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Origin of quartz-free gold nuggets and supergene gold found in laterites and soils — a review and some new observations

1984; Taylor & Francis; Volume: 31; Issue: 3 Linguagem: Inglês

10.1080/14400958408527933

1440-0952

Allan F. Wilson,

Heavy metals in environment

Although most quartz-bearing gold nuggets have been shed from quartz veins, supergene processes commonly upgrade the quality (fineness) of their gold. Similarly, some quartz-free nuggets can be shown to have been shed from ultramafic schists or carbonates where they were precipitated from hydrothermal fluids. On the other hand, other quartz-free nuggets of superior fineness (e.g. Au 96.5, Ag 3.5, wt.%) enclose pisolitic laterite concretions or other soil detritus. Earthy haematitic inclusions near the edge of some nuggets have expanded during hydration and burst open the otherwise solid gold. In several localities where nuggets of this type are found there is no obvious vein or lode source of the accreted gold. In many of these the source of the migrating gold which is liberated by weathering is presumably finely divided ‘invisible’ gold (atomic or colloidal), or gold-bearing sulphides or arsenides which are disseminated throughout the country rocks. At many Australian localities almost pure supergene gold is found in weathered gossanous zones but gives way beneath the water table to primary sulphide, sulph-arsenide or carbonate veins containing gold alloyed with much silver and/or some copper. Supergene gold has been found in the following forms: (1) octahedra of a very pure gold (0.5 mm diameter) set on felted surfaces of Mn oxide, or in puggy clays; (2) filamentous and arborescent forms in several types of decomposed rock; (3) dendritic or paint gold on cracks in weathered rock, clay and laterite; (4) gold impregnations of fossil wood and coaly matter; (5) some gold nuggets of mammillary form. Variations in the non-gold components of solid gold are thought to reflect the chemical properties of gold-bearing non-hydrothermal solutions.