The following text are extracts and summaries from: Grech, L.L., and da Silva, G., 2024, Mafic-ultramafic intrusion-hosted Ni-Cu-PGE deposits: a mineral system analysis: Geological Survey of Western Australia Record, In Prep.
Mafic-ultramafic intrusion-hosted Ni-Cu-PGE deposits are found worldwide, and make up some of the world’s largest nickel deposits (e.g., Fig. 4 and Fig. 5 in Hoatson et al., 2006). Globally they include world-class examples such as Jinchuan (China), Pechenga (Russia), and Voisey’s Bay (Canada), while Julimar, Nova- Bollinger, Savannah, and Nebo-Babel (West Musgraves Project) are examples of such deposits in Western Australia. Mafic-ultramafic intrusions can host several mineralization types including vanadium, chromite, and more PGE rich examples. However, they will not be discussed here.
Mineralization processes
Mineralization of Ni-Cu-PGEs in mafic-ultramafic intrusions involve the eruption of large volumes of hot magmas over a short period of time, which are emplaced into the crust via long-lived crustal structures. Magmas become sulfur saturated, which allows metals to be scavenged from silicate magmas and concentrated in sulfide droplets, then accumulated. Modification, weathering, and preservation of the ore body post-formation also needs to be factored in.
A range of different geometries for mineralized intrusions, including chonolith/elongate sills (Noril’sk (Nkomati) Type), tubular chonoliths (Nebo-Babel (Limoeiro) Type), blade shaped dykes (Expo-Savannah Type) and tube/funnel transitions (Eagle/Kalatongke Type) are recognised for these deposits Barnes et al. (2016). More information on specific deposit geometries and formation can be found in (Barnes et al., 2016). In general, intrusions that host Ni-Cu-PGE mineralization seem to have more of a horizontal rather than vertical extent (Barnes and Mungall, 2018). Barnes and Mungall (2018), also argue that blade-shaped dyke style intrusions perhaps represent an end member of intrusion geometries and could be precursors to other intrusion shapes. In examples such as tube-funnel transitions and blade-shaped dykes (and potentially tubular chonoliths), geometries could be influenced by the following factors, as stated in Barnes and Mungall (2018); “lateral propagation of dikes, widening of conduits due to preferential thermal erosion of country rocks, gravity flow of sulfide-silicate-xenolith slurries, and self-enhancing propagation of sulfide vein-dike networks into process zones in country rocks—coupled with post emplacement tilting and random intersection with present-day erosion surfaces”. There isn’t a particular deposit geometry that is more favourable in terms of hosting mineralization, although chonoliths have recently been popular exploration targets, and bladed dykes are the least identified to host such mineralization (Barnes et al., 2016).
Critical processes
Derived layers are grouped based on their critical features:
SOURCE – of mafic-ultramafic magmas
PATHWAY – location of lithospheric faults, ancient cratonic blocks and dyke/sill complexes, responsible for transport of mafic-ultramafic magmas through the crust
CHEMICAL TRAP – sulfur saturation of previously sulfur undersaturated magma
CHEMICAL AND PHYSICAL TRAP – sequestering metals into sulfides
PHYSICAL TRAP – concentration of metal-rich sulfides
PRESERVATION – of nickel orebodies
Mineral system analysis
The Mineral System Tree is the graphical display of a mineral systems analysis showing the link between critical/constituent processes and their recommended targeting features and GIS layers.
References
Barnes, SJ, Cruden, AR, Arndt, NT and Saumur, BM 2016, The mineral system approach applied to magmatic Ni-Cu-PGE sulphide deposits: Ore Geology Reviews, v. 76, p. 296–316, doi:10.1016/j.oregeorev.2015.06.012.
Barnes, SJ and Mungall, JE 2018, Blade-shaped dikes and nickel sulfide deposits: a model for the emplacement of ore-bearing small intrusions: Economic Geology, v. 113, no. 3, p. 789–798, doi:10.5382/econgeo.2018.4571.
Hoatson, DM, Subhash, J and Jaques, AL 2006, Nickel sulfide deposits in Australia: Characteristics, resources and potential: Ore Geology Reviews, v. 29, p. 177–241.
Recommended reading
Barnes, SJ 2023, Lithogeochemistry in exploration for intrusion-hosted magmatic Ni–Cu–Co deposits: Geochemistry: Exploration, Environment, Analysis, v. 23, no. 1, article no. geochem2022-025, doi:10.1144/geochem2022-025.
Barnes, SJ, Cruden, AR, Arndt, NT and Saumur, BM 2016, The mineral system approach applied to magmatic Ni-Cu-PGE sulphide deposits: Ore Geology Reviews, v. 76, p. 296–316, doi:10.1016/j.oregeorev.2015.06.012.
Schulz, KJ, Woodruff, LG, Nicholson, SW, Seal II, RR, Piatak, NM, Chandler, VW and Mars, JL 2014, Occurrence model for magmatic sulfide-rich nickel-copper-(platinum-group element) deposits related to mafic and ultramafic dike-sill complexes: USGS, U.S. Geological Survey Scientific Investigations Report 2010-5070-I, 80p.