Geology of the Broken Hill area.

The Broken Hill region is famous for hosting one of the largest Lead-Zinc-Silver ore deposits on the planet. The deposit is hosted within Early to Middle Proterozoic meta-sedimentary and meta-volcanic rocks of the Willyama Supergroup, which is exposed throughout most of the Broken Hill Inlier (Fig. 1). These rocks were deposited in a continental back-arc environment between ca. 1710-1640 Ma and were deformed during the Olarian Orogeny (ca. 1600-1580 Ma). Adelaidian sedimentary rocks (ca. 820-750 Ma) were unconformably deposited onto the polydeformed metamorphic rocks during the break-up of the supercontinent Rodinia.
The Adelaidian and Willyama Supergroups were then deformed during
the Delamerian Orogeny at ca. 520-500 Ma.

 

Figure 1. Simplified geological map of the Broken Hill Inlier
showing the location of the Eldee region.

Click Here for a link to the GoogleMaps image of the field area.

The Willyama Supergroup contains a basal succession of migmatite, and quartz albitite. These are overlain by dominantly psammitic and pelitic rocks of the Broken Hill and Sundown Groups. These are overlain by the schist and phyllite of the Paragon Group. The metamorphic grade of the Willyama Supergroup varies from amphibolite facies in the northern inlier to granulite facies in the southeastern inlier. The structural evolution of the Willyama Supergroup remains highly contentious and debated by many researchers.

The Eldee region contains rocks of the Broken Hill, Sundown, and Paragon Groups. The geology of the area is also hotly debated with various models proposed for the evolution of the region. It will be your task to help constrain the evolution of the Eldee region. We do not expect that you learn the entire stratigraphy of the Willyama Supergroup, nor do we expect you to adapt other structural interpretation of the region. We do, however, expect you to come up with a geological evolution of the Eldee region based on your observations, and applying the mapping techniques that you will learn during the course.

Some of the concepts that will be useful during the mapping exercise are vergence, kinematic indicators of shear zones, structural facing, younging criteria, and sedimentary structures. If you are not familiar with some of these concepts don’t panic, because you will be by the end of the camp.


Some suggested reading.
Stevens, B.P.J., 1986. Post-depositional history of the Willyama Supergroup in the Broken Hill Block, NSW. Australian Journal of Earth Sciences 33, 73-98.

Stevens, B.P.J., Barnes, R.G., Brown, R.E., Stroud, W.J., Willis, I.L., 1988. The Willyama Supergroup in the Broken Hill and Euriowie Blocks, New South Wales. Precambrian Research 40/41, 297-327.

Clark, G.L., Burg, J.P., & Wilson,C.J.L. 1986. Stratigraphic and structural constraints on the Proterozoic tectonic history of the Olary Block, South Australia. Precambrian Research, 34, 107-138.

Page, R.W., Laing, W.P., 1992. Felsic metavolcanic rocks related to the Broken Hill Pb-Zn-Ag orebody, Australia: Geology, depositional age, and timing of high-grade metamorphism. Economic Geology 87, 2138-2168.

Stuwe, K., Ehlers, K., 1997. Multiple metamorphic events at Broken Hill, Australia. Evidence from Chloritoid-bearing paragenesis in the Nine-mile region. Journal of Petrology 38, 1167-1186.

Willis, I.L., Brown, R.E., Stroud, W.J., ,Stevens, B.J., 1983. The Proterozoic Willyama Supoergroup: stratigraphic subdivisions and interpretation of high and low-grade metamorphic rocks in the Broken Hill
Block, New South Wales. Journal of the Geological Society of Australia 30, 195-224.

Wilson, C.J.L., Powell, R., 2001. Strain localization and high-grade metamorphism at Broken Hill, Australia: a view from the Southern Cross area. Tectonophysics 335, 193-210.

Vassallo, J.J. and Vernon, R.H., 2000. Origin of megacrystic felsic gneisses at Broken Hill. Australian Journal of Earth Sciences 47, 733-748.

Laing, W.P., 1978. Structure of the Broken Hill mine area and its significance to the genesis of the ore bodies. Economic Geology, 73, 1112-1136.

Nutman, A.P. and Ehlers, K. 1998. Evidence for multiple palaeoproterozoic thermal events and magnetism adjacent to the Broken Hill Pb-Zn-Ag orebody, Australia. Precambrian Research, 90, 203-
238.

Forbes, C.J., Betts, P.G., & Lister, G.S., 2004. Synchronous development of Type 2 and Type 3 fold interference patterns: evidence for recumbent sheath folds in the Allendale area, Broken Hill, NSW,
Australia. Journal of Structural Geology 25, 113-125.

Forbes, C.J., Betts, P.G., 2004. The development of Type 2 fold interference patterns in the Broken Hill Block: Implications for strain partitioning across a detachment during the Olarian Orogeny. Australian
Journal of Earth Sciences 51.

Gibson, G.M. and Nutman, A.P. 2004. Detachment faulting and bimodal magmatism in the Palaeoproterozoic Willyama Supergroup, south-central Australia: keys to recognition of a multiply deformed Precambrain metamorphic core complex. J. Geological Society of London , 161 , 55-66.

Hobbs, B.E. Archibald, N.J., Etheridge, M.A. and Wall, V.J. 1984. Tectonic history of the Broken Hill Block, Australia. In, Kroner, A. and Greiling, A (eds), Precambrian tectonics illustrated: Stuttgart Schweizerbart'sche.

Hughes, F.E. (Editor),1990. Geology of the Mineral Deposits of Australia and Papua New Guinea. Monograph 14, AIMM, vol. 2 (in particular, see section 5.2 describing the geology etc. of Willyama Block, South Australia and New South Wales, pp. 1065-1084.

Plimer, I.R., 1985. Broken Hill Pb-Zn-Ag deposit - A product of mantle metasomatism, Mineralium Deposits , 20 , 147-153.

Slack, J.F. et al., 1993 Origin and significance of tourmaline-rich rocks in the Broken Hill District, Australia. Ec. Geol., 88 , 505-541.

Willis et al., 1983. The Proterozoic Willyama Supergroup: Stratigraphic subdivision and interpretation of high- to low-grade metamorphic rocks in the Broken Hill Block. Geol. Soc. Aust. Journal , 30 , 195-224.

 


These papers are readily available on the web or in the Hargrave or Earth Science Librarys. These are some of many papers written on the Broken Hill Block. We will bring some other reading up to the camp for anyone who is interested.