4.1.2. Gold–Quartz Veins in Granitoids GIT Fineness of Native Gold

Gold–quartz veins GIT is characterized by native gold of medium and high purity with maxima of this value in the regions of 850–950‰ (Figure 5). Fineness histograms are predominantly unimodal with a higher dispersion of this indicator (from 500‰ to 1000‰) compared to gold–arsenic-sulfide GIT. Often, in the frame of this type of deposits, there is a sharp decrease in the fineness of native gold and the appearance of silver mineralization (stephanite, freibergite, polybasite, acanthite, pyrargyrite).

**Figure 5.** Fineness histograms of native gold for deposits of gold–quartz veins GIT along the abscissa axis—frequency of occurrence, %; along the *x*-axis, fineness intervals, ‰; in the numerator—fineness, ‰, in the denominator—the number of determinations.

#### Trace Elements

It has been established that native gold from deposits of this type constantly contains As in significant concentrations and Bi, Sb, Pb, Cu, and Fe are less common, while Hg, Sn are noted in rare cases (Table 2). As concentrations range from 0.8 to 300 g/t, all other impurities do not reach 50 g/t. Bi and Sn are classified by most researchers as granitogenic elements. Among the deposits of gold–quartz veins GIT, a gold-rare metal formation stands out [44–46], where Bi and Sn, and often W, form noticeable concentrations and are included, along with gold, in the list of useful components.

**Table 2.** Concentration of microimpurities in native gold from ores of gold–quartz veins GIT deposits.


Notes: —–below detection limit.

#### Mineral Intergrowths

The widespread association of gold–quartz veins GIT is naturally quartz, which was the reason for its name. In addition, the diversity of mineral associations of native gold is determined by its intergrowth with Sb and Bi minerals, and often Te (fahlore, joseite, native bismuth, bismuthine, etc.), as well as with Fe-Sb sulfosalts (Figure 6). Intergrowths with vein minerals are characterized by the features of host rocks—granitoids. At the Dorozhnoye deposit, native gold often forms intergrowths with muscovite (Figure 6h). Figure 6a–f shows mineral intergrowths of native gold in the ores of the Shkolnoye deposit for various horizons. Changes in the average fineness value with depth change in waves, but the dispersion of this indicator naturally falls from the upper horizon to the lower one (Table 3).

**Figure 6.** Intergrowths of native gold typical for gold–quartz veins GIT: (**a**–**f**) for different horizons of the Shkolnoye deposit: (**a**) native gold in the intergranular space of arsenopyrite, (**b**) in quartz with inclusions of small grains of arsenopyrite, (**c**) intergrowth with tennantite and jamesonite in quartz, (**d**) intergrowth with tetrahedrite in quartz, (**e**) inclusions in native gold of jamsonite and plagionite, (**f**) gold in the intergranular space of quartz; (**g**) (Maltan) inclusion of native gold in an intergrowth of bismuth, bismuthine, and joseite A; (**h**) (Dorozhnoye) intergrowth of native gold with muscovite; (**i**) (Butarnoye) small segregations of native gold in arsenopyrite intergrown with joseite A.


**Table 3.** Characteristics of native gold from ores of different horizons of the gold–quartz veins GIT Shkolnoye deposit.

### Internal Structures

For gold–quartz veins GIT deposits localized in granitoids, the structure of native gold is coarse-grained with rare simple and polysynthetic twins and patchy heterogeneity, presumably associated with the incorporation of microparticles of native Bi, which, unlike gold, undergoes intense etching even in air (Figure 7). A similar type of patchy heterogeneity for gold from deposits in granitoids was previously described in [8].

**Figure 7.** Polygonal-granular structure with simple polysynthetic twins and fine spotted heterogeneity in native gold from ores of gold–quartz veins GIT deposits: (**a**,**b**) Butarnoe; (**c**,**e**) Shkolnoye; (**d**) Maltan, (**f**) Dorozhnoye.
