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
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.