The role of aqueous silica concentration in controlling the mineralogy during high temperature contact metamorphism: A case study from Fuka contact aureole, Okayama, Japan

Abstract

The contact aureole at Fuka, Okayama, Japan is peculiar for the occurrence of extensive high-temperature skarn resulting from the intrusion of Mesozoic monzodiorite into Paleozoic marine limestone. The occurrence is also notable for the finding of ten new minerals, of which five are calcium-boron-bearing minerals, and scores of other rare minerals. Skarn formation at Fuka can be classified into three major types 1. Grossular-vesuvianite-wollastonite endoskarn, 2. Gehlenite-exoskarns, and 3. Spurrite-exoskarns. Grossular-vesuvianite-wollastonite endoskarn forms a narrow zone (few centimeter width) separating the exoskarn and the igneous intrusion. It is also found, developed independently, along contacts of the younger basic intrusive dykes and limestone in the region. The gehlenite-exoskarns, in most cases, are spatially associated with igneous intrusion and are extensive (decimeter to meter thick). However, exceptions of independent gehlenite dikes are also observed. Retrogression of the gehlenite endoskarns results in the formation of hydrogrossular and/or vesuvianite. Accessory phases include schrolomite and perovskite. The predominantly monomineralic spurrite-exoskarn was formed in the outer zone of the gehlenite-skarn parallel to the contact as well as independent veins, dikes and tongues. The spurrite-exoskarn may extend tens of meters. Spurrite coexists with tilleyite or rankinite, although larnite is absent. Idiomorphic gehlenite and vesuvianite are the most common accessory phases observed. Retrograde hydration of spurrite to foshagite, scawtite and hillebrandite is commonly observed as veins and alteration zones within the spurrite exoskarn.
    Petrogenetic grids were constructed using “THERMOCALC” for the observed mineral assemblage in the spurrite skarn. Mineral-fluid equilibrium in the CaO-SiO₂-CO₂ system was computed, considering the metasomatic input of aqueous silica. Phase diagram analysis in the form of T-X_CO2 grids with varying silica activity indicated that the stability field of spurrite is strongly controlled by the activity of silica in the fluid. Optimum silica concentration in the fluid was between 9.1 × 10⁻⁴ and 4.5 × 10⁻³ mol/liter, above which wollastonite becomes stable, whereas further reduced silica activity will generate larnite. Appropriate temperature condition for the formation of spurrite is between 850°C and 1080°C at an X_CO2 fluid composition of 0.05 to 0.42. At temperature conditions lower than 850°C, the spurrite stability field becomes narrow, with low CO₂ activity. The formation of extensive spurrite-exoskarn suggests that the silica activity, temperature and fluid composition remained within the spurrite stability field. Petrogenetic analysis of phase diagrams suggests that the exoskarn formation at Fuka contact aureole was robustly controlled by the activity of silica in the high temperature metasomatic fluid.