Geology and genesis of the mammoth Cu deposit, Mount Isa Inlier, Australia

Of the numerous structurally-controlled, sediment-hosted Cu deposits throughout the Proterozoic Western Fold Belt of the Mt lsa lnlier, the two most significant are the giant Mt Isa Cu deposit (255 Mt @ 3.3% Cu) and the Mammoth deposit (16.8 Mt @ 3.4% Cu). The deposits are hosted in deformed Proterozoic sediments that range in age from 1740 to 1652 Ma. Copper mineralisation is hosted in breccias and veins, which appear to be fault controlled and is interpreted as occurring approx. 150 Ma after deposition of the host rocks during regional deformation (1590 to 1500 Ma; Isar) Orogeny). The Mammoth deposit is located approximately 115 km to the north of Mt lsa, and copper mineralisation is hosted within the Palaeoproterozoic Whitworth Quartzite of the Myally Subgroup. Understanding the local geological and geochemical controls at Mammoth will assist in the development of a genetic model and exploration targeting of similar deposits in ancient, structurally deformed, sedimentary basins in Australia and elsewhere in the world. Structural studies using orientation and kinematic data from mineralised veins and bounding faults have provided a new understanding of the mechanisms that localised Cu mineralisation at the Mammoth deposit. The Mammoth deposit formed during E-W transpression localised at the intersection of the Mammoth and Portal Faults during periods of high fluid pressure (Pf), subsequent to slip episodes, when the principle stresses were sub-equal (0.3 :+.- cs2 al ). Dilation was achieved when fluid pressure (PO exceeded the tensile strength (T) of the host rock as indicated: 1) by the irregular development of syndeformational quartz-Cu sulfide extensional veins; and 2) by the regular development of syndeformational quartz-Cu sulfide extensional veins along bedding planes when a i a3 . The Mammoth deposit has 6 major ore lenses that extend over 1,100 metres vertically and have a strike extent of 100 to 300 m and widths from 10 to 75m. Within these lenses, Cu mineralisation is hosted in veins and breccias adjacent to the intersection of the Mammoth and Portal Faults. A new textural classification scheme, consisting of five categories was used to describe the variations of brecciation and fracturing of the Whitworth Quartzite during hypogene mineralisation. The grade of Cu mineralisation in these units is variable, but is high around zones of intense brittle deformation in the breccia units reflecting zones of enhanced permeability. Wall rock alteration, replacement and cementation of the breccia units by the hypogene mineral assemblage took place after and/or during the fragmentation. Hypogene Cu mineralisation consists of chalcopyrite, bornite and chalcocite. Supergene chalcocite (¬¨¬± covellite) has variably overprinted and enriched the hypogene assemblage. Hypogene and supergene chalcocite within the deposit are texturally and paragenetically distinct. A four stage paragenetic sequence has been defined across the Mammoth ore zones: Stage I) hypogene pyrite and quartz; Stage II) hypogene chalcopyrite, bornite, chalcocite and carrollite (trace) + chlorite, illite and quartz; Stage III) supergene hematite, chalcocite, covellite and wittichenite, and Stage IV) supergene kaolinite, chalcocite, covellite and wittichenite. Hypogene Cu sulfides are broadly zoned within the Mammoth ore zones from hangingwall to footwall and up dip. This zonation is: 1) Stage ll hypogene chalcocite ¬¨¬± bornite; 2) Stage II bornite ¬¨¬± hypogene chalcocite and 3) Stage II chalcopyrite ¬¨¬± bornite. Microanalytical trace element geochemistry of sulfide minerals from the Mammoth deposit confirmed paragenetic interpretations. These investigations involved the development of a new analytical technique to obtain high quality, sub-ppm, quantitative data from the in situ microanalysis of sulfide minerals using laser ablation (LA) ICP-MS analysis. Pyrite, chalcopyrite, bornite and both supergene and hypogene chalcocite were analysed. Stage III supergene chalcocite is enriched in Bi and depleted in both Ag, Co, Sb, and As compared to Stage ll hypogene chalcocite. No differences were detected in trace element composition between disseminated and veined styles of Stage I pyrite. Stage I pyrite has elevated contents of As, Co, Ni, Sb, and Pb compared to chalcopyrite, bornite and chalcocite. Bornite is anomalously enriched in Bi compared to the other sulfides minerals The processes of meteoric and supergene alteration have led to complex geochemical dispersion patterns overprinting geochemical patterns developed during hypogene mineralisation. Whole-rock geochemistry analysis across the deposit revealed a distinct metal zonation. Elevated Co, Ni, CaO and P205 contents form a narrow halo (4_ 2 to 5m) - around the hypogene ore zones at Mammoth. Along the ore equivalent structural horizon Zn, Co, and Ni are enriched distal (100 to 200m) to the ore zones. In contrast, Cu, Bi, As and Fe, initially at or below detection, all increase with proximity to the ore zones. Mineralised structures that have undergone supergene modification also have a similar suite of trace elements (Cu, Pb, Zn, Ni, Bi, Sb, and As). Their level of enrichment is significantly lower than their un-oxidised equivalent but more enriched than the enclosing host rocks. These suites of trace elements form potential vectors to differentiate mineralised and unmineralised structures and can be used to identify Mammoth style mineralisation in surface or near surface leached structures. Sulfur, 0 - D and Pb isotopic investigations allow characterisation of the ore fluids and identification of potential metal source reservoirs. Lead isotopic ratios from Mammoth ore samples lie along an isochron that originates from the Eastern Creek Volcanics at 1540 Ma, indicating that the Eastern Creek Volcanics were the source of Pb, and by inference Cu, contained in the Mammoth deposit. The 6 34S values of Stage I pyrite (- 15 to - 7 %o; mean = ‚ÄövÑvÆ 12 %o) and Stage II Cu and Cu-Fe sulfides (‚ÄövÑvÆ 19 to ‚ÄövÑvÆ 1 %o; mean = - 8 ¬¨‚àû/00) supports the conclusion that the Eastern Creek Volcanics were the source of metals and sulfur as they represent the only probable source of light sulfur in the Western Fold Belt. The calculated 8180 and 6D fluid composition in equilibrium with Stage I quartz and Stage ll chlorite yielded a range of ‚ÄövÑvÆ 10 to -2 Too 6 180 and ‚ÄövÑvÆ2 to + 11 Too 8 180 and ‚ÄövÑvÆ41 to ‚ÄövÑvÆ56 %o 8D respectively. These values suggest the fluid had two end members: 1) dominantly meteoric fluid and 2) fluid originating from metamorphic processes. Geological, isotopic, paragenetic, geochemical and structural data suggest the Mammoth Cu deposit formed synchronous with regional deformation when channelised flow of hot salineoxidised Cu-enriched fluid generated by metamorphic devolatilisation, leached Cu and S from the Eastern Creek Volcanics, and was focussed along the Esperanza, Mammoth and Mammoth Extended Faults. Upon reaching the Whitworth Quartzite, a build up of fluid pressure combined with the existing regional stress field, created the appropriate conditions for brittle failure in the quartzite forming dilational sites for sulfide precipitation. The hot metal and sulfur-bearing fluid mixed with a deep circulating methane-bearing hydrothermal fluid originally of meteoric origin. Changes in the physicochemical conditions upon this mixing destabilise the metal chlorine complexes resulting in sulfide precipitation. All of these attributes cumulatively broaden the classification of the metamorphogenic style of Cu deposit. Most notable of which is that the Mammoth deposit provides evidence that metamorphogenic Cu deposits can be hosted in host rocks other than meta-carbonates. This new model for the genesis of the Mammoth deposit has developed new criteria for the exploration of similar resources in the Western Fold Belt; including the world class Mt Ise Cu deposit. The exploration strategy currently in use throughout the Western Fold Belt of the Mt Ise Idler has traditionally focused on sites where structural geometries potentially gave rise to dilatational fault jogs during deformation. This research provides evidence that structural geometries that had the potential to create zones of E-W compression and/or N-S extension should now be included in exploration targeting.