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

Citation
Pj. Williams et al., Late orogenic alteration in the wall rocks of the Pegmont Pb-Zn deposit, Cloncurry District, Queensland, Australia, ECON GEOL B, 93(8), 1998, pp. 1180-1189
Citations number
37
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
ISSN journal
03610128 → ACNP
Volume
93
Issue
8
Year of publication
1998
Pages
1180 - 1189
Database
ISI
SICI code
0361-0128(199812)93:8<1180:LOAITW>2.0.ZU;2-F
Abstract
Pegmont is one of several significant Pb-Zn +/- Ag deposits in metamorphose d late Paleoproterozoic rocks of the Mount Isa Eastern fold belt of northwe st 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 sulfi de mineralization. Alteration of the wall rocks occurred after the amphibol ite facies peak regional metamorphism. Early alteration produced quartz +/- tourmaline I K feldspar +/- biotite veins and K feldspar-muscovite-biotite alteration together with bed-selective garnet (intermediate almandine-gros sular-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 r eveal that these fluids, at least locally, had high concentrations of Pb la s 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 pr oduced illite/phengite-chlorite-carbonate alteration and minor Fe-Cu-domina ted sulfide mineralization. Fluid access occurred largely through moderate to steeply dipping sheeted fracture systems that are commonly strongly disc ordant to both bedding and ductile tectonite fabrics. Fracture densities re veal a strong small-scale lithological control on alteration intensities an d broadly coincident development of the high- and low-temperature paragenes es. Data integrated over approximate to 100-m intervals show that alteratio n intensity generally increases toward the ironstone. These observations su ggest that some of the distinctive petrochemical features of the Pegmont de posit and its host rocks may have been produced through the introduction of exotic fluid components during the late-orogenic metasomatism.