Gold in ore

As discussed in previous chapters, gold deposition in epithermal systems and back-arc basins (Kuroko deposits) occurs in relatively higher /oj conditions than base metal 

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

It is noteworthy that bornite, chalcocite and tetrahedrite-tennantite which are common minerals in Kuroko deposits occur in gold bearing Besshi-type deposits. Although these minerals are considered to be secondary minerals, depositional environments of these minerals are characterized by higher /s, and foj conditions. It is also noteworthy that these deposits are rich in pyrite rather than pyrrhotite. Probably, Besshi-subtype deposits in Shikoku formed under the higher fo and /sj conditions than the deposits characterized by pyrrhotite (Maizuru, Hidaka, Kii, east Sanbagawa). Such typical Besshi-type deposits (Besshi-subtype deposits in Shikoku) are characterized by simple sulfide mineral assemblage (chalcopyrite, pyrite, small amounts of sphalerite). Inclusion of bornite in pyrite is also common in these deposits. 

Sato and Kase (1996) summarized lead i.sotope data of ores from 12 major Besshi-type deposits (11 Besshi subtypes and 1 Hitachi subtype) (Fig. 2.57). 

Cretaceous-Tertiary vein, skarn kuroko deposits in Japan 

---

The primary attraction of CL detection of inorganic compounds is the excellent sensitivity that can be obtained over a wide dynamic range using simple instruments. A detection limit of 40 pg/mL gold in ore samples reported by us is a striking example to indicate the advantage of CL detection [1],... 

An early study referred to the determination of traces of gold in ores by absorbing this element on the surface of foamed plastic [23], The sample containing the gold traces was first dissolved by a HC1-HN03 mixed solution and a piece of foamed plastic was then placed therein. After vibration for 30 min. on a vibrator, the solid material was removed, washed with tap water, and placed in a CL cell. A 2-mL volume of 1.0 mmol/L EDTA was added to the cell and the lid was closed then 2 mL of 0.01 mmol/L luminol solution was injected into... 

FI) preconcentration system for determination of trace gold in ore samples was studied by simplex... 

The ion associate of gold with Methylene Blue has been used in determination of gold in ore, anode slime, and cyclone dust [53], and in solders [73]. The Nile Blue method was used for determining gold in coal dust and in ores [54]. 

FIGURE 11. Use of mercury to dissolve gold in ore concentrates- The gold amalgam is then heated and mercury distills (Ercker, see Figure 5). 

Silver occurs native and in ores such as argentite (Ag2S) and horn silver (AgCl) lead, lead-zinc, copper, gold, and copper-nickel ores are principal sources. Mexico, Canada, Peru, and the U.S. are the principal silver producers in the western hemisphere. 

Figure 5 Mass-resolved secondary ion images of sulfur and gold in a pyrite ore sample.

These correlations mean that the HSAB principle could be a useful approach to evaluate the geochemical behavior of metals and ligands in ore fluids responsible for the formation of the epithermal vein-type deposits. Among the ligands in the ore fluids, HS" and H2S are the most likely to form complexes with the metals concentrated in the gold-silver deposits (e.g., Au, Ag, Cu, Hg, Tl, Cd), whereas Cl prefers to form complexes with the metals concentrated in the base-metal deposits (e.g., Pb, Zn, Mn, Fe, Cu, and Sn) (Crerar et al., 1985). 

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. 

Aq- (Fig. 1.189), while an increase in pH causes a decrease in wrAucr/ Au(Hsr It is apparent in Fig. 1.190 that Au bisulfide species are more abundant than Au cliloride species under the conditions common for ore fluids responsible for Japanese Au-Ag veins. However, Au chloride species may dominate Au bisulfide species in ore fluids responsible for the gold-quartz (auriferous) vein deposits, as shown in Fig. 1.189. 

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. 

Factors in controlling chemical compositions of gold in equilibrium with the ore fluids are temperature, pH, concentration of aqueous H2S and Cl in the ore fluids, concentration ratio of Au and Ag species in the ore fluids, activity coefficient of Au and Ag components in gold, and so on (Shikazono, 1981). In the Yamizo Mountains, as a result, Ag/Au ratios of gold are correlated with a kind of the host rocks and sulfur isotopic compositions of the deposits. This correlation could be used to interpret Ag/Au ratios of gold. 

It is concluded that, in either case, in situ interaction of the ore fluids with the host rocks controls the chemical compositions of gold in the Yamizo Mountains. 

Nishiwaki, C., Matsukuma, T. and Urashima, Y. (1971) Neogene gold-silver ores in Japan. Mining Geology Special Issue, The Society of Mining Geologists of Japan, 3, 409-417. 

Nysten, P. (1986) Gold in the volcanogenic mercury-rich sulfide deposit LSngsele, Skellefte ore district, northern Sweden. Mineralium Deposita, 21, 116-120. 

Watanabe, K. and Nagai, S. (1986) Regional aleration in and around the Rendaiji gold-silver ore deposits, Shizuoka Prefecture. J. Miner. Soc. Jpn., 17, 69-74 (in Japanese with English abst.). 

Yoneda, T. and Watanabe, T. (1981) Clay minerals in the gold silver ore of the Chuetsu-hi vein of the Todoroki mine, Hokkaido, Japan. Soc. Mining Geologists Special Issue, 10, 143-150 (in Japanese with English abst.). 

---