pXRF assessment of new magmatic fertility indicators in the Macquarie Arc

Abstract

Summary This research presents a new method for assessing magmatic fertility using pXRF in altered terranes using Zr and Y ratios. A study of global batholith related mineralised porphyries, highlights a low Zr fractionation trend. The depletion in Zr is associated with early crystallisation of titanite. Yttrium depletion is related to early crystallisation of both titanite and amphibole, an indication of hydrous melts. Previously differentiation of hydrous, potentially ore-forming (fertile) porphyries relied on Sr/Y from whole rock assay of least altered rocks. Finding samples in porphyry terranes where Sr can be demonstrated to be immobile is difficult given the ubiquity of hydrothermal alteration associated with porphyry emplacement. The requirement for unaltered samples is based on the mobility of Sr which precludes the widespread use of pXRF as quantitative assessment of LOI% (as a proxy for alteration) is beyond the current capabilities of the technology. Using comparatively immobile Zr and Y overcomes issues with alteration and provides a more reliable new indicator of magmatic fertility. In this case study, using a systematic workflow pXRF was successful in identifying the mineralising intrusions in the Northparkes intrusive complex using a Zr/Y vs. Y fertility indicator.