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Updated:
03 July 2007
Archived: 30th June 2000
BHEI Key Results
Summary
This section advises some of the key results, products, and conceptual advances made during the project, particularly towards understanding the regional structure, airborne geophysics (particularly aeromagnetics), and the timing of some base-metal mineralisation in the Broken Hill-Olary region. SHRIMP geochronological studies have provided further constraints on the depositional age of the Willyama Supergroup, timing of events and mineralisation.
Key Results
- Contrary to previous expectations, major shear zones in the Broken Hill region dip southeast rather than north-west as evidenced by new structural mapping and BHEI seismic reflection profiling. Although reactivated during younger events (eg. Delamerian orogeny), most of these structures retain their original geometry and form part of a D3 northwest-vergent fold and thrust belt. This fold and thrust belt was superimposed on an earlier episode of large-scale northeast-vergent recumbent folds (D2) which in turn overprint an earlier layer-parallel fabric of possible extensional origin. The recognition of deformational events (D2 and D3) with near orthogonal orientations has major implications for three-dimensional representations of regional structure and stratigraphy in the Broken Hill-Olary region and the ability to predict accurately what occurs at depth. Reactivation of the D3 shear zones during the Delamerian Orogeny and younger events is supported by Ar-Ar ages ranging from 500-400 Ma.
- The principal copper-gold (molybdenum) deposits and prospects in the Olary and Broken Hill Domains are epigenetic-syntectonic, early Mesoproterozoic (or late Palaeoproterozoic) in age, and share several characteristics of iron-oxide associated Cu-Au systems in the Mt Isa Inlier and elsewhere. Stable isotope studies demonstrate a clear difference in sources of fluids in Cu-Au mineralisation and regional syntectonic Na-Ca-Fe alteration that formed at deeper crustal levels. Deposit styles range from Cu-sulfide and Fe-oxide rich to Au-rich with sparse Fe-oxides; such variations have implications for exploration methodologies applied in the region.
- Mineralisation in the Broken Hill-Olary region is in part structurally controlled. Examples of shear-hosted mineralisation occur at Copper Blow south of Broken Hill (Cu-Au) whilst farther north in the Allendale region Pb-Zn-Ag mineralisation is contained within the regional D3 fabric. Less easily determined is whether Pb-Zn-Ag mineralisation, including the main Line of Lode, is also structurally controlled or whether it represent syngenetic/diagenetic mineralisation that was subsequently remobilised during later deformation and metamorphism.
- SHRIMP geochronological studies undertaken on zircons from rocks of volcanic and sedimentary origin in the Olary and Broken Hill Domains indicate a maximum depositional age for the Willyama Supergroup of ca 1710-1715 Ma whilst the youngest part of the sequence (upper Paragon Group) can be no older than 1642± 5 Ma. These data invite comparisons with the Mt Isa Inlier and McArthur Basin of northern Australia where Paleoproterozoic sequences of similar age were deposited in similar intra-cratonic settings. The results suggest that the stratigraphically higher parts of the Willyama Supergroup may be prospective for Mt Isa style mineralisation. Dating of a post-D3 granite (1591 ± 5 Ma) gives a minimum age for the cessation of major regional deformation (D3) and metamorphism.
- Collaborative SHRIMP U-Pb dating program with the Australian Geodynamics CRC successfully concluded. Results indicate an episode of bimodal magmatism (granites and mafic intrusives) contemporaneous with sedimentation at ca.1710-1675 Ma.
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