Search

Regional targeting is discussed for komatiite Ni-Cu-(PGE) deposits. The success of predictive targeting relies on identifying key characteristics that are identifiable prior to the use of detection techniques such as surface geochemistry, electromagnetic exploration and drilling. A review is presented of the different deposit styles or footprints. Discoveries in the last 15 years require a significant departure from the Kambalda style deposit model and of exploration targeting models. An analysis of first-order controls on komatiite Ni-Cu- (PGE) camps shows that the Kambalda deposits represent only part of the spectrum of deposit styles and genesis., Regional targeting is discussed for komatiite Ni-Cu-(PGE) deposits. The success of predictive targeting relies on identifying key characteristics that are identifiable prior to the use of detection techniques such as surface geochemistry, electromagnetic exploration and drilling. A review is presented of the different deposit styles or footprints. Discoveries in the last 15 years require a significant departure from the Kambalda style deposit model and of exploration targeting models. An analysis of first-order controls on komatiite Ni-Cu- (PGE) camps shows that the Kambalda deposits represent only part of the spectrum of deposit styles and genesis.

Wall rock alteration and gold concentration are found to precede peak regional metamorphism. Intense alteration of footwall and, in particular, hangingwall and large amounts of gold relative to base metal suggest concentration of gold was in an epithermal-like hydrothermal system, synchronous with burial and diagenesis of the host rocks., Wall rock alteration and gold concentration are found to precede peak regional metamorphism. Intense alteration of footwall and, in particular, hangingwall and large amounts of gold relative to base metal suggest concentration of gold was in an epithermal-like hydrothermal system, synchronous with burial and diagenesis of the host rocks.

A new style of komatiite-associated sulphide-poor Pt-group element mineralisation has been identified in the strongly NiS mineralised Agnew- Wiluna Greenstone Belt. The komatiite sequence is about 200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate- textured peridotite core, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic domains are common features. The presence of anomalous PGE-enriched horizons in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained PGMs. Not all PGMs are in paragenetic association with sulphides, and only sulphide-poor/free intervals contain significant PGM concentrations. The Pd-Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir-Ru depletion in the upper mafic section of the sequence may be the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulphide-undersaturated environment., A new style of komatiite-associated sulphide-poor Pt-group element mineralisation has been identified in the strongly NiS mineralised Agnew- Wiluna Greenstone Belt. The komatiite sequence is about 200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate- textured peridotite core, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic domains are common features. The presence of anomalous PGE-enriched horizons in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained PGMs. Not all PGMs are in paragenetic association with sulphides, and only sulphide-poor/free intervals contain significant PGM concentrations. The Pd-Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir-Ru depletion in the upper mafic section of the sequence may be the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulphide-undersaturated environment.

The sulphide deposits, in Western Australia, occur in Norseman-Wiluna greenstones associated with komatiite. The volcanic and sedimentary sequence is intruded by granitoid, metamorphosed to amphibolite facies and faulted and folded during four episodes of deformation. Recently discovered deposits in the area have important geological characteristics which differ significantly from the typical Kambalda-style deposit. These deposits are Helmut, Mariners, Blair, Coronet and Carnilya Hill. The area is the largest Ni producer in Australia, producing 40 000 t of Ni in concentrate in the year ending July 1996. As of July 1996, proved and probable reserves were 10 500 000 t at 3.1% Ni., The sulphide deposits, in Western Australia, occur in Norseman-Wiluna greenstones associated with komatiite. The volcanic and sedimentary sequence is intruded by granitoid, metamorphosed to amphibolite facies and faulted and folded during four episodes of deformation. Recently discovered deposits in the area have important geological characteristics which differ significantly from the typical Kambalda-style deposit. These deposits are Helmut, Mariners, Blair, Coronet and Carnilya Hill. The area is the largest Ni producer in Australia, producing 40 000 t of Ni in concentrate in the year ending July 1996. As of July 1996, proved and probable reserves were 10 500 000 t at 3.1% Ni.

The area consists of a tholeiite unit overlain by a komatiite unit with or without sedimentary rocks on the contact. The lower 50-200 m of the komatiite is composed of up to six komatiite flows and contains Ni sulphide ores or sulphidic sedimentary rocks at the base of the lowermost two or three flows. The upper 20-1 000 m consists of numerous, thin and less magnesian flows and generally lacks sulphides and interflow sedimentary units. The controls on sulphide saturation and concentration of the resulting sulphide in a limited zone to form ore are still uncertain. Two sulphur sources have been proposed: komatiite lava/magma and sedimentary material. The sedimentary models have recently been challenged on geochemical grounds, specifically using Re-Os and Sm-Nd isotopic and rare earth element composition., The area consists of a tholeiite unit overlain by a komatiite unit with or without sedimentary rocks on the contact. The lower 50-200 m of the komatiite is composed of up to six komatiite flows and contains Ni sulphide ores or sulphidic sedimentary rocks at the base of the lowermost two or three flows. The upper 20-1 000 m consists of numerous, thin and less magnesian flows and generally lacks sulphides and interflow sedimentary units. The controls on sulphide saturation and concentration of the resulting sulphide in a limited zone to form ore are still uncertain. Two sulphur sources have been proposed: komatiite lava/magma and sedimentary material. The sedimentary models have recently been challenged on geochemical grounds, specifically using Re-Os and Sm-Nd isotopic and rare earth element composition.

Coronet is a small deposit within the northwestern part of the Kambalda dome and is concealed beneath a faulted wedge of Lunnon Basalt, which also forms the footwall of the deposit. The orebody consists of four major ore surfaces, Coronet South, Coronet North, and two hanging-wall ore surfaces, SO1H and SO2H. The orientation of the hanging-wall ore surfaces is oblique to the trend of Coronet North. The SOIH is the main mineralised ore shoot and is spatially associated with the base of the second flow unit. The mineralisation is predominantly interspinifex in nature. The SO2H surface follows a similar trend to the SO1H and is inferred to represent the continuation of the SO1H. The SO2H ore is unusual in that the footwall is sedimentary. The distribution of sedimentary units, the nature of the contacts with the komatiite flows, the abundance and distribution of vesicles in the flows and the trend of primary volcanic facies are consistent with the emplacement of komatiite lavas by early open channel flow followed by a transition to laminar emplacement., Coronet is a small deposit within the northwestern part of the Kambalda dome and is concealed beneath a faulted wedge of Lunnon Basalt, which also forms the footwall of the deposit. The orebody consists of four major ore surfaces, Coronet South, Coronet North, and two hanging-wall ore surfaces, SO1H and SO2H. The orientation of the hanging-wall ore surfaces is oblique to the trend of Coronet North. The SOIH is the main mineralised ore shoot and is spatially associated with the base of the second flow unit. The mineralisation is predominantly interspinifex in nature. The SO2H surface follows a similar trend to the SO1H and is inferred to represent the continuation of the SO1H. The SO2H ore is unusual in that the footwall is sedimentary. The distribution of sedimentary units, the nature of the contacts with the komatiite flows, the abundance and distribution of vesicles in the flows and the trend of primary volcanic facies are consistent with the emplacement of komatiite lavas by early open channel flow followed by a transition to laminar emplacement.

The spectrum of volcanic and structural controls indicates a continuum of Kambalda ore shoots from a volcanologically controlled end member through structurally modified to a structurally controlled end member. Shape analysis of three-dimensional models indicates that the structurally modified and controlled ore shoots have prolate shapes and two of the volcanically controlled ore shoots have oblate shapes. Comparison of the prolate shapes with the shapes of strain indicators reveals the importance of constrictional D1 deformation in elongation of the ore shoots, particularly at the north and south ends of the dome. The oblate volcanically-controlled end members reflect low-strain settings adjacent to major fault zones. Comparisons with previous experimental systems suggest that the ore shoots and metakomatiite deformed preferentially by ductile flow. Exploration for Kambalda-style mineralisation should take into consideration the deformation history of terranes that host ore., The spectrum of volcanic and structural controls indicates a continuum of Kambalda ore shoots from a volcanologically controlled end member through structurally modified to a structurally controlled end member. Shape analysis of three-dimensional models indicates that the structurally modified and controlled ore shoots have prolate shapes and two of the volcanically controlled ore shoots have oblate shapes. Comparison of the prolate shapes with the shapes of strain indicators reveals the importance of constrictional D1 deformation in elongation of the ore shoots, particularly at the north and south ends of the dome. The oblate volcanically-controlled end members reflect low-strain settings adjacent to major fault zones. Comparisons with previous experimental systems suggest that the ore shoots and metakomatiite deformed preferentially by ductile flow. Exploration for Kambalda-style mineralisation should take into consideration the deformation history of terranes that host ore.

PGE-rich zones with discrete Pd, Pt and Rh mineral phases occur in three 80-130 m thick peridotite-gabbro flows. Along 2 km of the Boston Creek Flow ferropicrites, three zones of up to 1 000 ppb PGE+Au correlate most strikingly with lenticular-podiform concentrations of chalcopyrite and clinopyroxene + interstitial magnetite-ilmenite intergrowths, high concentrations of related metals (0.3% Cu and S, 0.1% V and 1.2 ppm Ag), strong PGE depletion in adjacent rocks and along strike, and lithological and textural complexity in the margins of the central gabbro-diorite layer. Similar occurrence, style and composition of mineralisation (110 ppb Ir+Pt+Pd+Au) is found within the Theo's Flow tholeiites. In both, PGE-rich Fe-Cu sulphides and Fe-Ti oxides may have formed simultaneously from flowing silicate liquid in the margins of internal lava channels. In the Fred's Flow komatiites, 340 ppb Ir+Pt+Pd+Au correlates with vesicle-filling intergrowths of pyrrhotite + pentlandite +/- chalcopyrite and high whole-rock Ni (0.25%), Cu (0.07%) and S (1.1%) in the upper chilled margin of the flow, suggesting ponding and coalescence there of PGE-enriched sulphurous vapour bubbles during olivine accumulation at the base of a dynamic lava channel., PGE-rich zones with discrete Pd, Pt and Rh mineral phases occur in three 80-130 m thick peridotite-gabbro flows. Along 2 km of the Boston Creek Flow ferropicrites, three zones of up to 1 000 ppb PGE+Au correlate most strikingly with lenticular-podiform concentrations of chalcopyrite and clinopyroxene + interstitial magnetite-ilmenite intergrowths, high concentrations of related metals (0.3% Cu and S, 0.1% V and 1.2 ppm Ag), strong PGE depletion in adjacent rocks and along strike, and lithological and textural complexity in the margins of the central gabbro-diorite layer. Similar occurrence, style and composition of mineralisation (110 ppb Ir+Pt+Pd+Au) is found within the Theo's Flow tholeiites. In both, PGE-rich Fe-Cu sulphides and Fe-Ti oxides may have formed simultaneously from flowing silicate liquid in the margins of internal lava channels. In the Fred's Flow komatiites, 340 ppb Ir+Pt+Pd+Au correlates with vesicle-filling intergrowths of pyrrhotite + pentlandite +/- chalcopyrite and high whole-rock Ni (0.25%), Cu (0.07%) and S (1.1%) in the upper chilled margin of the flow, suggesting ponding and coalescence there of PGE-enriched sulphurous vapour bubbles during olivine accumulation at the base of a dynamic lava channel.

The study documents the distribution of the PGE (Ir, Pt and Pd), Au and Ag and non-precious metals (Co, Cr, Ni, Cu, S, Se, As, Sb, V, Zr and Ti) relative to primary and secondary mineral assemblages and alteration and deformation in the flow. It was found that the mineralisation is not explained convincingly by models based on the settling of segregated sulphide, upwardly mobile and Cl-rich late-stage magmatic volatiles and hydrothermal fluid activity during metamorphism. Mineralisation is explained in the main by in situ segregation of sulphide accompanying the crystallisation of Fe-Ti oxide from upwardly mobile, volatile-charged interstitial melt that evolved in clinopyroxenite and basal gabbro and subsequently formed anorthsite. Ni, Co, Se, S, Ag and Cu were mobilised relative to the PGE in the mineralised rocks by hydrothermal fluids and reconcentrated in zones of shear deformation and brittle failure during greenschist facies contact metamorphism. It is proposed that Ir and Pd were enriched in the basal peridotite by coprecipitation of a sulphide-alloy phase during olivine+chromite fractionation., The study documents the distribution of the PGE (Ir, Pt and Pd), Au and Ag and non-precious metals (Co, Cr, Ni, Cu, S, Se, As, Sb, V, Zr and Ti) relative to primary and secondary mineral assemblages and alteration and deformation in the flow. It was found that the mineralisation is not explained convincingly by models based on the settling of segregated sulphide, upwardly mobile and Cl-rich late-stage magmatic volatiles and hydrothermal fluid activity during metamorphism. Mineralisation is explained in the main by in situ segregation of sulphide accompanying the crystallisation of Fe-Ti oxide from upwardly mobile, volatile-charged interstitial melt that evolved in clinopyroxenite and basal gabbro and subsequently formed anorthsite. Ni, Co, Se, S, Ag and Cu were mobilised relative to the PGE in the mineralised rocks by hydrothermal fluids and reconcentrated in zones of shear deformation and brittle failure during greenschist facies contact metamorphism. It is proposed that Ir and Pd were enriched in the basal peridotite by coprecipitation of a sulphide-alloy phase during olivine+chromite fractionation.

Metallogenesis correlates strongly with the respective host lithologies and is characterised by Fe-Ni-Co-Zn-Mo sulphides in the Cr-rich skarn and Fe-Zn-Cu-Ni-Co sulphides in the Fe-rich skarn. Textural evidence indicates that most of the polymetallic sulphides crystallised during the earlier stages of the skarn development, but Mo (as molybdenite) in the Cr-rich skarn was probably introduced during late calc-silicate alteration. Gold mineralisation is spatially related to the REE-enriched calc-silicates and was most probably emplaced during the late calc-silicate alteration. It differs from most Phanerozoic gold skarn deposits by its geologic setting in a brittle-ductile shear zone and association with carbonate-free host rocks, and from Archaean mesothermal lode gold deposits by a lack of prominent quartz veins and involvement of a hydrothermal fluid of moderate salinity. Although the brittle-ductile shear zone almost certainly provided necessary permeability for the fluid responsible for gold mineralisation and the REE-enriched calc-silicates, the localisation of gold deposition was probably controlled largely by a preexisting pyritic zone., Metallogenesis correlates strongly with the respective host lithologies and is characterised by Fe-Ni-Co-Zn-Mo sulphides in the Cr-rich skarn and Fe-Zn-Cu-Ni-Co sulphides in the Fe-rich skarn. Textural evidence indicates that most of the polymetallic sulphides crystallised during the earlier stages of the skarn development, but Mo (as molybdenite) in the Cr-rich skarn was probably introduced during late calc-silicate alteration. Gold mineralisation is spatially related to the REE-enriched calc-silicates and was most probably emplaced during the late calc-silicate alteration. It differs from most Phanerozoic gold skarn deposits by its geologic setting in a brittle-ductile shear zone and association with carbonate-free host rocks, and from Archaean mesothermal lode gold deposits by a lack of prominent quartz veins and involvement of a hydrothermal fluid of moderate salinity. Although the brittle-ductile shear zone almost certainly provided necessary permeability for the fluid responsible for gold mineralisation and the REE-enriched calc-silicates, the localisation of gold deposition was probably controlled largely by a preexisting pyritic zone.