Non-magmatic sulfur source of hydrothermal colloform pyrite: Insights from the Jinshandian iron skarn deposit, Daye district, eastern China

Abstract

Colloform pyrite occurs widely in many ore deposits in the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in eastern China, but its genesis is still debated. In this study, we present detailed textures and geochemical compositions of newly identified colloform pyrite (Py1) and late coarse-grained pyrite (Py2) from the Jinshandian iron skarn deposit in the Daye district to decipher the genesis of the pyrite grains. Colloform pyrite is composed of nanometer-sized (<1 μm) grains, showing typical colloform textures, and occurs as interstitial fillings between magnetite grains. In contrast, Py2 commonly has a large grain size (up to 200 μm) and is usually distributed along the margins of colloform pyrite. These two types of pyrite have relatively high Co/Ni (1.24–13.36) ratios. δ34SCDT values of Py1 (17.22‰–20.63‰, mean 19.42‰) and Py2 (18.96‰–19.95‰, mean 19.43‰) are close to those of gypsum (anhydrite) from evaporites in the Daye district, indicating the incorporation of evaporite-derived materials into the formation of colloform pyrite. On the basis of the textures, trace-element compositions, and sulfur isotope characteristics of the colloform pyrite, it is inferred that this pyrite from the Jinshandian iron deposit is of hydrothermal origin. The sulfur isotope composition of colloform pyrite from Jinshandian is heavier relative to other magmatic-hydrothermal colloform pyrites, which indicates colloform pyrite could formed in diverse hydrothermal fluids (magmatic to no-magmatic fluid). Our study further confirmed that hydrothermal colloform pyrite has widely occurred in Fe and Cu-Au deposits associated with the Cretaceous magmatism in the MLYRMB.