Typomorphic Features of Native Gold and Magnetite from Placers in the Rangkul Area (Eastern Pamirs, Tajikistan): Interrelationship and Assessment of Possible Indigenous Sources

We have studied the typomorphic features of native gold and magnetite of black and gray heavy mineral concentrates from the placers of the Rangkul region (Eastern Pamirs, Tajikistan), stored in the collections of the Vernadsky Museum (Moscow). Optical microscopy, scanning electron microscopy with SEM-EDS microanalysis, and high-resolution X-ray computed tomography (micro-CT) were used to study these minerals. It was revealed that the gold grains are flattened, semi- or medium-rolled, and they correspond to small and medium size fraction. Native gold from the black concentrate is characterized by a heterogeneous structure and variations in the fineness from 900‰ in the center to 1000‰ in the marginal parts. On the surface, it contains goethite and a mixture of Fe–Ca–Mg–Si–Al–O phases. In two gold grains from the gray concentrate with a fineness of 918–987‰ and 923–1000‰, mineral microinclusions were detected: one contains glaucodot with impurities of Fe and Ni, galena, kaolinite, and goethite, and the other contains muscovite, plagioclase, Ni-pyrrhotite (Ni 0.3–0.5 wt.%), and goethite. The third gold grain of the gray concentrate is characterized by a lower fineness ranging from 746‰ (in the center) to 914‰ (in the margins). Secondary minerals, goethite and Fe–Si–Al–O phases, are present in the inner zone of this gold grain. Magnetite is represented by octahedral crystals of 100–200 microns in size. They are slightly rounded. Their surface is often covered with goethite and clay minerals, rarely Au-Ag particles. Inclusions of monazite-(Ce), xenotime-(Y), ilmenite, rutile, chloritoid, garnet (almandine), tourmaline (schorl), apatite, quartz, and barite were found in the magnetite crystals. Significant amounts of magnetite in the placer concentrates containing native gold may indicate common and (or) different primary sources of these minerals. Orogenic gold deposits in the shear zones with a veined–disseminated type of ores and gold–sulfide–magnetite skarns or gold–magnetite (iron ore) deposits are predicted as possible common potential sources of native gold and magnetite in the Rangkul area. The presence of carbonate–terrigenous rocks and magmatic rocks (diorites) in this area together with the occurrence of ore bodies in dikes suggest the skarn type of primary source of both gold and magnetite. The typomorphic features of magnetite and the widespread occurrence of metamorphic schists in the Rangkul ore district indicate that these ore-bearing rocks could serve as an additional source of these minerals in heavy concentrates. The obtained results can be useful for planning exploration and mining activities of gold deposits in the Rangkul area and adjacent territories.