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Magmatic-vapor expansion and the formation of high-sulfidation gold deposits: Chemical controls on alteration and mineralization

Henley, Richard; Berger, Byron

Description

Large bulk-tonnage high-sulfidation gold deposits, such as Yanacocha, Peru, are the surface expression of structurally-controlled lode gold deposits, such as El Indio, Chile. Both formed in active andesite-dacite volcanic terranes. Fluid inclusion, stable isotope and geologic data show that lode deposits formed within 1500. m of the paleo-surface as a consequence of the expansion of low-salinity, low-density magmatic vapor with very limited, if any, groundwater mixing. They are characterized by...[Show more]

dc.contributor.authorHenley, Richard
dc.contributor.authorBerger, Byron
dc.date.accessioned2015-12-10T23:10:05Z
dc.identifier.issn0169-1368
dc.identifier.urihttp://hdl.handle.net/1885/63572
dc.description.abstractLarge bulk-tonnage high-sulfidation gold deposits, such as Yanacocha, Peru, are the surface expression of structurally-controlled lode gold deposits, such as El Indio, Chile. Both formed in active andesite-dacite volcanic terranes. Fluid inclusion, stable isotope and geologic data show that lode deposits formed within 1500. m of the paleo-surface as a consequence of the expansion of low-salinity, low-density magmatic vapor with very limited, if any, groundwater mixing. They are characterized by an initial 'Sulfate' Stage of advanced argillic wallrock alteration ± alunite commonly with intense silicification followed by a 'Sulfide' Stage - a succession of discrete sulfide-sulfosalt veins that may be ore grade in gold and silver. Fluid inclusions in quartz formed during wallrock alteration have homogenization temperatures between 100 and over 500 °C and preserve a record of a vapor-rich environment. Recent data for El Indio and similar deposits show that at the commencement of the Sulfide Stage, 'condensation' of Cu-As-S sulfosalt melts with trace concentrations of Sb, Te, Bi, Ag and Au occurred at > 600 °C following pyrite deposition. Euhedral quartz crystals were simultaneously deposited from the vapor phase during crystallization of the vapor-saturated melt occurs to Fe-tennantite with progressive non-equilibrium fractionation of heavy metals between melt-vapor and solid. Vugs containing a range of sulfides, sulfosalts and gold record the changing composition of the vapor. Published fluid inclusion and mineralogical data are reviewed in the context of geological relationships to establish boundary conditions through which to trace the expansion of magmatic vapor from source to surface and consequent alteration and mineralization. Initially heat loss from the vapor is high resulting in the formation of acid condensate permeating through the wallrock. This Sulfate Stage alteration effectively isolates the expansion of magmatic vapor in subsurface fracture arrays from any external contemporary hydrothermal activity. Subsequent fracturing is localized by the embrittled wallrock to provide high-permeability fracture arrays that constrain vapor expansion with minimization of heat loss. The Sulfide Stage vein sequence is then a consequence of destabilization of metal-vapor species in response to depressurization and decrease in vapor density. The geology, mineralogy, fluid inclusion and stable isotope data and geothermometry for high-sulfidation, bulk-tonnage and lode deposits are quite different from those for epithermal gold-silver deposits such as McLaughlin, California that formed near-surface in groundwater-dominated hydrothermal systems where magmatic fluid has been diluted to less than about 30%. High sulfidation gold deposits are better termed 'Solfataric Gold Deposits' to emphasize this distinction. The magmatic-vapor expansion hypothesis also applies to the phenomenology of acidic geothermal systems in active volcanic systems and equivalent magmatic-vapor discharges on the flanks of submarine volcanoes.
dc.publisherElsevier
dc.sourceOre Geology Reviews
dc.subjectKeywords: Enargite; Magmatic vapor; Solfatara; Sulfidation; Sulfosalts; Tennantite; Antimony; Bismuth; Crystallography; Deposits; Expansion; Fluids; Fracture; Geothermal fields; Gold; Gold coatings; Groundwater; Heat losses; Inclusions; Isotopes; Mineralogy; Minera Enargite; Gold; High-sulfidation; Magmatic vapor; Silica-alunite; Solfatara; Sulfosalt melt; Tennantite
dc.titleMagmatic-vapor expansion and the formation of high-sulfidation gold deposits: Chemical controls on alteration and mineralization
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume39
dc.date.issued2011
local.identifier.absfor040307 - Ore Deposit Petrology
local.identifier.ariespublicationf2965xPUB823
local.type.statusPublished Version
local.contributor.affiliationHenley, Richard, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationBerger, Byron, US Geological Survey
local.description.embargo2037-12-31
local.bibliographicCitation.startpage63
local.bibliographicCitation.lastpage74
local.identifier.doi10.1016/j.oregeorev.2010.11.003
local.identifier.absseo840105 - Precious (Noble) Metal Ore Exploration
dc.date.updated2016-02-24T08:33:09Z
local.identifier.scopusID2-s2.0-79952445692
CollectionsANU Research Publications

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