Petrographic and geochemical evolution of pervasively altered Bushveld granites at the Zaaiplaats tin mine

Abstract

Tin with associated tungsten and rare earth element mineralization at the Zaaiplaats mine is hosted by alkali feldspar granites of the Lebowa Granite Suite which intrude granophyric granites of the Rashoop Granophyre Suite. From the upper contact downward the alkali feldspar granites include a marginal pegmatite (stockscheider), the fine-grained Lease Granite, and the coarse-grained Bobbejaankop Granite. The major mineralization styles consist of subhorizontal, tabular zones (low-grade orebodies) containing disseminated cassiterite in the Lease Granite, a zone of disseminated cassiterite mineralization (ZDM) in the Bobbejaankop Granite approximately 50 m below the Lease Granite, and a series of shallowly plunging and branching pipe orebodies which occur in both granite types and which show a close spatial relationship to zones of disseminated mineralization.The granites contain miarolitic cavities generally less than 5 mm in diameter which range from more than 10 vol percent in the upper facies of the Lease Granite to less than 1 vol percent in the deeper part of the Bobbejaankop Granite. Miarolitic cavities are filled or partially filled by hydrothermal minerals including quartz, albite, chlorite, sericite, fluorite, hematite, and calcite. The zones of disseminated mineralization contain high abundances of miarolitic cavities and appear to reflect zones of hydrothermal fluid accumulation during crystallization. In addition to the small cavities within the disseminated cassiterite mineralization zone, the Bobbejaankop Granite contains a series of large cavities (to approx 1 m) and patches of strongly miarolitic granite. Additional infill phases in mineralized zones include cassiterite, scheelite, synchisite, arsenopyrite, chalcopyrite, galena, and other sulfide phases. Pervasive alteration of the granites is expressed mainly by the replacement of primary biotite by chlorite, sericite, and hematite, by ordering of alkali feldspar (maximum microcline, low albite), and by substantial replacement of alkali feldspar by chlorite, sericite, hematite, and other hydrothermal phases.In the Bobbejaankop Granite below the disseminated cassiterite mineralization zone amounts of Al 2 O 3 , K 2 O, Na 2 O, Ba, Rb, Nb, Ta, and U increase with increasing elevation whereas SiO 2 and Ti decrease. This is compatible with upward crystallization in an evolving F-rich magma, but it may partly reflect upward transport of alkalies and incompatible elements via a fluid phase. The ZDM is a zone of enrichment in SiO 2 , Ba, Cu, Pb, Zn, and Sn, whereas it is depleted in Al 2 O 3 , K 2 O, Na 2 O, Rb, Ta, and U compared to the underlying granite.The increasing importance of sericite as an infilling and replacement mineral as the Alpha pipe is approached suggests that part of the alteration and cavity filling in the Lease Granite occurred at the same time as alteration and mineral precipitation in the Alpha pipe. This increasing importance of sericitic alteration is reflected in the geochemical data by increasing CO 2 , Rb, and Cs contents and decreasing Na 2 O, F, and Ba contents with proximity to the pipe.The distribution of disseminated and pipe-style mineralization at Zaaiplaats reflects a combination of processes including (1) early separation of an H 2 O-NaCl-CO 2 fluid enriched in boron to produce pipelike conduits by collection and upward flow beneath crystallization fronts, supplemented by later dissolution of granite, (2) accumulation of fluorine-rich fluids evolved at a more advanced stage of crystallization beneath downward-advancing crystallization fronts to produce strongly miarolitic zones of the Lease Granite containing disseminated cassiterite (low-grade orebodies), (3) accumulation of fluorine-rich hydrothermal fluids, possibly between upward- and downward-advancing crystallization fronts to produce the disseminated cassiterite mineralization zone, and (4) pervasive alteration of the granites accompanied by filling of miarolitic cavities and pipes. The lack of significant hydraulic or tectonic fracturing during crystallization of the granites at Zaaiplaats was probably the most crucial factor in the development of these unusual styles of mineralization.