Molleturo Project
Geology
Ecuador is made up of five tectono-stratigraphic sub-divisions or terranes. These terranes are separated from each other by major north-northeast striking fault zones that follow the Andean trend. These terranes are briefly described, from west to east, as follows:
- Piñon Terrain: This terrane is typified by accreted Cretaceous oceanic crust which underlies the Pacific coastal plain region or La Costa.
- Cordillera Occidental (Western Cordillera), Piñon/Macuchi Terrane: This zone is typified by accreted Cretaceous to Eocene oceanic assemblages.
- Valle Interandino (Inter-Andean Graben), Chaucha Terrane: This terrane consists of mainly epicontinental volcanic, volcanoclastic and sedimentary rocks ranging in age from Jurassic to Oligocene deposited upon Upper Paleozoic (Carboniferous-Devonian) metamorphic basement. This zone is also marked by sub-aerial volcanic activity that spans the time period from the Oligocene to the present.
- Cordillera Real (Eastern Cordillera), Alao, Loja & Salado Terranes: The Eastern Cordillera represents a mixture of oceanic and continental crustal suites. In the western sector the Alao and Loja Terranes respectively comprise Jurassic oceanic crust/ophiolitic assemblages and Paleozoic epicontinental basement. In the east the Salado Terrane is made up of Jurassic submarine lavas and sediments.
- Sudamerica Autoctona (El Oriente) Terrane: This terrane consists of PreCambrian gneisses, deformed upper Paleozoic volcanic and sedimentary assemblages and a thick sequence of undeformed flat-lying Mesozoic to Tertiary sediments. The tectono-stratigraphic terranes are overlain in the southwestern part of the country by Eocene to Recent calc-alkaline volcanic units (Saraguro Group) with numerous Tertiary granitoid intrusions. The Tertiary volcanic rocks, in particular widespread Early Miocene ignimbrites (dated at 20-22Ma), host many metallic mineral deposits.
The Molleturo Project is near a major regional fault, the north-northeast trending Pujili Fault, that juxtaposes the Piñon-Macuchi Terrane oceanic crust and the Chaucha Terrane continental crust.
The oldest rocks in the Molleturo region lie to the south of the Molleturo mining concessions, where a fault bounded inlier of upper Paleozoic basement is exposed. These rocks are Devonian semi-pelitic schists and gneissic rocks of the La Victoria Unit (PZV). These rocks correspond to the oldest lithologies outcropping in the Ecuadorian Sierra.
The north-western sector of the Project and region adjacent to the Pujili Fault is underlain by Cretaceous submarine basaltic lavas, tuffs and breccias intercalated with occasional ultramafic (ophiolitic) units of the Piñon Formation (KP). These rocks are the basal units of the Piñon Terrane and the Western Cordillera in Ecuador.
To the west of the Piñon Formation in the extreme northwest corner of the Project is a fault bounded belt of Cretaceous (Cenomanian) volcano-sedimentary deposits composed of lutites, cherts and sandstone rocks of the Cayo Formation (KK). Regionally this formation overlies the Piñon Formation.
To the west of the Cayo Formation there are outcropping Paleocene-Eocene rocks of the Macuchi Unit (PCEM) composed of volcanogenic lithologies of calc-alkaline affinity possibly related to an emerging island arc. These rocks are mainly represented by tuff sandstone, andesite, basalt and volcanic breccias. Younger granodiorite and quartz diorite bodies have intruded the Macuchi volcanic assemblage. Small rhyolitic stocks are mineralized in several places.
The island arc sequences culminate with the deposition of the turbiditic rocks of the Medium to Upper Eocene Apagua Unit (EA) composed of lutites and greywackes. These rocks overlain transitionally the Macuchi Formation described previously.
Most of the mining concession areas of the Molleturo Project are covered by the regionally prospective Oligocene Saraguro Formation (OS) which correspond to andesitic volcanic lavas, rhyolitic flows and pyroclastic rocks of sub-aerial continental origin. In the Miocene-Pliocene the volcanic activity evolves to more acidic giving origin to the Pisayambo Volcanics (MPLP) which cover the eastern sector of the region.
The above stratigraphic units are intruded by Eocene granodiorite, diorite, and related porphyries of the Chaucha Batholith (CZ) and related stocks. The batholith has been dated at 12 Ma (Serravallian) on the basis of K-Ar dating. These porphyritic intrusions gave place to copper-molybdenum deposits such as Chaucha Project (Ascendant Copper Corporation).