I. A. Baksheev

Tourmaline as a prospecting guide for the porphyry-style deposits

Tourmaline-supergroup minerals are proposed as efficient indicators of porphyry-style Cu, Au and Sn deposits and therefore are considered as useful for both the prospection and exploration of these important mineral deposits. The tourmaline-supergroup minerals from the porphyry deposits exhibit some general features: (1) sector and oscillatory chemical zoning of crystals; (2) presence in several generations; (3) Fe → Al coupled substitution; (4) evolution from Fe-rich to Mg-rich varieties resulting from sulphide deposition at the late stage; (5) Li concentration ranging from a few to ~30 ppm. On the other hand, there are distinguishing features for tourmalines from different deposits: (1) total Mg content is ~2 apfu for tourmaline in the Cu deposits, 1–2 apfu in the Au deposits, and 0–1 apfu in Sn deposits; (2) maximum Fe tot ~3 apfu (Cu, Sn) and ~5 apfu (Au), (3) maximum F content up to 0.1 apfu (Cu, Au) and up to 0.5 apfu (Sn); (4) Fe 3+ /Fe tot ~0.5–0.8 (Cu, Au) and ~0.2 (Sn).

Composition, formation conditions, and genesis of the Talatui gold deposit, the Eastern Transbaikal Region, Russia

The mineral composition of the Talatui gold deposit has been studied with modern methods. Previously unknown minerals (ilmenite, siegenite, glaucodot, wittichenite, matildite, hessite, pilsenite, zircon, tremolite, cummingtonite, hercynite, and goethite) have been identified in the ore. A high Re content has been detected in molybdenite. The spatiotemporal separation of Au and Ag is caused by different mineral species of these elements and their diachronous precipitation during the ore-forming process. Gold crystallized along with early mineral assemblages, beginning from virtually pure gold (the fineness is 996). Silver precipitated largely at the end of the process as hessite (Ag2Te) and matildite (AgBiS2). The temperature of ore deposition varied from 610 to 145°C, the pressure was 3370–110 bar, and the salt concentration ranged from 56.3 to 0.4 wt % NaCl equiv. The heterogeneous state (boiling) of fluid at the early stages has been documented. The chemical and isotopic compositions of the fluid testify to its magmatic nature and the participation of meteoric water at late stages in the ore-forming process. Thermodynamic modeling reproduces the main specific features of ore formation, including separation of Au and Ag. A physicochemical model of the gold mineralization in the Darasun ore district has been proposed. On the basis of several attributes, the Talatui deposit has been referred to the prophyry gold-copper economic type.

Tourmaline in the Vetka Porphyry Copper-Molybdenum Deposit of the Chukchi Peninsula of Russia

Three generations of tourmaline have been identified in propylite in the Vetka porphyry copper-molybdenum deposit of the Chukchi Peninsula of Russia. Tourmaline-I is characterized by its Fetot/(Fetot + Mg) value, which ranges from 0.33 to 0.49. Tourmaline-II, which crystallizes at a lower temperature, overgrowing tourmaline-I or occurring as isolated crystals, is distinguished by a higher Fetot/(Fetot + Mg), which varies from 0.46 to 0.72. The Fetot/(Fetot + Mg) ratio in tourmaline-III, which overgrows tourmaline-II is lower (0.35–0.49), and is identical to that of the first tourmaline generation. This is probably caused by the beginning of sulfide deposition. Tourmalines in the deposit characterized by complex isomorphic substitutions can be attributed to the intermediate members of the dravite—“hydroxy-uvite”-“oxy-uvite” and schorl-“hydroxy-feruvite”-“oxy-feruvite” series. Tourmaline starts to crystallize at temperatures above 340°C. The fluid responsible for the tourmaline deposition was magmatic, with a significant admixture of meteoric water (δ18OH2O = −0.85 to −0.75‰). The high Fe3+/Fetot ratio (0.50) indicates high oxygen activity when the tourmaline precipitated. It has been established that the isomorphic substitution Fetot → Al is typomorphic of tourmalines from porphyry copper deposits worldwide.

Gold-telluride mineralization of the Western Chukchi Peninsula, Russia: Mineralogy, geochemistry, and formation conditions

Mineralogy, geochemistry, and formation conditions of the Sentyabr’sky prospect—the first economic occurrence of Au-Te mineralization in the Chukchi Peninsula—was studied. Gold occurs in native form and as telluride compound (petzite). Petzite and hessite are the major ore minerals of the gold-telluride assemblage; native gold is superimposed on them. Altaite, coloradoite, and paratellurite have been also identified. The study of fluid inclusions in sphalerite and quartz associated with ore minerals shows that the Au-Te mineralization of the Sentyabr’sky prospect and the low-sulfide Au-Ag ore of the adjacent Dvoinoi deposit were formed from different fluids and under different conditions. The multistage hydrothermal process developed in the frames of long-living porphyry-epithermal system functioned in the Ilirnei district. The Au-Te mineralization at the Sentyabr’sky prospect is related to alkaline magmatism. The high-salinity (above 5 wt % NaCl equiv) fluid inclusions in hydrothermal quartz can be indicators of such mineralization. Mineralogy and geochemistry of ore at the Sentyabr’sky prospect provide evidence for its deposition at the middle level of porphyry-epithermal system and testify to prospectivity of deep levels.

Gold–Silver Mineralization in Porphyry–Epithermal Systems of the Baimka Trend, Western Chukchi Peninsula, Russia

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.

Quartz-sericite and argillic alterations at the Peschanka Cu-Mo-Au deposit, Chukchi Peninsula, Russia

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 H2S 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 CO2 followed by the decreasing activity of H2S 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.

The crystallization temperature of vein quartz estimated from the concentration of the titanium paramagnetic center in quartz: A case study of the Peschanka porphyry copper–molybdenum–gold deposit, Western Chukchi Peninsula, Russia

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 (<2 at. ppm). The [TiO4-/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.

Minerals of the Au-Ag-Pb-Te-Se-S system of porphyry-copper-molybdenum deposits from the Nakhodka ore field, Chukchi Peninsula, Russia

The Nakhodka ore field located 220 km south of the Bilibino town, Chukchi Peninsula, Russia comprises Cu-Mo-porphyry (Malysh and Vesennii) and Mo-Cu-porphyry (Nakhodka, III Vesennii) deposits. The late epithermal mineralization with native gold of low fineness (498–766) of the first group deposits refers to the IS (intermediate sulfidation) type, which is characterized by the occurrence of petzite; stutzite; acanthite; pearceite; and minerals of the Pb-Bi-Ag-Se-Te, Ag-Te-Se, and Ag-Bi-Se systems, as well as by native tellurium. The epithermal mineralization forms at fTe2(−19...−18) and fS2(−14...−13) and temperatures <200°C. The second group deposits do not exhibit epithermal mineralization; telluride mineralization is present as only native tellurium and altaite.

Tourmaline from quartz lenses of the Urtui granite pluton, Strel’tsovka orefield, Ttansbaikal krai

Quartz-tourmaline lenses, around which host granite is impregnated by uraninite, have been found among porphyritic granite with large phenocrysts of the Urtui pluton in the Ttansbaikal krai framing the Strel’tsovka volcano-tectonic structure. Two generations of tourmaline are distinguished. Most individual crystals belong to the first generation attributed to “fluor-schorl”; tourmaline-II attributed to schorl occurs as thin rims overgrowing tourmaline-I. The major type of cation isomorphic substitution in both tourmalines is Fe2+ → Mg. The Fe3+/Fetot value and Li content in the average sample are 2% and 80 ppm, respectively. The high F content, comparatively high Li, low Fe3+/Fetot value, and character of cation isomorphic substitution indicate that the tourmaline relates to greisens. The combination of these features allows one to distinguish greisen-type tourmaline-bearing rocks. The impregnated uranium mineralization in granite of the Urtui pluton, one of the probable sources of uranium in economic U ore of the Strel’tsovka deposit, is suggested to be caused by greisenization and the formation of quartz-tourmaline lenses.

Tourmaline from deposits of the Birgil’da-Tomino ore cluster, South Urals

Tourmalines from the Kalinovka porphyry copper deposit with epithermal bismuth-gold-basemetal mineralization and the Michurino gold-silver-base-metal prospect have been studied in the South Urals. Tourmaline from the Kalinovka deposit occurs as pockets and veinlets in quartz-sericite metasomatic rock and propylite. The early schorl-“oxy-schorl” [Fetot/(Fetot + Mg) = 0.66−0.81] enriched in Fe3+ is characterized by the homovalent isomorphic substitution of Fe3+ for Al typical of propylites at porphyry copper deposits. The overgrowing tourmalines of the second and third generations from propylite and quartz-sericite metasomatic rock are intermediate members of the dravite-magnesio-foitite solid solution series [Fetot/(Fetot + Mg) = 0.05−0.46] with homovalent substitution of Mg for Fe2+ and coupled substitution of X▭ + YAl for XNa + YMg. These substitutions differ from the coupled substitution of YAl + WO2− for YFe2+ + WOH− in tourmaline from quartz-sericite rocks at porphyry copper deposits. At the Michurino prospect, the tourmaline hosted in the chlorite-pyrite-quartz veins and veinlets with Ag-Au-Cu-Pb-Zn mineralization is an intermediate member of the dravite-magnesio-foitite solid solution series [Fetot/(Fetot + Mg) = 0.20−0.31] with homovalent substitution of Mg for Fe2+ and coupled substitutions of X▭ + YAl for XNa + YMg identical to that of late tourmaline at the Kalinovka deposit. Thus, tourmalines of the porphyry and epithermal stages are different in isomorphic substitutions, which allow us to consider tourmaline as an indicator of super- or juxtaposed mineralization.