The Gold Canyon gold deposit in the Gold Bar district in the
southern Roberts Mountains has a resource of 140k oz. Au (average
grade 0.056 opt), 41k oz recovered. It is a sedimentary rock-hosted
disseminated gold deposit in the lower plate of the Roberts Mountain
thrust (RMT). Host rocks are Unit 2 of the Devonian Upper Denay
Limestone, and jasperoid breccia. A large pipe-like jasperoid at the
center of the orebody produced gold grades ranging from 0.1-0.3 opt.
Structure, lithology and alteration mapping show a spatial correlation
between the high-grade jasperoid breccia, north-northwest-trending
normal faults, and a zone of thrust faults and overturned folds. This
correlation suggests that pre-mineralization structure of Paleozoic or
Mesozoic age controlled the location of Tertiary normal faults and
related fluid flow and mineralization.
Whereas bedding mostly dips gently to the northeast, a 90-meter
wide fault block between two northeast-trending faults contains
inclined to overturned east-northeast-vergent, subhorizontal folds,
low-angle thrust faults, jasperoid breccia, and high-grade Au ore. The
geometry and asymmetry of folds is not consistent with the direction
of movement along the normal faults. Rather, the folds are faultpropagation
folds typically associated with thrust faults.
A structural model for the Gold Canyon deposit is proposed.
Paleozoic or Mesozoic thrust faulting and associated east-northeastvergent
fault propagation folding occurred in lower plate rocks below
the RMT. During Tertiary extension, normal faults developed on
either side of this folded and faulted zone, essentially reactivating the
weak imbricate structure. Hydrothermal fluid flow, brecciation, silicification
(jasperoid formation) and Au mineralization were thus
focused along this pre-Tertiary structural zone. Mineralization along
northwest-trending faults was amplified where northeast structural
feeders intersected. The important conclusion from this structural
analysis is that Paleozoic or Mesozoic contractional structures (thrust
faults, fault-propagation folds) in the lower plate are important controls
on northwest-trending mineralization. Exploration efforts in the
lower plate of the RMT should focus on Paleozoic or Mesozoic contractional
structures where they may have been reactivated by
Tertiary normal faults.