Calcic hydrosilicate metasomatic rocks at the Gumeshevsk skarn-porphyry copper deposit in the central Urals, Russia

Abstract

The body of hydroxylellestadite metasomatic rock penetrated by a borehole drilled at the Gumeshevsk deposit at depths of 530–534 m includes a thin interval of younger lower temperature tobermorite-plombierite metasomatic rock with subordinate amounts of Ca-Si gel, tacherenite, cubic lime, and thaumasite. Hydroxylellestadite has never before been found in calc skarns. The hydroxylellestadite metasomatic rock is cut by gypsum and fukalite veinlets, and the tobermorite-plombierite metasomatic rock is intersected by thaumasite veinlets. The pristine rock of the metasomatics was marble, and the metasomatic rock replaced andradite-bearing wollastonite skarn (with wollastonite replaced by foshagite). The ore minerals (chalcopyrite, valleriite, sphalerite, and others) were formed after the hydroxylellestadite metasomatite but most probably before the tobermorite-plombierite metasomatic rock and the veinlets of calcic minerals. The metasomatic rock was produced at significant variations in the oxygen, sulfur, and carbon dioxide fugacities. The composition of the hydroxylellestadite is, according to its microprobe analysis, as follows (wt %): SiO2 17.10, TiO2 0.01, Al2O3 0.02, FeO 0.20, MnO 0.00, MgO 0.04, CaO 55.40, Na2O 0.14, K2O 0.09, P2O5 0.12, CO2 1.90 (chemical analysis), SO3 21.60, F 0.16, Cl 0.14, total 96.92. The plombierite (SiO2 43.8–44.1 wt %, CaO 30.5–31.1 wt %) in the metasomatic rock notably differs from rare plombierite (SiO2 48.18 wt %, CaO 39.19 wt %) contained in the veinlets of thaumasite (SiO2 12.70 wt %, CaO 30.69 wt %, SO3 17.78 wt %).