The Bronzewing lode-gold deposit, Western Australia: P-T-X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins
Al. Dugdale et Sg. Hagemann, The Bronzewing lode-gold deposit, Western Australia: P-T-X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins, CHEM GEOL, 173(1-3), 2001, pp. 59-90
The Bronzewing lode-gold deposit is located in the Yandal greenstone belt i
n the Yilgarn Craton of Western Australia. Cold mineralization is hosted in
tholeiitic basalt that is metamorphosed to greenschist facies. Individual
ore bodies are controlled by a complex, gold-bearing quartz vein system tha
t comprises shear and extension veins formed during a progressive D-2 defor
mation event. Gold is localized in quartz along fractures and deformed grai
n boundaries, and is interpreted to have formed late in the formation of th
e veins.
Detailed petrography and microthermometry on primary, pseudosecondary and s
econdary fluid inclusions trapped in gold-bearing shear and extension veins
revealed five types of fluid inclusions: (1) CO2 + CH4-H2O-NaCl inclusions
of variable salinity (0.1 to 17.5 eq, wt.% NaCl) containing between 10 and
99 mol% CO2 and molar volumes that range from 22 to 76 cm(3); (2) H2O-NaCl
inclusions of variable salinity (0.4 to 22.1 eq, wt.% NaCl); (3) CO2 +/- C
H4 inclusions with up to 58 mol% CH4 and molar volumes between 54 and 73 cm
(3); (4) CH4-H2O inclusions with CH4 ranging from 80 to 90 mol%; and (5) CH
4 inclusions with low molar volumes of 19 to 23 cm(3)
Types 1, 2 and 3 constitute a fluid inclusion assemblage that occurs consis
tently in primary, pseudosecondary and secondary trails and clusters within
the gold-bearing quartz vein system. These co-existing fluids are interpre
ted to have formed by fluid immiscibility of a low-salinity, homogeneous pa
rent fluid at about 300 degreesC and 1500 bars. Locally, Type 2 fluid inclu
sions exhibit total homogenization temperatures that are on average 100 deg
reesC less than the co-genetically trapped Type 1 aqueous-carbonic inclusio
ns. This discrepancy is interpreted to have involved CO2 effervescence in r
esponse to fluid pressure fluctuations. Types 4 and 5 fluid inclusions are
rare, and are only present locally in secondary trails and clusters in exte
nsion veins.
Significant pressure fluctuations, but relatively constant homogenization t
emperatures for Types 1, 2 and 3 fluid inclusions, suggest that cyclic deco
mpression of the hydrothermal fluids, due to seismic activity along the she
ar zones that host the gold-bearing veins, triggered fluid immiscibility. T
he process of fluid immiscibility and subsequent lowering of gold solubilit
y is interpreted to be the most efficient precipitation mechanism for gold
in the D-2 shear zone hosted vein system at Bronzewing. (C) 2001 Elsevier S
cience B.V. All rights reserved.