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Australian Zn-Pb-Ag ore-forming systems; a review and analysis

David L. Huston, Barney Stevens, Peter N. Southgate, Peter Muhling and Lesley Wyborn
Australian Zn-Pb-Ag ore-forming systems; a review and analysis
Economic Geology and the Bulletin of the Society of Economic Geologists (October 2006) 101 (6): 1117-1157

Abstract

Zn-Pb-Ag mineral deposits are the products of hydrothermal ore-forming systems, which are restricted in time and space. In Australia, these deposits formed during three main periods at approximately 2.95, 1.69 to 1.58, and 0.50 to 0.35 Ga. The 1.69 to 1.58 Ga event, which accounts for over 65 percent of Australia's Zn, was triggered by accretion and rifting along the southern margin of Rodinia. Over 93 percent of Australia"s Zn-Pb-Ag resources were produced by four ore-forming system types: Mount Isa (56% of Zn), Broken Hill (19%), volcanic-hosted massive sulfide (VHMS; 12%), and Mississippi Valley (8%). Moreover, just 4 percent of Australia"s land mass produced over 80 percent of its Zn. The four main types of ore-forming systems can be divided into two "clans," based on fluid composition, temperature, and redox state. The Broken Hill- and VHMS-type deposits formed from high-temperature (>200 degrees C) reduced fluids, whereas the Mount Isa- and Mississippi Valley-type deposits formed from low-temperature (<200 degrees C), H (sub 2) S-poor, and/or oxidized fluids. The tectonic setting and composition of the basins that host the ore-forming systems determine these fluid compositions and, therefore, the mineralization style. Basins that produce higher temperature fluids form in active tectonic environments, generally rifts, where high heat flow produced by magmatism drives convective fluid circulation. These basins are dominated by immature siliciclastic and volcanic rocks with a high overall abundance of Fe (super 2+) . The high temperature of the convective fluids combined with the abundance of Fe (super 2+) in the basin allow inorganic sulfate reduction and leaching of sulfide from the country rock, producing reduced, H (sub 2) S-rich fluids. Basins that produce low-temperature fluids are tectonically less active, generally intracratonic, extensional basins dominated by carbonate and variably mature siliciclastic facies with a relatively low Fe (super 2+) abundance. In these basins, sediment maturity depends on the paleogeography and stratigraphic position in an accommodation cycle. Volcanic units, if present, occur in the basal parts of the basins. Because these basins have relatively low heat flow, convective fluid flow is less important, and fluid migration is dominated by expulsion of basinal brines in response to local and/or regional tectonic events. Low temperatures and the lack of Fe (super 2+) prevent in-organic sulfate reduction during regional fluid flow, producing H (sub 2) S-poor fluids that are commonly oxidized (i.e., Sigma SO (sub 4) >Sigma H (sub 2) S). Fluid flow in the two basin types produces contrasting regional alteration systems. High-temperature fluid-rock reactions in siliciclastic-volcanic-dominated basins produce semiconformable albite-hematite-epidote assemblages, but low-temperature reactions in carbonate-siliciclastic-dominated basins produce regional K-feldspar-hematite assemblages. The difference in feldspar mineralogy is mostly a function of temperature. In both basin types, regional alteration zones have lost, and probably were the source of, Zn and Pb. The contrasting fluid types require different depositional mechanisms and traps to accumulate metals. The higher temperature, reduced VHMS- and Broken Hill-type fluids deposit metals as a consequence of mixing with cold seawater. Mineralization occurs at or near the sea floor, with trapping efficiencies enhanced by sub-surface replacement or deposition in a brine pool. In contrast, the low-temperature, oxidized Mount Isa- and Mississippi Valley-type 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 petroleum traps, or just below the sea floor in pyritic, organic-rich muds.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 101
Serial Issue: 6
Title: Australian Zn-Pb-Ag ore-forming systems; a review and analysis
Affiliation: Geoscience Australia, Canberra, A.C.T., Australia
Pages: 1117-1157
Published: 200610
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 254
Accession Number: 2007-021260
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. chart, sects., 9 tables, sketch maps
S22°00'00" - S18°00'00", E138°25'00" - E141°00'00"
Secondary Affiliation: Geological Survey of New South Wales, AUS, AustraliaCSA Australia Pty., AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200713
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