Fluid inclusion and stable isotope evidence for interaction between granites and magmatic hydrothermal fluids during formation of disseminated and pipe-style mineralization at the Zaaiplaats tin mine

Abstract

Tin-tungsten-rare earth element mineralization at Zaaiplaats is hosted by the Lease and Bobbejaankop Granites--miarolitic, alkali feldspar granites which form part of the Lebowa Granite Suite of the Bushveld Complex. Pervasive hydrothermal alteration of the granites resulted in replacement of original biotite by chlorite, sericite, carbonate and Ti oxides, development of coarse vein and patch perthite and antiperthite in alkali feldspar, and precipitation of hydrothermal minerals in miarolitic cavities. Cassiterite occurs as a cavity infill and replacement mineral in tabular, subhorizontal zones within the Lease and Bobbejaankop Granites and in shallowly plunging and branching pipe systems in the granites. Disseminated mineralization exhibits no relationship to fracture systems, and hydrothermal fluids appear to have evolved essentially in situ during crystallization to produce miarolitic cavities. Minor subvertical veins containing fluorite, calcite, quartz, and sulfides crosscut the granites and pipes.Three major types of fluid inclusions have been identified in granite quartz and hydrothermal minerals from pipes and cavities. High-salinity inclusions characterized by the presence of solid phases such as halite, sylvite, and Fe chloride occur as primary inclusions in (1) granite quartz, (2) cavity-filling quartz, fluorite, and scheelite, and (3) cassiterite from the Iota pipe. These inclusions homogenize by dissolution of solid phases in the temperature range 160 degrees to 600 degrees C. Liquid-vapor inclusions occur in (1) late quartz from cavities, (2) quartz, fluorite, and calcite from cavities in the marginal pegmatite, and (3) late quartz and calcite infill from the Iota pipe. These inclusions homogenize to the liquid phase between 88 degrees and 240 degrees C, while ice melting temperatures range from -11 degrees to -30 degrees C. H 2 O-NaCl-CO 2 inclusions occur as secondary inclusions in granite quartz and early cavity-filling quartz, and primary inclusions in quartz from the quartz-tourmaline zone of the Bobbejaankop pipe. The compositional range and homogenization behavior of these inclusions is consistent with immiscibility around 350 degrees C and 1,000 bars.The oxygen isotope composition of granite whole rocks ranges from 9.2 to 9.9 per mil. Two samples of low-grade ore (8.4 and 9.0ppm) are depleted relative to the granites, possibly reflecting higher abundances of alteration and infill chlorite, sericite, and cassiterite. The delta 18 O values of quartz from granites and the marginal pegmatite range from 7.3 to 9.4 per mil, whereas those of quartz from cavities, low-grade orebodies, pipes, and veins range from 7.8 to 13.5 per mil. Most granite and cavity samples have quartz-feldspar fractionations less than 0 per mil. Vein and pipe samples tend to have higher delta 18 O feldspar and much higher delta 18 O quartz compared to granites, with quartz-feldspar fractionations of -0.6 to 1.2 per mil.The disequilibrium quartz-feldspar fractionations observed in granites and cavities reflect pervasive hydrothermal fluid-rock interaction at subsolidus temperatures. Modeling isotopic exchange between granites and a magmatic fluid phase in isotopic equilibrium with average Bobbejaankop Granite quartz at 600 degrees C indicates that whole-rock compositions result from final exchange at temperatures less than 400 degrees C, with fluid compositions in the range 4.2 to 5.2 per mil at 300 degrees C and 0.8 to 1.8 per mil at 200 degrees C. These results are consistent with fluid compositions calculated from delta 18 O quartz and pressure-corrected fluid inclusion homogenization temperatures, and indicate that oxygen isotope variation at Zaaiplaats reflects exchange between the granite and the magmatic fluid phase at temperatures down to 200 degrees C.