Indicative features of placer gold for the prediction of the formation types of gold deposits (east of the Siberian Platform)

Abstract

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.

Typomorphic Features of Placer Gold from the Billyakh Tectonic Melange Zone of the Anabar Shield and Its Potential Ore Sources (Northeastern Siberian Platform)

“…The wide variation of gold fineness is probably due to the unstable physical and chemical environment of the ore body formation. The absence of high-grade fringes in native gold excludes its prolonged stay in exogenous environments [4,36]. This is also confirmed by the detection of a small homogeneous gold nugget with very thin and short fringes of the initial stage in the peripheral part of the polished shear ( Figure 6).…”

“…The discovery of a small gold nugget also signifies the proximity of the ore body. It is important to emphasize that metal finds of this size are very rare for placers in eastern Siberia, which is typically dominated by very small (−0.25 mm) gold grains, representing more than 95% [36]. Morphological features of native gold, such as a low ratio of roundness and a primary tubercular and dimpled surface, indicate its short stay in hydrodynamic environments and short-range water drift [37].…”

Mineralogical Criteria for the Prediction of Gold Mineralization in the Structures of the Siberian Craton

“…It is proved that placer gold carries enormous information, both about the endogenous origin of gold and about its change in exogenous conditions, depending on the environment [13]. Based on the study of morphology (size, shape, surface) of placer gold, chemical composition (fineness, impurity elements) and microinclusions, it became possible to predict the type of ore source and the stages of ore formation [14,15]. The use of this method made it possible, for the first time, to develop criteria for predicting the types of gold deposits and to link them with certain geological structures [16,17].…”

Material composition and geological-structural position of gold-bearing hydrothermal-metasomatic formations of the Anabar river basin (northeast of the Siberian Platform)

“…Никифоровой с соавторами на основании изучения минералого-геохимических особенностей самородного золота было обосновано два основных этапа рудообразования для всего востока Сибирской платформы, в том числе и исследуемой территории -докембрийский ПРИРОДНЫЕ РЕСУРСЫ АРКТИКИ И СУБАРКТИКИ, 2022, Т. 27, № 2 и мезозойский [6]. Однако, немногочисленные поисковые работы до последнего времени не привели к обнаружению коренных источников россыпей.…”