Precious metal placer




Native gold. Kalimantan, Indonesia


Click hereThis is a gold concentrate from a river placer. The variation in colour is related to the fineness of the gold. Darker, more yellow, gold is close to 980 fine and forms rims around whiter, more argentian cores. The cores of the grains are themselves heterogeneous (bottom right) with a yellow core within a whiter margin (left of grain); this is difficult to see, but both are surrounded by a spongy, deep yellow, fine gold rim. One grain (top left) shows a relict core within a non-porous fine gold rim. Dark grey area is silicate (bottom left), black areas are erupted air bubbles within the resin. The green-grey colour of the resin is due to the incomplete removal of carbon-coating.



Grain mount, plane polarized light, x80, air


Native gold and zircon. Kalimantan, Indonesia


Click hereThis is a gold concentrate from a river placer. Gold grains show variation in their core colour and hence composition. Fine gold cores are darker yellow (right centre) than those grains with higher reflectance cores (centre and top left). High fineness rims are either well polished (top left) or spongy (centre). The contact between core gold and gold rim is sharp (less than 2śm). Pounded zircon grains (grey, top right) retain some of their original morphology. Rounded black areas are erupted air bubbles within the resin. Incomplete removal of carbon-coating along scratches in the resin is visible as blue-grey areas.



Grain mount. plane polarized light, x 80, air


Native gold and zircon. Kalimantan, Indonesia


Click hereThis is a gold concentrate from a river placer. A gold grain with a silver-rich core (pale yellow, centre) is enclosed within a spongy gold enrichment rim (yellow). Other gold grains are finer and have very thin or no gold enrichment rims. Zircon grains (grey, bottom left) show faint internal reflections. The rounded black area is an air bubble within the carbon-coated resin (green-grey).



Grain mount, plane polarized light, x 80, air


Native gold, platinum-iron alloy and zircon. Kalimantan, Indonesia


Click hereThis is a gold concentrate from a river placer. Gold grains have well polished gold enrichment rims (bottom left and right) around high fineness gold cores (slightly higher reflectance and lighter surface colour). A spongy gold enrichment rim has developed around a silver-rich gold core (top left). A rounded grain of platinum-iron alloy (centre) is white when compared with silver-rich cores within gold grains (top left). Zircon grains are dark grey and are subhedral (centre top). Rounded black areas are erupted air bubbles within carbon-coated resin (green-grey).



Grain mount, plane polarized light, x80, air


Platinum-iron alloy, native gold, osmium-iridium-platinum alloy and osmium-iridium alloy. Kalimantan, Indonesia


Click hereAn irregular gold grain has high fineness (top) and looks folded. The platinum grain is a complex mixture of platinum-iron alloy (brown-white, centre left) which encloses small areas of an osmium-iridium-platinum alloy (cream, higher reflectance than platinum-iron, centre bottom) and has a rim of the same composition, namely approximately equal proportions of the three metals. The outermost rim (blue, bottom right) is an osmium-iridium alloy, iridosmine, with approximately 70% osmium. The fine scale 'banding' within the main osmium-iridium-platinum alloy (bottom left) is partly due to thin parallel intergrowths of osmium-iridium within the main 'host'. The parallel striations (oriented north-south, centre right) in the resin are damage due to the electron microprobe beam. The round area (top left) showing colours is due to an air bubble in the immersion oil.



Grain mount, plane polarized light, x180, oil


Native gold and osmium-iridium alloy. Kalimantan, Indonesia


Click hereAll three native gold grains have gold enrichment rims about a high fineness core (bottom left), and a low fineness core (light yellow, higher reflectance, right); the latter rim is spongy. An angular grain of osmium-iridium alloy (white, centre left), iridosmine, has an incomplete gold rim which either has been mechanically applied onto the alloy or has precipitated on it. At higher magnification a radiating habit (right-hand margin of alloy) suggests precipitation.



Grain mount, plane polarized light, x80, air