The effect of alteration and metamorphism on wall rocks to the Balcooma and Dry River South volcanic-hosted massive sulfide deposits, Queensland, Australia

Abstract

Two types of visibly hydrothermally altered rocks are present in the Cambro-Ordovician Balcooma volcanic-hosted massive sulfide (VHMS) district of northeast Queensland: (1) quartz-chlorite ± biotite schist, and (2) pyritic quartz-muscovite schist. The quartz-chlorite ± biotite schist, which has gained Mg, Fe, Mn, Si, and S, but lost K, Na, Ca, Rb and Sr, occurs in a crosscutting pipe below Cu-rich massive sulfide lenses at the Balcooma deposit. Pyritic quartz-muscovite schist, which has gained Fe, Si, K, Rb, S, Cu, Zn and Pb, but lost Na, Ca and Sr, occurs in extensive zones below the Dry River South sulfide lens and Balcooma Zn-Pb-rich lenses as well as in stratiform zones west of Balcooma. With the possible exceptions of Nb and Y at Dry River South, Al, Ti, Zr, Nb and Y are all immobile in these alteration zones. An outer zone of weak Fe, S, Cu and Zn enrichment, and Na, Ca and Sr depletion surrounds the Balcooma deposit and extends up to distances of 200 m beyond visibly altered rocks. Evidence for syn-tectonic composition changes to these alteration zones exist as 0.1–10 m thick gradational zones between quartz-chlorite ± biotite schist and the outer weakly altered zone at Balcooma. These zones lack muscovite and contain highly poikilitic staurolite and biotite porphyroblasts. These zones gained Mg, Fe, Mn, Si, Ca, Cu and Sr, but lost K and S. These zones also gained Zr, which is considered to be immobile in most geological environments. Major and minor element concentrations in metasediments from the Balcooma district vary systematicaly with Zr/TiO 2 as a result of weathering and deposition during turbitidic sedimentation. Deviations from trends relating Na and Fe to Zr/TiO 2 can be used to define peripheral weak alteration zones not identifiable from geological mapping.