Summary:
The Nifty mineralisation is a strata-bound copper deposit, is structurally controlled, with fresh mineralisation being chalcopyrite-quartz-dolomite replacement of carbonaceous and dolomitic shales within a folded sequence.
The Nifty mineralisation is hosted within the folded late-Proterozoic Broadhurst Formation, part of the Yeneena Group. The Broadhurst Formation is between 1,000 m to 2,000 m thick and consists of a stacked series of carbonaceous shales, turbiditic sandstones, dolomite, and limestone.
The dominant structural feature is the Nifty Syncline which strikes approximately southeast-northwest and plunges approximately 6°-12° to the southeast. The bulk of the mined sulphide mineralisation is largely hosted within the keel and northern limb of the Syncline.
Weathering and oxidation extend down to a maximum depth of 200 m vertically. – Oxide copper mineralisation is identified by the presence of azurite and malachite, as well as minor cuprite and native copper.
A lower saprolite zone is a sub-domain of the oxide zone, where the rock mass has more than 20% altered minerals but with identifiable remnant rock textures, which may contain oxide copper mineralisation.
There is development of a sub-horizontal chalcocite blanket within the transitional zone.
The transitional zone marks the gradual transition from chalcocite to fresh chalcopyrite mineralisation.
Fresh mineralisation consists of chalcopyrite, with minor covellite and bornite.
Mineralisation has a strong lithostratigraphic control, with carbonate-rich rocks preferentially mineralised relative to carbonate-poor rocks and silica-dolomite alteration typically accompanying the copper mineralisation, which has a positive correlation with alteration intensity.
From youngest to oldest, the stratigraphic units and their relative state of mineralisation are as follows:
• Pyrite Marker Bed (PMB): The PMB is a 5m to 7m thick, vuggy carbonate and silty-shale unit containing abundant pyrite. The unit only occasionally displays silica-dolomite alteration with associated copper mineralisation.
• Upper Interbedded Shale Unit (ISHU): The ISHU is 25m to 50m thick and consists of interbedded siltstones and shales. This unit contains copper mineralisation associated with silica-dolomite veining and alteration.
• Lower Interbedded Shale Unit (ISHL): The ISHL is a 10m to 25m thick unit consisting of interbedded siltstones, dolomitic shales, and laminated carbonates. The carbonate component increases towards the base, as does the copper mineralisation, which occurs as disseminations along bedding and in the matrix of breccias.
• Middle Carbonate Unit (MCU): The MCU is the uppermost of four units comprising the Nifty Carbonate member (NCM) and is 20m to 40m thick, consisting of algal carbonate with minor shale interbeds. This unit is strongly altered and, along with the LCU, hosts the majority of the sulphide mineralisation.
• Barren Algal Carbonate (BAC): The BAC is a barren wedge ranging from 5m to 20m in thickness, being thickest in the east.
• Shale Unit (SH): The Shale Unit is 2m to 10m thick and is generally poorly mineralised.
• Lower Carbonate Unit (LCU): The LCU is 15m to 30m thick and consists mostly of marine algal limestone with common siltstone and shale interbeds, which becomes more numerous towards the basal contact with the underlying FWS, especially in the east. The LCU is strongly mineralised, particularly within a ~10m band just below the SH unit, with once again an association with strong silica-dolomite alteration. The copper mineralisation is less intense where there are shale interbeds.
• Footwall Shale (FWS): The FWS is gradational with the overlying LCU, and the contact is defined below the last appearance of 1m thick silica-dolomite alteration. The unit is poorly mineralised, and a 1m to 5m thick laterally extensive massive pyrite bed occurs 5m to 10m below the LCU contact.
The primary chalcopyrite copper mineralisation has been modified by weathering and oxidation down to a depth of 200m:
• The oxide copper mineral species are dominantly malachite and azurite, with some cuprite and native copper, which can extend down to 100m below the surface.
• The Lower Saprolite zone potentially contains oxide mineralisation.
• The supergene or chalcocite zone generally extends from 100m to 200m below the surface, consisting dominantly of chalcocite.
• The unweathered, hypogene fresh or primary sulphide zone is dominated by chalcopyrite associated with silica-dolomite alteration. Minor covellite and bornite are also present. Pyrite is a common gangue mineral but only occurs with chalcopyrite on the margins of the deposit.
Dimensions
The Nifty copper deposit occurs over a 1,200m down plunge distance; units vary individually between from 0 m to 30 m in true thickness. The limbs of the sequence are variously mineralised and up to 400 m in vertical extent.