CHLORINE-ENRICHED TOURMALINES IN HYDROTHERMALLY ALTERED DIORITE PORPHYRY FROM THE BIELY VRCH PORPHYRY GOLD DEPOSIT (SLOVAKIA)

摘要

Chlorine-enriched tourmaline was found in a hydrothermally altered Miocene diorite porphyry at the Biely Vrch porphyry gold deposit (Central Slovakia) as a relatively rare alteration mineral forming radial aggregates composed of thin prismatic crystals in the central part of quartz veinlets filled by calcite. Zoned tourmaline with Cl-enriched central zones is accompanied by allanite-(Ce) to epidote, fluorapatite, Al-rich titanite, magnetite, rutile, ilmenite, and pyrite. The tourmaline chemical composition is controlled by exchange of Fe2+ for Al at relatively constant Mg content. It produces a trend from the earliest X-site vacant Al-rich oxy-tourmaline in the central part of tourmaline aggregates to the latest feruvite on their margins. Tourmalines belong to three groups: X-site vacant ("vacancy-Mg-O root name" tourmaline, magnesio-foitite, and foitite), alkali (oxy-schorl, schorl), and calcic (feruvite) tourmaline. The variation in Al and Fe content is the result of concomitant action of AlOFe-1(OH)(-1), Al-x square Fe-1Na-1, and FeCaAL(-1)Na(-1) substitutions. Enrichment in Ca likely results from FeCaAL(-1)Na(-1) substitution. The Cl content in tourmaline from Biely Vrch attains 0.45 wt.%, which is equivalent to 0.126 apfu. The chlorine content increases with decreased Fe and increased octahedral Al and X-site vacancy. Bond-length calculations yield six bond-valence and bond-length probable local structural arrangements with Cl-w, and these are all characterized by dominance of Al at the Y sites and O at the V sites. Based on the analytical data and bond-length calculations, the chemical composition of Cl-enriched tourmaline is a result of crystallographic factors as well as petrologic environmental conditions. The enrichment in Cl is most likely related to the unusually Cl-rich hot hydrothermal fluid responsible for the origin of the host Biely Vrch porphyry gold deposit. The Cl incorporation in tourmaline is influenced by Fe availability in the genetic environment; increased Fe2+ substitution for Al reduces the ability of tourmaline to incorporate Cl.