Geological Settings

Regional Geological Setting

The general geological setting is described in Billingsly (1926), Van Thournout et al. (1996), and Spencer et al. (2002). Simplified Geological Map of Ecuador (Left image)

The Zaruma-Portovelo Mining District consists of a 15 km long vein system that lies immediately north of the Piñas Fault, a major regional structure that separates a metamorphic terrain to the south from a Tertiary volcanic sequence to the north. The metamorphic rocks, sub parallel to the Piñas Fault, show that this structure represents a suture dated as Jurassic to early Cretaceous (Aspden et al., 1995). Tertiary volcanic rocks preserved on the northern side of the Piñas Fault unconformably overlie metamorphic rocks facies of continental origin, implying that the suture marks a site of continent-continent collision in the late Jurassic to early Cretaceous.

Preserved volcanic units show southward-thickening that define growth sequences formed in a half graben during normal movement on the Piñas Fault (Coward, 2001 in Spencer, R.M. et al., 2002) and as a consequence of deceleration in the rate of subduction of the Nasca plate beneath the South American plate at 37 Ma. North-south directed compression increased the rate of the plate convergence at 20 Ma and caused the hanging wall of the Piñas Fault to bulge into an open, regional-scale antiform. Host rocks of the Zaruma – Portovelo vein system are late Oligocene to early Miocene (28.4 – 21.5 Ma) in age (Dunkley and Geybor, 1997 in Spencer, R.M. et al., 2002) and range in composition from calc-alkaline to alkaline, compatible with their emplacement in an extensional cordillera setting. Four porphyry intrusions of dioritic to granodioritic composition have intruded the hinge of the Cangrejos antiform; one has been dated at 16 Ma (Pratt et al., 1997). On one of these, the Fierro Urcu porphyry, drilling revealed a mass of coalescing sills which is compatible with intrusion into a compressive stress field (PRODEMINCA, 2002 in Spencer, R.M. et al., 2002). Regional Geological Map of the Zaruma – Portovelo District (Right image)