Genesis of copper mineralization in the polymetallic tin deposits from the Herberton Mineral Field, Queensland, Australia

Abstract

The Herberton Mineral Field hosts multiple small-scale, polymetallic Sn-Cu and Sn-Zn deposits. The Baal Gammon and Isabel polymetallic deposits in the Herberton Mineral Field contain early Sn that is overprinted by sulfides. At Baal Gammon, the sulfide overprinting is Cu-In-rich, whereas Isabel is a Zn-Pb-In-rich system. These deposits are hosted in the meta-sedimentary rocks of the Hodgkinson Formation and the porphyry dikes and volcanic rocks of the Kennedy Igneous Association. Primary Sn in these deposits occurs as cassiterite, which was altered to stannite during sulfide mineralization. The sulfide ores from Baal Gammon consist of chalcopyrite, pyrrhotite, and minor sphalerite, and sphalerite, galena, and minor chalcopyrite are observed at Isabel. Chalcopyrite from Baal Gammon contains on average 609 ppm Ag, 1194 ppm In, and 1410 ppm Sn, whereas chalcopyrite from the Isabel deposit contains on average 2302 ppm Ag, 725 ppm In, and 1042 ppm Sn. Sulfur isotope (δ34S) measurements of in-situ chalcopyrite-pyrrhotite mineral pairs show limited variation and low values that are indicative of a magmatic sulfur source with limited interaction with sulfur from connate and meteoric fluids during mineralization. The chalcopyrite and pyrrhotite δ34S values at the Baal Gammon deposit vary between 0.99–1.91‰ and 1.35–2.48‰, respectively. The δ34S values at the Isabel deposit vary between 0.91–1.45‰ for chalcopyrite and 1.12–2.11‰ for pyrrhotite. The trace element composition of major sulfides and sulfur isotopes of chalcopyrite and pyrrhotite combined with thermodynamic modeling indicates that the mineralizing fluids at the Baal Gammon and Isabel deposits have an igneous source, where the metals were transported as metal-chloride complexes at low pH (< 5) and below ~ 300 °C. The source of these sulfide-rich mineralizing fluids is most likely derived from the magmatic activity associated with the emplacement of the Slaughter Yard Creek Volcanics during a period of crustal thinning between 300 and 280 Ma.