Two important aspects of the aqueous geochemistry of gold are its chemistry And the particular properties of the matrix solution (e.g. acidity, pH and Oxidation potential, EH). Salinity can arise from various processes including rock Weathering and dissolution of previously deposited halite, evaporation, sea water And aerosol deposition of seawater. Acidity, which is usually measured as pH And factors such as pH, Eh, and salinity have major effects on the speciation and olubility of gold.

 Eh (electrical conductivity) values less than 200 mV indicateReducing solutions, which tend to be rich in reduced species such as Fe²⁺. OrSH^-. Values higher than 500mV indicate oxidizing solutions, which generally Contain high concentrations of oxidized species e.g., UO2 ²⁺. Or AuCl4^- . The oxidation of pyrite and other sulphide minerals plays an important role in the generation of hydrogen ions (acidity) during weathering. Particular complexing anions and/or solution processes are required to enable ground water mobility and the various complexes to become important under different chemical conditions as specific complexes are thio-complexes, halide complexes and organic complexes.

 -Thio-complexes:

Sulphur forms a number of species with varying oxidation states from ÿ2 to .6.

Depending upon the concentration of reduced sulphur, the most important species for gold mobilizations appear to be (from lowest to highest oxidation-state):

· Hydrogen sulphide (SH^-)

· Solid sulphur, which does not mobilize gold

· Thiosulphate (S₂O₃^2-).

· Sulphite (SO3^2-).

· Sulphate (SO4^2-), which does not complex gold.

The most important sulphur species for Au mobilization appear to be hydrogen Sulphide and thiosulphate.

 -Halide complexes:

The dissolution of Au chloride (AuCl2) requires highly acid, saline and oxidizing

Conditions: 2Au_(S)  +  4Cl^-  + ½O2  +  2H⁺  ?  2AuCl₂^-  +  H2O .

-Organic complexes:

Organic/biologically based complexes important for the mobility of Au in soils include cyanide complexes, organic complexes, and colloidal gold and biological effects.

-Cyanide complexes:

Organic complexes capable of mobility of Au in soil profiles include cyanide omplex Au (CN)₂^-. A highly organic horizon can contain high levels of cyanide nd produce Au mobility.

-Colloidal gold:

Gold readily forms molecular aggregations up to 5 _m in size (colloids or sols)

And such chemical species have been known for centuries. Where stabilized by Organic matter.

 Gold grain morphology:

The morphology of a grain of gold is inherited from its primary state and to a Large extent, irregularities of gold grains in source rocks predetermine grain Morphology in an alluvial setting. Gold is one of the last minerals to crystallize Out under hydrothermal conditions of deposition and thus tends to fill cracks and Spaces between other minerals with which it comes into contact. The gold grains Are moulded by the geometry of the opening into aggregates of irregular shape And size and commonly contain inclusions of quartz and other rock forming Minerals.

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