GSA talk and AGM: Mesoproterozoic shear-hosted REE-enriched IOCG mineralisation in the Broken Hill Block: new insights

The upper greenschist to granulite facies field array of the Broken Hill Block of NSW is crosscut by a network of biotite-muscovite-rich retrograde schist zones that are the primary record of post Olarian Orogeny (<1600 Ma) evolution in the eastern Curnamona Province. The Copper Blow Schist Zone south of Broken Hill township is unique in that it hosts an allanite-rich REE-IOCG prospect (Copper Blow) that preserves an unprecedented history of shear zone reactivation to the Carboniferous. Allanite U-Pb dating and trace element chemistry from the mineralised shear zone records initial LREE-rich allanite crystallisation associated with IOCG mineralisation between 1430-1340 Ma. Subsequent recrystallisation/neocrystallisation of allanite (±titanite and mica) records events at ~800 Ma, ~500 Ma, 440 Ma and 320 Ma, with younger events associated with limited, but distinctive HREE-rich allanite as rims surrounding Mesoproterozoic allanite. The initial Mesoproterozoic (1430-1340 Ma) timing of LREE-rich allanite and IOCG mineralisation is previously unknown in the Broken Hill region, while ~800 Ma coincides with Neoproterozoic magmatism/sedimentation in the Adelaide Rift Complex, ~500 Ma with the Delamerian Orogeny, 440 Ma with the Benambran Orogeny and 320 Ma with the Kanimblan/Alice Springs Orogeny.

Yu et al (2024) have recently detailed magnetite-Cu IOCG’s in the eastern Gawler Cration extending to ~1450 Ma, which is considerably younger than the magmatic hematite IOCG systems of the region. The authors interpreted these young IOCG’s to have formed during Nuna breakup, defining a new opportunity for copper systems in southern Australia. The Copper Blow REE IOCG falls on the younger end of the Gawler IOCG’s and may represent expression of the same Nuna break up event in the eastern most Curnamona Province. Subsequent crystallisation of limited, but distinctive HREE-rich allanite primarily at ~500 Ma times extensive biotite-muscovite retrogression of the garnet-rich host sequences and the main phase of schist zone formation during the Delamerian Orogeny, with recrystalised granulite facies garnet representing the mostly likely, local HREE source.

You're invited to dinner at The Bank Hotel after the talk. 

Speaker Dr Joel Fitzherbert has worked with the Geological Survey of NSW for the past 20 odd years, initially as a regional mapper, followed a brief stint in the seamless geology team, and then as a member of mineral systems team. Joel has worked on most of the geological elements of NSW, but his post graduate research focused on high grade metamorphism has fuelled a continual interest in the metamorphic evolution of the Broken Hill region, with recent work focusing of the late-stage development of IOCG mineralisation in the region.