The magmatic-hydrothermal transition recorded by trace elements in quartz: a case study from the Zaaiplaats Tin Field, South Africa

Abstract

The composition of quartz has historically been considered unimportant for mineral exploration, although this perspective is changing with the advancement of analytical techniques. The ability to measure trace element variations in quartz provides a unique window into the evolution of mineral deposits. Granites are currently of interest as they can host late-stage magmatic-hydrothermal mineralisation, such as Sn and other critical metals. The Nebo, Bobbejaankop, and Lease granites in the Zaaiplaats Tin Field of the Bushveld Complex represent well-exposed expressions of endogranitic Sn-mineralisation. These granites display an upward increase in their degree of hydrothermal alteration. Disseminated Sn-mineralisation is restricted to the Bobbejaankop and Lease granites and high-grade cassiterite-bearing tourmaline-quartz hydrothermal pipes that radiate upwards through these granites, terminating below the roof contact. Trace element compositions of the quartz from the Zaaiplaats Tin Field shows evidence that supports the suggested fractionation and fluid-saturation models of ore genesis. The Al/Ti and Ge/Ti ratios in quartz increase from the base to the roof and illustrate the sequential fractionation and increase in the degree of fluid-rock interaction. The trace element data display a shift from a magmatic fractionation-controlled evolution to a hydrothermally-controlled system influenced by the saturation of a late-stage magmatic-hydrothermal fluid. Thus, trace element variations in quartz can record the point of fluid-saturation and the magmatic-hydrothermal transition. Therefore, the recognition of the most evolved, fluid-saturated facies indicates lithologies with the best mineralisation potential for cassiterite. The use of trace elements in quartz extends beyond granite-hosted deposits and is potentially applicable to various mineralised systems.