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TitleComparative mineralogy and geochemistry of gold-bearing sulfide deposits on the mid-ocean ridges
AuthorHannington, M; Herzig, P; Scott, S; Thompson, G; Rona, P
SourceMarine Geology vol. 101, 1991 p. 217-248,
LinksAbstract - Résumé
Alt SeriesGeological Survey of Canada, Contribution Series 30690
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region
AreaMarianas; Galapagos Islands; Okinawa; Snake Pit
Subjectsmarine geology; metallic minerals; mid-ocean ridges; oceanographic surveys; sulphide deposits; sea sediment geochemistry; mineral enrichment; gold; hydrothermal deposits; barite; pyrite; chalcopyrite; pyrrhotite; cubanite; sphalerite; trace element analyses; Endeavour Ridge; Southern Juan de Fuca Ridge; East Pacific Rise; Galapagos Rift
Illustrationslocation maps; tables; schematic cross-sections; histograms; plots; ternary diagrams
ProgramNSERC Natural Sciences and Engineering Research Council of Canada
ProgramGerman-Canadian Cooperation Project on Shallow Submarine Hydrothermal Systems
ProgramFisheries and Oceans Canada, Funding Program
AbstractA comparative study of the mineralogy and geochemistry of sulfide deposits on mid-ocean ridges in the Northeast Pacific
and the Mid-Atlantic reveals common characteristics associated with primary gold enrichment. Average gold contents of 0.8
to 5 ppm Au occur in sulfides from Southern Explorer Ridge and Axial Seamount (Northeast Pacific) and from the TAG
hydrothermal field and Snakepit vent field (Mid-Atlantic Ridge). The enrichment of gold in these deposits is consistently
related to a phase of late-stage, low-temperature (< 300°C) venting. Concentrations > I ppm Au occur exclusively in pyritic
assemblages and commonly with abundant Fe-poor sphalerite and a suite of complex Pb-Sb-As sulfosalts. Amorphous silica
and, locally, barite or carbonate are important constituents of the gold-rich precipitates but do not contain gold themselves.
High-temperature (350°C) black smoker assemblages, consisting dominantly of pyrite, chalcopyrite, pyrrhotite, isocubanite
and abundant anhydrite are uniformly gold-poor (<0.2 ppm Au). To the extent that individual sulfides can be mechanically
separated, chemical analyses by neutron activation indicate that gold is most abundant in sphalerite (up to 5.7 ppm Au) but
also occurs in pyrite and marcasite. Samples of sphalerite with abundant inclusions of fine-grained sulfosalts locally contain
up to 18 ppm Au, suggesting that sulfosalts may be repositories for gold. No free gold has been observed at 4000 × magnification
of polished specimens, indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent
within the sulfides. Samples from gold-rich deposits in the Northeast Pacific and Mid-Atlantic are compared with similar but
relatively gold-poor sulfides from the Galapagos Rift and 13°N on the East Pacific Rise (EPR), and with barren sulfides from
11°N EPR, 21°N EPR, the Endeavour Ridge, and the Southern Juan de Fuca Ridge. Trace element analyses of more than
170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of Ag, As, Sb, Pb
and Zn, and locally with high Cd, Hg, T1, and Ga. In contrast, gold is typically depleted in samples with high Co, Se, and
Mo. The close association of Au with Ag, As, Sb, and Pb may reflect the common behavior of these metals as aqueous sulfur
complexes (e.g., [Au(HS)2]) at low temperatures. Similar mineralogical and geochemical associations are observed in sulfide
deposits from modern back-arc settings and in the ancient geologic record.