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.

The Ranger deposit (Northern Territory, Australia) is one of the largest uranium deposits in the world. Uranium mineralisation occurs in crystalline basement rocks and is thought to belong to the unconformity-related category. In order to address the sources of magnesium and boron, and the temperature of the fluids related to boron and magnesium metasomatism that occurred shortly before and during the main uranium stage, in situ analyses of chlorite and tourmaline were carried out. The chemical composition of tourmaline shows an elevated X-site vacancy and a low Fe tot /(Fe tot + Mg) ratio typical of Mg-foitite. Uranium-related chlorite has relatively low Fe content (0.28-0.83 apfu) and high Mg content (3.08-3.84 apfu), with Si/Al = 1.08−1.22 and Mg/(Mg + Fe tot ) = 0.80−0.93 indicating a composition lying between the clinochlore and Mg-amesite fields. Chlorite composition indicates crystallisation temperature of 101-163 • C. The boron isotopic composition of tourmaline shows a range of δ 11 B values of~1-9% . A model is proposed involving two boron sources that contribute to a mixed isotopic signature: (i) evaporated seawater, which is typically enriched in magnesium and boron (δ 11 B~40% ), and (ii) boron from the crystalline basement (δ 11 B~−30 to +10% ), which appears to be the dominant source. Collectively, the data indicate similar tourmaline chemistry but significant differences of tourmaline boron isotopic composition and chlorite chemistry between the Ranger deposit and some of the Canadian unconformity-related uranium deposits. However, lithogeochemical exploration approaches based on identification of boron-and magnesium-enriched zones may be usefully applied to uranium exploration in the Northern Territory.

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