Carrick Range Au and Sb mineralisation in Caples Terrane, Otago Schist, Central Otago, New Zealand

Abstract

Mineralised zones of the Carrick Range, bearing gold and stibnite veins, constitute one of the rare economically significant mineralised fields in the Caples Terrane. The mineralised zones have a polyphase hydrothermal history. Early low‐angle shears are filled by milky quartz, with minor carbonate, sulphides, and gold. These are cut by steeply dipping hydrothermal breccias and veins with prismatic and fine‐grained chalcedonic quartz ± carbonate (calcite and ankerite). Gold is associated with some high‐angle mineralised zones, and stibnite with spatially separated but texturally similar high‐angle zones. Fluid inclusions in early milky quartz homogenise mainly between 145° and 210°C, with some higher temperature homogenisation up to 300°C. Late prismatic quartz has inclusions that homogenise at 150–170°C, and ice‐melting temperatures suggest low salinity (2.4–4.8 wt% NaCl equivalent). Oxygen isotopic ratios of early milky quartz range from δ¹⁸O = +15.4 to +17.6%_o, whereas fine‐grained late quartz ranges from +13.4 to +22.8%_o. Vein carbonates have δ¹⁸O between +13.8 and + 19.9%_o, and δ¹³C between ‐1.6 and ‐3.6%_o. The isotopic data, combined with other geological and mineralogical evidence, imply that mineralisation occurred over a wide range of temperature (140^°‐400°C) with a fluid of constant isotopic composition similar to typical Otago Schist metamorphic fluid. Sulphide sulphur isotopic ratios fall in a narrow range of δ³⁴S = ‐1.6 to +2.6%_o, consistent with a homogenised crustal sulphur source. Late stage mineralisation occurred at shallow crustal levels (upper 2 km?), but earlier mineralisation was probably deeper. Mineralisation may have occurred sporadically, or as a continuous event during progressive uplift, during Cretaceous and/or early Miocene extensional tectonics.

Keywords:

• gold
• stibnite
• Carrick Range
• Otago Schist
• Caples Terrane
• isotopes
• fluid inclusions