Abstract

Tin mineralization at Zaaiplaats is hosted in alkali feldspar granites of the Lebowa Granite Suite which intrude the Rashoop Granophyre Suite. From the upper contact downward, the alkali feldspar granites include the marginal pegmatite, fine-grained Lease Granite and the coarse-grained Bobbejaankop Granite. The major mineralization styles consist of subhorizontal, tabular, low-grade orebodies in the Lease Granite, a lower zone of disseminated cassiterite mineralization in the Bobbejaankop Granite and shallowly plunging and branching pipe orebodies in both granites. Fluid inclusion data show a continuous evolution of the magmatic hydrothermal system from >600 degrees to 200 degrees C, with cassiterite and scheelite precipitation above 400 degrees C. Mineral compositions and oxygen isotope fractionations indicate reequilibration of minerals within the granites during pervasive alteration at temperatures generally between 200 degrees and 300 degrees C.The emplacement age of these and other Bushveld granites is ca. 2050 Ma, yet Pb-Pb whole-rock isochron ages for the different, variably mineralized granite bodies at Zaaiplaats show a relatively restricted range from 961 + or - 129 Ma for the Rashoop Granophyre to 1187 + or - 51 Ma for the Lease Granite. Thus the analyzed sample suites consistently record an event approximately 1 b.y. after the emplacement of the granites. Modeling their Pb isotope evolution indicates that each suite evolved independently between 2050 and ca. 1100 Ma, suggesting that hydrothermal fluid circulation was essentially confined within each granite body and/or fluid/rock ratios were low, and that the overall 238 U/ 204 Pb within each suite from 2050 to ca. 1100 Ma was not unusually high or low for fractionated granites, suggesting largely closed-system behavior of U and Pb within each suite during this interval. The ca. 1100 Ma event which caused closure of the U-Pb systems at Zaaiplaats is interpreted to reflect a blocking temperature age related to uplift and cooling of a long-lived hydrothermal system and precipitation of the latest stage minerals (e.g., galena) at 200 degrees to 300 degrees C.The high steady-state temperature required to maintain the hydrothermal system at Zaaiplaats at temperatures above 200 degrees C for ca. 1 b.y. was provided by radioactive decay. Current heat production characteristics calculated from present-day U, Th, and K 2 O concentrations range from 4.2 mu W/m 3 for the Rashoop Granophyre to 11.5 mu W/m 3 for the Lease Granite. Preweathering heat production of 30.3 mu W/m 3 is inferred for the mineralized Bobbejaankop Granite after accounting for dramatic Recent losses of U and Th. The low level of hydrothermal fracturing combined with stable and Pb isotope evidence effectively rules out convective cooling by external fluids as a significant heat loss mechanism at Zaaiplaats. Fluid pathways within the granites were controlled dominantly by grain boundaries, microfractures, and vugs, which allowed internal circulation and elevated subsolidus temperatures to be maintained indefinitely under a thermal blanket of an estimated 4 km of postemplacement cover. Uplift and erosion of the terrain caused cooling through the Pb exchange blocking temperatures at ca. 1100 Ma.

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