Abstract
Porphyry-related deposits are the fourth type of tin deposits in importance after pegmatite, greisen and granite-related hydrothermal veins. We have studied tourmalines and associated minerals from the several porphyry-tin occurrences related to rhyolite dikes in the Mramorny tin cluster, located in Chukotka, the second large tin province in Russia after the Far East province. Two types of tourmaline were identified in the occurrences of the Mramorny tin cluster: (1) early pre-ore tourmaline, and (2) ore-stage tourmaline, which is associated with chamosite. The early-stage tourmaline can be classed as an intermediate member of the schorl–foitite and “oxy-schorl” – “oxy-foitite” series. Its compositions are characterized by their values of X -site vacancy and Fe tot /(Fe tot + Mg) ranging from 0.147 to 0.665 apfu and from 0.85 to 1.00, respectively. The ratio Fe 3+ /Fe tot in the tourmaline is about 19–25%. The ore-stage tourmaline is characterized by a more complex composition distinguished by lower values of X -site vacancy and Fe tot /(Fe tot + Mg), ranging from 0.018 to 0.437 apfu and from 0.49 to 0.75, respectively. The Fe 3+ /Fe tot value in the tourmaline is about 11–23%. The early and ore-stage tourmalines are characterized by different types of Al substitutions: AlO[R(OH)] −1 and □Al(RNa) −1 in the former, and FeAl −1 , taking into account the higher Mg concentration, in the latter. The precipitation of cassiterite and the higher activity of H 2 S during the ore stage resulted in the deposition of Fe-bearing sulfides, and the Fe tot /(Fe tot + Mg) value in the ore-stage tourmaline is lower than that of the early tourmaline. The pattern is similar in the associated early and ore-stage chamosite, i.e., the Fe tot /(Fe tot + Mg) ratio decreases like in tourmaline. On the basis of fluid inclusions, the ore-stage tourmaline started to crystallize at temperature about 385°C and at a pressure of 23 MPa. The fluid responsible for tourmaline formation and ore deposition did boil and was similar to that of porphyry-copper deposits, confirming that the occurrences studied are related to a porphyry environment. The oxygen isotopic data indicate a significant admixture of meteoric water at the ore-stage tourmaline deposition (δ 18 O H2O from −8.4 to +0.6‰). The comparison of the tourmalines studied here with tourmalines from porphyry-copper and porphyry-gold deposits indicates that the FeAl −1 substitution is typical of tourmaline in the porphyry environment.
Published Version
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