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Geochemistry and geology mineralized and barren Komatiites-Western Australia

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McNeil, RD (1980) Geochemistry and geology mineralized and barren Komatiites-Western Australia. Research Master thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (Research Master)
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Date Deposited: 01 Jun 2012 05:46
Last Modified: 11 Mar 2016 05:56
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