Apatite as as indicator of porphyry fertility in the Northparkes district

Wells, Tristan1, Meffre, Sebastien1, Cooke, David R,1,2, Steadman, Jeffrey A.1, Goemann, Karsten

1CODES. Centre for Ore Deposits and Earth Sciences, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia. 2ARC Industrial Transformation Research Hub for Transforming the Mining Value Chain – TMVC, Private Bag 79, University of Tasmania – Hobart, Tasmania, 7001. 3Central Science Laboratory, University of Tasmania, Hobart Tasmania, 7001

The resistate nature of apatite in the weathering profile, combined with its potential to record physical and chemical information about magmatic and hydrothermal systems, makes it a useful mineral for assessing magmatic fertility. Magmatic apatite trace element compositions can reflect the degree of fractionation, sulphur content and oxidation of the host rock, whereas hydrothermal or recrystalised apatite also has the potential to record hydrothermal fluid evolution.

Apatite from the Northparkes district, NSW were analysed by colour cathodoluminescence, scanning electron microscopy, electron micro-probe and laser ablation inductively coupled mass spectrometry. Colour cathodoluminescent imaging of apatite from Northparkes highlights complex zonation and differing luminescent colours that are linked to variations in mineral trace element abundance and crystal origins. At Northparkes, magmatic apatites have a lavender or blue luminescent colour, whereas apatites that have complex internal geometries and a yellow-green to brownish luminescent colour are associated with hydrothermal alteration and proximity to mineralised centres. Magmatic and hydrothermal apatite have similar crystal forms, making them virtually indistinguishable from each other without the aid of colour cathodoluminescence. Depletion of light rare earth elements in apatite is associated with hydrothermal alteration across the Northparkes district. Hydrothermally altered apatite from the mineralising intrusions at the Endeavour 26 deposit have pronounced LREE depletion and MREE enrichment, concurrent with a strong positive Eu anomaly. The detection of apatite with these characteristics can be used to infer the proximity to mineralization in porphyry systems in the Northparkes district.


Biography

Tristan is a part-time PhD student researching the fingerprint of magmatic fertility in the Northparkes district and broader Macquarie Arc. His research uses a combination of whole rock and mineral chemistry to define the signature of fertile porphyry intrusions and vectors towards them.

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