Structural and lithological variation around Serpentine Diggings, Central Otago, New Zealand
A.P. Martin, D.J. MacKenzie, C. Blundell, S.C. Cox and S. Henderson
ABSTRACT: The Serpentine Diggings on Rough Ridge, Central Otago, are an area of historic, hard rock and alluvial gold mining. These diggings, and the area around the southern end of Rough Ridge, have been targeted by recent mapping to understand enigmatic responses in the aeromagnetic data set flown by Glass Earth in 2007 and to evaluate the potential prospectivity for orogenic-style gold mineralisation. In this region, aeromagnetic highs correspond to magnetite-bearing greenschist layers inter-fingered with magnetically unresponsive greyschist rock. Greenschist layers mapped at the surface are a few metres wide at most, but by comparison with anomalies elsewhere in Central Otago, may be tens of metres thick at depth. The greenschist rocks have a mafic igneous protolith. The map area straddles textural zones III and IV of the Otago Schist and is dominated by outcrop of metamorphosed Permian-Triassic greyschist. The major Rakaia-Caples terrane boundary is inferred close to the main area of the diggings. Quaternary cover of alluvium and colluvium is common and has been mined historically. A complex interplay of different late metamorphic fold generations around the Serpentine Diggings can be traced in outcrop and is reflected in the complex aeromagnetic pattern around this region. Structural mapping, in combination with geophysical interpretation and lithological mapping of greenschist units, is unravelling the tectonic evolution of this part of the Otago Schist, which will aid future mineral exploration efforts. The Serpentine Diggings remain a prospective area for orogenic-style gold.