Porphyry fertility in the Northparkes district: indicators from whole-rock geochemistry |
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| Authors: | T J Wells S Meffre D R Cooke J A Steadman J L Hoye |
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| Institution: | 1. Centre for Ore Deposits and Earth Sciences—CODES, University of Tasmania, Hobart, Australia;2. tjwells@utas.edu.au;4. ARC Industrial Transformation Research Hub for Transforming the Mining Value Chain—TMVC, University of Tasmania, Hobart, Australia;5. CMOC-Northparkes Mines, Life of Mine and Exploration Department, Parkes, Australia |
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| Abstract: | AbstractFour 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 < ～17?ppm) 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.- KEY POINTS
Mineralising intrusions in the Northparkes district have distinct Zr vs Y concentrations. The Zr vs Y indicator of magmatic fertility is less sensitive to alteration than Sr-based indicators. The Zr vs Y magmatic fertility indicator identified at Northparkes is not unique and identifies mineralising intrusions in other porphyry fields.
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| Keywords: | Macquarie Arc Northparkes porphyry fertility Ordovician intrusive rocks prospectivity indicators |
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