Structure, alteration and mineralisation of the Nifty Copper Deposit, Western Australia: Implications for ore genesis

The Nifty Cu deposit is located approximately 450km east of Port Hedland, Western Australia in subgreenschist grade rocks of the Proterozoic Paterson Orogen. The deposit consists of a secondary oxide resou~ce of 12.2 Mt@ 2.52% Cu and a primary sulphide ore body of94Mt @1.63% Cu (0.5% cut-off). Mineralisation is hosted in plunging syncline of carbonaceous and dolomitic shales of the upper Broadhurst Formation. The depositional age of the Broadhurst Formation is constrained to between 1132¬±21 and 816¬±6Ma. A reinterpretation of the local stratigraphy proposes that the mine sequence consist of the footwall beds, Nifty member, pyrite marker bed, and hanging wall beds. The footwall beds consist of chloritic and pyritic shales. The Nifty member is sub-divided into the lower unit, intermediate shale, upper unit and upper shale. The lower unit contains 40-70m thick package of interbedded finegrained, pale grey, dolomitic mudstone and blue-black, carbonaceous shale. Separating the lower and upper units is a prominent 1-4 metre thick bed of dark-grey to black, very fine-grained, chloritic shale. The upper unit (25-60m) has a similar composition to that of the lower unit and consists of alternating beds of laminated carbonaceous shale and dolomitic mudstone. The upper shale (20-40m) consists of dark-grey to black, laminated, carbonaceous shale with thin to massive framboidal pyrite scams and blebs, disseminated very fine-grained framboidal pyrite and minor dolomitic mudstone. Overlying the Nifty member is the pyrite marker bed, which consists of 1-22m of framboidal pyrite in carbonaceous shale. Above the pyrite marker bed is a 20-60m thick package of carbonaceous and pyritic shales of the hangingwall beds and 5-60m finely laminated dolomitic and calcareous silty shale of the upper carbonate bed. Five deformation events are recognised. 0 1 includes rare variably oriented folds of unknown significance. 0 2 produced regional folding and cleavage during north-east to south-west compression. Folds tend to be doubly plunging with axes trending north-east, south-west. 0 2 deformation has been at dated at Maroochydore as occurring at 717¬±6Ma (Reed, 1996). 0 3 is an upright folding and faulting event that folds S2. Associated with D3 is a north-north-east striking, slaty cleavage that is less well developed than S2. 0 4 is a complex folding event with sub-horizontal axial planes and fold axis coaxial to 0 2. A brittle deformation event, D5 has also been recognised and this event has resulted in fault offsetting of the ore body. There is significant structural complexity in the Nifty syncline area but most of the complexity is due to small scale, local, fault related events. The Nifty Cu deposit occurs as a structurally controlled, chalcopyrite-quartz-dolomite replacement of carbonaceous and dolomitic shale. The main phase of Cu mineralisation occurred during 0 2 deformation and is associated with a zoned hydrothermal alteration system. Six mineralisation textures were observed; veins, vein networks, breccia matrix, and a progressive sequence of isolated chalcopyrite spots and blebs, to bedding parallel bands, and finally replacement of host rock. Early framboidal pyrite with later chalcopyrite, euhedral pyrite, sphalerite, and galena are the dominant sulphide species. Sphalerite and galena occur in isolated zones in siliceous and pyritic beds of the pyrite marker bed and deposit footwall. Minor diagenetic alteration occurs as ¬£1-Fe-Mg Carbonate in several prominent bands in hangingwall shales 2-5m above the pyrite marker bed. Two main stages of alteration occur at Nifty. A pre-D2‚ unmineralised, siliceous alteration, ¬£2-Green Quartz, wilh minor chlorite+ pyrite+ hematite+ sericite+ stilpnomelane, replaces interbedded carbonaceous shale and dolomitic mudstone beds of the upper and lowe!' unit of the Nifty member. The second phase (syn-D2) of alteration is associated with Cu mineralisation and occurs as a zoned quartz-dolomite system. Distal alteration consists of S2-Silicified Pyritic Shale where quartz+ sphalerite+ galena+ chalcopyrite and cuhedral pyrite ~verprint framboidal pyrite beds in the pyrite marker bed and immediate deposit footwall. Inward is a progressive sequence of interbedded/banded SJ-Hydrothermal Quartz-Dolomite and unaltered shale beds. The SJ-Hydrothermal Quartz-Dolomite alteration first appears as spots and veins with 2-4mm light grey alteration margins in lhe footwall followed by coalesced spots. An Fe-rich variant of hydrothermal dolomite occurs where minor SJ-Chloritic Shale was observed on the distal margin of Fe-rich quartz-dolomite altered rocks. Further inward and proximal to the highest ore grade is S4-Silicified Dolomitic Shale (dolostone) lhat grades into S5-Biack Silica in the centre of the ore body. S5 alteration consists of fine-grained quartz + chalcopyrite + apatite + carbonaceous material and replaces host rock and earlier siliceous alteration. Cu grade has a strong positive correlation with intensity of alteration. Minor concentrations of Pb and Zn are associated with bedding parallel quartz flooding + chalcopyrite + euhedral pyrite of S2-Silicified Pyritic Shale. Sphalerite and galena replace pyrite framboids. A new method of calculating mass balance change in compositions during alteration has been developed. This method uses a log transform of the element of interest divided by an immobile element. The logratio method is scale invariant and therefore avoids the unit sum problem. This melhod has been applied to Nifty multi-element data from host rock lithologies and alteration phases. Mass balance calculations suggests that during intense alteration (S4 and S5) the central core of the deposit was enriched in Cu, Si02, As, Zn, Pb, Sn, Ni, U, Fez03 andY. Depletion has occurred in Ba, Sr, and Rb. The outer halo of the deposit experienced similar additions but of a smaller magnitude. An implication of the mass balance analysis is that there has been a significant volume change during alteration and mineralisation. The highest concentration of Cu occurs within hydrothermally altered rocks of the upper and lower units of the Nifty member while Pb and Zn occur in silicified pyrite shales in the pyrite marker bed and footwall beds. High concentrations of Zn also occur in shale above the upper carbonate beds. This distribution of base-metals has resulted in a vertical metal zonation of Cu~Pb~Zn. New galena Pb isotope data from the Nifty Cu deposit and other regional prospects have been combined with existing galena data to constrain the source of Pb. Pb isotope data from Throssell Group deposits and prospects plot as a linear trend in 206PbP04Pb vs 206PbP04Pb space. The Warrabarty Pb-Zn prospect plots at the least radiogenic end, Maroochydore Cu deposit at the most radiogenic end, and lhe Nifty Cu deposit plots towards the least radiogenic end of the trend. The Throssell Group linear trend suggests mixing between two Pb sources. Potential sources of Pb are I) Rudall Complex, 2) Pilbara Craton, or 3) internally sourced Pb leached from Throssell Group sediments. The Rudall Complex Pb signature is poorly defined as shown by initial Pb-Pb ratios of K-feldspars separates. Galena Pb isotope data from deposits in the Pilbara Craton plot on a Pb evolution trend similar to the growth curve of Cumming and Richards (1975). A source-mixing model is proposed where Pb from a magmatic source (J.t:=9.88) is mixed with crustal Pb (J.t:=l0.55): The position of deposits and prospects along the linear trend suggests that the Pb in the Warrabarty Pb-Zn prospect was dominated by magmatic Pb and that Pb at the ‚ Maroochydore Cu deposit has a crustal source. Pb in the Nifty Cu deposit appears to have a mixed source. Syn-mincralisation fluid inclusions are small (commonly <5-lOJ.lm) and consist a simple two phase petrology of liquid and vapour. Fluid inclusion trapping temperatures of mineralising fluids at Nifty range between 270-457¬∞C (median 358¬∞C) and salinity of 8-27 eq. wt.% NaCl (median 15 eq. wt.% NaCI). Post mineralisation inclusions have trapping temperatures between 18I-269¬∞C (median 190¬∞C) and salinity of 11-15 eq. wt.% NaCI (median 13 eq. wt.% NaCl). Sulphur isotope data from framboidal pyrite ranges between o34ScoT of -27 and + 16%o, suggesting that the source of framboidal pyrite sulphur is a combination of reduced seawater sulphate and evaporite sulphate. Chalcopyrite o34ScoT ranges between -6 and +6%o and o34ScoT for euhcdral, syn-mineralisation pyrite ranges between - 12 to+ 12%o. The range of syn-mineralisation sulphide data suggest that source of sulphur at Nifty was homogenous and that syn-mineralisation sulphides did not utilise existing sulphur from sedimentary pyrite. The o34S range of -6 to +6%o for chalcopyrite also suggests a reduced magmatic hydrothermal source of sulphur. Carbon and oxygen isotopic data from host rock carbonates, pre-, syn-, and post-mineralisation vein stages have lighter oxygen and carbon values than other deposits in the region. Results from the various vein stage carbonates overlap, with o13Cvpo8 values from approximately 6 to -13%o and o180vsMow values from approximately 15 to 20%o. Carbon and oxygen isotope trend modelling suggests that fluid mixing and fluid/rock interaction are potential precipitation mechanisms. A model for the formation of the Nifty Cu deposit is proposed whereby compression associated with the Miles Orogeny caused the expulsion hydrothermal fluids from deep in the Yeneena Basin. Fluid expulsion was driven by 1ithostatic loading due to overriding thrust sheets. Hydrothermal fluids travelled along a basement decollement and then ascend D2 thrust faults toward zones of low pressure. During tightening of the Nifty syncline thrust faulting penetrated carbonaceous and dolomitic shales allowing hydrothermal fluids to enter the Nifty member. Early fluid pulses were weakly oxidised and contained low concentrations of base ...