Evolution of Native Gold under Exogenous Conditions

Abstract

––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.

Internal Structures of Placer Gold as an Indicator of Endogenous and Exogenous Processes

“…It has been revealed that such thick high-grade shells are characteristic of type I flake gold, which is very widespread in the east of the Siberian Platform. Thick high-grade shells (20-30 microns) on low-grade flake gold were found both in the northeast of the Siberian Platform in the basin of the Udzha river and in its central part in the Chebyda, Namana, Tonguo rivers and in its southern part in the basin of the middle Lena river (Figure 2d) [9,10]. The identified high-grade shells on gold particles with deformation lines are extremely rare (Figure 2e,f).…”

“…The internal structure of type II placer gold is practically unchanged, characterized mainly by mono-grain and coarse-medium-grained structures; sometimes, very thin highgrade shells are noted (Table 1; Figure 3). In addition, it has been found to contain unclearzonal and interblock structures, as well as a porous, spongy structure characteristic of gold from near-surface deposits, found at the mouth of the Bolshoi Patom river, indicating the proximity of the primary source [9,10]. The placer gold of the Mesozoic stage of ore formation is also characterized by granulation, disintegration structures (Figure 4а-c) and unclear-zonal structures (Figure 4d,e), found in the basin of the middle Lena, along with gold, whose internal structures are the structures of primary recrystallization and secondary recrystallization.…”

“…Patom), Sensky (Tokko and Torgo rivers), activated in the Mesozoic. Internal structures in exogenous conditions were changed, depending on the conditions where native gold was located [10].…”

“…Internal structures in exogenous conditions were changed, depending on the conditions where native gold was located [10].…”

“…The indicators of placer gold (morphology, impurity elements, microinclusions, internal structures, etc.) carry extensive information about the nature of native gold, both in endogenous and exogenous conditions [1][2][3][4][5][6][7][8][9][10][11]. In exogenous conditions, the evolution of placer gold occurs, but it retains the "memory" of its primary sources.…”