A novel mineralization style associated with evaporite diapirism through ophiolite assemblages: Muscat, Sultanate of Oman

摘要

The geochemical attributes of Tertiary/Maastrichtian ophiolitic conglomerates, altered during the ascent and emplacement of evaporite diapirs, were studied in Muscat, Oman. The breccia-conglomerates form the ca 200 m thick basal unit to a Tertiary carbonate sequence and occupy a ca 300 m wide, wedge-shaped outcrop, defined by two intersecting faults. To the west the conglomerates are juxtaposed against footwall Triassic dolomites (Mahil Formation, Hajar Supergroup) by a high-angle, northeast-southwest-trending fault (Aulitic). To the south both hanging-wall and footwall rocks are truncated against a crosscutting east-west fault (Tethyan), which juxtaposes the two sets of strata against sheared peridotites.The breccia-conglomerates host decametre-size open folds that show symmetrical vergence and which are cored by a lime-green coloured evaporite assemblage. This consists of essential halite (<35 percent mode) and gypsum with dolomite, with accessory nickel-rich nontronite, and trace potassium and magnesium salts. The evaporite is host to numerous inclusions (<10 cm in diameter) of mesh-textured serpentinite. To varying extents (though generally limited) the inclusions are replaced by anastomosing networks of microscopic iron(III) veins and euhedral dolomite crystals. Three lithologies occur in the basal sequence overlying the evaporite diapirs. The rock types are arranged systematically with respect to fold structure. Intercalated bands (<1 m long) of chalcedony (with accessory to trace ankerite and hematite/ magnetite) and a dolomite-goethite-rich unit constitute and define fold limbs. The apical sections of the folds are defined by a strongly magnetic, magnetite-rich unit (>45 percent Fe_2O_3, with essential SiO_2). Micro-fractured chromite and intensely replaced serpentinite mesh textures are ubiquitous to the three lithologies, suggesting a common, serpentinite-rich protolith.