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Geochemistry: Exploration, Environment, Analysis
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Geochemistry: Exploration, Environment, Analysis; 2009; v. 9; issue.1; p. 9-17;
DOI: 10.1144/1467-7873/07-169
© 2009 Geological Society of London

research-article

Lithogeochemistry of the Collahuasi porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster, northern Chile: Pearce element ratio vectors to ore

Esteban Urqueta1,2, T. Kurt Kyser1,*, Alan H. Clark1, Clifford R. Stanley3 & Christopher J. Oates4

1 Department of Geological Sciences and Geological Engineering, Queen's University , Kingston, Ontario, K7L 3N6, Canada
2 Present address: Exploration Division , Anglo American Chile, Santiago, Chile
3 Department of Geology, Acadia University , Wolfville, Nova Scotia, B4P 2R6, Canada
4 Anglo-American plc, 20 Carlton Terrace, London, SW1Y 5AN, UK

* Corresponding author (e-mail: kyser{at}geol.queensu.ca)

Lithogeochemical vectors to ore-bodies are obscured by the variety of lithological units and the alteration events that have affected them. In the magmatic-hydrothermal context, Pearce element ratio (PER) analysis can be a cost-effective lithogeochemical technique that identifies and eliminates non-hydrothermal sources of variation, thus permitting the definition of material transfer related only to hydrothermal alteration. Dedicated PER diagrams for mafic and felsic lithologies in the world-class porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster of the Collahuasi district, I Región, northern Chile, effectively model background variability and discriminate between fresh or propylitically altered rocks and those with a hydrolytic alteration overprint. Furthermore, PER diagrams allow for the definition of an alteration index (AI) that quantifies the degree of metasomatic exchange during hydrolytic alteration of a particular rock. Plots of different PER values against the AI show that potassium enrichment during hydrolytic alteration was followed by potassium, calcium and sodium depletion as a result of the destruction of feldspars during sericitic and later argillic alteration. Although PER plots record no mass transfer processes involving major elements during propylitic alteration, the integrated AI for both mafic and felsic units spatially defines the major alteration centres within the district.

Key Words: lithogeochemistry • Pearce element ratio analysis • Collahuasi • porphyry Cu–Mo • hydrothermal alteration • metasomatism