Ekaterina V. Nagornaya

This paper is focused on the results of the Rb–Sr isotope dating of gold-bearing quartz–sericite–dolomite altered wall-rock (beresite) at the Kekura deposit, Western Chukotka. The results obtained constrain the temporal relationship between ore formation and the emplacement of granodiorite. Due to the fine-grained nature of the metasomatic rock, a density fractionation was used to separate mineral sample for subsequent isotope study. The Rb–Sr isochron yields an age of 111.4 ± 1.1 Ma for altered wall-rock, which is consistent with the U–Pb zircon age of 110.6 ± 1.2 Ma obtained from the same sample. The established age of the rocks indicates an absence of a significant time gap between the granodiorite intrusion and the hydrothermal alteration processes associated with gold mineralization at the Kekura deposit. These findings provide a strong evidence for a relation of the studied processes within a single ore-forming system.

Scheelite of the Kekura gold deposit in the Western Chukchi Peninsula is reported for the first time. Three generations of the mineral have been identified

The concentration of the Al and Ti paramagnetic impurity centers in pre-ore and ore-stage quartz at the Peschanka porphyry copper–molybdenum–gold deposit in the Western Chukchi Peninsula, Russia were determined using electron paramagnetic resonance spectroscopy (EPR). The [AlO₄^-/h⁺]⁰ concentration in pre-ore and ore-stage quartz varies from 29 to 124 and from 13 to 101 at. ppm, respectively. The contents of the [TiO₄^-/Li⁺]⁰ and [TiO₄^-/H⁺]⁰ centers reach 20 and 6.3 at. ppm, respectively. Pre-ore quartz associated with the formation of biotite–potassium feldspar–quartz alteration and ore-stage quartz associated with the formation of quartz–sericite rocks followed by the ore deposition differ considerably in the Ti center content, especially the [TiO₄^-/H⁺]⁰ center. The [TiO₄^-/H⁺]⁰ concentration is much higher in the pre-ore quartz (>2 at. ppm) than that in the ore-stage quartz related to copper mineralization (4^-/Li⁺]⁰ concentration also decreases from pre-ore to ore-stage quartz. Taking the data we obtained into account, the formation temperature of pre-ore and ore-stage quartz estimated from a titaniumin-quartz geothermometer is 590–470°C (weighted average 520°C) and 510–310°C (weighted average 430°C), respectively. The obtained temperature range of 590 to 310°C is similar to that determined from homogenization of fluid inclusions in quartz.

Mineralogical, fluid inclusion, and geochemical studies of precious metal mineralization within the Baimka trend in the western Chukchi Peninsula have been preformed. Porphyry copper–molybdenum–gold deposits and prospects of the Baimka trend are spatially related to monzonitic rocks of the Early Cretaceous Egdygkych Complex. Four types of precious metal-bearing assemblages have been identified: (1) chalcopyrite + bornite + quartz with high-fineness native gold enclosed in bornite, (2) low-Mn dolomite + quartz + sulfide (chalcopyrite, sphalerite, galena, tennantite-tetrahedrite) ± tourmaline with low-fineness native gold and hessite, (3) rhodochrosite + high-Mn dolomite + quartz + sulfide (chalcopyrite, sphalerite, galena, tennantite- tetrahedrite) with low-fineness native gold, electrum, acanthite, Ag and Au–Ag tellurides, and Ag sulfosalts, and (4) calcite + quartz + sulfide (chalcopyrite, sphalerite, galena) with low-fineness native gold, Ag sulfides and selenides, and Ag-bearing sulfosalts. Study of fluid inclusions from quartz, sphalerite, and fluorite have revealed that hydrothermal ores within the Baimka trend precipitated from fluids with strongly variable salinity at temperatures and pressures ranging from 594 to 104°C and from 1200 to 170 bar, respectively. An indicator of vertical AgPbZn/CuBiMo geochemical zoning is proposed. The value range of this indicator makes it possible to estimate the erosion level of the porphyry–epithermal system. The erosion level of the Baimka deposits and prospects deepens in the following order: Vesenny deposit → Pryamoi prospect → Nakhodka prospect → Peschanka deposit → III Vesenny prospect.