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The role of metamorphic fluids in the formation of ore deposits
Author(s) -
B. W. D. Yardley,
James S. Cleverley
Publication year - 2013
Publication title -
special publication - geological society of london/geological society, london, special publications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.673
H-Index - 132
eISSN - 2041-4927
pISSN - 0305-8719
DOI - 10.1144/sp393.5
Subject(s) - metamorphic rock , geochemistry , geology
Many ore deposits are hosted by metamorphic rocks, and metamorphic fluids have been invoked as a source for various deposits, especially gold deposits. Metamorphic fluid compositions reflect original sedimentary environment: continental shelf sequences yield saline metamorphic fluids with little dissolved gas while metasediments from accretionary and oceanic settings host less saline fluids with significant CO 2 contents. The principal difficulty in reconciling ore deposits with a metamorphic origin is that many form quickly ( c. 1 Ma), whereas metamorphic heating is slow ( c. 10–20 °/Ma). Gravitational instability means that fluid cannot be retained. Metamorphic ores may nevertheless form by: (a) segregation leading to enrichment of pre-existing concentrations; (b) infiltration of water-rich fluids from schists into marbles at high temperature overstepping decarbonation reactions and allowing fast reaction that locally draws down temperature; and (c) rapid uplift driving dehydration reactions owing to pressure drop. Some orogenic lode gold deposits fit well with a purely metamorphic origin during rapid uplift, but others are problematic. At Sunrise Dam, Western Australia, anomalies in Sr-isotope ratios and in apatite compositions indicate a partial mantle/magmatic source. Low salinity, H 2 O–CO 2 fluids commonly associated with hydrothermal gold reflect the effect of salt on gas solubility, not the origin of the fluid.