Blue Spur Conglomerate: Auriferous Late Cretaceous fluvial channel deposits adjacent to normal fault scarps, southeast Otago, New Zealand

Abstract

Latest Cretaceous Blue Spur Conglomerate at Gabriels Gully was deposited in a northeast‐southwest‐trending valley incised >200 m into metamorphic basement within the Tuapeka Fault Zone. Clast imbrication unequivocally indicates paleoflow towards the southwest, across the Tuapeka Fault Zone from the uplifted block. Early schist‐and quartz‐rich fluvial sediments in the valley and transverse mass flow deposits derived from the valley sides are preserved on a strath terrace on the southeast side of the valley. These sediments were incised by the paleoriver and partially reworked into the base of a deeper channel cut into the underlying schist before deposition of greywacke‐rich fluvial conglomerates. There is a strong enrichment of gold on and just above schist at the base of the incised channel, and subordinate enrichment at and above the basal scours of the overlying fluvial conglomerate packets. Imbricated clasts at less well exposed Blue Spur Conglomerate at Wetherstons, Forsyth, and Waitahuna Gully suggest that southwest‐flowing rivers in incised paleovalleys crossing the Tuapeka Fault Zone also deposited those outliers. The four incised paleovalleys were probably fed by a single river that successively abandoned the valleys in favour of a new course around the propagating tip of the Tuapeka Fault Zone. Structural and geomorphic relationships along the Tuapeka Fault Zone suggest that it is multi‐stranded and segmented on the scale of a few kilometres, rather than being a single continuous structure. Independent and concomitant extension on collinear and overlapping strands, respectively, was the primary control on fluvial incision into intrafault zone blocks and on gold concentration within the valleys. Transverse topographic highs across the fault zone, between sediment sinks on the downthrown side, are probably the result of distributed deformation amongst and between strands. These relative highs controlled the location, size, and orientation of the depositional systems, and prevented the development of axial drainage along the Tuapeka Fault Zone itself. Valley incision and associated gold‐concentrating processes will not have extended significantly downstream from the Tuapeka Fault Zone. The best gold concentrations will lie on intrazone blocks that have undergone the most favourable combination of sediment accumulation, fluvial incision, and re‐concentration cycles, and not necessarily close to the absolute footwall.