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

Abstract

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 laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) that in situ rubidium–strontium (Rb–Sr) dating has become a more commonly applied and powerful tool to date K-rich or Rb-bearing minerals. Here, we date 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 as well as additional phengite associated with the younger 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, rhenium–osmium (Re–Os) pyrite and uranium–lead (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 are overdispersed for a single coeval population of phengite and show 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 as part of 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.

In situ Rb–Sr dating by collision cell, multicollection inductively-coupled plasma mass-spectrometry with pre-cell mass-filter, (CC-MC-ICPMS/MS)

“…The value of n was chosen to result in a minimum 2 σ z total / z of 0.14%, which equals the best reproducibility reported for in situ spot analysis of NIST SRM 610 using single collector CC-ICPMS/MS in previous in situ Rb–Sr dating studies. 17 This uncertainty was comparable to the minimum 87 Sr/ 86 Sr uncertainty achieved using the iCAP TQ™. The minimum theoretical 87 Sr/ 86 Sr precision calculated for Proteus in the model did not exceed observed best reproducibility, therefore no additional uncertainty was applied to Proteus 87 Sr/ 86 Sr ratios.…”

“…The best reported reproducibility achieved for in situ 87 Sr/ 86 Sr spot analysis of NIST SRM 610 using single collector CC-ICPMS/MS when in situ Rb–Sr dating is 0.14% (ref. 17 ) (2RSD, n = 30). Comparatively, Proteus achieves a minimum reproducibility of 0.009% for 87 Sr/ 86 Sr measurements of NIST SRM 610 (2RSD, n = 11, see Section 3.2 and ESI S2 † ).…”

“…87 Rb/ 86 Sr ratios >500) or samples that are particularly old (>1 Ga). 6,11,[15][16][17] Here, we illustrate the potential of collision cell mass spectrometry for a wider range of geological targets using a novel multicollection system.…”

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 AE 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, 2002aStipp et al, , 2002b.…”

“…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).…”