Structure and geochemistry of the South Reefton Shear Zone, Reefton Goldfield
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The South Reefton Shear Zone (SRSZ) is a 11km long mineralised shear zone in the Reefton Goldfield. The SRSZ strikes parallel to the fold hinges of the pervasive tight folds in the Reefton Goldfield. In outcrop the SRSZ is defined by a zone of discrete brittle shears and a close association with sulphide and carbonate minerals.
A shear sense obtained from rotated carbonate and sulphide porphyroclasts, shows the SRSZ had an oblique sinistral movement, east side down in its current position. Upon comparison with regional tight folds the SRSZ appears to have been rotated since its formation, and was initially a reverse oblique shear zone. Shearing may have begun during the tight folding event, between layers that progressively steepened, or after the tight folding event along the axial planar cleavage of tight folds.
A possible paragenesis for mineralisation of the SRSZ begins during diagenesis or early metamorphism of the Greenland Group when traces elements, including Au and As concentrated in mud layers and formed framboidal and small euhedral arsenian pyrite (SF1). Shearing on the SRSZ began and focused hydrothermal fluids, enriched in trace elements that have been released from metamorphic reactions at depth and in the surrounding rocks. This second stage of mineralisation (SF2) formed the euhedral disseminated grains of arsenopyrite and arsenian pyrite which is abundant in the southern portion of the SRSZ. In a later event, parts of the SRSZ were hydrothermally brecciated and stibnite was deposited, SF3.
The age of the SRSZ can not be well constrained, but must have formed after deposition of the host Greenland Group rocks (490 Ma) and finished before the unmineralised Reefton Group was deposited (Devonian). Compared with structures in the Reefton Goldfield shearing occurred during or just after the tight folding event which formed the pervasive tight fold seen around the Reefton Goldfield (D2 of Maw (2000)).
A shear sense obtained from rotated carbonate and sulphide porphyroclasts, shows the SRSZ had an oblique sinistral movement, east side down in its current position. Upon comparison with regional tight folds the SRSZ appears to have been rotated since its formation, and was initially a reverse oblique shear zone. Shearing may have begun during the tight folding event, between layers that progressively steepened, or after the tight folding event along the axial planar cleavage of tight folds.
A possible paragenesis for mineralisation of the SRSZ begins during diagenesis or early metamorphism of the Greenland Group when traces elements, including Au and As concentrated in mud layers and formed framboidal and small euhedral arsenian pyrite (SF1). Shearing on the SRSZ began and focused hydrothermal fluids, enriched in trace elements that have been released from metamorphic reactions at depth and in the surrounding rocks. This second stage of mineralisation (SF2) formed the euhedral disseminated grains of arsenopyrite and arsenian pyrite which is abundant in the southern portion of the SRSZ. In a later event, parts of the SRSZ were hydrothermally brecciated and stibnite was deposited, SF3.
The age of the SRSZ can not be well constrained, but must have formed after deposition of the host Greenland Group rocks (490 Ma) and finished before the unmineralised Reefton Group was deposited (Devonian). Compared with structures in the Reefton Goldfield shearing occurred during or just after the tight folding event which formed the pervasive tight fold seen around the Reefton Goldfield (D2 of Maw (2000)).
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112 p. : ill., maps, ; 30 cm
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2011Smith
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Smith, James Andrew., “Structure and geochemistry of the South Reefton Shear Zone, Reefton Goldfield,” Otago Geology Theses, accessed June 8, 2026, https://theses.otagogeology.org.nz/items/show/541.