Late orogenic alteration in the wall rocks of the Pegmont Pb-Zn deposit, Cloncurry District, Queensland, Australia

Abstract

Pegmont is one of several significant Pb-Zn + or - Ag deposits in metamorphosed late Paleoproterozoic rocks of the Mount Isa Eastern fold belt of northwest Queensland. It occurs in a package of feldspathic psammites and pelites that display fracture-controlled hydrothermal alteration as much as several 100 m away from a 1- to 6-m-thick ironstone, composed largely of fayalite, garnet, hedenbergite, and apatite, which carries all the significant sulfide mineralization. Alteration of the wall rocks occurred after the amphibolite facies peak regional metamorphism. Early alteration produced quartz + or - tourmaline + or - K feldspar + or - biotite veins and K feldspar-muscovite-biotite alteration together with bed-selective garnet (intermediate almandine-grossular-spessartite)-biotite alteration in some units close to the ironstone. This involved infiltration of hot (>500 degrees C), very saline, Na-K-Fe-Ca-Mn-Cl-rich fluids. Proton microprobe (PIXE) analyses of fluid inclusions reveal that these fluids, at least locally, had high concentrations of Pb (as great as 1.45 wt %) and Zn (to 0.8 wt %), and therefore, that metals were mobile beyond the boundaries of the mineralized ironstone. Later fluids produced illite/phengite-chlorite-carbonate alteration and minor Fe-Cu-dominated sulfide mineralization. Fluid access occurred largely through moderate to steeply dipping sheeted fracture systems that are commonly strongly discordant to both bedding and ductile tectonite fabrics. Fracture densities reveal a strong small-scale lithological control on alteration intensities and broadly coincident development of the high- and low-temperature parageneses. Data integrated over [asymp]100-m intervals show that alteration intensity generally increases toward the ironstone. These observations suggest that some of the distinctive petrochemical features of the Pegmont deposit and its host rocks may have been produced through the introduction of exotic fluid components during the late-orogenic metasomatism.