Resolving multiple geological events using in situ Rb-Sr geochronology: implications for metallogenesis at Tropicana, Western Australia

Abstract

<jats:p>Abstract. Dating multiple geological events in single samples using thermochronology and geochronology is relatively common but it is only with the recent advent of triple quadrupole LA-ICP-MS that in situ Rb-Sr dating has become a more commonly applied and powerful tool to date K- and Rb-bearing minerals. Here, we date, for the first time, two generations of mineral assemblages in individual thin sections using the in situ Rb-Sr method. Two distinct mineral assemblages, both probably associated with Au mineralization, are identified in samples from the Tropicana gold mine in the Albany–Fraser Orogen, Western Australia. For Rb-Sr purposes, the key dateable minerals are two generations of biotite, and additional phengite associated with the second assemblage. Our results reveal that the first, coarse-grained generation of biotite grains records a minimum age of 2535 ± 18 Ma, coeval with previous 40Ar/39Ar biotite, Re-Os pyrite and U-Pb rutile results. The second, fine-grained and recrystallized generation of biotite grains record an age of 1207 ± 12 Ma across all samples. Phengite and muscovite yielded broadly similar results at ca. 1.2 Ga but data is overdispersed for a single coeval population of phengite and shows elevated age uncertainties for muscovite. We propose that the ca. 2530 Ma age recorded by various geochronometers represents cooling and exhumation, and that the age of ca. 1210 Ma is related to major shearing associated with the regional deformation associated with Stage II of the Albany–Fraser Orogeny. This is the first time that an age of ca. 1210 Ma has been identified in the Tropicana Zone, which may have ramifications for constraining the timing of mineralization in the region. The in situ Rb-Sr technique is currently the only tool capable of resolving both geological events in these rocks. </jats:p>

From microanalysis to supercontinents: Insights from the Rio Apa Terrane into the Mesoproterozoic SW Amazonian Craton evolution during Rodinia assembly

“…Because titanite from one sample retains the protolith age (1959 ± 25 Ma, FMR101), we infer that this deformational event occurred at temperatures lower than that of average Pb closure in titanite (~600–700°C), and higher than the average Pb closure temperature of apatite (~450–550°C) (Chamberlain & Bowring, 2000; Cherniak, 2010; Oriolo et al, 2018; Schoene & Bowring, 2006). We infer that this deformational event occurred at temperatures of ~450–550°C, consistent with reset of the apatite U–Pb and white‐mica Rb–Sr systems (e.g., Eberlei et al, 2015; Jepson et al, 2021; Kirkland et al, 2017; Odlum & Stockli, 2020; Olierook et al, 2020; Ribeiro, Mulder, et al, 2020). The deformation temperature is qualitatively supported by the dominance of quartz SGR and GBM (Figure 3a–d), commonly developed at medium‐grade thermal conditions in strain rate regimes for natural faults (Faleiros et al, 2010; Stipp et al, 2002a, 2002b).…”

“…This interpretation is supported by low temperature microstructures including pervasive quartz BLG overprint and fractured feldspar (Figure 3d). Additionally, this suggests that the white‐mica Rb–Sr system can be affected by thermal events with temperature conditions below ~450°C (e.g., Olierook et al, 2020), pushing the lower limit of the closure temperature beyond what was previously suggested (<500°C; Eberlei et al, 2015).…”

“…In situ Rb–Sr geochronology combined with textural and chemical constraints can resolve the timing of deformation and the role of fluids (Olierook et al, 2020; Van Daele et al, 2020). Coarse‐grained white‐mica from the NW‐trending Porto Murtinho Complex (FMR104 and FMR105) has microstructures indicating crystal‐plastic deformation (Bell et al, 1986; Wilson & Bell, 1979) and lower phengite contents in comparison with white‐mica from the Caracol Complex and Alto Tererê Formation (Figure 5f).…”

“…In-situ Rb-Sr geochronology combined with textural and chemical constraints, can resolve the timing of deformation and the role of fluids (Olierook et al, 2020;Van Daele et al, 2020).…”

LA‐ICP‐MS/MS Single‐Spot Rb‐Sr Dating

“…Being cost and time efficient, it is possible to collect large 87 Rb/ 86 Sr and 87 Sr/ 86 Sr data sets from multiple grains and mineral species, which makes it an attractive new tool in geochronology (e.g., Gorojovsky and Alard 2020, Olierook et al . 2020, Redaa et al . 2021, Tillberg et al .…”