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Formation of Willemite in Hydrothermal Environments

2003; Volume: 98; Issue: 4 Linguagem: Inglês

10.2113/gsecongeo.98.4.819

1554-0774

Joël Brugger, Derry McPhail, Malcolm W. Wallace, Jonathan M. Waters,

Radioactive element chemistry and processing

Research Article| June 01, 2003 Formation of Willemite in Hydrothermal Environments Joël Brugger; Joël Brugger Department of Geology and Geophysics, University of Adelaide, 5000 South Australia, and South Australian Museum, Adelaide, 5000 South Australia, Australia †Corresponding author: e-mail, Joel.Brugger@adelaide.edu.au Search for other works by this author on: GSW Google Scholar D. C. McPhail; D. C. McPhail School of Geosciences, Monash University, Victoria 3800, Australia, and Cooperative Research Centre for Landscape Environments and Mineral Exploration & Department of Geology, The Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Malcolm Wallace; Malcolm Wallace School of Earth Sciences, University of Melbourne, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar John Waters John Waters School of Geosciences, Monash University, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Joël Brugger Department of Geology and Geophysics, University of Adelaide, 5000 South Australia, and South Australian Museum, Adelaide, 5000 South Australia, Australia D. C. McPhail School of Geosciences, Monash University, Victoria 3800, Australia, and Cooperative Research Centre for Landscape Environments and Mineral Exploration & Department of Geology, The Australian National University, Canberra, ACT 0200, Australia Malcolm Wallace School of Earth Sciences, University of Melbourne, Victoria 3010, Australia John Waters School of Geosciences, Monash University, Victoria 3800, Australia †Corresponding author: e-mail, Joel.Brugger@adelaide.edu.au Publisher: Society of Economic Geologists First Online: 02 Mar 2017 Online ISSN: 1554-0774 Print ISSN: 0361-0128 Economic Geology Economic Geology (2003) 98 (4): 819–835. https://doi.org/10.2113/gsecongeo.98.4.819 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Joël Brugger, D. C. McPhail, Malcolm Wallace, John Waters; Formation of Willemite in Hydrothermal Environments. Economic Geology 2003;; 98 (4): 819–835. doi: https://doi.org/10.2113/gsecongeo.98.4.819 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract Willemite (zinc silicate) is the main zinc mineral in some carbonate-hosted ore deposits (e.g., Franklin, New Jersey; Vazante, Brazil; Beltana, South Australia; Kabwe, Zambia). Recent interest in these unconventional zinc deposits has increased because of high zinc grades that exceed 40 wt percent, relatively low environmental impact of ore processing owing to the lack of acid-generating sulfides in the waste, and advances in ore processing technologies. In the past, most metallogenic studies proposed formation of willemite deposits by supergene or hypogene alteration of preexisting sulfide deposits. However, recent data on the Vazante, Beltana, and Kabwe deposits indicate willemite crystallization at temperatures in excess of 150°C, raising the possibility of primary precipitation from hydrothermal fluids.We use numerical geochemical modeling to examine the formation of willemite under hydrothermal conditions. Activity-activity diagrams reveal that, in the presence of dissolved sulfur and quartz, willemite instead of sphalerite will precipitate under oxidizing (e.g., hematite-stable, sulfate-predominant) and alkaline (pH higher than K feldspar-muscovite-quartz) conditions. Willemite also becomes more stable, relative to sphalerite, at high temperature, and willemite can coexist with magnetite at 300°C. The stabilities and solubilities of sphalerite, willemite, smithsonite, hydrozincite, and zincite were calculated for wide ranges of temperature (25°–300°C), chloride concentration, dissolved sulfur and carbon concentrations, pH, quartz saturation, and oxidation potential. Plots of the solubility of the different minerals as a function of two variables (e.g., temperature and redox state; pH and redox state) allow us to predict the effects of changing chemical conditions, which in turn permits an estimate of the efficiency of particular precipitation processes. Cooling is an effective process for precipitating sphalerite but not willemite, whereas pH increase (e.g., by acidic fluids reacting with carbonates) is effective for precipitating willemite but not sphalerite.Dynamic geochemical models that simulate physicochemical processes are used to understand the formation of the Beltana willemite deposit in the Adelaide geosyncline of South Australia. This small, high grade deposit (850,000 t at 36% Zn) is hosted in dolomite of the Cambrian Ajax Limestone, next to a tectonic contact with the diapiric, halite-bearing clastic sediments of the Callanna Group. The orebody is associated with hematite alteration and is characterized by the total absence of sulfides; willemite is the only zinc ore mineral, and the arsenate hedyphane (Ca2Pb3[AsO4]3Cl) is the main lead mineral. The model results show that willemite will precipitate in response to water-rock interaction and fluid mixing processes at temperatures above 120°C. The presence of arsenate in the hydrothermal fluid is likely to have been important at Beltana; in arsenate-absent models sulfate is reduced to sulfide by the precipitation of ferrous iron as hematite, resulting in the precipitation of sphalerite and galena. In contrast, in models including arsenate the reduction of sulfate to sulfide is inhibited and willemite is predicted to precipitate. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.