Interpretation of post-entrapment fluid-inclusion re-equilibration at the Three Mile Hill, Marvel Loch and Griffins Find high-temperature lode-gold deposits, Yilgarn Craton, Western Australia
J. Ridley et Sg. Hagemann, Interpretation of post-entrapment fluid-inclusion re-equilibration at the Three Mile Hill, Marvel Loch and Griffins Find high-temperature lode-gold deposits, Yilgarn Craton, Western Australia, CHEM GEOL, 154(1-4), 1999, pp. 257-278
Fluid inclusions that are interpreted to be related to vein filling of quar
tz veins at the Three Mile Hill and Marvel Loch gold deposits in amphibolit
e-facies rocks, and the Griffins Find deposits in granulite-facies rocks in
the Yilgam Craton of Western Australia are dominantly low-salinity H2O-CO2
+/- CH4 mixtures of CO2-CH4 fluids. At each deposit, inclusions have a wid
e range of compositions with respect to both the aqueous,carbonic ratio and
the CO2:CH4 ratio of the carbonic phase, and most fluids could not have be
en in equilibrium with the mineral assemblage in the vein and adjacent rock
. Inclusion densities suggest a range of P-T conditions of entrapment, and
are in general not consistent with the conditions of vein formation indicat
ed by vein assemblages. Diffusional addition of H-2 into inclusions, diffus
ional loss of H2O, and reduction of inclusion volume are possible during co
oling and uplift along the inferred P-T path. The inclusion populations cou
ld have been derived from an originally uniform population of low-salinity,
aqueous dominated H2O-CO2 inclusions by a combination of these processes.
Inclusion modification as the cause of the complex inclusion populations is
supported by relations of molar volume to composition, and, to an extent,
by variations in fluid salinity. If inclusion re-equilibration is the cause
of inclusion variability, it was of variable intensity within a single vei
n system, and within individual clusters of inclusions in some samples, and
is suggested to have been a function of the local petrological and textura
l environment. (C) 1999 Elsevier Science B.V. All rights reserved.