Abstract

Hydrothermal fluids responsible for the formation of volcanogenic massive sulfides may transport gold in solution as Au(HS) 2 (super -) at low temperatures (<350 degrees C), near neutral pH, and high concentrations of H 2 S (10 (super -3) -10 (super -2) m). To a lesser extent gold may be transported as AuCl 2 (super -) at higher temperatures (> or =350 degrees C), low pH, and elevated salinities (> or =1 m NaCl). The documented fluid chemistry of active hydrothermal vents on the modern seafloor confirms that gold transport is mainly due to Au(HS) 2 (super -) , and the calculated solubility of gold as Au(HS) 2 (super -) is highest in low-temperature (150 degrees -250 degrees C) vent fluids at elevated oxygen and sulfur activities. Sulfides which coprecipitate with gold commonly have properties which can be related to the temperature and sulfidation state of the hydrothermal fluids and therefore reflect conditions which were favorable for the transport or deposition of gold as Au(HS) 2 (super -) .A comparison of gold grades with sulfide mineral equilibria and the FeS contents in sphalerite from a variety of deposits indicates that gold enrichment is closely related to the temperature-a (sub S 2 ) conditions in the ore-forming fluids. Sea-floor sulfides range from sulfur-rich, pyrite-marcasite assemblages to relatively sulfur-deficient, pyrrhotite-isocubanite assemblages, with coexisting sphalerite containing 0 to 55 mole percent FeS. Significant gold enrichment occurs exclusively in low-temperature (<300 degrees C) pyritic sulfides, together with sulfur-rich trace minerals and Fe-poor sphalerite. Bulk gold contents range from <0.5 ppm Au in sulfides with sphalerite containing 10 to 55 mole percent FeS up to 6.7 ppm Au in sulfides with sphalerite containing <10 mole percent FeS. Fluid inclusions in Fe-poor sphalerite (<5 mole % FeS) from one gold-rich deposit (avg 4.9 ppm Au) have trapping temperatures of 235 degrees + or - 13 degrees C (Hannington and Scott, 1988). Low-temperature ores with high gold contents are also common in Phanerozoic Zn-Cu-Pb deposits in Japan (Kosaka, Furutobe, Shakanai) and similar deposits in British Columbia (HW mine, Lynx, Seneca). Gold-bearing polymetallic sulfides contain sulfur-rich mineral assemblages (e.g., bornite + pyrite) and recognizable trace mineral equilibria which indicate a high a (sub S 2 ) (e.g., argentite-electrum, tennantite-enargite, covellite-digenite). Zn-Cu-Pb ores formed at 200 degrees to 300 degrees C typically contain Fe-poor sphalerite (<1 mole % FeS) and gold grades of i to 3 ppm Au, whereas Cu-rich ores formed at >300 degrees C contain Ferich sphalerite (1-5 mole % FeS) and gold grades < 1 ppm Au. Archean Cu-Zn deposits at Noranda, Quebec, consist mainly ofpyrite-pyrrhotite ores containing 0.5 to 1 ppm Au together with Fe-rich sphalerite (10-12 mole % FeS). However, pyritic ores which occur stratigraphically up-section commonly contain 1 to 3 ppm Au and relatively Fe-poor sphalerite (5 mole % FeS). Different sulfide mineral equilibria and FeS contents in sphalerite are interpreted to reflect the same physical and chemical conditions which influence gold grades and suggest that petrologic indicators of the sulfidation state may be useful guides to gold mineralization in volcanogenic massive sulfides.

First Page Preview

First page PDF preview
You do not currently have access to this article.