Geobarometry in Archean Lode-Gold Deposits
STEFFEN G. HAGEMANN and PHILIP E. BROWN
Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, WI 53706 USA
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Abstract: A compilation of geobarometric data from Archean lode-gold deposits worldwide suggests that these deposits display a continuum of formation pressures from <50 MPa to over 700 MPa. The calculated fluid pressures have been utilized, using the fluid inclusion program MacFlinCor, to constrain crustal depths assuming litho- and hydrostatic-pressure gradients. The inferred crustal depths of mineralization in lode-gold deposits from the Archean Yilgarn Craton of Western Australia, the Superior Province in Canada, and cratonic areas in South Africa and Brazil display a continuum of depths for the formation of these late-orogenic, epigenetic lode-gold deposits. Three major pressure ranges and related depth-levels can be identified: (1) epizonal levels (at <150 MPa corresponding to < 6 km e.g. Wiluna and Racetrack), mesozonal levels (at >150 to 300 MPa corresponding to >6 to <12 km, e.g. Golden Mile and Hollinger-McIntyre), and hypozonal deposits (at > 300 MPa corresponding to > 12 km e.g. Marvel Loch and Griffin's Find). The large crustal extent of epigenetic Archean lode-gold deposits appears to be an important characteristic when compared to other ore deposit classes such as porphyry copper and Mississippi-Valley type (MVT) deposits that are constrained to have formed at upper crustal levels only.
The crustal depths of late orogenic, epigenetic Archean lode-gold deposits from Western Australia display a continuum from shallow to deep whereas lode-gold deposits from the Superior Province in Canada show a distinct clustering of depths at the mesozonal level (>6 km to <12 km). The geobarometric data for the Western Australian deposits are compatible with the model of Groves et al. (1991, 1992) that predicts a crustal continuum for Archean lode-gold deposits in Western Australia. The lack of evidence (to date) for large epi- and hypozonal deposits in the Canadian Superior Province may be a result of a combination of a different crustal make-up, scant fluid inclusion studies, and/or lack of exploration in terrains of high- and low-grade metamorphic grade.
Key-words: Archean lode-gold deposits, fluid pressure, geobarometry, crustal depths, fluid inclusions