Abstract
Abstract Giant hydrothermal ore deposits form where fluids carrying massive amounts of metals scavenged from source rocks or magmas encounter conditions favorable for their localized deposition. However, in most cases, the ultimate origin of metals remains highly disputed. Here, we show for the first time that two metal sources have provided, in comparable amounts, the 8 Mt of lead of the giant McArthur River zinc-lead deposit (McArthur Basin, Northern Territory, Australia). By using high-resolution secondary ion mass spectrometry (SIMS) analysis of lead isotopes in galena, we demonstrate that the two metal sources were repeatedly involved in the metal deposition in the different ore lenses ca. 1640 Ma. Modeling of lead isotope fractionation between mantle and crustal reservoirs implicates felsic rocks of the crystalline basement and the derived sedimentary rocks in the basin as the main lead sources that were leached by the ore-forming fluids. This sheds light on the crucial importance of metal tracing as a prerequisite to constrain large-scale ore-forming systems, and calls for a paradigm shift in the way hydrothermal systems form giant ore deposits: leaching of metals from several sources may be key in accounting for their huge metal tonnage.
Highlights
More than a thousand giant ore deposits worldwide are recognized as containing exceptional accumulations of metals in restricted volumes
We show for the first time that two metal sources have provided, in comparable amounts, the 8 Mt of lead of the giant McArthur River zinc-lead deposit (McArthur Basin, Northern Territory, Australia)
By using high-resolution secondary ion mass spectrometry (SIMS) analysis of lead isotopes in galena, we demonstrate that the two metal sources were repeatedly involved in the metal deposition in the different ore lenses ca. 1640 Ma
Summary
More than a thousand giant ore deposits worldwide are recognized as containing exceptional accumulations of metals in restricted volumes (i.e., they store the metals equivalent in 1011 tons of continental crust in mean crustal or “Clarke” concentration; Laznicka, 2014). In order to address the number and the nature of metal source(s) involved with the formation of a true giant hydrothermal ore deposit, we have targeted the McArthur River zinc-lead deposit (Northern Territory, Australia) and carried out a detailed in situ Pb isotope study of galena. This widely used method is a powerful tool for tracing metal sources and ages based on model ages because it combines three radioactive decay systems (238U → 206Pb, 235U → 207Pb, and 232Th → 208Pb; e.g., Deloule et al, 1986)
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