SHRIMP U–Pb geochronology of magmatism and thermal events in the Archaean Marymia Inlier, central Western Australia

Abstract

Zircons from porphyry and granitoid samples collected in and around the Marymia Gold Mine in the Marymia Inlier, Western Australia, record a complex history. The results of U-Pb studies confirm that the Plutonic Well greenstone belt, and the surrounding granitoid envelope (including a 2,721±6 Ma intrusion), represent an Archaean terrain, which was intruded by high-level, felsic to intermediate porphyries at 2,694±7 Ma and potentially also at 2,660±4 Ma. Zircon xenocrysts (≥ca. 3.35, 2.93 and 2.74 Ga) indicate that there was older crust within, or below, the greenstone belt at the time of porphyry emplacement. Zircons from the granitoid envelope and intrusions within the greenstone belt record subsequent metamorphism and/or hydrothermal activity coeval with magmatism in the Late Archaean (ca. 2.66-2.63 Ga), and peak metamorphism, magmatism and gold mineralisation in the Yilgarn Block. A later period of metamorphism and hydrothermal activity at ca. 1.72 Ga is coeval with orogenesis in the southern Capricorn Orogen. Both the Late Archaean and Palaeoproterozoic thermal events have altered zircons, redistributed trace elements and caused zircon recrystallisation, which is distinctive in its isotope chemistry (in particular Th/U ratios >1) and morphology (e.g. homogeneous in transmitted light and back-scattered electron images, but sector-zoned in cathodoluminescence).

New age constraints on metamorphism, metasomatism and gold mineralisation at Plutonic Gold Mine, Marymia Inlier, Western Australia

“…This age has a reasonably strong common Pb correction that may affect its reliability; however, it closely correlates with a pre-existing age for a metasomatic event dated at 1719 ± 14 Ma by U-Pb ages of zircon overgrowths in a sample from the Marymia Deposit in the north of the PWGB (Vielreicher & McNaughton, 2002). …”

“…The metabasaltic rocks preserve pillow structures in low-strain areas, demonstrating that deposition of the Mine Mafic Package occurred in a subaqueous environment. Granitoid plutons surrounding the greenstone belt are dated at ca 2720 Ma (U-Pb dating of zircons; Vielreicher & McNaughton, 2002), and appear to have been overthrust onto the generally NE-dipping greenstone sequence. The entire sequence is cross-cut by at least two generations of sub-vertical dolerite dykes (Gazley, Tutt, Brisbout, Fisher, & Duclaux, 2014a).…”

“…Based on U-Pb sensitive high resolution ion microprobe (SHRIMP) analyses of zircon from samples collected both at Plutonic and farther northeast in the belt, peak amphibolite-facies metamorphism in the PWGB is dated at 2660-2630 Ma (McMillan, 1996;Vickery, 2004;Vielreicher & McNaughton, 2002). The main Au mineralising event was interpreted to have accompanied or shortly followed peak metamorphism in the PWGB (McMillan, 1996;Rowe, Awan, McCuaig, Sauter, & Vickery, 2002;Vickery, 2004;, although there is no geochronological evidence from Plutonic to support this.…”

“…One of the most significant episodes of Au mineralisation in the Yilgarn Craton (Figure 1) is recorded at ca 2650 Ma (2670-2600 Ma; e.g., Groves et al, 2000), including the richly mineralised Eastern Goldfields (ca 2640 Ma; e.g., Vielreicher, Groves, Snee, Fletcher, & McNaughton, 2010), as well as in parts of the Marymia Inlier (ca 2650 Ma; Vielreicher & McNaughton, 2002). Previously, the major episode of Au mineralisation in the Plutonic Well Greenstone Belt (PWGB) of the Marymia Inlier ( Figure 1) was interpreted to have taken place at about the same time as elsewhere in the Yilgarn Craton, specifically the Eastern Goldfields (e.g., McMillan, 1996;Rowe, Awan, McCuaig, Sauter, & Vickery, 2002;Vickery, 2004).…”

“…Previously, the major episode of Au mineralisation in the Plutonic Well Greenstone Belt (PWGB) of the Marymia Inlier ( Figure 1) was interpreted to have taken place at about the same time as elsewhere in the Yilgarn Craton, specifically the Eastern Goldfields (e.g., McMillan, 1996;Rowe, Awan, McCuaig, Sauter, & Vickery, 2002;Vickery, 2004). That interpretation was based on geochronological data from the Plutonic Gold Mine (Plutonic) and the wider Marymia Inlier (McMillan, 1996;Vickery, 2004;Vielreicher & McNaughton, 2002;Vielreicher, Ridley, & Groves, 2002), but included very little data that might specifically constrain the timing of Au mineralisation at Plutonic. Here we present fourteen Pb isotope model ages for pyrrhotite, arsenopyrite, pyrite and sphalerite, and two new RbSr ages for muscovite and biotite, all from Au-bearing samples from Plutonic, as well as a Pb-Pb age for titanite from a late-stage chlorite vein.…”