Tracing metal sources for the giant McArthur River Zn-Pb deposit (Australia) using lead isotopes

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.

Uranium incorporation in fluorite and exploration of U–Pb dating

“…In Pierre-Perthuis, the geodic fluorite crystals Fl geo yield an Sm-Nd age of 130 ± 15 Ma (Gigoux et al, 2015). Here, we show that later fluid flows took place during the Cenozoic period, recorded as rims overlaying Fl geo .…”

“…In the absence of elemental fractionation correction against a fluorite RM, the age of the last generation of fluorite in Pierre-Perthuis ore (Fl og 2), 40.0 ± 1.7 Ma, may be overestimated or underestimated. Considering an age offset of 20 % as an extreme case (Guillong et al, 2020), this age estimate still significantly differs from the Sm-Nd age of 130 ± 15 Ma (Gigoux et al, 2015) for the prior geodic fluorite crystals Fl geo . Bergerat (1984) proposed an N-S compressive stress field during the Eocene through microstructural analysis in the studied area, interpreted as the consequence of the Pyrenean compression.…”

“…The uranium concentration in fluorite is highly variable (Piccione et al, 2019;Walter et al, 2018;Wolff et al, 2016) and is often insufficient for U-Pb dating (Piccione et al, 2019). In the Pierre-Perthuis fluorite ore, the geodic fluorite Fl geo , dated at 130 ± 15 Ma by Sm-Nd (Gigoux et al, 2015), could not have been tested for U-Pb geochronology because it contains no U. Likewise, Fl og 1 is not amenable to U-Pb dat-L. Lenoir et al: Uranium incorporation in fluorite and exploration of U-Pb dating ing.…”

“…Sedimentary rocks lying unconformably on a crystalline basement may host large concentrations of authigenic F, Ba, Pb, Zn and U minerals that are of economic interest (Boiron et al, 2002;Gigon et al, 2020;Gigoux et al, 2015;Leach et al, 2005;Sizaret, 2006). These unconformity-related deposits are usually the result of multiple episodes of fluid flow, inducing mineral dissolution-recrystallization, remobilization and precipitation (Chi et al, 2018;Walter et al, 2018).…”

“…The unconformityrelated F-Ba ore in Pierre-Perthuis (Morvan massif, France) was chosen as a case study. This ore has been investigated in previous studies (Gigoux et al, 2015(Gigoux et al, , 2016, and a generation of macroscopic geodic fluorite has been dated at 130 ± 15 Ma by Sm-Nd geochronology (Gigoux et al, 2015). As in many other F-Ba deposits, the most commonly used geochronometers are lacking in Pierre-Perthuis, making it an interesting target to gain knowledge on fluorite precipitation ages through fluorite U-Pb dating.…”