Alteration Halo of the Macraes Gold Deposit Within the Hyde-Macraes Shear Zone, Southern New Zealand

Abstract

The Hyde-Macraes shear zone at Frasers is surrounded by an alteration halo more than a kilometer wide. More strongly altered rocks within an approximately 150-m-thick slice of schist, which spans the Footwall fault and Hanging-Wall shear, are partly replaced by siderite, W-bearing rutile, kaolinite, illite, and graphite in microshears, seams, veins, and patches of alteration. Gold occurs within trace amounts of associated disseminated pyrite and arsenopyrite as well as coincident quartz-sulfide veinlets. Less-intense alteration, mostly around the minor Eastern Lode shears, is marked by the incoming of calcite, ankerite, Ca-light rare earth element (LREE)-Y-Th carbonate, smectite, W-free rutile, pyrite, and relict metamorphic epidote, coupled with a decrease in siderite, kaolinite, W-rich rutile, arsenopyrite, and illite like those present in more strongly altered schist. Incipiently altered schist 10s to 100s of meters from strands of the Hyde-Macraes shear zone includes trace amounts of (Ni)pyrite, relict metamorphic epidote, and titanite—the titanite partly replaced by smectite, rutile, calcite, ankerite, carbonaceous material, and traces of Ca-LREE-Y-Th carbonate.

The presence of hydrothermal clays indicates the Macraes orogenic gold deposit formed under relatively low-temperature subgreenschist facies conditions. Coincident changes in carbonate, clay, mica, and sulfide mineralogy, as well as the W and Nb contents of rutile, provide several potential vectors toward more prospective parts of the Hyde-Macraes shear zone that may also apply in other comparable shear zones. Closer proximity to mineralized shoots is likely to be marked by an increase in hydrothermal siderite, kaolinite, illite, arsenopyrite, and W-bearing rutile relative to ankerite, smectite, pyrite, W-absent rutile as well as relict metamorphic phengite, epidote, and titanite.