Native gold

Main opaque minerals include native gold, electrum, pyrite, pyrrhotite, chalcopy-rite, cubanite, sphalerite, arsenopyrite and tellurobismutite. The amounts of these sulfide minerals are poor, compared with those in epithermal Au-Ag vein-type deposits. It is noteworthy that silver minerals are abundant in epithermal Au-Ag vein-type deposits, whereas they are poor in gold-quartz veins. 

It is essential to know the mode of transport of Au and Ag in ore fluids to consider the factors which control the Ag/Au ratio of native gold and electrum. Many studies on Au and Ag complexes in ore fluids have been conducted and reviewed by several workers (Barnes and Czamanske, 1967 Barnes, 1979 Seward, 1981 Shenberger, 1986). 

According to these previous studies, the most dominant dissolved states of Au and Ag in ore fluids are considered to be bisulfide and chloride complexes, depending on the chemistry of ore fluid (salinity, pH, redox state, etc.). However, very few experimental studies of Au solubility due to chloride complex and Ag solubility due to bisulfide complexes under hydrothermal conditions of interest here have been conducted. Thus, it is difficult to evaluate the effects of these important species on the Ag/Au of native gold and electrum. Other Au and Ag complexes with tellurium, selenium, bismuth, antimony, and arsenic may be stable in ore fluids but are not taken into account here due to the lack of thermochemical data. 

Assuming that AgCl and Au(HS)2 are the predominant Ag and Au species, the following reaction may be used to derive the relationship between the Ag/Au ratio of native gold or electrum and temperature or other variables ... 

Equation (1-87) implies that the a g/aAn of native gold and electrum is controlled by temperature, Oq-, <3ih2S. pH, and Agci /" AuCHS)/ ... 

The curves representing the relationship between temperature and OAg/oAu (ttAg, activity of Ag component in native gold or electrum oau, activity of Au component in native gold or electrum) are shown in Fig. 1.190. It is assumed that aK+/ H+ is controlled... 

In fluids where precious metal transport is dominated by AgCl and AuCl, the following reaction may be written to consider the compositional variation of native gold or electrum ... 

As suggested by Henley et al. (1984) and Brown (1986), it is possible that Ag(HS)2 is dominant in low-salinity and near neutral ore fluids. Under this condition the Ag/Au ratios of electrum and native gold are controlled by the following reaction ... 

It is expected from the equilibrium ratios for equation (1-90) that the Ag/Au ratios of native gold and electrum are controlled by temperature and EAg/EAu in ore fluids, although the equilibrium constant for equation (1-90) has not experimentally been determined. 

Czamanske, G.K., Desborough, G.A. and Geoff, F.E. (1973) Annealing history limits for inhomogeneous native gold grains as determined from Au-Ag diffusion rates. Econ. Geol, 68, 1275-1288. 

Shikazono, N. and Shimizu, M. (1987) The Ag/Au ratio of native gold and electrum and the geochemical environment of gold vein deposits in Japan. Mineralium Deposita, 22, 309-314. 

Main opaque minerals are chalcopyrite, pyrite, pyrrhotite, sphalerite and bornite (Table 2.22). These minerals commonly occur in massive, banded and disseminated ores and are usually metamorphosed. Hematite occurs in red chert which is composed of fine grained hematite and aluminosilicates (chlorite, stilpnomelane, amphibole, quartz) and carbonates. The massive sulfide ore bodies are overlain by a thin layer of red ferruginous rock in the Okuki (Watanabe et al., 1970). Minor opaque minerals are cobalt minerals (cobaltite, cobalt pentlandite, cobalt mackinawite, carrollite), tetrahedrite-tennantite, native gold, native silver, chalcocite, acanthite, hessite, silver-rich electrum, cubanite, valleriite , and mawsonite or stannoidite (Table 2.22). 

Native gold is found as minute grains in the Okuki, Sekizen and Yokei deposits (Horikoshi, 1959 Takeda and Sekine, 1960 Tsunori, 1962). 

The (compositionally) simplest mineral class comprises the native elements, that is, those elements, either metals or nonmetals that occur naturally in the native state, uncombined with others. Native gold, silver, and copper, for example, are metals that naturally occur in a ductile and malleable condition, while carbon - in the form of either graphite or diamond -and sulfur are examples of nonmetallic native elements. Next in compositional complexity are the binary minerals composed of two elements a metal or nonmetallic element combined with oxygen in the oxides, with a halogen - either fluorine, chlorine bromine, or iodine - in the halides, or sulfur, in the sulfides. The oxide minerals, for example, are solids that occur either in a somewhat hard, dense, and compact form in mineral ores and in rocks, or as relatively soft, unconsolidated sediments that melt at moderate to... 

Throughout the ages humans have used sand for many and the most varied uses. Some river sand, known as auriferous sand, contains native gold... 

The gravity preconcentration method, which is used mainly for recovery of gold from placer deposits that contain coarse native gold. Gravity is often used in combination with flotation and/or cyanidation. 

Native gold and its alloys, which are free from surface contaminants, are readily floatable with xanthate collectors. Very often however, gold surfaces are contaminated or covered with varieties of impurities [4], The impurities present on gold surfaces may be argentite, iron oxides, galena, arsenopyrite or copper oxides. The thickness of the layer may be of the order of 1-5 pm. Because of this, the flotation properties of native gold and its alloys vary widely. Gold covered with iron oxides or oxide copper is very difficult to float and requires special treatment to remove the contaminants. 

Gold in the porphyry copper ore may appear as native gold, electmm, cuproaurid and sulphosalts associated with silver. During the flotation of porphyry copper-gold ores, emphasis is usually placed on the production of a marketable copper-gold concentrate and optimization of gold recovery is usually constrained by the marketability of its concentrate. 

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