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Stefan Marincea, Geological Institute of Romania (Romania)
Delia-Georgeta Dumitras, Geological Institute of Romania (Romania)
Nicoleta Anitai, Geological Institute of Romania (Romania)
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The occurrences of high-temperature, gehlenite-bearing skarns are quite exotic: fewer than forty such occurrences were reported so far in the world. Four of them are known in the Banatitic Magmatic and Metallogenetic Belt from Romania, being associate with monzodioritic or quartz-monzodioritic bodies of Late Cretaceous age. These occurrences are those at Cornet Hill (CH), Magureaua Vatei (MV), Ciclova - Ogasul Tiganilor (OT) and Oravita - Ogasul Crisenilor (OC). A very rich mineral association characterizes the high-temperature skarn areas in Romania: gehlenite, wollastonite 2M, calcic garnet, vesuvianite and locally spurrite and tilleyite are the most representative species. Accessory minerals include hydroxylellestadite, perovskite, monticellite, ettringite, pyrite. Subsequent hydrothermal and weathering overprinting of the primary assemblages resulted in the formation of three secondary parageneses (1) an early hydrothermal one that includes scawtite, xonotlite, hibschite and calcite; (2) a late hydrothermal one that includes 11 Å tobermorite, riversideite, thomsonite, gismondine, aragonite, calcite and (3) a weathering paragenesis that includes plombièrite, portlandite and allophane.
The inner skarn zones have a massive metasomatic texture and are mainly composed by medium to coarse gehlenite (up to 98% from the volume of the rock). The solid solutions toward kermanite vary from Ak 22.90 to Ak 36.28 (mean Ak 29.80) at CH, from Ak 33.64 to Ak 38.13 (mean Ak 36.22) at MV and from Ak 31.56 to Ak 49.59 (mean Ak 39.88) at OC and OT respectively. The garnet in equilibrium with gehlenite covers a large interval of grossular - andradite solid solution and has low Ti content (up to 1 wt.% TiO2). It generally occurs as inclusions in gehlenite and rarely within the gehlenite crystals as atoll-disposed star-shaped crystals. It generally occurs as inclusions in gehlenite and rarely within the gehlenite crystals as atoll-disposed star-shaped crystals. Zoning is rare and analyses vary chiefly between Grs 74.81 and Grs 88.52 at CH, between Grs 67.48 and Grs 87.16 at MV, between Grs 34.22 and Grs 69.67 at OC and between Grs 58.26 and Grs 72.14 at Ci. Subsequent generations of Ti-rich garnet characteristically occur at CH and MV, but are located in the outer skarn zones. These zones are characteristically spurrite- and tylleyite-bearing at CH, wollastonite-bearing at MV and vesuvianite and wollastonite-bearing at OC and OT. In the outer skarn zones, secondary, residual or recrystallized calcite occurs on the fissures affecting the whole mass and is interstitial to or penetrates on an extremely fine scale along the cleavages and crystal boundaries of the wollastonite, tylleyite or spurrite.
The compositional data together with experimental ones account for crystallization of the main parageneses in the endocontact zones, at 750oCelsius, accordingly with dioritic magma estimated temperature.
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