Abstract
Propylitic alteration is typically the most common and laterally extensive alteration facies in porphyry ore systems; and with exploration increasingly focused on searching under cover, it may be the only visible sign of hydrothermal activity. Consequently, knowledge of propylitic assemblages and how they form is crucial to effective exploration. Despite this, propylitic alteration is relatively poorly understood, with deposit models generally assuming an isochemical process driven principally by heated meteoric water. We present a detailed study of the Northparkes porphyry cluster to test this idea and establish whether systematic changes in whole rock and alteration mineral chemistry are developed that could assist with exploration targeting.
Northparkes, located in central NSW, consists of several small alkalic Cu-Au porphyry bodies, host to both high grade and sub-economic mineralisation. These multiphase quartz monzonite intrusions are particularly narrow (∼200 to 300 m diameter) but highly elongate (>1.4 km vertically), emplaced within and above a larger monzonite pluton. Field observations show each porphyry is surrounded by a small scale alteration halo, with progression from inner potassic alteration, characterised by K-feldspar + biotite, through a magnetite + biotite halo, passing outwards to a widespread propylitic epidote + chlorite zone.
The flux of metals (e.g. Cu, Au, Zn, Mn, Pb, V, Co) and ore-related elements (e.g. As, S, K, Ca, Sr) passing through the principal alteration zones was quantified using bulk rock geochemistry and chemical mass transfer analysis. The mineralogical residence of these elements was also mapped throughout the propylitic zone, with an extensive epidote and chlorite LA-ICP-MS mineral chemistry dataset. Results show propylitic alteration is not isochemical; rather elements lost from the potassic zone are typically gained in the propylitic zone (e.g. Ca, Fe and Co), and extensive outwards dispersion of other elements is observed. This is reflected either as a decreasing outward concentration (e.g. Cu, Si, Ti and S), or in a halo of higher values (e.g. Zn and Mn). Similar trends exist, but often with a greater magnitude of variation, in the chemistry of epidote and chlorite from the innermost to outermost propylitic zone.
Such distribution patterns for metals and ore-related elements, in particular their residence within propylitic alteration minerals, indicate a close link in space and time between intense propylitic alteration and ore formation. This is further supported by the δ18O and δD composition of epidote, chlorite and quartz separates from the E48 deposit. These indicate propylitic alteration was driven by a magmatic fluid during equilibration with the country rock at progressively lower water:rock ratios while cooling through 350–200°C.
We contend that propylitic alteration is generated principally by lateral and upward infiltration of magmatic fluids into the host rock sequence. For exploration, it is critical to examine propylitic mineral chemistry and record details such as mineral assemblages, alteration intensity and texture.