Australian Zn-Pb-Ag mineral systems: a review and analysis  

Authors / CoAuthors

Huston, D.L. | Stevens, B. | Southgate, P.N. | Muhling, P. | Wyborn, L.A.I.

Abstract

Zn-Pb-Ag mineral deposits, which are the products of specific types of hydrothermal "mineral systems", are restricted in time and space in Australia. These deposits formed during three main periods: ~2.95 Ga, 1.69-1.58 Ga, and 0.50-0.35 Ga. The 1.69-1.58 Ga event, which was triggered by accretionary and rifting events along the southern margin of Rodinia, is by far the most significant, accounting for over 65% of Australia's Zn. With the exception of the 0.50-0.35 Ga event, major Australian Zn-Pb-Ag events do not correspond to major events globally. Over 95% of Australia's Zn-Pb-Ag resources were produced by just four mineral system types: Mt Isa-type (MIT: 56% of Zn), Broken Hill-type (BHT: 19%), volcanic-hosted massive sulfide (VHMS:12%), and Mississippi Valley-type (MVT: 8%). Moreover, just 4% of Australia's land mass produced over 80% of its Zn. The four main types of mineral systems can be divided into two groups, based on fluid composition, temperature and redox state. BHT and VHMS deposits formed from higher temperature (>200?C), reduced fluids, whereas MIT and MVT deposits formed from low temperature (<200?C), oxidized (H2S-poor) fluids. These fluid compositions and, therefore, the mineralization style are determined by the tectonic setting and composition of the basins that host the mineral systems. Basins that produce higher temperature fluids form in active tectonic environments, generally rifts, where active magmatism (both mafic and felsic) produces high heat flow that drives convective fluid circulation. These basins are dominated by immature siliciclastic and volcanic rocks with a high overall abundance of Fe2+. The high temperature of the convective fluids combined with the abundance of Fe2+ in the basin allows sulfate reduction, producing reduced, H2S-rich fluids. In contrast, basins that produce low temperature fluids are tectonically less active, generally intracratonic, extensional basins dominated by carbonated and mature siliciclastics with a relatively low abundance of Fe2+. Volcanic units, if present, occur in the basal parts of the basins. Because these have relatively low heat flows, convective fluid flow is less important, and fluid migration is dominated by expulsion of basinal brines in response to local and/or out-of-area tectonic events. Low temperatures and the lack of Fe2+ prevent inorganic sulfate reduction during regional fluid flow, producing oxidized fluids that are H2S-poor. The contrasting fluid types require different depositional mechanisms and traps to accumulate metals. The higher temperature, reduced VHMS and BHT fluids deposit meatls as a consequence of mixing with cold sewater. Mineralization occurs at or near the seafloor, with trapping efficiencies enhanced by sub-surface replacement or deposition in a brine pool. In contrast, the low temperature, oxidized MIT and MVT fluids precipitate metals through thermochemical sulfate reduction facilitated by hydrocarbons or organic matter. This process can occur at depth in the rock pile, for instance in failed petroeum traps, or just below the seafloor in pyritic, organic-rich muds. Mass balance calculations indicate that the size of a metal accumulation, although controlled at the first order by the mineral system container size, also depends on the efficiencies at which metals are extracted from the source and retained at the trap site. The shear size of minerals systems required to form giant deposits may partly explain why these deposits commonly occur by themselves, without significant satellite deposits. In addition to the size of the mineral system container, metal retention efficiency appears to be the most important determinant of the size of metal accumulations.

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nonGeographicDataset

eCat Id

49046

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Keywords

• External PublicationScientific Journal Paper

( Theme )

• economic geology

( Theme )

• geochemistry

( Theme )

• metallogenesis

( Theme )

• mineral deposits

• AU

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

• Published_Internal

Publication Date

2003-06-30T00:00:00

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Australian Government Security ClassificationSystem    

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Creative Commons Attribution 4.0 International Licence    

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unknown

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geoscientificInformation

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[-44.0, -10.0, 111.0, 153.0]

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