Discussion
Discussion on ‘Geochemistry and geodynamic setting of
volcanic and plutonic rocks associated with early Archean
volcanogenic massive sulfide mineralisation, Pilbara Craton’
by Vearncombe, S.E., Kerrich, R., 1999. Precambrian
Research 98, 243 – 270
Carl W. Brauhart *, Peter Morant
Sipa Resources International NL,West Perth 6872 Australia
Received 1 February 2000; accepted 25 April 2000
www.elsevier.com/locate/precamres
1. Introduction
Vearncombe and Kerrich (1999) present major-and trace-element data for host rocks to the Panorama volcanogenic massive sulfide (VMS) deposits, based mainly on work presented in the senior author’s PhD thesis (Vearncombe, 1995). At the end of their introduction, they state that ‘the data presented here and in Vearncombe et al., (1995;1998) provide a complete record of an an-cient parallel of the modern geodynamic setting of VMS mineralisation’. A considerable body of other work, both published and unpublished, has been completed since the work of Vearncombe (1995), but apart from an incorrect reference to Van Kranendonk and Morant (1998), Vearn-combe and Kerrich (1999) do not refer to these contributions. The contributions by other workers
are relevant to the arguments presented by Vearn-combe and Kerrich (1999) and demonstrate that their claim of ‘a complete record’ is incorrect. The following discussion outlines where the paper by Vearncombe and Kerrich (1999) is at odds with data in the public domain and where as-yet un-published data impacts upon their work.
2. Nomenclature and stratigraphic — structural setting
Vearncombe and Kerrich (1999) have created considerable confusion by using informal nology where formal and widely accepted termi-nology exists. The ‘Strelley VMS deposits’ are referred to as the Panorama VMS deposits by all other workers in the district (e.g. Brauhart et al., 1998; Morant, 1998; Buick and Doepel, 1999). The ‘Strelley Belt’ is equivalent to the Soanesville Belt (Hickman, 1983; Van Kranendonk, 1998).
* Corresponding author. Fax: +61-8-93223047. E-mail address:[email protected] (C.W. Brauhart).
Vearncombe and Kerrich (1999) incorrectly refer to a volcanic-dominated succession and an unconformably overlying sedimentary-dominated succession as comprising the Kangaroo Caves Formation of the Sulphur Springs Group. The lower volcanic-dominated succession is part of the Kangaroo Caves Formation and the upper sedi-mentary-dominated succession is part of the Gorge Creek Group (Van Kranendonk and Morant, 1998).
Vearncombe and Kerrich (1999) also state that the Soanesville Belt is allochthonous relative to bounding belts. However, the Sulphur Springs Group has also been mapped in the Pincunah and East Strelley Belts to the west and the Euro Basalt of the Warrawoona Group extends west from the North Pole Dome into the Soanesville Belt (Van Kranendonk, 1998; Van Kranendonk and Morant, 1998). Structural evidence (Van Kranen-donk and Collins, 1998) and unpublished SHRIMP zircon ages (R Buick, pers. comm, 1999) also suggest that the Soanesville Belt is not allochthonous with respect to surrounding belts.
3. Geology and alteration
Brauhart et al., (1998) published geological and alteration maps of the Panorama VMS district which are based on regional and detailed mapping by company and academic geologists. This work, together with the recently published 1: 100 000 geological mapping by the Geological Survey of Western Australia (Van Kranendonk, 1998; Van Kranendonk and Morant, 1998), supercedes the description of regional geology and alteration by Vearncombe and Kerrich (1999), which is based on an interpretation of Landsat and aeromagnetic data (Vearncombe, 1995), rather than field mapping.
Vearncombe and Kerrich (1999) describe the volcanic pile as bimodal, but other geological mapping and geochemical studies have clearly shown that it contains a spectrum of compositions ranging from basalt through andesite, dacite and rhyolite (Morant, 1998; Brauhart, 1999). Vearn-combe and Kerrich (1999) interpret the volcanic succession as lava flows, but a volcanological
study by McPhie and Goto (1996) concluded that laterally extensive dacite units are subvolcanic sills in the upper Kangaroo Caves Formation. Much of the andesitic and rhyolitic components of the Kangaroo Caves Formation are also now inter-preted as subvolcanic sills.
The description by Vearncombe and Kerrich (1999) of the three phases of the Strelley Granite appears to be based on mapping by the Geologi-cal Survey of Western Australia in the mid-1970s (Hickman and Lipple, 1978). More detailed map-ping (Brauhart et al., 1998) shows that the Strelley Granite comprises a porphyritic biotite – blende inner phase and an equigranular horn-blende – biotite outer phase which grades upward into a quenched granophyric carapace in concor-dant contact with the overlying volcanic pile.
The paper by Brauhart et al., (1998) focuses on the regional VMS hydrothermal alteration system and demonstrates clearly that, at a regional scale, the predominant alteration underlying the Sul-phur Springs and Kangaroo Caves deposits is not sericitic, as described by Vearncombe and Kerrich (1999), but chloritic. Furthermore, the alteration zones are not just semiconformable but include km-scale transgressive zones of feldspar — de-structive alteration that underlie the Sulphur Springs and Kangaroo Caves VMS deposits.
4. Trace-element geochemistry
Vearncombe and Kerrich (1999) claim, based on the analysis of 13 samples, that the footwall dacite at the Sulphur Springs and Kangaroo Caves VMS deposits is ‘compositionally complex’ and refer to covariant trends of increasing Y, Al2O3 and TiO2 with Zr and variable LaN/YbN
ratios. Brauhart (1999) provides data for 38 dacite samples taken across the district and Young (1997) provides data for a further 53 dacite sam-ples taken at the Sulphur Springs deposit. They concluded that covarying trends amongst immo-bile elements (including those cited above) are a product of net mass transfer associated with hy-drothermal alteration. Young (1997) concluded that variable LaN/YbN ratios in his data were a
mobil-ity similar to that described in other VMS districts (e.g. Campbell et al., 1984; Liaghat and MacLean, 1995). Thus we interpret the variation in trace element geochemistry in Vearncombe and Kerrich (1999) dacite data to reflect hydrothermal alter-ation rather than complex heterogeneity in the initial composition of the unit.
Vearncombe and Kerrich (1999) refer to a more mafic sample (SG113) taken from the centre of the Strelley Granite and suggest that this sample may represent a parental magma to the more evolved rocks in the Strelley Granite. Sample SG113 is texturally and compositionally identical to a suite of ‘intermediate hybrid’ rocks mapped
and sampled by Brauhart (1999) and which are interpreted to represent magma mingling between the inner phase of the Strelley Granite and doler-ite. Evidence in support of magma mingling in-cludes; (1) complex fluidal textures between dolerite, granite and the intermediate hybrid; (2) disequilibrium textures in the intermediate hybrid such as resorbed quartz phenocrysts mantled by hornblende; and (3) samples of intermediate hy-brid lie on a geochemical mixing line between granite and dolerite end members.
On the basis of seven andesite – basalt samples, Vearncombe and Kerrich (1999) infer that nega-tive Nb, P and Ti anomalies on priminega-tive-mantle normalised trace element diagrams characterise the source region of the Kangaroo Caves Forma-tion magmas. However, much more extensive sampling and a better understanding of the geol-ogy (Brauhart, 1999) are used here to dispute this assertion with respect to P and Ti. Two Vearn-combe and Kerrich (1999) andesite – basalt sam-ples belong to a more fractionated andesite unit at the base of the volcanic pile which is characterised by Ti/Zr ratios between 20 and 30. This leaves five samples to characterise the most primitive rocks exposed at Panorama. Trace element data for 131 andesite – basalt samples from the Kangaroo Caves Formation (Huston and Brauhart unpub-lished data; Brauhart, 1999) show that these rocks have very small to non existent negative P and Ti anomalies, but pronounced negative Nb anoma-lies on primitive-mantle normalised trace element diagrams (Fig. 1). As noted by Vearncombe and Kerrich (1999), more evolved samples have pro-gressively greater negative Ti and P anomalies, consistent with the fractionation of magnetite and apatite. Thus, we interpret the source region of the Kangaroo Caves Formation magmas to be characterised by the retention of Nb in the source, but not by the retention of Ti and P.
Vearncombe and Kerrich (1999) use trace ele-ment data to draw analogies between the Strelley Granite and (1) two mica peraluminous granite suites from the Archean Abitibi Belt, (2) Archean low-Al TTG suites; and (3) post-Archean subduc-tion-related granites. We argue that, although they point out some similarities between these disparate granite types and the Strelley Granite,
Fig. 1. Chondrite-normalised extended trace-element diagrams for (a) the average composition of 131 andesite – basalt ples, and (b) the average composition of andesite – basalt sam-ples with Ti/Zr\60 (i.e. the 35 least-fractionated samples).
Table 1 Sm–Nd dataa
SiO2 Sm Nd 147Sm/144Nd 143Nd/144Nd
Lithology 2s
Sample T(Ma) oNd Ages(Ma)
wt.% ppm ppm TDM T(2-stage)
203366 Outer-phase granite 76.1 5.64 23.94 0.14247 0.511438 5 3239 −1.0 3658 3507 75.2 5.14 27.54 0.11272 0.510827
Inner-phase granite 6
203368 3238 −0.5 3496 3472
Mafic microdiorite
207186 50.8 3.42 12.98 0.15911 0.511824 6 3238 −0.4 3704 3467 75.3 6.18
207334 Rhyolite 26.97 0.13844 0.511378 6 3238 −0.5 3583 3472
a2s, 2 standard deviation error for143Nd/144Nd ratio; T, independently determined SHRIMP U-Pb zircon age of sample;oNd, {(143Nd/144Nd)
i/ItCHUR×104, where (143Nd/l44Nd)i, the initial143Nd/144Nd ratio of the sample, and ItCHUR,143Nd/144Nd ratio of chondrite uniform reservoir at the age of the sample (modern chondrite uniform reservoir values used here are143Nd/144Nd, 0.51265 normalised to146Nd/144Nd=0.7219, and147Sm/144Nd, 0.1967); T
DM, depleted mantle model age; IT(2-stage), two-stage model age calculated using the measured147Sm/144Nd from the present back to 3.24 Ga, and147Sm/144Nd, 0.11 before 3.24 Ga.
they are unable to demonstrate a close link with any of them. The Strelley Granite is a magnetite-series hornblende – biotite bearing granitoid (Brauhart et al., 1998) and thus it is clearly unre-lated to two-mica peraluminous granitoids. As indicated by Vearncombe and Kerrich (1999), least-altered Strelley Granite samples contain be-tween 4 and 5 wt.% K2O and high Th (generally
\20 ppm Th) and therefore, are not low-Al
TTGs. The Strelley Granite may be chemically similar to some post-Archean granites but the evidence presented by Vearncombe and Kerrich (1999) is superficial and not convincing.
We argue that the evidence presented by Vearn-combe and Kerrich (1999) for assimilation frac-tional crystallisation in the Strelley Granite is very weak. They claim that the extreme Sr and Eu depletion of the granite indicates high degrees of assimilation fractional crystallisation. However, they model neither of these elements, nor any of the incompatible elements, to demonstrate that fractional crystallisation alone cannot account for these patterns. Further, they present no isotope data that could be used to test the hypothesis of assimilation fractional crystallisation.
Vearncombe and Kerrich (1999) argue strongly for crustal contamination in the Kangaroo Caves Formation and Strelley Granite magmas, based on LREE and LILE enrichment and Ta – Nb de-pletion. However, preliminary Sm – Nd isotopic work on four samples (inner and outer phase granite, rhyolite and mafic microdiorite) shows
that these rocks have initial oNd values at 3240
Ma ranging from −1.0 to −0.4 (Table 1; Sun and Brauhart unpublished data). The tight clus-tering of these data, coupled with similar extended trace element profiles for the four samples, sug-gests that they were derived from the same, or similar sources. The samples plot below the mantle growth curve and therefore, the source region must have been contaminated by older crust or have been isotopically modified mantle. Crustal contamination requires that the older crust had the same Sm – Nd isotopic composition as the source region it contaminated, or that mafic and felsic melts were contaminated to the same degree. We consider both of these explanations unlikely and suggest that a subduction event which affected the mantle source prior to the generation of the Kangaroo Caves Formation and Strelley Granite magmas, would explain both the Sm – Nd isotopic data and LREE-LILE enrichment.
5. Conclusion
omis-sions. We acknowledge that some of the data presented here was not available to Vearncombe and Kerrich (1999) when they wrote their paper, but much of it was. Our greatest criticism of Vearncombe and Kerrich (1999) is that it at-tempts to constrain the geodynamic setting of the Panorama VMS district without the support of detailed petrography or regional mapping, relying almost solely on trace element geochemistry on a limited number of samples. We feel that ‘a com-plete record of an ancient parallel of the modern geodynamic setting of VMS mineralisation’ will only emerge by properly integrating other con-straints, including regional mapping and detailed petrography, with trace-element data.
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