Abstract

The Platreef, on the basal contact of the Rustenburg Layered Suite, has been studied on the farm Overysel, 30 km north of Potgietersrus. The major element, trace element, and Sr isotope geochemistry of the Platreef feldspathic pyroxenite and the adjacent floor rocks is presented. The immediate floor rocks consist of a suite of highly metamorphosed, banded tonalitic gneisses with leucotonalitic veins. To the south the floor rocks are metamorphosed sediments of the Transvaal Sequence. These were intruded by granites, which show signs of metamorphism close to the layered suite. One facies of the granite yielded an age of 2.504 + or - 0.109 b.y., with an initial 87 Sr/ 86 Sr ratio of 0.7035 + or - 6.The cumulate-textured pyroxenites of the Platreef are geochemically different from other units of the Rustenburg Layered Suite in having a high and variable initial 87 Sr/ 86 Sr ratio ranging from 0.7104 to 0.7227, with an average of 0.7160 (calculated at 2.05 b.y.), whereas overlying gabbro has a ratio of 0.7079. Other ratios such as Rb/Sr and Ba/Zr are anomalously high in the Platreef, ranging from 0.05 to 0.26 and 16 to 44, respectively. These data and the presence of relatively sodic plagioclase, biotite, and quartz suggest substantial assimilation of siliceous material. The sedimentary Transvaal Sequence, forming the footwall to the Platreef to the south of Overysel and consisting of dolomites and banded iron-formation, because of their low 87 Sr/ 86 Sr ratios and low K 2 O, Rb, and SiO 2 , cannot produce the observed anomalies. The banded tonalitic gneisses and associated veins are an unusual suite of rocks. Pyroxene geothermometry indicates that they have been metamorphosed to + or -900 degrees C. Rb/Sr ratios range from 0.02 to 0.21, and 0 to 1.8 percent K 2 O. 87 Sr/ 86 Sr ratios (at 2.05 b.y.) range from 0.7024 to 0.7258. Total assimilation of these rocks cannot have produced the anomalous features of the Platreef. However, their low K 2 O contents and high 87 Sr/ 86 Sr ratio in relation to their present low Rb/Sr suggest that they have undergone a partial melting or loss of fluid component as a result of contact metamorphism. However, there are isotopic grounds for rejecting this as the sole source of contamination.The granite close to the Platreef has a high Rb/Sr ratio, initial 87 Sr/ 86 Sr ratios of 0.73 to 0.79 at 2.05 b.y. ago, and 3.9 percent K 2 O. Total assimilation of these rocks could produce the trace element characteristics of the Platreef but only if over 60 percent assimilation has occurred. Major element considerations preclude this. A partial melt or fluid phase generated from these granites could have been the contaminant. A composition calculated to produce the observed trace element and isotopic features of the Platreef with less than 25 percent contamination has 150 ppm Sr, 87 Sr/ 86 Sr of 0.8, 9,000 ppm Ba, 600 ppm Rb, and 10 ppm Zr.

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