The Geometry and evolution of the boiling zone that produced the Favona Ore Bodies in the Hauraki Goldfield, and the implications for exploration

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Simmons SF. 2019. The Geometry and evolution of the boiling zone that produced the Favona Ore Bodies in the Hauraki Goldfield, and the implications for exploration. Lower Hutt, N.Z.: GNS Science. GNS Science report 2019/02., 29 p.; doi:10.21420/393T-G091.

Abstract: The Favona-Moonlight vein extends for ~1000 m, covers a vertical interval of at least 400 m, and is up to 25 m in width. The structure forms a dense network of sub-parallel veins that anastomose and branch upward and pinch out at 50–100 m below the current surface. The structure hosts three discrete ore zones, which are approximately 150 m high and about 200 m in length, and which occur in the widest segments of the vein system. In these ore zones, veins are primarily filled with quartz that show a wide range of textures (banded, breccia, massive) resulting from pulses of very strong to quiescent fluid flow associated with particle transport and sedimentation. On the fringes of ore zones, vein fill is predominantly made of lattice quartz, and precious metal mineralisation is poorly developed to non-existent.

The vein mineral zonation pattern is interpreted in the light of boiling fluid flow in geothermal wells, where the same minerals deposit and the same mineralogic textures develop. Lattice calcite and its quartz pseudomorph equivalent are key indicators of the base level of the boiling zone, which clearly fluctuated along the Favona-Moonlight structure during episodes of vein filling. Deep levels of boiling prevailed beneath ore zones during their formation, whereas outside the ore zone, shallow levels of boiling predominated. This interpretation is consistent with patterns of mineralisation in geothermal wells, where the gold-silver interval occurs several hundred metres above the base of the boiling zone. Accordingly, gold-silver grades correlate poorly with occurrences of lattice textures along the Favona-Moonlight structure. This finding should give explorers confidence to fully test the along strike potential of structures even where vein intervals are dominated by quartz lattice textures and barren or poorly mineralised in precious metals.