Discussion
Reply to discussion on ‘Geochemistry and geodynamic
setting of volcanic and plutonic rocks associated with early
Archaean volcanogenic massive sulfide mineralisation,
Pilbara Craton’ (Vearncombe, S.E., Kerrich, R., 1999.
Precambrian Research 98, 243 – 270) by C.W. Brauhart, P.
Morant
S. Vearncombe
a,*, R. Kerrich
baVearncombe and Associates Pty Ltd.,Geological Exploration Consultants,14A Barnett Street,Fremantle 6160, Australia bDepartment of Geological Sciences,Uni
6ersity of Saskatchewan,Saskatoon, Canada
Received 28 March 2000; accepted 25 April 2000
www.elsevier.com/locate/precamres
1. Introduction
We welcome the opportunity to respond to the discussion by Brauhart and Morant (2000), to clarify some issues and refute their arguments. As is explicit in the title of the paper, our objectives were to use the geochemistry of volcanic and intrusive rocks proximal to the Strelley VMS de-posits to interpret the geodynamic setting of this base-metal system. We then addressed the larger question as to whether the geochemistry of these arc magmas was typical of Archaean greenstone belts, or post-Archaean counterparts, and how these might relate to secular variations of magma compositions and mineralisation styles. Brauhart and Morant (2000) take a reductionist approach
in their discussion by focussing on nomenclature and alteration.
2. Addressing comments by Brauhart and Morant
2.1. Introduction
Brauhart and Morant maintain that our in-troductory comment is not a complete record because we have not referred to ‘a considerable body of other work, both published and unpub-lished’. In our paper we said, ‘the data presented here and in Vearncombe et al. (1995), Vearn-combe et al. (1998) provide a complete record of an ancient parallel of the modern geodynamic setting of VMS mineralisation’. In these earlier papers and in our 1999 paper, we have examined the geochemical, structural, mineralisation, sul-phide texture, and hydrosphere implications of the Strelley VMS deposits, and the analogy with
* Corresponding author. Fax: +61-8-93351845. E-mail address:[email protected] (S. Vearncombe).
VMS settings is complete (Vearncombe, 1995; Vearncombe et al., 1995, 1998; Vearncombe and Kerrich, 1999). The papers referenced by Brauhart and Morant do not consider the conjunction of multiple geological parameters.
Brauhart and Morant refer to work not refer-enced by us and criticise us for not using it. This includes: Buick pers comm, 1999; Brauhart un-published PhD thesis; 1999, Huston and Brauhart, unpublished data; Sun and Brauhart, unpublished data; Young, unpublished Hons Thesis, 1997. All of these data are both unpublished and unavail-able. The Theses remain uncatalogued in the rele-vant university library. Published data includes that of Brauhart et al. (1998), Buick and Doepel (1999), McPhie and Goto (1996), Morant (1998), Van Kranendonk (1998). Of these only Brauhart et al. (1998) on regional alteration is published in a peer review journal. This is an interesting study of the relationship of regional alteration and VMS mineralisation, but is not of direct relevance to our study. The paper Buick and Doepel (1999) avail-able in late 1999 is a general summary paper with no relevance to our geochemistry. McPhie and Goto (1996) is a brief conference abstract, and the paper Van Kranendonk (1998) is part of the progress review in the Geological Survey Annual Report. Morant (1998) is an industry-oriented review of exploration history, drilling and geology. It is difficult for us to take into consideration data that is neither public domain nor available to us, as yet unreferenced in the libraries, or, as indicated in their discussion, preliminary in the case of the Sm – Nd isotopes. Given that a draft of our manuscript was sent to Sipa Resources invit-ing comments, Brauhart and Morant could easily have mentioned and discussed this unpublished research then.
Clearly, Brauhart, Morant and colleagues have unpublished data. We look forward to seeing these published in peer review scientific journals.
3. Nomenclature and stratigraphic — structural setting
The first proposed formalisation of nomencla-ture in the literanomencla-ture as related to Strelley is by
Van Kranendonk and Morant (1998) to whom we have referred. In our introductory geological framework we attempted to avoid confusion by maintaining a consistent terminology relative to existing literature. The East Pilbara stratigraphy and its nomenclature are evolving as a result of recent dating (mostly unpublished) and mapping, and this evolution in understanding is expected to continue. We agree that the stratigraphic terminol-ogy needs to be established, and then used consis-tently.
We challenge Brauhart and Morant’s assertion of our use of informal terminology instead of ‘widely accepted terminology’ as used by ‘all other workers in the district (e.g. Brauhart et al., 1998; Morant, 1998; Buick and Doepel, 1999; Van Kranendonk, 1998)’. For brevity, we refer Brauhart and Morant back to the exact papers they quote where the proposed terminology of Van Kranendonk and Morant (1998) is rarely used, and instead informal, new and undefined terminology appears. As one example of several, Brauhart et al. (1998), refer to the Panorama District — a new terminology, with the informal, Strelley succession, and the formal, Gorge Creek Group. Under the Van Kranendonk and Morant (1998) scheme this should read the Soanseville Belt, the Sulphur Springs Group and the Gorge Creek Group, respectively.
The proposed change in name from Strelley deposits, as used by us, to Panorama deposits is confusing to uninitiated readers, considering that Panorama is located 27 km NE across the geolog-ical grain from the Strelley area (1:100 000 North Shaw Topographic Survey, Sheet 2755). The Panorama Belt lies east of the North Pole Dome and is an entirely different tectono-stratigraphic terrain (Van Kranendonk, 1998).
4. Geology and alteration
However, the maps in these publications contain few substantive or critical geological differences to our Fig. 2, excepting Brauhart et al., (1998) which includes a granite subdivision and a summary of their alteration mapping, the specific emphasis of their paper. The maps in Van Kranendonk (1998) and Van Kranendonk and Morant (1998) of the same Strelley area as ours show no alteration or mineralisation and considerably less structural in-formation than our Fig. 2. Our paper is concerned with the volcanic and granite geochemistry, not regional geology and alteration. Hence the alter-ation section has been kept brief, and the map is a summary, aimed to assist the reader understand the geological setting. It is not a presentation of all known facts.
Brauhart and Morant’s comment that our de-scription of the Strelley Granite is based purely on Geological Survey mapping is incorrect. Our de-scriptions are based on traverse sampling and mapping from the core to the rim of the Strelley Granite and detailed petrography of these sam-ples. A comparison of our description of the granite phases (Vearncombe, 1995; Vearncombe and Kerrich, 1999) with those Brauhart and Morant’s reveals no appreciable differences.
Whether the volcanic succession comprises lavas (Vearncombe, 1995; Buick and Doepel, 1999) or extensive sub-volcanic sills is a matter of interpretation. Brauhart and Morant’s position on sub-volcanic sills seems somewhat overstated given the reliance on an abstract (McPhie and Goto, 1996). In this abstract McPhie describes features of the interpreted subvolcanic sills, such as columnar jointing, massive structure, lobe structure and layering. These features apply equally to extrusive lavas and indeed McPhie concedes that ‘similar lobe structure also occurs in some extrusive subaerial and subaqueous domes’. Brauhart and Morant criticise the use of the term bimodal for the volcanic sequence. In use of this term there is implicitly the caveat ‘domi-nantly’ bimodal. In their book on the continental crust, Taylor and McLennan (1985) describe Ar-chaean arc magmas as bimodal. Similarly, Thurston’s classic paper is entitled ‘Archaean bi-modal volcanism’ (Thurston, 1985). For these and other authors, it is understood that bimodal does not signify the total absence of andesites.
In terms of the regional alteration system, our paper did not set out to characterise this, nor would it have been useful for our objectives. Accordingly, distal chloritic alteration to Ar-chaean VMS deposits, as described by numerous authors (e.g. Costa et al., 1983) is not germane to the issue we addressed. Footwall volcanic rocks mapped, sampled and analysed by the senior au-thor have up to 3.76 wt.% K2O present as sericite (Vearncombe and Kerrich, 1999; Table 3), notwithstanding distal chlorite alteration.
5. Trace element geochemistry
In the original paper, Vearncombe and Kerrich (1999) concluded that compositional variations of alteration insensitive elements in the footwall vol-canic rocks were due to fractional crystallisation given covariant inter-element trends as expected during evolution of arc magmas. We cite several studies documenting the alteration insensitive na-ture of HFSE (Th, Nb, Ta, Zr, Hf), Ti, Al2O3, V and rare-earth elements (REE) in footwall vol-canic rocks to VMS deposits (Finlow-Bates and Stumpfl, 1981; MacLean and Barrett, 1993; Vance and Condie, 1987; see also example in Costa et al., 1983).
For the footwall basalts – andesites, REE pat-terns are coherent, increasing in absolute abun-dance and fractionation towards more evolved compositions. Ti/Zr decreases whereas Al2O3/ TiO2 increases during progressive fractional crystallisation.
Ti/Zr correlates as follows:
Ho −0.90; Yb −0.95; Hf −0.99; Th −0.96; La/Sm −0.78; Nb/Ta +0.71; Ti/Sm +0.93; Sc/
Yb +0.80; P/Ti −0.79.
Al2O3/TiO2 correlates as follows:
Zr 0.93; Ho 0.85; Yb 0.94; Hf 0.98; Th 0.92; Ta 0.82; La/Sm 0.79; Nb/Ta −0.91; Ti/Sm −0.90; Sc/Yb −0.87.
mimics magmatic fractional crystallisation, nor has this been found in thorough studies of alter-ation associated with VMS deposits (Fig 1; e.g. Costa et al., 1983). Based on ionic radius and mineral fluid distribution coefficients, Pearce and Peate (1995) have shown that there is differing mobility within and between the REE and HFSE. Brauhart and Morant incorrectly cite Campbell et al. (1984) for REE mobility in VMS alteration systems. Numerous studies have demonstrated the near-isochemical behaviour of REE and HFSE in VMS deposits, except in the most intensely altered ‘feeder pipes’. Costa et al. (1983; Fig. 5, Table 5) showed immobility of REE, Ti, Al, Y, Zr and Hf in the footwall, but not the intensely altered ore zone of the Mattagami Late Archaean ZnCu
deposit. Campbell et al. (1984) state that ‘there is no evidence of REE mobility in the pervasive low-grade envelopes to VMS deposits’. In fact REE systematics is the basis of a proven method for discriminating barren from prospective mag-matic sequences for VMS deposits (Lesher et al., 1986; Barrie et al., 1993; Vearncombe and Ker-rich, 1999, Fig 10).
In their discussion, Brauhart and Morant inter-pret small negative Nb but zero P and Ti anoma-lies in the footwall basalts-andesites, as a characteristic of the magma source region based on data that is not in peer reviewed scientific publications. Here they introduce a logical im-passe. On the one hand they represent Nb, P and Ti normalised to neighbouring REE as a primary signature of immobile elements to characterise the source, and at the same time conclude that covari-ant trends are a product of ‘net mass transfer associated with hydrothermal alteration’. Neither of the conflicting standpoints are quantitatively evaluated.
Brauhart and Morant have not represented our paper accurately when claiming that we draw analogies between the Strelley granite and (i) two mica peraluminous granites from the Archaean Abitibi belt, (ii) Archaean low Al-TTG suites, and (iii) post-Archaean subduction-related granites. Rather we compare them. It is stated ‘the chon-drite normalised REE patterns for the Strelley granite are within the range for FIIIa rhyolites of Lesher et al. (1986)’, and ‘Significant differences
between the Strelley granite and low Al type TTG include higher K2O (average 4 wt.%) and greater Rb/Y ratios…’. The paper on Archaean grani-toids we cite (Feng and Kerrich, 1992) is about differences between six granitoid series.
De Paolo (1981) has demonstrated that frac-tional crystallisation in granitoids is invariably accompanied by assimilation fractional crystalli-sation (AFC), from thermal budgets and trace element modelling. Thus the coherent REE pat-terns in the Strelley granite over a large range of absolute abundances is plausibly AFC (Vearn-combe and Kerrich, 1999; Fig. 6).
We specifically avoided a Sm – Nd isotope study given the insensitivity of this approach to crustal contamination: any potential contaminant must have a long time integrated difference in Sm/Nd, but this is not known. It is possible that both the mantle source was contaminated, and the grani-toid liquid crustally contaminated during em-placement. Endogenous contamination of the Archaean mantle by recycling has been discussed recently by Sylvester (1994) and Kerrich et al., 1999a,b).
In summary, we find that Brauhart and Morant appear not to have understood the focus of our paper. Their interpretation of the geochemistry of altered rocks appears flawed, and they have missed some critical scientific literature on this subject. They have not evaluated clear patterns of magmatic fractional crystallisation, nor con-tributed to the larger issues our paper addressed.
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