Uranium metallogeny in the North Flinders Ranges region of South Australia.

Abstract

The geological province of the Mount Painter in the North Flinders Ranges (South Australia) is well-known for its uranium mineralisation, and uraniferous granites. The presence in the nearby Cenozoic sediments of the Lake Frome basin of uranium mineralisations (Beverley deposit) and the recent discovery of the Four Mile deposit has triggered the interest of explorers. Based on extensive laser-ablation inductively-coupled-plasma-mass-spectrometry (LA-ICPMS) U-Pb geochronological data and mineralogy of U-Th-bearing minerals, rock geochemistry and petrography, we present a global study on the mobility of U, Th and REE in the Mount Painter Domain, including a detailed reconstitution of the Beverley deposit genesis. Seven significant stages of U-Th-REE mobility are recognised: 1. The possible presence U-enriched ~1600 Ma lower crust under the MPD 2. Intrusion of two A-type Mesoproterozoic granites suites (~1575, and ~1560 Ma respectively) with high HFSE contents and crustal origin; the porphyritic biotite K-rich highly-enriched Yerila granite belongs to the youngest suite and hosts magmatic allanite-(Ce), potassic-hastingsite, ilmenite, fergusonite-(Y), chevkinite, molybdenite, zircon, uranothorite, uraninite and titanite and fluorite 3. Late-magmatic or post-magmatic metasomatism in the same granites; evidenced by F-rich annite, zircon, Y-bearing Al-F-titanite (< 6 kbar, >400°C), Y-rich fluorapatite, synchysite-(Ce) and fluorite. Early ilmenite, molybdenite, allanite-(Ce) and oligoclase reacted with an alkaline oxidising F-rich melt or fluid. The latemagmatic to post-magmatic metasomatism is also recorded at the intrusion contact in regional rocks, forming allanite-, magnetite-, uranothorite-, zircon- (1501 ± 6 Ma), and uraninite-bearing calcsilicate skarns. The spreading of zircon ages in the Yerila granite (~1565 to ~1521) relates to the mixing of magmatic and metasomatic crystals. 4. the MPD was subject to the Delamerian orogeny and related metamorphism (amphibolite facies); most Mesoproterozoic granitic assemblages present signs of recrystallisation or stress; recrystallisation of monazite-(Ce) and xenotime-(Y) during Paleozoic (Cambrian) (490-495 Ma). U-Th-rich minerals also bear Delamerian ages (polycrase-(Y), euxenite-(Y), davidite-(La) and uraninite). 5. Anatexis of local basement during Ordovician and generation of peraluminous granite (British Empire granite) with low Th/U. The granite is enriched in U and Y. We provide the first robust ages on it: 456 ± 9 and 459 ± 9 Ma on zircon, 453.3 ± 4.6 on xenotime-(Y). 6. Very active hydrothermal/pegmatitic uranium remobilisation along active faults; brannerite-quartz veins formation (367 ± 13 Ma), further signs of remobilisation or hydrothermal event during Permian (284 ± 25 Ma in thorite) and around the Mt Gee (~290 Ma radiogenic gain in davidite) which agrees with the previous data (paleomagnetic ages of 250-300 Ma). 7. Cenozoic supergene uranium remobilisation in MPD and migration of U-rich oxidised groundwaters into the Lake Frome. The uranium is precipitated in the sandy formation of the lake and in the top layer of the underlying organic-matter-rich clays and silts. The micro-environment of reduction efficiently trap U but also REE, fingerprinting the REE-rich MPD granite source. Coffinite and carnotite give concordant Pliocene ages (6.7 to 3.4 Ma). Provenance studies on the sands hosting the Beverley mineralisations suggest a reworking of Early Cretaceous glacial or glacio-lacustrine sediments originally sourced in Eastern Australia (Lachlan Fold Belt). The youngest recorded zircon (130 Ma) doesn’t constrain the sediment age but refines the provenance region (New England Orogen).