Cr-Zoning in pyroxene as a prospectivity indicator for magmatic Ni-Cu sulfide deposits

Schoneveld, Louise1; Barnes, Steve1, HV Makkonen5, M Le Vaillant1, D Paterson6, V Taranovic1, K-Y Wang2,3, Y-J Mao4

1CSIRO, Kensington, Australia, 2Chinese Academy of Sciences, China, 3University of Chinese Academy of Sciences, China, 4Chinese Academy of Sciences, China, 5Boliden FinnEx, Finland, 6Australian Synchrotron, ANSTO, Clayton, Australia

Small intrusions dominated by olivine- and pyroxene-rich cumulates are common in a variety of settings around the world, but only a very small proportion contain economically exploitable sulfides. We aim to provide a new tool for distinguishing these fertile intrusions from sparse exploration drilling.

Cumulate and poikilitic pyroxenes in strongly mineralised intrusions have complex grain-scale Cr zonation. We separate the zonations into three distinct types: 1) abrupt zoning; 2) sector zoning 3) oscillatory zoning. This combination of zoning patterns is likely to indicate high magma flux and fluctuating cooling rate that accompanies wall rock assimilation in the dynamic conduits where sulphide liquid forms and accumulates. As the diffusion of chromium is extremely slow, these zoning patterns can last thousands of years within hot terranes. 

We have investigated pyroxene-bearing samples from small intrusions containing magmatic sulphide deposits including the Noril’sk-Talnakh camp in Siberia, the Kotalahti Nickel Belt in Finland, Ntaka Hill in Tanzania, Nova-Bollinger in the Albany-Fraser Orogen and Savannah in the Halls Creek Orogen of Australia, Jinchuan in central China, Xiarihamu in Tibet and Huangshanxi in the east Tianshan Ni province of NW China. To compare, we analysed samples from the weakly mineralised or barren intrusions in four of these regions along with four mafic intrusions that are not associated with any economic sulfide mineralisation.

Cumulus orthopyroxene with a combination of abrupt zoning, sector zoning and resorbed olivine inclusions has so far only been detected in mineralised intrusions. Desktop XRF mapping instruments easily image this distinctive zoning pattern in large pyroxenes,which provides a useful fertility indicator for exploration of new magmatic Ni-Cu-(PGE) deposits.


Biography

Louise obtained her bachelors and honours in geology from James Cook University, Townsville and in 2018 graduated with a PhD from the Australian National University (ANU).  Currently she is focussed on investigating the trace element signatures in minerals as possible indicators for economic mineralisation.

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