Porphyry fertility in the Northparkes district: indicators from whole-rock geochemistry
Wells, TJ and Meffre, S and Cooke, DR and Steadman, JA and Hoye, JL, Porphyry fertility in the Northparkes district: indicators from whole-rock geochemistry, Australian Journal of Earth Sciences ISSN 0812-0099 (2020) [Refereed Article]
Four economic porphyry Cu–Au deposits and several prospects have been investigated in the
Northparkes district, part of the Ordovician to early Silurian Junee–Narromine Belt of the
Macquarie Arc, New whole-rock geochemical data from the Northparkes porphyry Cu–Au district,
NSW, indicate that the mineralising intrusive complexes exhibit distinct arc signatures that are
transitional from high-K calc-alkaline to silica-saturated alkalic. Based on ratios of Sr/Y vs Y (e.g. Sr/
Y > -20 and Y < -17ppm) the mineralising intrusions are interpreted to have crystallised from
fractionated hydrous melts indicating the suppression of plagioclase crystallisation in favour of
hydrous mineral phases. This interpretation is supported by listric-shaped rare earth element
curves and the presence of primary hornblende phenocrysts indicating elevated magmatic water
contents. There is an association of mineralising intrusions with a low Zr trend both in the mineralised
Northparkes district intrusive rocks and in mineralised porphyry-related intrusive rocks globally.
A newly developed fertility indicator ratio Zr/Y -10% is more accurate at identifying the
mineralised rocks at Northparkes than the conventional Sr/Y vs Y fertility indicator diagram, successfully
identifying 92% of the mineralising intrusions, mainly owing to the fact that it is less
affected by hydrothermal alteration. The insensitivity of Zr–Y to alteration makes this indicator a
useful new tool that may lead to enhanced probabilities for future discoveries in the Northparkes
district, broader Macquarie Arc and altered rocks globally.
1. Mineralising intrusions in the Northparkes district have distinct Zr vs Y concentrations.
2. The Zr vs Y indicator of magmatic fertility is less sensitive to alteration than Srbased
3. The Zr vs Y magmatic fertility indicator identified at Northparkes is not unique and identifies
mineralising intrusions in other porphyry fields.