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.
