Bulletin of the Geological Society of Malaysia, Volume 20, August 1986, pp. 537-577
Camborne School of Mines, Pool, Redruth, Cornwall, U.K.
"This is not the end. It is not even the beginning of the end.
But it is, perhaps, the end of the beginning." (Winston Churchill)
Abstract This paper aims at bringing together and commenting on the scattered published and other data relating to the topic under review and highlighting that which still remains to be done before a really comprehensive treatment of the subject can be attempted. At the outset it is stressed that many of the problems relating to the geneses of some of the material described have not been solved because, of necessity, a considerable proportion of the samples examined were obtained from secondary sources, such as beneficiation plants.
The following authigenic species are discussed:-
pyrite, marcasite, sphalerite, galena, copper-bearing sulphides, siderite and native copper. Authigenic iron oxides, ‘chlorite‘, calcite, silica and phosphates have been passed over, largely because they have not yet been sufficiently researched, although they are all known to occur in the alluvial deposits in question.
All the sulphides discussed in this paper were generated in swamps, some of which may well have been of the fresh-water type, during early diagenesis. At Lenggang, Belitung, where all the sulphides occur, the paragenesis is pyrite and marcasite, sphalerite, galena, copper-bearing sulphides.
In the Southeast Asian stanniferous alluvials both pyrite and marcasite are very common. Both species occur in organic-rich deposits as individual crystals of a variety of habits, but pyrite crystals are by far the more abundant. Marcasite, however, is usually the dominant species replacing and/or occupying cells in fossil plant material, and then pyrite framboids are often included in it. At Lenggang, galena rims voids in marcasite replaced wood.
Slabs of pyrite and of marcasite, both with included framboids, have been found on the bedrock of some opencast mines, as have sandstones and conglomerates cemented by one or both of the sulphides.
Not surprisingly, framboids and polyframboids are commonly found in organic-rich deposits but, in addition, framboids have been located in voids of blocks of primary ore lying on the bedrock, in a supergene colloform chalcopyrite coating of a lode, in siderite dripstone, and as a late component within the hard-rock ore of Sungai Goh where, possibly, it is of hypogene origin. In the mill at Ayer Hitam Tin Dredging Property, Malaysia, an ornamented polyframboid was found whose ornamentation may have developed in the mill.
In addition to the modes of occurrence of marcasite, noted above, spherulites of the species have been recovered from Perak, but they are probably of rare occurrence.
Siderite, a rather common mineral in the alluvials, occurs, as noted above, as dripstone, and also as deposits on pebbles, and within sandy deposits. However, siderite is most commonly found as spherulites, either as isolated bodies or as aggregates. These display radial crystallisation and concentric colour zones, some of which are due to incipient oxidation during development. They may be locally slightly replaced peripherally and/or along fractures, by pyrite. They developed in sandy clays and similar deposits, essentially by replacement, in an environment which was somewhat less reducing than that in which the sulphides developed, and so, not surprisingly, in Perak, siderite-rich dredging areas have been, on occasion, observed to give way laterally to sulphide-rich ones.
Sandy inclusions not uncommonly occur in the sulphides and siderite. These inclusions are often fragmented. The fragments are but little displaced and are cemented by the authigenic host. These facts suggest that the fragmentation was probably largely or entirely due to the force of crystallisation of the host, perhaps during its conversion from a gel state. However, on occasion, the pressure of overlying sediments may have been a contributory factor.
Crystals of native copper have been recorded from some of the Perak alluvial deposits. They occurred at a site visited by the writer in a shallow hematite-rich stanniferous deposit. Most of the crystals showed no deformation such as might be expected were they transported in water. Possibly they developed in the placer from copper-bearing solutions seeping down the shallow valley slope: that they were washed out of a hard-rock oxidised copper deposit is much less likely. Native copper is also reputed to have been found in some of the stanniferous placers in Belitung.