Fertile, alkalic intrusions, Karari–Whirling Dervish gold deposit, Yilgarn Craton, Western Australia

Abstract

Some Archean and Phanerozoic gold deposits demonstrate a close spatial relationship with alkalic intrusions, although the genetic connotations of this association are contentious. Only a few of the alkalic intrusions in the Kurnalpi Terrane of the Archean Kalgoorlie–Kurnalpi Rift are associated with economic gold mineralisation (e.g. at Wallaby, Karari). The subvolcanic syenitic intrusions at the 2 Moz Karari–Whirling Dervish gold deposit are markedly different from most other alkalic rocks in the Kalgoorlie–Kurnalpi Rift. Although preserving pristine to slightly modified igneous textures, the syenitic intrusions are thoroughly altered by hypogene hydrothermal fluids, such that igneous minerals, comprising apatite micro-phenocrysts and relicts of Ba-rich K-feldspar, Ba-rich biotite, sodic amphibole and Cr-rich magnetite, total <5 modal %. Elevated SO3 (up to 1 wt%) in igneous apatite shows that the alkalic magmas were S-bearing and oxidised, and a high oxidation state carried through to the hydrothermal stages that followed, including potassic (biotite) alteration, at 650 °C (apatite–biotite geothermometry). Oxidised hydrothermal activity was interrupted by periods of sodic and sodic–calcic alteration in structurally controlled zones, interpreted to record the influx of reduced, external fluids at around 400–450 °C. A second, volumetrically minor stage of potassic (K-feldspar–pyrite–hematite) alteration in the intrusions is nevertheless significant, as it matches the proximal alteration assemblage in volcaniclastic country rock-hosted gold lodes. Overall, the hydrothermal system in the intrusions evolved from pervasive, high-temperature to more focussed, structurally controlled, mesothermal alteration. The source of the high-temperature, oxidised, alkaline hydrothermal fluid is considered to be the oxidised, alkalic magma itself, including an alkalic magma chamber underlying the gold deposits. Based on evidence from Karari–Whirling Dervish, gold fertility indicators for alkalic intrusions include: (i) widespread high-temperature alkaline metasomatism, including coarsening of alkali-feldspar intergrowths, abundant hydrothermal magnesian biotite and advanced to total replacement of igneous minerals; (ii) low SiO2 and high K2O, relative to unmineralised alkalic rocks in the region; (iii) alteration of igneous titanite to hydrothermal rutile with concomitant depletion of Nb; (iv) low rare earth element/P owing to depletion of rare earth element in apatite by the potassic (biotite) alteration fluid; and (v) high V/Th relative to regional alkalic rocks.