Disseminated tin mineralization in the roof of the Bushveld granite pluton at the Zaaiplaats Mine, with implications for the genesis of magmatic hydrothermal tin systems

Univ. Pretoria, Inst. Geol. Res. Bushveld Complex, Pretoria, South Africa

The Bobbejaankop Granite is a hydrothermally altered facies equivalent of the typical Bushveld (Nebo) granite. On a regional scale, the Bobbejaankop usually occurs in the upper part of the sheetlike Nebo pluton, but it is particularly well developed in a roof cupola at the Zaaiplaats tin mine. Several styles of endogranitic tin mineralization occur at the Zaaiplaats mine, but disseminated deposits hosted by the Bobbejaankop Granite form the main producing ores.Cassiterite, commonly associated with scheelite and sulfides, is interstitial to the major silicate minerals in the disseminated ores. The ores are confined to a favorable zone that can be defined on geochemical-statistical grounds as a tabular body of varying thickness (55-66 m) with an upper contact that is parallel to, and some 66 to 83 m below, the pluton roof contact. Geochemical halos around the ore-bearing zone are absent or poorly developed and, except for the anomalous tin contents in this zone, there are few obvious differences between mineralized and barren Bobbejaankop Granite.The Zaaiplaats rocks are more heavily altered than equivalent granites in the southeastern Bushveld Complex, and granite in the disseminated tin zone is more altered than barren Bobbejaankop Granite. A complex history of sequential alteration can be recognized, beginning with high-temperature feldspathic (albitization and microclinization) alterations and followed by chloritization, sericitization, and lastly, silicification. These events induced many chemical changes. Compared with Bobbejaankop variants in the southeastern Bushveld, the entire Zaaiplaats suite is enriched in K, Ca, Rb, Cu, and Sn, and depleted in Na. Within the mine area, granite from the mineralized zone contains higher Na, Sn, W, As, S, and F, but lower K and Rb contents than the barren granite. These differences reflect increasing degrees of fluid-controlled precipitation and alteration.For the most part, the Bushveld granites are not metallogenically specialized in the manner of other tin mineralized plutons. Ore genesis cannot be explained by geochemical heredity, or contamination during or after emplacement. Fractional crystallization cannot directly produce significant tin concentrations; the Bushveld magma contained relatively low (<5 ppm) primary tin contents, which probably underwent less than a twofold increase during crystallization. Late saturation and exsolution of a Cl-rich fluid represents by far the major process responsible for the Zaaiplaats deposits and other endogranitic tin ores. At Zaaiplaats, the ore minerals crystallized at high temperatures from relatively saline fluids that exsolved into interstitial pore spaces in the cooling crystal mush. This process formed an intrinsic part of the normal cooling history of the pluton, illustrating that the Bushveld granites define a temporal continuum between magmatic crystallization, fluid exsolution, and water-rock reaction.

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