Green grossular in reaction skarns of the San Antonio quarries, Córdoba Ranges, Argentina: Mineral chemistry and genesis

Abstract

This work presents mineral chemistry and paragenetic data on green grossular from marbles of the San Antonio quarry, Valle de Punilla, Córdoba. In the studied samples two mineral assemblages were recognized; one composed of calcite(1)-wollastonite(1)-clinopyroxene±fluorapatite, representative of medium grade regional metamorphism, and the second one, representative of a reaction skarn, composed of garnet (Grt-1 and Grt-2)-wollastonite(2)-calcite(2)-quartz. The metasomatic assemblage overprints the regional metamorphic association as centimeter-sized single mineral bands associated to a few square millimetres sized  irregular patches of scarce recrystallized calcite(2) and wollastonite(2), and residual quartz veinlets. Reaction skarn garnet occurs as green garnet (Grt-1, ~ 98 to 99.5 % molar grossular) in @ 2 cm thick banded polygranular crystalline aggregates, narrowly distant from reddish brown garnet (Grt-2, ~ 91- 97.5% molar grossular). Grt-1 is slightly richer in Al^VI and depleted in TiO₂ (@ 25 %); in analytical profiles, Fe and Ti gradually increase from Grt-1 to Grt-2. The total Fe content as FeO increases in average from 0.03 wt. % in Grt-1 to 1.2 wt. % in Grt-2, being Fe³⁺ practically absent in Grt-1. ICP-MS analyses yielded negligible contents of potential chromophorous cations such as Cr³⁺, Ni²⁺ and V³⁺. The chondrite-normalized rare earth elements (REE) show a normal distribution pattern, slightly enriched in light REE, with a notorious negative Eu anomaly and an increase of Ho, Er and Tm. Compared with representative values of a regional marble, such pattern mimics the marble pattern though enriched in total REE, with the exception of La, which is diminished. The “gooseberry green” color of grossular (@ apple green) is interpreted as the result of the reduced content of Fe²⁺ in the dodecahedral X-site of practically pure grossular end member (> 98 % molar). Contrasting color and chemical compositions between both garnet types, separated each other by no more than 1 cm, likely reflect the compositional variations of marble, after a very low fluid/rock ratio (W<<R) during diffusional metasomatic processes. The contrasting presence of green and brown garnet at less than 1 cm of separation distance is the response to small-scale diffusional mechanisms between a wollastonite-bearing calcitic marble and layers of Fe-Mg-bearing accessories.