Please use this identifier to cite or link to this item: http://hdl.handle.net/1959.13/33964

OpenURL Link
Title
Physical conditions of gold deposition at the McPhees Deposit, Pilbara Craton, Western Australia: fluid inclusion and stable isotope constraints
Author/Creator
Baker, Darcy E. L.;  Seccombe, Philip K.
Institution
The University of Newcastle. Faculty of Science & Information Technology, School of Environmental and Life Science
Description
Fluid inclusions in mineralized quartz veins from the ca. 2890 Ma McPhees gold deposit, Pilbara Craton, Western Australia, were examined to determine the composition of the mineralizing fluid and to constrain the physical conditions of gold deposition. The composition of inclusions was determined by standard microthermometric and laser Raman analyses of selected assemblages of fluid inclusions. The veins contain: (1) abundant, Type-Ia monophase, CO₂-rich inclusions and coeval Type-Ib two-phase, mixed H₂O + CO₂ inclusions; (2) secondary, two-phase, Types II and III, respectively, low- and high-salinity aqueous inclusions, and (3) secondary Type-IV monophase, mixed CO₂–CH₄–N₂ inclusions. Type-Ib inclusions have highly variable bulk-compositions (10–100 vol.% CO₂–CH₄) and variable salinity (3–12 eq. wt.% NaCl). Types II and III inclusions have similar morphologies and modes of occurrence, but have distinctly different salinities (0.3–12.6 eq. wt.% and >21.0 eq. wt.%, respectively). Heating experiments indicate minimum temperatures of trapping of 350 641°C for Type-I inclusions, 207 660°C for Type-II inclusions, and 143 <637°C for Type-III inclusions. Oxygen isotope compositions of quartz–actinolite and albite–actinolite pairs indicate a temperature of gold-associated alteration of ~350°C, consistent with arsenopyrite thermometry, which indicates gold mineralization at <480°C. The early CO₂-rich fluid inclusions have densities that range from 0.6 to 1.05 g/cm3 which, at 350°C, correspond to 1–2 kbar pressure, consistent with geological relations indicating that the McPhees deposit formed at <7 km. Type-I inclusions are interpreted to contain early vein-related fluids that carried gold, but this assemblage (nearly pure CO₂ and subordinate, coexisting H₂O-rich fluid inclusions) is unusual for orogenic gold deposits. It is most likely a result of fluid mixing that may have played a role in gold deposition in veins; however, host-rock lithology seems to have been a first-order control in localizing the gold.
Relation
The Canadian Mineralogist Vol. 42, Issue 5, p. 1405-1424
Publisher Link
http://dx.doi.org/10.2113/gscanmin.42.5.1405
Date
2004
Publisher
Mineralogical Association of Canada
Keyword(s)
orogenic gold deposit;  McPhees deposit;  CO₂-rich fluid inclusions;  oxygen isotopes;  fluid mixing;  Pilbara Craton;  Archean;  Australia
Resource Type
journal article
Identifier
http://hdl.handle.net/1959.13/33964
Identifier
ISSN:0008-4476
Reviewed
Reviewed
46 Visitors 48 Hits 0 Downloads
Save/E-mail Citation
Citation Format
E-mail Address
Subject
"The Canadian Mineralogist"
 
OR