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Discussion on detachment faulting and bimodal magmatism in the Palaeoproterozoic Willyama Supergroup, south-central Australia: keys to recognition of a multiply deformed Precambrian metamorphic core complex

^{1 1}Geological Survey, Primary Industries and Resources, South Australia, 101 Grenfell St, Adelaide, South Australia 5000, Australia
^{2 2}Predictive Minerals Discovery CRC, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
^{3 3}Geological Survey New South Wales, Department of Primary Industries, 32 Sulphide St, Broken Hill, NSW 2880, Australia
^{4 4}School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia
^{5 5}Earth Sciences, University of New England, Armidale, NSW 2531, Australia
^{6 6}Geoscience Australia, Canberra, ACT 2601, Australia
^{7 7}Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The first 20% of the full text of this article appears below.

	Introduction

 
C. H. H. Connor, W. V. Pries, R. W. Page, B. P. J. Stevens, I. R. Plimer and P. M. Ashley write: Gibson & Nutman (2004) postulate that the Willyama Supergroup in the southern Curnamona Province contains a 1690–1670 Ma metamorphic core complex, and imply that this is relevant to the genesis of the Broken Hill Pb–Zn–Ag deposit. We contend that this is a model-driven, speculative interpretation predicated upon unsupported assertions that conflict with substantial stratigraphic and geochronological data. Not only does the paper fail to demonstrate an early high-grade event and formation of a metamorphic core complex at 1690–1670 Ma, but it is also factually incorrect in several critical geological and geochronological aspects. The detachment concepts of Gibson & Nutman may appear persuasive, but only by omitting reference to a huge body of published and recent mapping and laboratory research, by companies, universities, and geological surveys, some of which are cited here. Surprisingly, Gibson & Nutman ignore much recent work (e.g. Page et al. 2004) to which one of them has contributed.

The essence of the model is that a low-angle, extensional ‘detachment’ formed along the boundary between the Broken Hill and Sundown Groups in the Broken Hill Domain, and between the Bimba and Plumbago Formations in the Olary Domain (Gibson & Nutman, fig.1). The ‘detachment’ was alleged to have occurred as part of a sillimanite-grade D₁ deformation between 1690 and 1670 Ma, and to have provided a channel for hydrothermal fluids that could have been responsible for regional scale Na (–Fe) alteration and formation of Pb–Zn mineral deposits, including the Broken Hill orebody. As pointed out below, most of the evidence is misinterpreted, misleading and/or ambiguous, and the geochronological case advanced by Gibson & Nutman for such an early (1690–1670 Ma) high-grade event has no foundation.

Ambiguities in the model.
. . . [Full Text of this Article]

This article has been cited by other articles:

				R. W. Page, B. P. J. Stevens, and G. M. Gibson Geochronology of the Sequence Hosting the Broken Hill Pb-Zn-Ag Orebody, Australia Economic Geology, June 1, 2005; 100(4): 633 - 661. [Abstract] [Full Text] [PDF]