Sodic(–calcic) alteration in Fe-oxide–Cu–Au districts: an origin via unmixing of magmatic H2O–CO2–NaCl?±?CaCl2–KCl fluids |
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Authors: | Peter J Pollard |
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Institution: | (1) Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia e-mail: peter.pollard@jcu.edu.au Tel.: +61-7-47815050; Fax: +61-7-47251501, AU |
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Abstract: | Iron-oxide–Cu–Au deposits, particularly those formed in deeper level (plutonic) environments, are commonly characterized
by regional scale sodic(–calcic) alteration, which typically formed pre- or syn-Cu–Au mineralization. The sodic(–calcic) assemblages
include albite, scapolite, pyroxene, actinolite, apatite, titanite, epidote and calcite. The consistent presence of coexisting
hypersaline aqueous and CO2-rich fluids in minerals from sodic(–calcic) alteration and associated Fe-oxide–Cu–Au deposits is the result of unmixing of
H2O–CO2–NaCl ± CaCl2–KCl magmatic fluids. Experimental evidence indicates that the Na/(Na + K) ratio of fluids in equilibrium with two alkali
feldspars in CO3
2−-bearing parent fluids would be significantly higher than in unmixed chloride-bearing aqueous fluids. Therefore, fluid unmixing
caused by decreases in temperature and/or pressure, will result in albitization of wall rocks, as is observed in most deeper
level Fe-oxide–Cu–Au deposits. This alteration style may be succeeded by K-feldspathization with decreasing temperature because
of the increase in equilibrium Na/(Na + K) in chloride-bearing fluids buffered by alkali feldspars.
Received: 26 May 1999 / Accepted: 8 June 2000 |
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