Обобщены результаты исследований Баимской рудной зоны (БРЗ) на Западной Чукотке, полученные при проведении поисково-оценочных и разведочных работ в 2008–2016 гг., и показаны основные особенности ее строения и развития. Формирование рудной минерализации порфировых и эпитермальных рудных систем БРЗ происходило в раннемеловое время в зоне глубинного правого сдвига северо-западного простирания. Меридиональные структуры растяжения и диагональные сколы в зоне сдвига контролировали позицию и морфологию интрузивных тел монцонитоидов и парагенетически связанных с ними рудных штокверков с медно-порфировой и золото-серебряной эпитермальной минерализацией. Рудные штокверки прослеживаются бурением на глубину до 700 м и прогнозируются глубже по геофизическим данным. Описана зональность аномальных геохимических полей вторичных ореолов и первичная геохимическая зональность месторождения Песчанка и Находкинского рудного поля (НРП). Эрозионный срез месторождений и проявлений различный. Для месторождения Песчанка установлен верхне-среднерудный срез, для проявлений НРП эрозионный срез изменяется от верхнерудного до нижнерудного. Выявлены новые перспективные участки в пределах БРЗ, где прогнозируется промышленное медно-порфировое и золото-серебряное эпитермальное оруденение.

The results of modern studies of the Baimka Ore Zone (BOZ) in Western Chukotka obtained during prospecting and exploration in 2008–2016 are summarized, and the main features of its structure and development are shown. The porphyry–epithermal ore systems of the BOZ were formed within the NW-trending regional-scale dextral strike-slip fault in the Early Cretaceous time. Meridional extensional structures and diagonal strike-slip faults in the strike-strip fault zone controlled the position and morphology of intrusive bodies of monzonites and paragenetically related with them ore stockworks with porphyry copper and gold–silver epithermal mineralization. Ore stockworks were traced to 700-m depth by drilling, and, accordingly to the geophysical data, mineralization is forecasting deeper. The zoning of soil anomalies and the primary geochemical zoning of the Peschanka deposit and the Nakhodka ore field are described. An erosion levels of deposits are different. For the Peschanka deposit, an upper-middle erosion level has been established. For the deposits of the Nakhodka Ore Field, the erosion grade changes from the upper to the lower level. New prospects have been identified within the BOZ, where economical porphyry copper and gold–silver epithermal mineralization is predicted.

The Nakhodka ore field (NOF) is situated in the Baimka Trend, Chukotka, Russia, and comprises the Vesenny epithermal Au–Ag, and Malysh, Nakhodka, Vesenny III, and Pryamoy porphyry Cu-Au ± Mo deposits. Porphyry and epithermal mineralization of the NOF are hosted by Early Cretaceous diorite and monzonite intrusions, which are dated at 139–141 Ma (U–Pb zircon). The NOF mineralization is structurally controlled. The prevailing stress field during the evolution in the Baimka dextral shear zone (also known as Baimka Trend) has led to the formation of extensional and strike-slip structures that control distinct zones with strong quartz-sericite alteration and sheeted high-grade quartz–sulfide veining; characteristics that are similar to the world-class Peschanka porphyry Cu-Au deposit located about 20 km to the NW of the NOF. Four types of hydrothermal alteration are documented in the NOF: (1) potassic, (2) propylitic, (3) quartz-sericite, and more rarely (4) argillic. Two phases of porphyry-style mineralization are distinguished: (1) early-stage quartz-magnetite veining associated with potassic alteration and (2) sheeted quartz-sulfide (bornite, chalcopyrite, molybdenite, pyrite) veining that is spatially associated with a strong quartz-sericite alteration assemblage. Epithermal Au–Ag mineralization belongs to the intermediate-sulfidation type and consists of gold-bearing polymetallic quartz-dolomite ± rhodochrosite veins and veinlets. The NOF is defined by a distinct geochemical zonation. Geophysical data show that the high-grade stockwork zones at the Vesenny III porphyry Cu-Au deposit are defined by pronounced magnetic anomalies reflecting abundant hydrothermal magnetite veining, while the Vesenny epithermal Au–Ag deposit is defined by a strong negative magnetic anomaly due to strong silicification and magnetite-destructive quartz-sericite to argillic alteration.

Porphyry Cu-Au +/- Mo mineralization at Peschanka is hosted by monzodiorite and monzonite intrusions with high-K calc-alkaline to shoshonitic compositions and dated at about 144.1 +/- 1.5 Ma, using U/Pb zircon ages. The Cretaceous intrusions are emplaced in a melange of Cretaceous island arcs, a tectonic setting comparable with other world-class porphyry Cu-Au deposits, such as Oyu Tolgoi, Mongolia and Pebble, Alaska. Abundant primary magnetite contents of the Peschanka intrusions, as well as numerous gypsum and anhydrite veins, reflect the high oxidation states of their parental magmas. This mineralogical interpretation is confirmed by high whole-rock Fe2O3/FeO ratios and high V/Sc ratios of the rocks of up to 1.27 and up to 21.9, respectively. The whole-rock Eu/Eu* ratios of the Peschanka intrusions are >= 1 which is also typical for potassic igneous rocks with high oxidation states. Abundant amphibole and biotite phenocrysts of the intrusions as well as their high whole-rock Sr/Y ratios of up to 225 document significantly high H2O contents of the highK magmas. Peschanka contains a resource of >9.5 Mt of copper at an average grade of 0.43 wt% and 16.5 Moz of gold at a high average grade of 0.23 g/t and thus represents one of the largest undeveloped greenfield copper projects worldwide. The vicinity of Peschanka still offers significant brownfield exploration potential. The hypogene vein-related and disseminated Cu-Au +/- Mo sulfide mineralization at Peschanka is structurally controlled by significant NE-trending strike-slips that acted as the conduits for the hydrothermal fluids. The central part of the orebody consists of high-grade north-south-trending sheeted quartz-bornite veining with unusually high vein densities. The highest Cu and Au grades are directly correlated with high vein densities. Peschanka is defined by distinct hydrothermal alteration zones including potassic, phyllic, propylitic and argillic assemblages, but a distinct lack of advanced argillic alteration. The mineralization itself is also zoned ranging from a central Mo-Cpy-Bn sulfide assemblage to a peripheral Py-Mt-dominated zone ('pyrite-shell'). Late-stage polymetallic assemblages overprint and surround the main stockwork zone.

The paper presents the results of geological and structural analysis and identification of distribution patterns and spatial orientation of gold-bearing quartz-sulfide veins within quartz stockworks in the central part of the Drazhnoe deposit (Taryn gold field). The results of direct observations in outcrops and exploration drillholes oriented core were used as input data. Based on the results of the analysis, high thickness and high gold content systems of quartz veins and veinlets were identified. The orientation of tectonic stress axes during the ore phase, during which gold-bearing quartz stockworks were formed, was reconstructed.

Orogenic gold-quartz deposits have a clear structural control and are accompanied by wallrock metasomatic alteration. However, in detailed modeling of such deposits, there is often a mismatch between the structural plans for high-grade ore zone distribution and metasomatite zones, and the latter are not always associated with faults. This is explained by the evolution of the hydrothermal process and the pulsating nature of the development of the territory. In the early stages of the mineral deposit study, it is very important to reliably determine the distribution of ore zones, since the correct targeting of the drilling program and the economic deposit assessment depend on it. The problem can be solved using the method of X-ray computed tomography (СТ) in the core study. This paper presents the methodology of studying fullsize core samples of gold deposits by using CT. A core sample characterizing the central part of ore body of Drazhnoye deposit (Tarynskoye ore field, Republic of Sakha (Yakutia)) was used as the study material. The sample studied was scanned by a SIEMENS Somatom Perspective tomograph at two energies (80 and 130 keV). As a result, a detailed three-dimensional stereological model of the core was obtained, which made it possible not only to study the distribution of ore minerals in the volume of the entire sample, but also to identify vein bodies of different ages, as well as to study their morphology and trace the distribution patterns of ore mineralization in them. Based on the study results, we can offer a preliminary interpretation of ore mineralization and vein formation sequence.

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 [AlO 4 - /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 [TiO 4 - /Li+]0 and [TiO 4 - /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 [TiO 4 - /H+]0 center. The [TiO 4 - /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 (<2 at. ppm). The [TiO 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.

The Al and Ti paramagnetic impurity center concentrations in pre-ore and ore-bearing quartz at the Peschanka porphyry copper-molybdenum-gold deposit in the Western Chukchi Peninsula, Russia were determined using electron paramagnetic resonance (EPR). The [AlO4-/ h+]0 concentration in pre-ore and ore-bearing quartz varies from 29 to 124 and from 13 to- + 0- + 0101 at. ppm, respectively. The contents of [TiO4 /Li ] - and [TiO4 /H ] -centers reach 20 and6,3 at. ppm, respectively. Pre-ore stage quartz associated with the formation of biotite-potassiumfeldspar-quartz alteration and quartz of the ore stage associated with the formation of quartz-sericite rocks followed by the ore deposition are substantially in content of the titanium centers,- + 0- + 0especially [TiO4 /H ] -centers: the [TiO4 /H ]concentration is much higher in the pre-orestage quartz (>2 at. ppm) than that in quartz related to copper mineralization (<2 at. ppm).+ 0The [TiO4-/Li ]concentration also decreases from pre-ore to ore-bearing quartz. Taking intoaccount the data obtained, the formation temperature of pre-ore and ore-bearing quartz estimatedfrom a titanium-in-quartz geothermometer is 590-470 °C (average 520 °C) and 510-310 °C (average 430 °C), respectively. The obtained temperature range of 590 to 310 °C is similar to that determined from homogenization of fluid inclusions in pre-ore and ore-bearing.

The Peschanka deposit and Nakhodka ore field occurring in the Baimka Cu-Mo-Au porphyry-epithermal trend in the western Chukchi Peninsula, Russia are spatially related to monzonitic rocks of the Early Cretaceous Egdykgych Complex. Tetrahedrite solid solution (Tt(s)) was recognized at the deposit and within the ore field in the following assemblages: (1) porphyry stage bornite, chalcopyrite, and molybdenite, (2) transitional (subepithermal) stage sphalerite, galena, chalcopyrite, and As-free pyrite, (3) HS epithermal stage enargite, chalcopyrite and high-fineness native gold, and (4) IS epithermal stage As rich pyrite, galena, sphalerite, and low -fineness native gold and electrum. The porphyry and subepithermal Its crystals are oscillatory zoned because of variable contents of Sb and As. The literature data show that similar zoning has been recognized in the Tt(s) crystals in the other porphyry Cu-Mo-Au and transitional assemblages and differs from the other type deposits. Therefore, such a zoning is considered to be a guide to these two mineralization types. No zoning was found in Tts referred to the HS and IS epithermal assemblages within the Baimka trend. The porphyry stage Tts evolves from Fe-rich tennantite ((sb = Sb/(Sb + As) below 0.01, fe = Fe/(Fe + Zn) 0.60-0.80) through oscillatory zoned tennantite enriched in Sb and Zn (sb 0.19-0.37, fe 0.56-0.66) to tetrahedrite enriched in Zn (sb 0.51-0.70, fe 0.39-0.66). This trend is caused by the Sb accumulation and increased f(s2), The review of published data shows that such trend is typical of the other porphyry deposit worldwide. Therefore, it is considered to be a guide to distinguish porphyry deposits from the other type deposits containing fahlores and to distinguish porphyry stage fahlores. The composition of the transitional stage Tts evolves from Zn-rich tetrahedrite (sb 0.56-0.82, fe 0.03-0.05) through Zn-rich tennantite (sb 0.03-0.19, fe 0.11-0.13) followed by Zn-rich oscillatory zoned solid solution (sb 0.03 to 0.69, fe 0.09 to 0.11) to goldfieldite. This evolution testifies to increased f(Te2) to the end of transitional stage. The HS stage Tt(s) corresponds to Fe -rich tennantite (sb below 0.05, fe 0.65-1.00) containing high Cu-excess (1.43 apfu). The IS stage Tt(s) evolves from Zn-rich tennantite (sb = 0, fe = 0.44) to Zn-rich tetrahedrite (sb = 0.97, fe = 0.03). The latest Zn-rich tetrahedrite of this assemblage is enriched in Ag (up to 4.1 wt.%) testifying to increasing Ag activity to the end of mineralizing process. The review of literature data shows the similar trend for the transitional and IS Tts in the other porphyryepithermal systems. Therefore the Tt(s) evolution trend is a criterion to separate porphyry, transitional, IS, and HS Tt(s). (C) 2017 Elsevier B.V. All rights reserved.

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.

Au-Mo-Cu-порфировое месторождение Песчанка и рудное поле Находка, входящие в состав Баимской рудной зоны в Западной Чукотке, пространственно связаны с монцонитоидами раннемелового егдыкгычского комплекса. На месторождениях и проявлениях установлены два типа кварц-серицитовых метасоматитов (КСМ): (I) хлорит-кварц-мусковитовые породы с борнитом и халькопиритом (мезотермальный или порфировый тип); (II) турмалин-кварц-карбонат-мусковит ± фенгитовые породы, сопровождающиеся жилами с полисульфидной минерализацией (субэпитермальный или переходный тип) и карбонат-кварц-иллитовые аргиллизиты – породы с жилами, содержащими благороднометальное оруденение (эпитермальный тип). Хлорит кварц-серицитовых пород эволюционирует от шамозита к клинохлору, что обусловлено ростом активности сульфидной серы в минералообразующем флюиде и осаждением сульфидных минералов. Клинохлор КСМ-I существенно обеднен кремнеземом по сравнению с клинохлором аргиллизитов. Химический состав мусковита кварц-серицитовых метасоматитов I и II типа не различается. Установлена эволюция состава карбонатов кварц-серицитовых пород II типа (кальцит доломит сидерит), обусловленная сначала увеличением активности CO2, а затем снижением активности сульфидной серы в минералообразующем флюиде. По содержанию Mn эти карбонаты близки к таковым в плутоногенных золотоносных березитах-лиственитах. Иллит аргиллизитов менее глиноземистый по сравнению с иллитом вулканогенных Au-Ag-месторождений. Однако карбонаты этих пород – родохрозит и высокомарганцовистый доломит – идентичны карбонатам кварц-иллитовых пород вулканогенных Au-Ag-месторождений.

The porphyry Peschanka copper-molybdenum-gold deposit and the Nakhodka ore field located in the Baimka ore trend on the western Chukchi Peninsula are spatially related to monzonitic rocks of the Early Cretaceous Egdykgych Complex. Two types of quartz-sericite metasomatic rocks (QSR) have been identified at both the deposits and the ore field: (I) chlorite-quartz-muscovite rock with bornite and chalcopyrite (porphyry type) and (II) tourmaline-quartz-carbonate-muscovite ± phengite rock accompanied by veins with base-metal mineralization (subepithermal or transitional type), as well as carbonate-quartz-illite rock (argillic alteration) accompanied by veins with precious metal mineralization (epithermal type). The QSR I chlorite evolves from chamosite to clinochlore, which is caused by increasing H 2 S activity in mineralizing fluid and precipitation of sulfide minerals. The QSR I clinochlore is significantly depleted in silica as compared with that from the rocks affected by argillic alteration. The chemical composition of muscovite from both quartz-sericite alterations is similar. The QSR II carbonates evolve from calcite through dolomite to siderite, which results from the increasing activity of CO 2 followed by the decreasing activity of H 2 S in mineralizing fluid. The Mn content in dolomite is similar to that in beresite (quartz-muscovite-carbonate-pyrite metasomatic rock) of the intrusion-related gold deposits. Illite from argillic alteration is depleted in Al as compared with that of postvolcanic epithermal Au-Ag deposits. However, carbonates from the discussed argillic alteration rhodochrosite and Mn-rich dolomite are similar to those from quartz-illite rock at postvolcanic epithermal Au-Ag deposits.