Galena-bearing grains from the Lenggang stanniferous placers, Belitung, Indonesia

Hosking, K. F. G. and Rabelink, T. B. A. (1978) Galena-bearing grains from the Lenggang stanniferous placers, Belitung, Indonesia. Bulletin of the Geological Society of Malaysia, 10. 63 -72. ISSN 2637-109X

Abstract

In the rejects from the beneficiation of the now completely disorganized shallow stanniferous placers of Lenggang, Belitung, Indonesia, galena occurs in grains, which, as a result of examination in polished section, may, according to their mineral content, be classified as galena grains, galena/sphalerite grains, galena/copper-bearing sulphide grains, and galena/iron sulphides grains. The galena grains may consist of primary and/or secondary sulphide, and when the latter is present the bodies are commonly botryoidal, and in polished section display a banded texture. In one instance what are just possibly galena framboids were noted. The galena/iron sulphides grains are angular fragments of originally woody tissue whose lumina have been largely infilled with pyrite and/or marcasite, and which may contain pyrite framboids, and whose voids have provided sites for deposition of galena. All but the primary galena grains may contain pyrite framboids, which are sometimes disrupted, and woody fragments whose lumina are infilled either with iron sulphides, or galena, or possibly sphalerite. In addition, inclusions of organic-rich mud are common, as are fragments of resistates, such as quartz, which may be fractured in such a way as to suggest that the fragmentation was due to forces that were generated during the crystallization of the sulphide gel host. The earliest of the secondary sulphides to be formed were pyrite and marcasite, and these were followed, in the order given, by sphalerite, galena, and copper-bearing sulphides, but a wholly satisfactory reason for this paragenesis has not been advanced. The evidence suggests that these ‘secondary‘ sulphides were all generated by supergene processes in the organic-rich, oxygen deficient environment of a freshwater swamp, and in the vicinity of fragmented galena that had been shed from a lode or from a collapsed portion thereof. Furthermore, it is thought that the lead, copper and zinc required for the formation of the supergene sulphides were derived from those parts of an undiscovered but neighbouring orebody that were in an oxidising environment. It is believed that mobilisation of the elements was in part effected by direct oxidation and in part by solution of oxysalts by organic acids. The iron was largely derived from the swamp waters, whilst the sulphide and elemental sulphur components were produced, by biogenic means, either from sulphate ions liberated from the oxidising part of the ore-body and/or from sulphur-containing proteins. As the sulphides under review were the products of a somewhat unusual combination of circumstances, it is held that the content of this paper provides but little in support of any likely supergene theory of genesis of large stratabound deposits such as those of Rammelsberg and the Kupferschiefer.

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