Abstract
The stanniferous granites of the Zaaiplaats Tin Field are part of the A-Type Lebowa Granite Suite, within the greater Bushveld Igneous Complex of northeast South Africa. The tin field comprises three granites: (1) the Nebo, a leucocratic, equigranular biotite granite; (2) The brick-red hypidiomorphic Bobbejaankop granite, which is extensively microclinized with chloritized biotite and characteristic synneusis-textured quartz; and (3) The variably altered roof facies of the Bobbejaankop granite known as the Lease microgranite. The Bobbejaankop and Lease granites were both extensively mined for cassiterite until 1989. The cassiterite is hosted in disseminations, miarolitic cavities, and within large hydrothermal, tourmalinized, and greisenized pipes and lenticular ore-bodies. An extensive petrological and whole-rock XRF and ICP-MS geochemical study, has provided new insight into the magmatic and magmatic-hydrothermal mineralization processes in these granites. Trace elements and Rayleigh Fractionation modelling suggest the sequential fractionation of the Nebo granite magma to be the origin of the Bobbejaankop granite. Incompatible elemental ratios, such as Zr/Hf and Nb/Ta, record the influence of internally derived, F-rich, hydrothermal fluid accumulation within the roof of the Bobbejaankop granite. Thus, the Lease granite resulted from alteration of the partially crystallized Bobbejaankop granite, subsequent to fluid saturation, and the accumulation of a magmatic-hydrothermal, volatile-rich fluid in the granite cupola. The ratio of Nb/Ta, proved effective in distinguishing the magmatic and magmatic-hydrothermal transition within the Bobbejaankop granite. Elemental ratios reveal the differences between pre- and post-fluid saturation in the mineralizing regimes within the same pluton. Thus highlighting the effect that the location and degree of hydrothermal alteration have had on the distribution of endogranitic tin mineralization.
Highlights
The Paleoproterozoic Bushveld Complex, in the northeast of South Africa, is the largest known layered igneous complex on Earth
Trace element fractionation proxies and Rayleigh Fractionation display a linear trend from the Nebo to the Bobbejaankop and Lease granites, supporting the origin of the Bobbejaankop by fractionation of the Nebo granite (Figure 15)
Trace element geochemistry indicates that the Bobbejaankop granite resulted from extensive fractionation of the Nebo granite, whereas the Lease granite represents the hydrothermal alteration of the semi-crystallized Bobbejaankop magmatic mush by an incompatible element rich, acidic, magmatically derived hydrothermal fluid
Summary
The Paleoproterozoic Bushveld Complex, in the northeast of South Africa, is the largest known layered igneous complex on Earth. It exceeds an area of 90,000 km and has a magma volume estimated at 450,000 km , hosting some of the world’s largest mineral deposits [1,2,3]. Its origin is still debated [3,4]. Recent work by Zeh et al [5] on the origin of the Rustenburg. Layered Suite has made a significant contribution, the origin of the granites remains poorly understood. The Lebowa Granite Suite of the Bushveld is the largest A-Type batholith on
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