both as late infills and/or replacing phases in both the Chingolo scheelite mine and Casa la Plata. Primary fluid inclusions from helvine of both localities suggest that helvine deposited from moderate to high temperature and salinity aqueous fluids of likely magmatic origin. In both localities a late influx of gas-rich, CO2-bearing, moderate temperature, and moderate to low salinity fluid was trapped as secondary fluid inclusions. At the Chingolo scheelite mine, CO2-bearing fluids likely originated from decarbonation along the skarnification reaction front distanced from the prograde zone at lower temperature. In both studied areas, the latest trapped secondary fluids were of lower temperature and lower salinity. Pressure-corrected homogenization temperatures between 1.8 and 2.0 kbar sustain trapping temperatures for primary fluid inclusions within the 510–610 °C range for both localities. In the Chingolo scheelite mine, helvine formed at the contact zone between Ordovician aplite-pegmatites and Cambrian marbles that differentially reacted with infiltrated distal metasomatic-hydrothermal Be-bearing fluids fractionated from evolved granitic facies and/or pegmatites of the neighboring Devonian to Carboniferous Achala Batholith. Calculated δ18OH2O in equilibrium with garnet at ∼575 °C yielded δ18OH2O = 11.1 or 12.2‰. These heavy δ18OH2O values may derive from a magmatic fluid source possibly enriched in 18O from marbles or any other metasedimentary country rock during skarnification. This interpretation is supported by δ18OH2O values (8.3 to 10.1‰) and heavy δD values (−21.6 to −20.6‰) from retrograde epidote of other neighboring scheelite mines related to the Chingolo scheelite mine. The presence of beryl partially replaced by bertrandite + K-feldspar, non-paragenetically associated with helvine, confirms that early prograde crystallization conditions switched from a high temperature, subaluminous environment to a lower temperature, aluminous and increasingly acidic environment toward the retrograde skarn stage. At Casa la Plata, Cambrian schists and marble-amphibolite were also skarnified after the circulation of Be-bearing fluids derived from an epizonal Carboniferous granite (Capilla del Monte pluton) and its pegmatite dike swarm, which share parental links with the Achalian magmatism. The major Be supply for both studied localities should be attributed to Devonian and Carboniferous postorogenic to extensional peraluminous A-type granitic magmatism, which is the major source of beryllium not only for the Pampean Ranges but for the whole country.