Pyrite–sulfosalt reactions and semimetal fractionation in the Chinkuashih, Taiwan, copper–gold deposit: a 1 Ma paleo-fumarole

Geofluids
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Abstract

The mineralized fracture system that underlay paleo-fumarole field at Chinkuashih, Taiwan has been exposed by copper–gold mining to depths of about 550 m below the paleo-surface. Its mineralogy and systematic variations in metal and semimetal (Fe, Cu, As, Sb, Bi, Hg, Cd, Sn, Zn, Pb, Se, Te, Au, Ag) concentrations provide insights into the chemical responses of a magmatic-vapor phase as it expands through fracture arrays to the surface and discharges as fumaroles associated with more extensive solfatara. At Chinkuashih, following initial sealing of the fractures by silica-alunite alteration, brittle failure reestablished discharge from an underlying reservoir of magmatic vapor. Crystalline pyrite was deposited first in the fractures and was succeeded and replaced by ‘enargite’ (Cu3(As,Sb)S4) as sulfosalt encrustations (‘sublimate’) on fracture surfaces and in extensional cracks. Subsequent recrystallization resulted in complex exsolution intergrowths with antimony fractionation to the evolving crystal–vapor interface. Heavy metal fractionation between sulfosalt and vapor enriched the vapor phase in heavy metals that subsequently precipitated as complex Bi–Hg–Sn sulfosalts in discrete areas (paleo-fumaroles) close to the paleo-surface in a manner analogous to modern-day fumaroles on active volcanoes such as Vulcano, Italy. As in similar paleo-fumaroles (e.g., El Indio, Chile and Lepanto, Philippines), the most characteristic reaction sequence is the partial replacement of the early pyrite by enargite and Fe-tennantite. It is proposed that this reaction tracks the decrease in the pressure of the underlying magmatic-vapor reservoir because of the sustained discharge of vapor to the surface.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Pyrite–sulfosalt reactions and semimetal fractionation in the Chinkuashih, Taiwan, copper–gold deposit: a 1 Ma paleo-fumarole
Series title Geofluids
DOI 10.1111/j.1468-8123.2012.00367.x
Volume 12
Issue 3
Year Published 2012
Language English
Publisher Elsevier
Contributing office(s) Crustal Geophysics and Geochemistry Science Center
Description 16 p.
First page 245
Last page 260
Country Taiwan