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

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 degrees 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 degrees 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.