GOLD

Oxidation and Mobility of gold

General principles

This commonly takes place in the deeper parts of gossans and oxidized gold deposits. The native gold is commonly adsorbed or absorbed by the hydrous manganese oxide in a very finely divided form and is rarely visible (see 4. below).

4. Adsorption and/or coprecipitation negatively charged gold complexes and colloids by positively charged gels such as hydrous ferric oxides (limonite). This mechanism appears to be particularly effective since most gelatinous iron oxides in or near gold-bearing deposits are generally enriched in gold. Gelatinous manganese oxides, likewise, adsorb and/ or coprecipitate gold, but the mechanism is not entirely clear. Since the gold complexes and colloids are negatively charged the negative hydrous manganese oxides should repel them. However, these gels invariably contain considerable iron and other elements that may reverse the overall charge giving the complex a positive charge, which is effective in precipitating the negative gold complexes and colloids. Alternatively, anionic gold complexes may be adsorbed on the positively charged hydrous manganese (II) oxides.

The writer's observations on natural gold- and silver-bearing hydrous iron and manganese oxide gels suggest that initially the reaction is essentially an adsorption phenomenon, that is, the gels exist as a random arrangement of micelles of the oxides with adsorbed gold and silver ions. No gold or silver can be observed in these dried gels even under the highest powers available with the optical microscope. With aging of the gel some of the gold and silver may be desorbed and split out of the complex yielding small spangles and filaments of native gold in the vicinity of the oxides (limonite and wad). In other cases much of the gold and silver is retained by the oxides as an adsorbed phase or in an extremely finely divided form inextricably mixed with the oxides.

Clay minerals and gelatinous silicate complexes also tend to markedly concentrate gold. One frequently finds seams, bunches and patches of clay minerals in the oxidized zones of gold deposits greatly enriched in gold. Gouge and puggy clay along fractures and faults, likewise, tends to concentrate gold in the oxidized zones. The reason for this is evidently largely due to adsorption processes.

Numerous other natural gel-like substances tend to precipitate gold readily. Among these may be mentioned humic gels, humic-limonitic gels, humic-limonite-wad-alumina gels, bismuth ochres, tellurium ochres and amorphous antimony and arsenic ochres. The precipitation mechanism is evidently largely due to adsorption on these substances.

5. Coagulation and/or precipitation of gold colloids by various charged ions, sols and gels. Metallic gold colloids carry a negative charge and hence positively charged clay minerals, hydrous iron oxide and various other positively charged ions, sols and gels should effectively coagulate or precipitate the gold. The importance of this mechanism is unknown since we have no data on the transport of gold as a colloid in the oxidized zones of gold deposits.

6. Precipitation due to the presence of various natural reductants. Various field and laboratory observations show that native gold is readily precipitated from gold-bearing solutions by practically all sulphides, silicates, carbonates, native metals, clay minerals, carbonaceous substances, and a great variety of supergene hydrous oxides, arsenates, antimonates, etc. The gold is only rarely found coating these various substances; more generally the dissolved metal is precipitated on nearby minute nuclei of native gold, and these continue to grow to give relatively coarse platelets, wires, sprigs, flakes, slugs and nuggets in the oxidized zones. In some veins, however, there is a general precipitation of gold in the form of finely divided particles throughout such materials as disintegrated sulphides, limonite, wad, puggy clay and various ochres in the oxidized zones.

7. Coprecipitation and/or adsorption by numerous supergene minerals. Among these may be mentioned bismutite, chlorargyrite, the jarosites, beudantite, bindheimite, malachite, azurite, gypsum, native sulphur, pyromorphite, scorodite, and various other phosphates, vanadates, arsenates and antimonates. The exact nature of gold in some of these materials is unknown. Some gold is evidently in lattice positions in these minerals, but most is probably present in a very finely divided form (dust). In some occurrences in chlorargyrite and the analogous bromide and iodide the gold may be coarse and often well crystallized.

Gold in: Primitive Classic Medieval Renaissance post-Renaissance period.

Gold: Deposits Transport 1 2 3 4 5 6

Rafal Swiecki, geological engineer email contact
February, 2006

This document is in the public domain.