Geology of the Silsilah ring complex, and associated tin mineralization, Kingdom of Saudi Arabia-a synopsis

A significant tin deposit has been identified in the northeast part of the Late Proterozoic Arabian Shield. Cassiterite-topaz-quartz greisens are distributed over 16 km2 in flat-topped cupolas of a highly evolved, zinnwaldite-bearing alkali-feldspar granite which composes part of the silsilah ring complex (lat 26"06'N, long 4240'E). The 587 m.y. old alkali-feldspar granite is overlain by a carapace of aplite and pegmatite. The carapace acted as an impermeable boundary, beneath which fluids accumulated and caused greisenization of the alkalifeldspar granite and deposition of disseminated cassiterite. Subsequently emplaced quaftzwolframite veins cut the alkali-feldspar granite and the aplite-pegmatite carapace. The rest of the Silsilah ring complex is composed of peralkaline granite, its hypabyssal equivalent, and alkaline dacite. The occurrence of alkaline dacite, peralkaline granite, and alkali-feldspar granite (oldest to youngest) in a single ring fracture suggests that these rocks form a single differentiation series. Major, trace, and rare-earth element data support this hypothesis. The alkaline dacite evolved to the peralkaline granite by fractionation of sodic plagioclase and Fe-Ti oxides. The peralkaline granite continued differentiation by fractionation of anorthoclase, Na-pyriboles, and zircon. This process yielded the peraluminous composition, incompatible trace element enrichment, and flat REE patterns with large negative europium anomalies that are characteristic of the alkali-feldspar granite. This unusual differentiation trend may be typical of other, highly evolved intrusive suites and associated, yet undiscovered, tin-tungsten deposits in the Arabian Shield.