Mineral Associations of Concentrates from Gold-Bearing Placers of the Miass Placer Zone (South Urals) and Possible Primary Sources of Gold

Abstract

—The placer deposits of the Miass valley in the Southern Urals have been known since the beginning of the 19th century. Over two centuries of mining more than 120 tons of gold have been extracted from alluvial and alluvial–deluvial placers. Despite the industrial significance of this territory, native gold of alluvial deposits is practically unstudied and there is no data on the mineral composition of concentrates. Native gold and heavy fraction minerals from seven placers of the Miass placer zone of deluvial–alluvial and alluvial types were researched. The study of the typomorphic features of native gold and distribution of heavy fraction minerals in loose deposits makes it possible to determine the genetic types of primary sources of placer formation. Chrome-spinel and ilmenite prevail in the concentrates, magnetite, epidote, rutile, titanite, clinopyroxene, hematite, zircon, garnets, and monazite are less common. Chromespinelides of various placers are represented by chromite and subferrichromite, they are characterized by a high average content of Cr2O3 (50.9 wt.%). Chromespinelide chemical composition features show evidence of its penetration into placer sediments from rocks of the ophiolitic association. The composition and morphological features of phosphates of rare earth elements (monazite and xenotime) indicate their metamorphogenic nature and penetration into placers from the Riphean strata. Native gold is characterized by a low degree of roundness. Mineral inclusions in native gold are represented by sulfides, arsenides, bismuthides, cuprous gold, platinoids. Composition of native gold from the Miass River valley placers varies over a wide range. The main admixtures are silver (4.5–23.4 wt.%) and mercury (up to 5.14 wt.%), copper (up to 2.03 wt.%) was noted in several grains. The EBSD study of native gold internal structure showed predominance of primary crystallization structures, a low degree of deformation and growth of gold-rich rims with a fine-grained mosaic structure. Formation of gold-rich rims in gold grains in placers is explained by recrystallization of the most deformed parts of gold during transportation. Placers occurred due to the destruction of primary sources located in the upper reaches of streams or near them, which is confirmed by a low degree of grains’ roundness. The composition of native gold in combination with a set of micro-inclusions indicate the gold–sulfide-quartz type of mineralization of most primary sources.