Indicators of, and vectors to fertile magmas in the Northparkes District and broader Macquarie Arc

Exploration for metallic mineral resources in New South Wales has primarily focused on the Ordovician to Early Silurian volcanic rocks and related sub-volcanic intrusions of the Macquarie Arc. The Northparkes and Cadia Cu-Au porphyries are the largest economic mineral deposits in the Macquarie Arc and exploration for analogues to these deposits is the focus of numerous exploration companies. However, exploration efforts are hampered by the fact that no geochemical fingerprint for magmatic fertility of porphyry style mineralisation in the Macquarie Arc has been defined previously. This research identifies new fertility indicators that are applicable in the Macquarie Arc and on a global scale using whole rock and mineral geochemistry.

Four economic porphyry Cu-Au deposits and several prospects were investigated in the Northparkes district, located in the Junee-Narromine Belt of the Macquarie Arc. New whole rock geochemical data from the Northparkes district indicate the mineralising intrusive complex exhibits distinct arc signatures that are transitional from high-K calc-alkaline to silica-saturated alkalic. Mineralising intrusions are interpreted to have crystallised from fractionated hydrous melts, based on ratios of Sr/Y vs. Y (e.g. Sr/Y >20 and <17 ppm Y), indicating the suppression of plagioclase crystallisation in favour of hydrous mineral phases. This interpretation is supported by the listric-shaped rare-earth element curves and the presence of primary hornblende phenocrysts, both of which are indicative of elevated amounts of magmatic water. A study on Northparkes and global Cu-Au porphyries as a part of this thesis highlighted low Zr and Y trends associated with mineralising intrusions. Yttrium depletion has been shown to be a product of hydrous magmatic conditions that cause hornblende to crystallise early. The low Zr concentrations in progenitor porphyries is a product of the early saturation of zircon which occurs at low Zr concentrations owing to low temperature crystallisation depressed by high dissolved H₂O in the melt.

Previous geochemical differentiation of hydrous, potentially ore-forming (fertile) porphyries relied on ratios of Sr/Y and Sr/MnO from geochemical and/or pXRF analysis of least altered rocks. However, Sr and MnO are both highly mobile which limits their use in hydrothermally altered rocks. Finding samples in porphyry terranes where Sr can be demonstrated to be retained is difficult given the ubiquity of hydrothermal alteration associated with porphyry emplacement. This precludes the widespread use of pXRF, as quantitative assessment of alteration is beyond the capabilities of pXRF technology. Using immobile elements overcomes issues with alteration and provides more reliable indicators of magmatic fertility. A systematic pXRF workflow including cross plots of Zr vs Y and Zr*Y vs Al₂O₃/TiO₂ was successful in identifying 100% of the mineralising intrusions in the Northparkes intrusive complex and provide a proxy for melt hydration state. The ratios of Zr/Nb vs Y/Nb are insensitive to hydrothermal alteration and near-surface weathering and are indicative of the evolution and dissolved H₂O content of magmatic sources. Moving from bivariate plots to ratio plots shifts the data into a real number space and provides a linear function to assess against magmatic fertility. Application of fertility indicators to a regional dataset from New South Wales highlighted areas of known porphyry mineralisation indicating that the method has potential to be a useful indicator of fertility beyond the scope of the Northparkes pilot study. Application of the fertility fields defined in the Zr vs Y and Zr*Y vs Al₂O₃/TiO₂ space from the Northparkes pilot study is demonstrated to successfully identify >71% of fertile intrusions from a global porphyry data set.

In order to assess magmatic fertility of intrusive complexes it is important to be able to locate them spatially. Outcrop of Ordovician rock in the Macquarie Arc is relatively limited, with the majority of the arc buried beneath Siluro-Devonian cover sequences. Geophysical methods such as magnetic intensity and gravity have been useful in identifying the likely location of buried intrusive complexes but they remain elusive drill targets at depth. Using mineral chemistry, this study has defined associations between propylitic alteration minerals epidote and chlorite and the proximity of intrusive complexes. Propylitic alteration associated with porphyry style mineralisation can extend kilometres beyond the causative intrusion. Anomalous concentrations of pathfinder elements (Sb, As, and Mn) can be detected in epidote far beyond the traditional whole rock pathfinder elements, and are demonstrated to be useful in identifying the location of intrusive complexes. Despite regional variation, the highest values of Sb + As in epidote in each district within the Macquarie Arc are well correlated with location of known intrusive complexes. Northparkes in particular has anomalously high concentrations of Mn that increases with proximity to the porphyry deposits within the district. Regional sampling has also indicated that intrusive complexes are likely to be located where they have previously been inferred from geophysics but remain untested by drilling. In close proximity to the intrusive complexes chlorite mineral chemistry was demonstrated to be a useful tool in vectoring to the loci of heat within the intrusive complexes.

Anomalous concentrations of temperature sensitive elements in chlorite (Ti, Sr, Co, and Tl) provide useful vectors to hydrothermal centres. Concentrations of Ti/Sr and Ti/Co are demonstrated to increase with increasing proximity to porphyry, and epithermal mineralisation. Elevated concentrations of Tl in chlorite are correlated with increased proximity to VMS style mineralisation in the Lachlan Fold Belt. The combination of epidote as an indicator and chlorite as a vector provides a useful tool for mineral explorers in the Macquarie Arc, particularly when the samples are considered in regard to the surrounding alteration, mineral morphology, the target resource and likely metallogeny.

Investigation of apatite mineral chemistry also demonstrated its potential to be a useful vector to porphyry mineralisation. The resistate nature of apatite in the weathering profile makes it a useful mineral for assessing magmatic fertility particularly in weathered terrane and transported cover. Apatite with low LREE concentration are indicative of alteration related to porphyry mineralisation. Depletion of light rare earth elements (LREE) in apatite is associated with hydrothermal alteration across the Northparkes district. Hydrothermally altered apatite from the mineralising intrusions at the Endeavour 26 deposit (175 Mt at 0.6% Cu and 0.15 g/t Au) have a pronounced LREE depletion and MREE enrichment, concurrent with a strong positive Eu anomaly. These characteristics are useful indicators of mineralisation-related hydrothermal alteration at Northparkes.

Colour cathodoluminescent response of apatite was shown to be a valid indicator of proximity to mineralised centres. Imaging of apatite from Northparkes highlights complex zonation and differing luminescent colours that are linked to variations in mineral trace element geochemistry. At Northparkes, variations in luminescent colour are associated with hydrothermal alteration and proximity to mineralised centres. The count of apatite with complex zonation and bright yellow-brown cathodoluminescence increased with proximity to known porphyry mineralisation. The increased abundance of complex zoned apatite provides a simple measure to assess proximity to mineralised centres in highly weathered terranes where bulk rock alteration is obscured. Apatite provides a useful new tool for assessing the fertility of the numerous intrusive phases, and the hydrothermal alteration footprint associated with them in the Northparkes Cu-Au porphyry district, NSW.

The novel suite of whole rock and mineral geochemical analyses used in this research provide a method for identifying areas of potential porphyry mineralisation. Regional investigation of epidote geochemistry can be utilised to identify the likely location of intrusive complexes at depth, and chlorite geochemistry can be used to vector to the hottest area of the intrusive complex. Apatite mineral chemistry and colour cathodoluminescence can be used to identify the hydrothermal alteration footprint and fertility of a porphyry system. Whole rock geochemistry can be used to assess the alteration zonation and ore-forming potential of the intrusive complexes. Magmatic fertility assessment can be carried out by means of traditional geochemical analyses or by pXRF. These novel geochemical indicators and vectors when combined with traditional geological mapping, logging, structural analysis and geophysics have the potential to improve exploration outcomes.