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Геология России

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Phase relations of arsenian pyrite and arsenopyrite
(2021) Stepanov A.S.; Large R.R.; Kiseeva E.S.; Danyushevsky L.V.; Goemann K.; Meffre S.; Zhukova I.; Belousov I.A.
Arsenian pyrite containing above 1 wt% As plays a crucial role in deposition and deportment of Au and other chalcophile elements. The importance of arsenian pyrite led to theoretical and experimental studies that examined properties and genesis of the mineral; however, the interpretation of the phase relations between arsenian pyrite and arsenopyrite is conflicting. In this contribution, we present the results of a review of the experimental studies that have investigated the crystallisation of pyrite in As-bearing systems, a summary of As content in pyrite coexisting with arsenopyrite in 37 deposits and the composition of arsenian pyrite in deposits with little or no arsenopyrite. The review demonstrates that the previous experimental studies that attempted to achieve equilibrium between pyrite and arsenopyrite observed from <1 to 4.7 wt% As in pyrite. The literature survey of the assemblages of pyrite and arsenopyrite shows that pyrite crystallising together with arsenopyrite commonly has a very heterogeneous composition with As content varying from below detection to about 10 wt% As and no clear discontinuities were observed across this range. In the deposits without arsenopyrite, arsenic content in pyrite can reach 20 wt% As. We consider three principal scenarios of the relations of arsenian pyrite and arsenopyrite: (A) Pyrite with high As content is stable in equilibrium with arsenopyrite. Low-As pyrite coexisting with arsenopyrite is a product of disequilibrium crystallisation; (B) a scenario of control of As content in pyrite coexisting with arsenopyrite by thermodynamic parameters including temperature, pressure, the activity of components and fluid composition and (C) a scenario where arsenian pyrite is a metastable mineral. The observations are inconsistent with a model of 5–6 wt% As in pyrite coexisting with arsenopyrite in equilibrium (scenario A). The stability range of the assemblage of pyrite and arsenopyrite constrains the thermodynamic control on the composition of pyrite crystallising in equilibrium with arsenopyrite (scenario B). The scenario of metastable crystallisation of arsenian pyrite (C) proposes formation of the mineral by fast growth from oversaturated fluids with As content controlled by surface adsorption and can explain such features as sector zoning of the mineral and the apparent negative temperature dependence of the solubility. The data phase relations of arsenian pyrite highlight the need for new experimental studies, and suggest that the scenario of disequilibrium phase relations of arsenian pyrite should be considered for interpretation of natural assemblages.
Petrogenesis of Zr–Nb (REE) carbonatites from the Arbarastakh complex (Aldan Shield, Russia): Mineralogy and inclusion data
(2021) Prokopyev I.R.; Doroshkevich A.G.; Zhumadilova D.V.; Starikova A.E.; Nugumanova Ya.N.; Vladykin N.V.
The Arbarastakh Neoproterozoic ultramafic carbonatite complex is located in the southwestern part of the Siberian Craton (Aldan Shield) and contains ore-bearing Zr–Nb (REE) carbonatites and phoscorites. Carbonatites are mainly represented by calcite and silicocarbonatite varieties. The primary minerals composing the carbonatites are calcite and dolomite, as well as phlogopite, clinopyroxene, fluorapatite, amphibole, fluorite, K-feldspar and feldspathoids. Olivine (forsterite), Ti-magnetite, apatite, phlogopite, calcite, dolomite and the minor spinel group minerals form the primary phoscorites. The ore-bearing Zr–Nb mineral assemblages of the phoscorites and carbonatites include accessory zircon, zirconolite, perovskite, pyrochlore and baddeleyite. The Ba–Sr–REE hydrothermal mineralisation consists of ancylite-(Ce), bastnaesite-(Ce) and burbankite, as well as barite-celestite, strontianite, barytocalcite, and rare Cu–Fe sulphides. The silicocarbonatites and carbonatites formed in multiple stages from a single alkaline Ca–Na–K–silicocarbonatite melt, while the phoscorites are products of differentiation of the carbonatitic melt and were crystallised from an Fe-rich phosphate–carbonate melt at temperatures of more than 720 °C. The silicate–phosphate–carbonate melts were responsible for the Zr–Nb mineralisation of the carbonatites at temperatures of more than 540–575 °C; the hydrothermal REE-bearing mineral assemblages crystallised from saline (60–70 wt%) carbonatitic fluids of Na–Ca–Mg–F–carbonate composition at a minimum temperature range of 350–300 °C. The Ca–Sr-carbonate as well as the Na–hydro–carbonate fluids were responsible for the Ba–Sr–REE mineralisation of the phoscorites at ~500–480 and 450–430 °C.
Platinum mineralization and geochemistry of the Matysken zoned Ural-Alaskan type complex and related placer (Far East Russia)
(2021) Kutyrev A.V.; Sidorov E.G.; Kamenetsky V.S.; Chubarov V.M.; Chayka I.F.; Abersteiner A.
Ural-Alaskan type complexes are the sources for unique placer platinum deposits and contain platinum-group minerals (PGM) within lodes. Such complexes occur along modern and ancient convergent tectonic margins and are comprised of dunite, wehrlite, clinopyroxenite and gabbro, which gradually interchange towards the marginal parts of the complex. In-situ PGM are largely restricted to chromitites, which are comprised of small veinlets and schlieren of chromian spinel in the dunite unit. PGM mineralization in chromitites can be extremely rich; however, their distribution is very sporadic, without any apparent regularity. Moreover, the role of accessory chromian spinel and post-magmatic overprinting of platinum group element (PGE) concentrations is poorly defined. Detailed studies of PGM assemblages, including those in chromitites, serpentine veinlets in dunites, and wehrlites, combined with PGE geochemistry could provide insights into the distribution and processes responsible for PGE accumulation. The Matysken complex in the Koryak Highlands (Far East Russia) is comprised of dunite, wehrlite, clinopyroxenite and gabbro units, and has a classic zoned structure, rendering it an excellent example of an Ural-Alaskan type complex, ideal for this case study. This locality contains mm-scale isoferroplatinum (Pt3Fe) nuggets that cement chromitites and occur as µm-scale euhedral inclusions in chromian spinel. Placer PGM assemblages in the nearby Matysken River consist of exactly the same assemblages of minerals that are found in dunites and chromitites but with larger compositional scatter, reflecting a complex history of alluvium accumulation, which was sourced from different eroded parts of the complex. Detailed textural investigations discovered a diverse array of hydrous silicate–related mineralization, including euhedral isoferroplatinum grains in chlorite matrix, isoferroplatinum–amphibole intergrowths, and a wide range of PGE, Fe and Cu alloys, sulfarsenides and antimonides, which formed in serpentine veinlets together with awaruite (Ni3Fe) and base metal sulfides. This provides further evidence that isoferroplatinum, which is widely accepted to be a magmatic mineral, may form under a wide range of conditions, including during serpentinization. Dunites have fractioned primitive mantle-normalized PGE patterns, typical of Ural-Alaskan type complexes, with strong enrichments in Pt and depletions in Ir, Ru and Pd. PGE, with an exception of Pd, are quite uniformly distributed in dunite containing accessory chromian spinel, with good correlations between PGE and Cr. This correlation, together with data from previous studies, allowed us to calculate the concentration of Pt in accessory chromian spinel (probably hosted as minute PGM inclusions), which is of the same grade as for chromitites (ppm levels). This suggests that the formation of chromitites does not concentrate PGE over Cr, but only assembles them into larger aggregates, causing the observed unevenness in the PGM distribution.
Typomorphic features of placer gold from the Bystrinsky ore field with Fe-Cu-Au skarn and Mo-Cu-Au porphyry mineralization (Eastern Transbaikalia, Russia)
(2021) Savichev A.A.; Nevolko P.A.; Kolpakov V.V.; Redin Yu.O.; Mokrushnikov V.P.; Svetlinskaya T.V.; Sukhorukov V.P.
Eastern Transbaikalia is one of the oldest gold-producing regions in Siberia, and for over 300 years it has been the largest source of the most important raw minerals in Russia. A large number of gold, gold-bearing complex, antimony, mercury and other mineral deposits are known within this region. The orogenic, intrusion-related, and epithermal deposits are important in the balance of gold reserves and gold production. Au-Cu-Fe-skarn deposits are located mainly in the northeastern and southeastern parts of the area, wherein the skarn Kultuma, Lugokan, and skarn- porphyry Bystrinsky deposits are the largest deposits. Herein, we studied the the Bystraya River gold placers and tributaries, which are located to the east of the Bystrinsky massif near the Bystrinsky deposit. In this study, a microchemical characterization technique was applied to gold grains to facilitate the classification of alluvial gold localities in terms of the style or styles of source mineralization, thereby permitting a comprehensive interpretation of regional gold mineralization. In total, 55 samples were collected from four placers and processed for more than 1500 analyses of the native gold. In the alluvial placers of the Bystrinsky deposit areas, weakly rounded native gold either prevails or is significantly present. A low portion of supergenically transformed of gold grains in the placers indicates that the placers formed directly from endogenous gold mineralization. The native gold from the Bystraya River catchment can be divided into 3 types and 1 subtype: (1) Cu-bearing gold grains with a fineness of 800 − 995‰, and a copper impurity of up to 0.73%; (2) Hg-bearing gold grains with a fineness of 800 − 995‰, and an impurity of Hg up to 4.68%; (2.1) Hg-poor gold grains with a fineness of 800 − 995‰, containing the same mineral microinclusions assemblages as the Type 2 gold grains; and (3) gold with a fineness of 400 − 770‰, containing up to 8.5% Hg. The distinguished types of gold grains are manifested to varying degrees at the Bystraya, Left, and Right placers. At at the Yakovlevsky placer, there is no Hg-bearing low fineness and poorly occurring Cu-bearing native gold. Comparing of the obtained data with gold grain composition data from the hypogene deposits allows us to infer bedrock sources. The Cu-bearing high-fineness type of placer gold can be compared with the Cu-bearing native gold of the Bystrinsky, Lugokan, and Kultuma deposits. Hg-bearing low-fineness placer gold grains related to erosion of the Au-bearing base metal mineralization occurred at the Novoshirokinsky and Kultuma deposits. However, Hg-bearing high-fineness gold grains, which most widely occurring in the studied placers, are absent in the in-situ mineralization. Therefore, we suppose that the bedrock source of this gold grain type is linked to the completely eroded upper levels of the Bystrinsky deposit. Thus, our study shows that it is possible to speculate on the nature of eroded material from the gold particle signature. Hense, this generic approach informs regional metallogeny in a way that even detailed field-based classical geology/geochemistry cannot.
The Kirganik alkalic porphyry Cu-Au prospect in Kamchatka, Eastern Russia: A shoshonite-related, silica-undersaturated system in a Late Cretaceous island arc setting
(2021) Soloviev S.G.; Kryazhev S.G.; Shapovalenko V.N.; Collins G.S.; Dvurechenskaya S.S.; Bukhanova D.S.; Ezhov A.I.; Voskresensky K.I.
The Kirganik prospect is a silica-undersaturated, shoshonite-related, alkalic, porphyry Cu-Au system developed within the Late Cretaceous island arc of Central Kamchatka. It is associated with a potassic volcano-plutonic suite of rocks that range from monzogabbro- and monzodiorite- to monzonite- and syenite-porphyry, followed by a second, compositionally overlapping magmatic cycle of trachytic syenite-porphyry, and possibly other, more differentiated monzonitic phases. The rocks exhibit a subduction-related, island arc rock affinity, with the magma generation involving low degree (~1–3 vol%) partial melting of metasomatized, K-rich lithospheric mantle, followed by amphibole fractionation in a deep, probably lower crustal, magma chamber, with subsequent fractionation of accessory minerals, clinopyroxene and possibly biotite in shallower magma chambers. An early calc-potassic alteration assemblage of K feldspar-pyroxene-apatite-magnetite is closely related to small syenite intrusions. This stage is overprinted by more extensive potassic alteration zones of similarly quartz-free biotite-K feldspar-magnetite, some of which contain abundant Cu-sulfides (mostly bornite and chalcopyrite, with minor chalcocite), minor native gold and PGE minerals. Subsequent sodic-potassic to calcic-sodic-potassic alteration comprising albite-magnetite-calcite-epidote, and propylitic assemblages of chlorite-magnetite-epidote ± pyrite are much more weakly developed. These hydrothermal alteration assemblages form lens-shaped, replacement-style zones, rather than stockworks of well-defined veinlets/stringers, particularly quartz-sulfide veinlets, which are absent. Apatite from calc-potassic and potassic alteration assemblages contains low salinity (5–10 wt% NaCl-eq.) liquid-gaseous fluid inclusions indicating a homogenous, low salinity, supercritical aqueous fluid exsolved directly from a cooling and degassing pluton, with no boiling and unmixing into hypersaline and gaseous phases. Under inferred high temperatures of calc-potassic and potassic alteration (450–550 to 400–420 °C) and significant (2.1–2.2 to 1.5–1.8 kbar) pressure, this suggests a deep (>5 km?), possibly root-level of a larger, shallower, since eroded, porphyry Cu-Au system. The current geological setting and distribution of alteration and mineralization can be explained by a sharp, up to 90°, post-mineral tilting of the initial narrow, subvertical, 'finger-like' intrusions and related mineralized system, followed by deep exhumation. As a consequence, a significant part (>1.5 km) of the deep porphyry system is currently exposed at surface.