The gold mineralogy in each ore deposit is unique, due 10 the variations in the following:
• Mineralogical mode of occurrence of gold
• Gold grain size distribution
• Host and gangue mineral type
• Host and gangue mineral grain size distribution
• Mineral associations & Mineral alterations
• Variations of the above within a deposit or with time
Gold gravity concentration processes
Because gold ore inert at ambient temperatures and pressures, there are very few naturally occurring compounds of the metal. The average concentration of gold in (the earth’s crust is 0.005 g/t, which is much lower than most other meials, for example, silver CO.07 g/t) and copper (50 g/t). The low concentration of gold in primary rocks means that upgrading by a factor of 3,000 to 4,000 is usually required during ore formation processes to achieve commercial concentrations. This may be possible by naiural gravity concentration processes or by the leaching of gold with natural fluids from the host rock; for example, by highly oxidizing, acidic and complexing (e.g., chloride) solutions, followed by redeposition in a more concentrated form. Owing to its siderophile properties , gold tends to concentrate in residual hydrothermal fluids and subsequent metallic or sulfidic phases, rather than silicates, which form at an earlier stage of magma cooling.
Gold ores Placer
Gold ores contain alluvial, eluvial, or colluvial material in active ore-deposit-forming systems and have been classified here as deposits where diagcnetic processes have occurred to only a limited extent. Ore crushing and grinding are unlikely to be required in the treatment of such ores. Hard rock palaeoplacers (e.g., Witwatersrand ores of South Africa) have been classed as free-milling ores (Section 2.4), in keeping with convention throughout the industry.
The evaluation of ore grade in placers is difficult because of the low grade and unusually coarse gold grain size. Extremely large samples of several hundreds of tons must be treated in a mineral concentration sampling plant for the evaluation of a placer ore grade and for flowsheet design. Commonly, this is a gravity concentration plant composed of spirals, jigs, and centrifugal concentrators. Estimates of gold grade by this method may differ greatly from the eventual average gold concentration recovered from operations, with ratios of recovered-to-expected gold content varying from 32% to 149% in one study.
Gold extraction processing
From a mineral processing point of view, palaeoplacers differ from young alluvial placers, as the gold is unliberated and the ore is consolidated. Crushing and grinding is therefore required to liberate the gold to an extent that allows efficient gold extraction. Because palacoplaccr gold deposits have been mined at depths of up to 3 km, mining costs are generally more than an order of magnitude greater than those for young placer deposits. Also, many of these quartz-rich ore types are very hard, resulting in high processing costs in some cases.
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