Epigenetic copper and tungsten mineralisation in the Aileron Province, central Australia: Examples from Molyhil, Bonya Hills, and the Jervois mineral field

Abstract

Palaeoproterozoic copper and tungsten deposits, prospects, and occurrences in the northeastern Aileron Province in HUCKITTA, central Australia, were studied to characterise the nature and origin of their mineralisation. The work focused on three areas:

- Molyhil area, Jinka: the Molyhil tungsten-molybdenum deposit, Pinnacle Hill tungsten prospect, and several nearby small tungsten occurrences

- Bonya Hills area, western Jervois Range: copper and tungsten mineralisation in the Bonya Hills (eg Bonya, Green Hoard, Marrakesh copper deposits/prospects; Samarkand and Jericho tungsten prospects)

- Jervois mineral field, central Jervois Range: copper and tungsten mineralisation within the Jervois mineral field (eg Anaconda copper prospect, Green Parrot scheelite prospect).

The research combined new and previously published petrological, structural, geochemical, chronological, and stable isotope data with field mapping to produce a district-scale mineralisation study. The results were used to develop a new genetic model for the copper and tungsten mineralisation in the northeastern Aileron Province. This mineralisation is interpreted to be epigenetic and to have formed after peak temperature and pressure metamorphism, and after regional deformation. Mineralisation is associated with magmatic-hydrothermal fluid flow and alteration related to intrusion of ca 1.73-1.70 Ga fractionated felsic magmas, including the Marshall Granite and Samarkand Pegmatite. This episode produced both vein and skarn mineralisation.

In the Molyhil area, tungsten-molybdenum skarns formed in a higher-temperature mineralising system close to intrusions of strongly fractionated and oxidised Marshall Granite at mid-crustal levels. In the Bonya Hills and Jervois mineral field, skarn-hosted tungsten mineralisation and quartz vein-related copper mineralisation formed in brittle, upper-crustal levels in a lower-temperature mineralising system further away from large felsic bodies such as the Samarkand Pegmatite.

Under our new genetic model, epigenetic copper and tungsten mineralisation in the northeastern Aileron Province formed contemporaneously in the same regional tectonic setting. The mineralisation has a distinct spatial association with pre-existing metamafic and metacarbonate rocks, and regionally widespread foliation planes and folds. Both copper and tungsten mineralisation is spatially associated with felsic intrusions and hydrothermal alteration.

We propose tungsten and molybdenum mineralisation was sourced directly from hot, variably saline, weakly acidic magmatic-hydrothermal fluids exsolved from strongly fractionated and oxidised felsic intrusions. Copper and sulfur were likely leached from pre-existing ca 1.79 Ga-aged metamafic intrusions and transported by the younger granite-derived magmatic-hydrothermal fluids. Several depositional controls are suggested: neutralisation of acidic fluids interacting with metacarbonate and metamafic rocks; fluid cooling and decompression through crustal conduits; and regional- and local-scale structural traps (eg fold hinges, shear zones, and foliation planes).

Based on the new genetic model, the relationships between geology and mineralisation provide a number of implications relevant for mineral exploration in the northeastern Aileron Province: ie sources, pathways, and traps for metals.

It is noted that the epigenetic mineralisation studied is distinct from metamorphosed polymetallic mineralisation in the Jervois mineral field; however, there is spatial overlap in some locations. Furthermore, hybrid multi-phase deposits that host post-metamorphic sulfides (ie Rockface) are interpreted to have formed when epigenetic copper-bearing fluids overprinted pre-existing and reactive lithologies (interpreted as meta-exhalites) in the hinge of a km-scale synform.

The epigenetic ore-forming processes share many similarities with the genesis of district-scale epigenetic mineralisation, which extends beyond the study area and include other parts of the northeastern Aileron Province (eg in ALCOOTA), as well as other districts in central Australia (eg Davenport Province, Tennant Creek mineral field and the wider Warramunga Province). Epigenetic mineralisation in these regions likely formed as part of an interprovincial-scale mineralising episode.