Geochemistry and geology mineralized and barren Komatiites-Western Australia

Western Australian Archaean komatiites which are associated with nickel sulphide mineralization can be separated into two groups - Mineralized or Barren, based on komatiite lithogeochemistry. Mineralized komatiites may host nickel sulphide deposits whereas Barren komatiites do not. Chemical relationships were determined from a data base of approximately 3300 samples of fresh komatiite ult.r.amafic from four nickel pro¥inces and other greenstone belts not known to contain nickel sulphides. Mean chemical values for each group of komatiites were: Category Mineralized (J) NiP - (2) 1027 Ni CuP Cu Al Ca 2220 36 42 1.6 2.2 !i9_ Zn Cr Mn Fe CoP 19.2 69 1617 1057 6.1 49 Co 119 Barren 429 1530 29 39 2.2 2.8 16.4 76 2260 1128 7.0 32 119 Discriminant analysis, using the above thirteen chemical determinations as variables, for each of 2775 samples from forty localities, indicated that samples could be classified as either Mineralized or Barren with an expected accuracy of greater than 80 percent. No single element or chemical determination is definitive, but collectively, Cr, Ni, Zn, Cu, Ni~, Mg, Fe and Co can distinguish between the two groups of ultramafics. Critical elements are Cr, Ni and Nif, assuming that values of Zn, Cu, Mg and Fe approximate the mean value for all West Australian komatiites. The Ni to Cr ratio is always greater than unity (1) in Mineralized komatiites and the Ni to NiP ratio is always less than 3.5. Sulphur is not a diagnostic element. NOTES: 1. f indicates a partial or sulphide analysis. 2. Al, Ca, Mg and Fe results are expressed in percentages; all others in parts per million. Increasing Ni/Cr ratios and decreasing Ni/Ni~ ratios within a komatiite can be regarded as indicative of increasing nickel sulphide potential. Mineralized komatiites contain less Cr within the silicate lattice structure and less chromite than Barren komatiites. However, the more important relationship appears to be the lesser amount of Cr attached to the silicate mineral lattice. Correlation analysis showed that: 1. most correlations are much stronger in Barren than in Mineralized ultramafics; 2. the chalcophile elements, Cu, Ni, Co and Fe (constituents of nickel sulphide deposits), show moderate to strong correlations with the rock forming elements, Mg, Mn, Ca, Al in Barren ultramafics, but only weak or no correlation in the Mineralized ultramafics; 3. copper has moderate positive correlation with Fe, Mn, Ca, Al and negative correlation with Mg in Barren ultramafics but shows no correlation with these same elements in Mineralized ultramafics. These correlation differences suggest that in Barren komatiites Ni, Cu, Co and Fe are contained in the silicate mineral lattice whereas in Mineralized komatiites they are presently partly as a separate sulphide fraction. In addition they may also suggest that these sulphides were added or removed from Mineralized komatiites after the formation of the komatiite magma, probably by concentration and removal in arl immissible sulphide-oxide melt. Komatiites can be divided into two separate suites called volcanic and intrusive. Volcanic suites such as those at Kambalda and Windarra South may contain many individual komatiite flows. The basal section of a komatiite volcanic pile consists of a small number of thick units which may contain sulphide mineralization whereas the central and upper parts of the pile consists of multiple thin units. Both thick and thin units consist of an olivine cumulate derived lower part overlain by a silicate liquid derived upper part. In thick units the olivine cumulate section is dominant whereas in thin units the silicate liquid section is dominant. Spinifex texture is characteristic of unmetamorphosed sequences. In metamorphosed sequences such as Windarra South it is not possible to identify individual komatiites using mineralogical or textural criteria but it can be accomplished using chemical data. Intrusive suite komatiite sequences such as Forrestania or Perseverance usually consist of a small number of high Mg, homogeneous peridotites and/or dunites. Equigranular, equant olivine textures are characteristic. These komatiites are often continuous over strike lengths of the order of tens of kilometers and contain relatively little internal chemical variation. Volcanic komatiites such as those at Windarra South and Kambalda are considered to be ultramafic lavas. Chemical differences between volcanic and intrusive sequences have been defined. Typical chemical values for the cumulate section of a volcanic komatiite and for intrusive komatiites, both with moderate to high mineralization coefficients are: Classification NiP- -Ni CuP & Cu Al !:19. Zn Cr Mn Fe CoP Co Volcanic Komatiites 1000 2100 30 - 90 1-2 17-24 60 1300 1000 5.5 55 120 Intrusive Komatiites 1200 2500 5 - 60 0.5 20-26 60 1000 900 6 60 125 In general, if Nif or Ni are less than 500 and 1800 ppm respectively, or Cr greater than 2100 ppm, a komati ite can be regarded as Barren. It has been possible to define sections of greenstone belts as prospective for nickel sulfides and other parts as unprospective. For example, the Forrestania section of the Forrestania-Southern Cross greenstone belt has a different chemical signature to the Southern Cross section. The latter section is unlikely to contain economic nickel sulphide accumulations.