SUN Wei-dong¹, LI He¹, LING Ming-xing², DING Xing², LI Cong-ying¹

 
1. CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, The Chinese Academy of Sciences, Guangzhou 510640, China;
2. State Key Laboratory of Isotope Geochemistry, Guangzhou 510640, China

Abstract: Magnetite crisis refers to the abrupt decrease of Cu, Au contents in convergent magin magmas induced by magnetite. The crystalization of magnetite in oxidized magmas causes sulfate reduction, forming hydrosulfide complexes that scavenge Cu and Au into hydrous phase, resulting in ore-forming fluids. This is one of the key processes that control porphyry and epithermal Cu, Au mineralizations. For porphyry deposits, the mineralization occurs in closed or semi-closed systems. Sulfate reduction occurs mainly in hydrothermal fluids, releasing large amount of H+, lowering the pH values and thus elevates the oxidiation potential of sulfate, reaching the magnetite-hematite oxygen fugacity buffer. Consequently, there is usually speculerite alteration at the late stage of porphyry mineralizations. For epithermal deposits, sulfate reduction occurs mainly in the magmas without releasing H+. Therefore, the oxygen fugacity tends to drop but does not change much. The initial Cu, Au contents, which are the main factors and essential to the mineralization potential of magmas, are controlled by oxygen fugacity and characteristics of source rocks. Subduction of young oceanic crust forms oxidized adakitic magmas with high initial Cu, Au contents, which are favorable for Cu, Au porphyry deposits. High sulfur epithermal deposits are more likely to be associated with porphyry Cu, Au deposits underneath.

Keywords: magnetite crisis plate subduction oxygen fugacity porphyry Cu-Au deposits epithermal deposit

E-Mail: weidongsun@gig.ac.cn

Bulletin of Mineralogy, Petrology and Geochemistry Vol.34, No.5, 2015, page 895-901