Late Cretaceous Blue Spur Conglomerate preserved as
isolated remnants at Gabriels Gully, Weatherstones, Forsyth, Waitahuna Gully and
Adams Flat was deposited during the intiation and early stages of extensional
movement on the Tuapeka Fault zone at each locality. The deposits represent
successive courses of a single fluvial system flowing across the Tuapeka Fault
zone from the up-thrown (northeast) block. Each course, beginning with the
channel at Gabriels Gully, was abandoned as a result of uplift-induced defeated
antecedence in favour of a new course farther to the southeast around the
southeast-propagating tip of the Tuapeka Fault zone.
Schist and quartz in the conglomerate was derived
ultimately from schistose rocks northeast of the Tuapeka Fault zone. Caples
Terrane greywacke detritus in the middle and upper beds of the conglomerate was
probably recycled from pre-existing sedimentary cover on the up-thrown block,
following initiation of uplift on the Tuapeka Fault zone at each locality. Most
of the gold is derived from vein sources in Otago Schist rocks to the north,
and/or associated placers. Hydrothermal veins at Waipori, Hindon, Barewood, and
Macraes are the most likely primary sources. There is no evidence to support
previous interpretations of deposition by river systems flowing from the
southwest, or axially along the Tuapeka Fault zone from northwest to southeast.
High gold grades within Blue Spur Conglomerate should have a linear trend more
or less similar to the orientation of the host channels, but are likely to drop
off rapidly to the southwest. Extensions of the conglomerate-filled channels are
unlikely to be preserved, but there is a strong probability that younger placers
recycled from the former channels exist.