The impact of hydrothermal mineral replacement reactions on the formation and alteration of carbonate-hosted polymetallic sulfide deposits: A case study of the Artemis prospect, Queensland, Australia

Abstract

Hydrothermal sulfidation of carbonate rocks and the reverse replacement of sulfides by carbonates are important processes responsible for the formation and subsequent alteration of ore deposits. These processes have been recognized in the case study of the Artemis Zn-Cu-Au prospect, which is located in the Cloncurry district within the Eastern Succession of the Mount Isa Inlier, NW Queensland, Australia. The high-grade polymetallic mineralization displays a complex association of massive sulfides and carbonates hosted in a vertical marble lens adjacent to garnet-bearing schist, staurolite-muscovite schist, and quartzo-feldspathic psammite. Petrographic analyses of drill core specimen by optical and electron microscopy, micro-X-ray florescence elemental maps, electron probe microanalyses, cathodoluminescence, and bulk geochemistry suggested three major stages of mineralization: (i) pre-ore stage marble (calcite-0); (ii) ore stage involving sulfidation of the marble and precipitation of calcite-1 and spherical sulfide inclusions in the first episode and the formation of calcite-2 and massive sulfide in the second episode; and (iii) post-ore stage involving replacement of sulfides by calcite-3 and garnet.Mineral replacement reactions are dominant processes in rocks of ore stage and post-ore stage. The ore stage involved the dissolution of coarse and pristine calcite-0 (Ca0.95Mn0.3Fe0.2CO3) and the precipitation of sulfides and calcite-1 (Ca0.96Mn0.3Fe0.1CO3) and calcite-2 (Ca0.96Mn0.3Fe0.1CO3). The sulfide assemblage consists of pyrrhotite, sphalerite, chalcopyrite, galena, arsenopyrite, cobaltite, costibite, Bi, cubanite ± Ag-sulfides and Au, in decreasing order of abundance. In episode 1, calcite-1 contains spherical sulfide inclusions while in episode 2, calcite-2 envelopes calcite-1. The replacement of Co-rich arsenopyrite (Fe0.75Co0.26As1.13S0.83) by relatively Co-poor arsenopyrite (Fe0.86Co0.15As0.99S0.99) via a coupled dissolution-precipitation reaction occurred in episode 2. Based on arsenopyrite geothermometry, the temperatures for the first and second episode are 650 ± 50 and 450 ± 70 °C, respectively. Micro-XRF elemental maps of the garnet-bearing schist, adjacent to the sulfidized marble, revealed the sulfidation front where pre-ore garnet and amphibole were replaced by pyrrhotite, chalcopyrite, Au, biotite, and muscovite. In the post-ore stage, calcite-2 was partially replaced by siderite and calcite-3 (Ca0.94Mn0.4Fe0.2Mg0.1CO3), which shows sectoral and oscillatory zoning. Galena was replaced by calcite-3 as documented by embayed lobate grain boundaries between these minerals, and pyrrhotite Fe(1-x)S, (x = 0.091–0.119) was pervasively replaced by garnet (Mn1.2Fe1.2Ca0.6)Al2Si3O12. The microscale investigations of the mineralization processes in the Artemis orebody document the impact of hydrothermal mineral replacement reactions on metal sequestration. Mineral replacement reactions facilitate the formation of ore-deposits and subsequent alteration processes, which may lead to the loss of economic value of the deposit.