Characterisation of Gold Mineralisation and Geophysical aided Geologic Mapping in the Old Man Range, Central Otago, New Zealand

Stephens, Samuel (Sam)

Six structurally controlled gold deposits are hosted within two different structural blocks in the Old Man Range area. The mineralised lodes are hosted in normal faults which cut steeply across the host schistosity. In the East structural block, mineralised faults and the prominent joint set strike northwest and cut steeply across greenschist facies TZ III Caples Terrane schist. In the West structural block, mineralised faults and prominent joint sets strike east-west and cut steeply across upper-greenschist facies TZ IV Wanaka lithologic association schist. These structural blocks are separated by the regional scale Old Man Fault. Orientation of hard rock gold deposits is closely linked to the prominent joints in host schist surrounding the deposits. Mineralised lodes formed along ~1m wide normal fault zones. They are discontinuous but can be traced for up to ~150m, with variable thickness along strike. The lodes comprise brecciated silicified schist and hydrothermal quartz breccia, and minor quartz veins with abundant arsenopyrite. Open cavities with euhedral quartz crystals are common. Euhedral arsenopyrite occurs in quartz and silicified schist clasts within mineralised zones. Gold occurs as micro-particulate blebs in partly oxidised arsenopyrite, and as coarser free grains within quartz, micaceous laminae, micro-faults, and micro-shears within mineralised rock. Hydrothermal alteration is minor, comprises addition of Si, Au and As, and extends only a few centimetres from the mineralised lodes. Mineralisation may have occurred within a few kilometres of the surface during mid-Late Cretaceous extension (~106-101Ma), with estimated formation temperatures between 200-350°C. The mineralised structures within the Old Man Range area are similar to other shallow level, post-metamorphic Otago gold deposits. Magnetic, magnetite bearing greenschist has a high magnetic response and can be successfully mapped using total magnetic intensity surveys over the Old Man Range area. Electromagnetic (EM) surveys can be used successfully to map post-metamorphic faults within the Old Man Range area, where they show up as linear conductive anomalies. These geophysical surveys are a useful tool for geologic mapping. However, there is no direct link between the geophysical features and gold mineralisation within the Old Man Range.

v, 112 pages A4 map in back pocket A1

2013Stephens

Open Access

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