Ferric-iron-rich tourmaline from the Darasun gold deposit, Transbaikalia, Russia

Iron redox reactions in the tourmaline structure: High-temperature treatment of Fe3+-rich schorl

“…Povondraite is a relatively low-temperature mineral occurring in fissures in metaevaporite (Ž áček et al, 2000): the geological position and mineral assemblage indicate temperature $500°C or less. Thus, povondraite from its type locality and povondraite-close tourmalines from other localities (Cavaretta and Puxeddu, 1990;Henry et al, 1999;Bačík et al, 2008;Baksheev et al, 2011) originated far from the PT conditions of our experiments and a LP-HT overprint is not probable at these localities.…”

“…Schorl from Silver Crater with 2.04 apfu Fe 3+ , 1.73 apfu Fe 2+ , 1.28 apfu Mg and 3.52 apfu Al tot , dravite from Bronson Station with 0.61 apfu Fe 3+ , 0.49 apfu Fe 2+ , 2.34 apfu Mg and 5.47 apfu Al tot -both from Hastings, Ontario (the same geological region as the Cancrinite Hill, Bancroft), dravite from Pierrepont, New York, with 0.69 apfu Fe 3+ , 0.31 apfu Fe 2+ , 2.70 apfu Mg and 5.29 apfu Al tot , and dravite from Montezuma, Mexico, with 0.98 apfu Fe 3+ , 0.70 apfu Fe 2+ , 1.86 apfu Mg and 5.47 apfu Al tot , all exhibit very high Fe 3+ contents. These high Fe 3+ contents in tourmaline probably reflect elevated activity of O 2 during crystallization rather than any late overprint and most likely result from fluids evolving from an oxidizing into a reducing environment (e.g., Baksheev et al, 2011). However, petrological data as well as zoning of the individual tourmaline samples are only poorly known.…”

“…Tourmaline analyses with elevated and reliable determination of Fe 3+ /Fe tot only rarely yield Fe 3+ PFe 2+ or even Fe 3+ /Fe tot P $30% (see e.g., Dyar et al, 1998;Taran and Rossman, 2002;Bačík et al, 2008;Baksheev et al, 2011;Novák et al, 2011), disregarding the studies on "buergerite" and povondraite from their type localities. Schorl from Silver Crater with 2.04 apfu Fe 3+ , 1.73 apfu Fe 2+ , 1.28 apfu Mg and 3.52 apfu Al tot , dravite from Bronson Station with 0.61 apfu Fe 3+ , 0.49 apfu Fe 2+ , 2.34 apfu Mg and 5.47 apfu Al tot -both from Hastings, Ontario (the same geological region as the Cancrinite Hill, Bancroft), dravite from Pierrepont, New York, with 0.69 apfu Fe 3+ , 0.31 apfu Fe 2+ , 2.70 apfu Mg and 5.29 apfu Al tot , and dravite from Montezuma, Mexico, with 0.98 apfu Fe 3+ , 0.70 apfu Fe 2+ , 1.86 apfu Mg and 5.47 apfu Al tot , all exhibit very high Fe 3+ contents.…”

“…Tourmalines (and other silicates) with a high content of Fe 3+ are not common in natural occurrences. They include "buergerite" (Mason et al, 1964;Donnay et al, 1966;Dunn, 1976; and povondraite Bačík et al, 2008;Baksheev et al, 2011), both very rare minerals known only from a few localities; and Fe 3+ -rich (about 90% of all Fe) dravite (Taran and Rossman, 2002). The origin of "buergerite" at its type locality at Mexique, San Louis Potosí, Mexico, is not revealed because the natural outcrops, where "buergerite" was found in early 1960s, are not known.…”

Tourmaline at diagenetic to low-grade metamorphic conditions: Its petrologic applicability

“…They suggest that the metal-rich, alkali, boron-bearing, "ultra-saline" fluids responsible for the deposits are derived from an evaporitic source. Tourmalines in a quartz-calcite-bearing vein associated with the porphyry-style Darasun Au deposits in the Chita region of Transbaikalia, Russia develop three generations of tourmaline representing separate stages of evolution of the ore system (Baksheev et al, 2011). The first generation tourmaline is highly enriched in Fe 3+ and deficient in Al following the O-P trend and is likely to develop as a consequence of boiling of a saline aqueous fluid phase at relatively low pressures (~0.02 GPa) and temperatures (~360°C) and at high f O 2 .…”