––A long-term study of the typomorphism of native gold under exogenous conditions gave an insight into its evolution in time and space. The morphology, internal structure, and chemical composition of native gold change depending on the duration of its occurrence under near-surface conditions and on the thermodynamic parameters of the environment. Along with the known facts of gold transformation in the exogenous environment, we consider some of the first identified aspects of the evolution of native gold in weathering crusts and the hydrodynamic, eolian, and anthropogenic environments. Additional attention is given to the transformation of gold in ancient gold-bearing conglomerates under lithostatic pressure and in metamorphic strata depending on the P–T environmental conditions. The paper is based on the results of field work, experiments, and analytical studies of the mineralogy of native gold. The objects of study were gold placers of the eastern Siberian Platform, Tuva, and Mongolia, gold-bearing conglomerates of the Timan Ridge, anthropogenic gold placers of the Yenisei Ridge, kaolinite and laterite weathering crusts of Salair, Kazakhstan, and the Republic of Guinea, and the collections of placer gold from the A.E. Fersman Museum (Moscow), the Central Research Institute of Geological Prospecting for Base and Precious Metals (Moscow), the Moscow Mining Academy, and ZAO VNESHMET (Moscow). In the course of experimental studies, we investigated for the fist time the mechanical transformation of gold particles under the impact of sand–air flow in the eolian conditions and under the lithostatic pressure of the overlying strata on ancient gold-bearing conglomerates. We also used a number of mineralogical and geochemical methods to study the typomorphic features of native gold. The evolution of gold under exogenous conditions depends on the ambient environment. Examination of weathering crust has revealed gold nano- and microparticles resulted from the decomposition of sulfides, tellurides, and other unstable gold-containing compounds. Newly formed gold nano- and microparticles in the form of finest crystals, dendrites, and globules are deposited on the surface of primary endogenous gold. The gold formed in weathering crust is spongy and nodular; the fineness of primary endogenous gold increases, the content of impurity elements in it decreases, and a high-fineness porous shell forms at the edges of the gold particles. In the hydrodynamic environment, placer gold, independently of its form (hemihedral, euhedral, interstitial, etc.), flattens and undergoes a simple deformation, but its chemical composition and internal structure change little; they depend on the stage of ore formation and on the mineragenic type of the gold ore source. We have established that the chemical composition and internal structure of gold change during its long occurrence in the environment and under its repeated redeposition from ancient (Precambrian) to younger (Quaternary) deposits. Based on the obtained results of experimental and mineralogical studies, we have proved that eolian processes change not only the shape of native gold but also its chemical composition and microhardness. In the eolian environment, placer gold of different shapes tends to become a globule with a film-fibrous surface. The change in the shape of gold is accompanied by an increase in its fineness, a decrease in the content of impurity elements, and, as a result, decrease in the gold microhardness. In ancient conglomerates (ancient fossil placers), placer gold subjected to the lithostatic pressure of overlying deposits transforms into pseudo-ore gold. In metamorphic strata with constant temperatures and pressures, gold becomes refined. The identified indicators of placer gold of different exogenous environments make it possible to reconstruct the geologic and geomorphologic conditions of gold placer formation, namely, to determine the genetic type of placers (related to weathering crusts, alluvial, eolian, etc.) and to define the source areas (intermediate or primary sources). This helps to find a more correct technique for the search for gold placer and ore deposits.
Prediction and search for gold deposits in the east of the Siberian Platform are problematic because the study area is overlain by a thick cover of MZ–KZ deposits. Search for gold deposits by the largest geological institutions using conventional methods have not yielded positive results, because the main attention was focused on the discovery of ancient gold-bearing conglomerates of the Witwatersrand type and on the evaluation of the gold ore potential of basic magmatism. Typomorphism of placer gold bears huge information about the genesis of native gold, both its primary endogenous origin and its exogenous transformation, which makes it possible to identify the formation type of mineralization, increases the reliability of the prediction of gold deposits, and ensures their purposeful search in the platform areas. The revealed indicative features of placer gold made it possible to substantiate the formation of the gold ore sources of Precambrian low-sulfide gold–quartz, gold–iron-quartzite, porphyry gold–copper, and gold–PGE mineralization and Mesozoic gold–silver, gold–rare-metal, and gold–sulfide-quartz mineralization in the east of the Siberian Platform. We have established that high-fineness placer gold with microinclusions of pyrite, arsenopyrite, quartz, and carbonates with recrystallized structures and lines of plastic deformation is specific to the ore sources of low-sulfide gold–quartz mineralization. A high content of Cu (up to 4%) in flaky high-fineness gold is one of indicators of porphyry gold–copper mineralization. The angular shape of gold grains, the fine fraction and high fineness of gold, its completely recrystallized and regrown internal structure, and the permanent presence of Fe, Bi, and Cu microimpurities and hematite, ilmenite, and corundum microinclusions are typical of gold–iron-quartzite mineralization. Flaky and laminated high-fineness gold particles with steady Pt, Pd, and Ni impurities and Pt-mineral phases and Au–Pt intergrowths in them testify to the ore sources of gold–PGE mineralization. Laminated and cloddy gold fractions of > 0.25–2.0 mm, the medium and low fineness of gold, its single-crystal or, sometimes, porous internal structure, the wide range of microimpurities (Pb, Zn, As, Sb, Cu, Te, etc.), and microinclusions of native Ag, adularia, Sr-barite, and calcite are indicators of gold–silver mineralization. Laminated, dendritic, and cloddy-angular gold grains, wide variation in gold fineness (307–950‰), and the presence of microinclusions of native bismuth, maldonite, arsenopyrite, and silver tellurides are indicative of gold–rare-metal mineralization. Laminated and cloddy gold grains, their size varying from dust to > 0.25 mm, their mono- and coarse-grained internal structure, wide variation in gold fineness (600–900‰), and the presence of Hg microimpurities (up to 6% and more) and microinclusions of quartz, calcite, pyrite, arsenopyrite, tellurides, selenides, and REE phosphates point to gold–sulfide-quartz mineralization. The established placer gold indicators of the particular formation types of ore sources in the east of the Siberian Platform made it possible to predict Precambrian gold deposits with low-sulfide–gold–quartz mineralization similar to the Kirkland Lake and Porcupine mines and gold deposits with Mesozoic gold–silver mineralization similar to the Cripple Creek mine. The developed criteria for determining the types of mineralization by indicative features of placer gold give an insight into the ore genesis and can be successfully applied to prediction and search for gold deposits and to evaluation of their gold resources.
Based on the identified typomorphic features of placer gold, a set of determined morphogenetic criteria is proposed to identify the genesis of placer gold content and different sources in the platform areas, which allow more correctly selecting search methods and improving the efficiency of forecasting ore and placer gold deposits.Goldparticles larger than 0.25 mm with signs of wind-worn processing indicate the formation of autochthonous aeolian placers.Gold particles with signs of wind-worn processing with a size of 0.1–0.25 mm, forming an extensive halo of dispersion, indicate the formation of allochthonous placers in Quaternary deposits.Deflationary (autochthonous) placers of native gold can be found by the halo of its distribution of toroidal and sphericalhollow forms, which, of course, are the search morphogenetic criterion of aeolian placers.The presence of disc-shaped and lamellar gold particles with ridgelike edges in alluvial placers is typical for placers of heterogeneous origin, formed due to deflation of proluvialplacers.The discovery of pseudo-ore gold in alluvial placers indicates the arrival of gold from intermediate gold-bearing sources of different ages and not from primary sources, which is a morphogenetic criterion for determining different sources of the placer.In modern gold placers, the presence of gold of a pseudo-ore appearance can serve as a search criterion for the discovery of gold-bearing conglomerates with high gold content. The developed method for diagnosing the genotype of placer gold by its morphological characteristics (alluvial, aeolian, pseudo-ore) can be successfully used by industrial geological organizations to search and explore ore and placer gold deposits.
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