The characteristics, geochemistry and origin of propylitic alteration in the northparkes porphyry Cu-Au system

Abstract

Propylitic alteration is typically the most extensive and peripheral alteration facies developed around porphyry ore deposits. Consequently, with exploration increasingly focused on searching under cover, it may be the only indicator of prospectivity. Understanding its relationship to mineralisation is therefore crucial. However, the genesis of propylitic alteration is poorly understood: deposit models generally assume isochemical processes driven by heated meteoric water. Here, the Northparkes porphyry deposits (NSW) are used to test this paradigm and show how alteration mineral chemistry could assist exploration targeting. Alteration surrounding each Northparkes deposit is zoned from an inner potassic (K-feldspar-biotite) assemblage, through a magnetite-biotite annulus, out into widespread and coeval propylitic (epidote-chlorite) alteration. The quantified net geochemical effects of alteration demonstrate that propylitisation is not isochemical, rather elements are mobilised and dispersed across various facies. Elements lost during potassic alteration are gained in the propylitic zone (e.g. Ca, Fe, Mg, Mn, Co, As) while others either decrease in abundance outwards (e.g. Cu, Au, S, K, Al, Si) or display a halo of elevated values (e.g. Zn, Mn, Pb, C). Investigation of epidote and chlorite chemistry reveals these minerals may host ore-related elements (e.g. Bi, As, Zn, Mn), the concentration of which, along with others (e.g. Ti, Al), is found to vary systematically around mineralised centres. Such trends indicate hypogene dispersion, controlled primarily by temperature and fS2; they have the potential to transform exploration in the propylitic domain. Oxygen isotope geothermometry suggests potassic alteration occurred between ~600-700oC in magmatic centres, persisting to ~450 oC upon transition into propylitic alteration. Across the propylitic facies temperature progressively decreases outwards to <250 oC. The oxygen, hydrogen and strontium isotopic composition of alteration minerals support the involvement of magmatic fluids in all alteration zones. Collectively, the results provide robust evidence that propylitic alteration is a distal equivalent of potassic alteration, driven by spent magmatic-derived ore fluids as they migrate away from thermal centres, cooling and equilibrating with country rocks.