A petrological study of the tin-tungsten deposit at Renosterkop, Augrabies, Northern Cape Province

Abstract

Renosterkop is a large low grade tin-tungsten-zinc deposit located 85km WSW of Upington in the northern Cape Province, South Africa. The mineralization is hosted by a number of shallow-dipping, sheeted greisen bodies that are surrounded by, and partly intercalated with a well foliated granite gneiss country rock. The gneiss is taken to belong to the intrusive Riemvasmaak gneiss of the Namaqualand Metamorphic Complex. The mineralized host (referred to as TBQ) is a grey, homogeneous, fine to medium grained rock composed predominantly of quartz, biotite and topaz with minor amounts of fluorite and accessory opaque minerals, zircon and secondary chlorite. The unmineralized granite gneiss country rock is medium-to coarse-grained, pinkish in colour and composed primarily of microcline, plagioclase, quartz and biotite, with or without hornblende. Rock types, transitional in mineralogy but with clearly distinguishable contacts, are present between the TBQ and the granite gneiss. A prominent chemical and mineralogical halo, 20m to 50m wide, envelopes the Renosterkop deposit. There is a gradational transition from an unaltered hornblende biotite gneiss, through gneiss containing greenish-brown biotite to an approximately 2 m wide transition zone, characterized by the partial replacement of the greenish-brown biotite by chlorite. The transition zone in turn yields to the TBQ in which reddish-brown biotite forms at the expense of the chlorite, and topaz, quartz and fluorite are formed at the expense of the feldspar. Major and trace element analyses show a spectrum of chemical compositions with coherent trends that support a gradational transition from the hornblende-bearing granite gneiss, through the transitional rock types to the TBQ. The mineralogical and chemical characteristics of the Renosterkop rock types are consistent with an origin by progressive greisenization of a "within plate" A- type granitoid host rock. A genetic model is proposed which involves the formation of the TBQ greisen during intense metasomatic alteration and replacement of the granite gneiss within a zone of structural weakness that provided conduits for migrating, F-rich, metal-bearing solutions, and thereby inherited the foliation and structural features present in the original granite gneiss.