Mineralogical and Geochemical Features of Native Gold From Placers of the Evota Gold-Bearing Region (Russia, Aldan Shield)

Evota gold-bearing region is located in south of Republic Sakha (Yakutia) within the Nimnyr terrane of Aldan shield. A large numbers of gold-bearing placers are known on studied territory, but the primary sources for them have not been established. In this work, based on the study of the mineralogical and geochemical features of gold from alluvial deposits of the Evota gold-bearing region, possible genetic types of primary sources are considered. Obtained data showed that native gold in the studied objects has a very high, high and medium fineness. The roundness of gold is different. Almost ore crystals with sharp edges and well-rounded individuals with polished faces were found. The fineness of the studied gold grains was determined by microprobe analyzer Cameca Camebax-micro and varies in the range from 812 to 1000 %⁰. A thin (up to 20 μm) high-grade rim was found in two grains (cr. Zolotoy). The central part of one of them has a fineness of 865 %o, and in the edge part it reaches 1000 %⁰, which indicates that this gold was in the hypergenesis zone. Admixture elements determined as traces and are presented Cu, Pd, Fe, Ni. Minerals-microinclusions - quartz, potassium feldspar, pyroxene, staurolite, maldonite, bismuthite revealed in gold grains. In some watercourses, for example, in the creek Sukhoi, only very high fineness gold (993-1000 %⁰) was found. The presence of high-grade gold (cr. Sukhoi), intergrowths of gold with bismuthite (cr. Zolotoy), as well as inclusions of maldonite (cr. Yagodny) gives opportunity for assuming that primary sources could be basic ores such presented in the P. Pinigin deposit. At the sites (cr. Elovyi, r. Evota), both medium-grade and very high-grade gold were found, the fineness range varied from 827 to 998 %⁰. The presence of gold with medium fineness and good roundness in studied watercourses probably indicates an additional supply of gold from primary sources formed as a result of the alkaline magmatism development of the Mesozoic age. Thus, for gold with high and very high fineness by admixture-elements, chemical composition and microinclusions, a genetic relationship with primary sources similar to the P. Pinigin deposit was found; for gold grains with medium fineness, formation in gold-ore mineralization characterized for the Mesozoic stage ore development such in deposits of Central-Aldan ore region is assumed.

An inorganic origin of the “oil-source” rocks carbon substance

On the basis of an inorganic concept of the petroleum origin, the phase relationships of crystalline kerogens of black shales and liquid oil at the physicochemical conditions of a typical geobarotherm on the Texas Gulf Coast are considered. At the conditions of the carbon dioxide (CO2) high fluid pressure, the process of oil transformation into kerogens of varying degrees of “maturity” (retrograde metamorphism) takes place with decreasing temperature and hydrogen pressure. Kerogen generation in black shale rocks occurs by the sequential transition through metastable equilibria of liquid oil and crystalline kerogens (phase “freezing” of oil). The upward migration of hydrocarbons (HC) of oil fluids, clearly recorded in the processes of oil deposit replenishment in oil fields, shifts the oil ↔ kerogen equilibrium towards the formation of kerogen. In addition, with decreasing of the hydrogen chemical potential as a result of the process of high-temperature carboxylation and low-temperature hydration of oil hydrocarbons, the “mature” and “immature” kerogens are formed, respectively. The phase relationships of crystalline black shale kerogens and liquid oil under hypothetical conditions of high fluid pressure of the HC generated in the regime of geodynamic compression of silicate shells of the Earth in the result of the deep alkaline magmatism development. It is substantiated that a falling of hydrogen pressure in rising HC fluids will lead to the transformation of fluid hydrocarbons into liquid oil, and as the HC fluids rise to the surface, the HC ↔oil ↔ kerogen equilibrium will shift towards the formation of oil and kerogen. It is round that both in the geodynamic regime of compression and in the regime of expansion of the mantle and crust, carboxylation and hydration are the main geochemical pathways for the transformation of oil hydrocarbons into kerogen and, therefore, the most powerful geological mechanism for the black shale formations.

Short-lived alkaline magmatism related to Réunion plume in the Deccan large igneous province: inferences from petrology, 40Ar/39Ar geochronology and paleomagnetism of lamprophyre from the Sarnu-Dandali alkaline igneous complex

Existing geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40Ar/39Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40Ar/39Ar age determinations of the phlogopites from this lamprophyre, reveal two distinct ages of 65.44±1.5 Ma and 68.17±1Ma. However, paleomagnetic results show a VGP at 32.31 N and 298.52 E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at ca. 65.44±1.5Ma. Analyzed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of ca. 68.17±1Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB- type enriched (metasomatized) mantle source with an involvement of phlogopite.This finding suggests that the pre-Deccan ages of ca. 68-69 Ma reported previously, may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at ca. ∼68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short-duration (occurred between 67-65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5490881

Ultramafic Alkaline Rocks of Kepino Cluster, Arkhangelsk, Russia: Different Evolution of Kimberlite Melts in Sills and Pipes

To provide new insights into the evolution of kimberlitic magmas, we have undertaken a detailed petrographic and mineralogical investigation of highly evolved carbonate–phlogopite-bearing kimberlites of the Kepino cluster, Arkhangelsk kimberlite province, Russia. The Kepino kimberlites are represented by volcanoclastic breccias and massive macrocrystic units within pipes as well as coherent porphyritic kimberlites within sills. The volcanoclastic units from pipes are similar in petrography and mineral composition to archetypal (Group 1) kimberlite, whereas the sills represent evolved kimberlites that exhibit a wide variation in amounts of carbonate and phlogopite. The late-stage evolution of kimberlitic melts involves increasing oxygen fugacity and fluid-phase evolution (forming carbonate segregations by exsolution, etc.). These processes are accompanied by the transformation of primary Al- and Ti-bearing phlogopite toward tetraferriphlogopite and the transition of spinel compositions from magmatic chromite to magnesian ulvöspinel and titanomagnetite. Similar primary kimberlitic melts emplaced as sills and pipes may be transitional to carbonatite melts in the shallow crust. The kimberlitic pipes are characterised by low carbonate amounts that may reflect the fluid degassing process during an explosive emplacement of the pipes. The Kepino kimberlite age, determined as 397.3 ± 1.2 Ma, indicates two episodes of ultramafic alkaline magmatism in the Arkhangelsk province, the first producing non-economic evolved kimberlites of the Kepino cluster and the second producing economic-grade diamondiferous kimberlites.