Using in situ SHRIMP U-Pb Monazite and Xenotime Geochronology to Determine the Age of Orogenic Gold Mineralization: An Example from the Paulsens Mine, Southern Pilbara Craton

Abstract

Paulsens is a mesothermal orogenic gold deposit located in the Wyloo Inlier on the southern margin of the Pilbara craton of Western Australia. Gold occurs in quartz-sulfide veins hosted within a folded and faulted gabbro dike, from which baddeleyite yields a U-Pb crystallization age of 2701 ± 11 Ma. Monazite and xenotime in the veins and from hydrothermally altered country rocks yield three distinct U-Pb dates of ca. 2400, 1730, and 1680 Ma. Textural relationships between euhedral xenotime and pyrite with rounded native gold inclusions from within the quartz-sulfide veins show that the primary gold mineralization was synchronous with xenotime crystallization at 2403 ± 5 Ma, and coeval with pervasive alteration of the host rocks, which yield monazite ages of 2398 ± 37 and 2403 ± 38 Ma. Regional-scale hydrothermal events at ca. 1730 and 1680 Ma are linked to the growth of monazite within phyllitic rocks at 1730 ± 28 and 1721 ± 32 Ma, carbonate veining at 1655 ± 37 Ma, and gold remobilization or introduction of new gold at 1680 ± 9 Ma. The ca. 2400 Ma age for mineralization and hydrothermal alteration does not correspond with any known deformation event in the region, indicating a significantly different and more complicated low-temperature tectonothermal evolution for the southern Pilbara region than previously recognized. The in situ secondary ion mass spectrometry dating of monazite and xenotime employed here will lead to better targeting of orogenic gold deposits in the northern Capricorn Orogen, and these techniques can be utilized for orogenic gold exploration worldwide.

Gold metallogeny of the northern Capricorn Orogen

“…Gold deposits in the northern Capricorn Orogen are intimately associated with the major crustal-scale structures identified in the seismic reflection survey (Johnson et al, 2013). In particular, the Nanjilgardy and Baring Downs Faults appear to be the most important as they both extend to the mantle and show a close association with gold mineralization ( Figure. 1 and 2; Fielding et al, 2017a, 2018, 2019, under review, Johnson et al, 2013. The largest gold deposits are associated with the Nanjilgardy Fault, and include orogenic gold mineralization at the Paulsens deposit, and Carlin-like gold mineralization at the Mount Olympus deposit, both with endowments of >1 Moz (Fielding et al, 2017a(Fielding et al, , 2019.…”

“…In particular, the Nanjilgardy and Baring Downs Faults appear to be the most important as they both extend to the mantle and show a close association with gold mineralization ( Figure. 1 and 2; Fielding et al, 2017a, 2018, 2019, under review, Johnson et al, 2013. The largest gold deposits are associated with the Nanjilgardy Fault, and include orogenic gold mineralization at the Paulsens deposit, and Carlin-like gold mineralization at the Mount Olympus deposit, both with endowments of >1 Moz (Fielding et al, 2017a(Fielding et al, , 2019. Numerous smaller orogenic gold deposits occur throughout the Ashburton Basin and Wyloo Inlier ( Figure 1; Seymour 1991, Thorne andTrendall 2001) and include the Belvedere (Fielding et al, 2018) and Star of the West gold deposits (Fielding et al, under review).…”

“…The Paulsens gold deposit is an orogenic gold deposit situated in the Wyloo Inlier adjacent to the Hardey Fault, which is a second order structure to the Nanjilgardy Fault. A mineralized quartz-carbonate-sulfide vein is hosted within the Paulsens gabbro which is a 40 m-thick, folded and faulted gabbroic dyke, which was emplaced into sedimentary and volcanic rocks at the base of the Fortescue Group at c. 2700 Ma (Fielding et al, 2017a). Gold mineralization occurs as two distinctive lodes focused along either margin of the auriferous vein, locally termed the 'upper zone' and 'lower zone'.…”

“…Lower Zone mineralization is characterized by laminated quartz-carbonate-sulfide veins, abundant carbonaceous stylolites and inclusions of altered siltstone are common and are associated with abundant free gold. Ore-stage alteration assemblages of muscovite-quartz-ankerite ± chlorite ± leucoxene surround the auriferous veins (Fielding et al, 2017a(Fielding et al, , 2017b). Monazite intergrown with ore-stage alteration minerals and euhedral xenotime crystals interlocking with auriferous pyrite provides an age for primary gold mineralization at c. 2400 Ma (Fielding et al, 2017a).…”

“…A recent deep crustal seismic reflection survey across the orogen, as well as numerous other geophysical surveys, have led to a growing understanding of the relationship between the deep crustal architecture and the location and setting of the ore deposits (Johnson et al, 2013). Gold mineralization in the Capricorn Orogen shows a close spatial relationship with pre-existing crustal-scale faults that show evidence for multiple reactivation and mineralizing events (Fielding et al, 2017a, 2019, under review, Johnson et al, 2013, making it an ideal area to assess the relationship between the timing of gold mineralization, crustal architecture and the fault reactivation history.…”