Exeter Resource Corporation

Advancing the world-class Caspiche project in Chile

Caspiche, Chile

Geology

View the Geology of the Caspiche Porphyry Gold-Copper Deposit (paper published in Economic Geology 2013 by Richard Sillitoe, Justin Tolman, Glen Van Kerkvoort)

Gravels (Quaternary alluvium) cover over 90% of the Caspiche property.

Host (basement) rocks at Caspiche comprise Tertiary age volcanic sandstone and siltstone. In the vicinity of Caspiche Porphyry this unit is overlain by a 500 to 700 m thick sequence of homogeneous polymict, volcanic breccia coeval with volcanism.

Two main porphyry intrusions, early and early inter-mineral phases, constitute the well mineralised Caspiche stock, with a third, late inter-mineral phase abutting it to the west and south. The well-mineralized stock measures roughly 300 x 400 m in plan view, with a vertical extent of over 1,200m. The early porphyry is blind, being concealed beneath an average of 100 m of pre-mineral volcanic breccia.

A late stage diatreme breccia occurs on the western side of the Caspiche porphyry system, dipping westward at approximately 30º. The breccia extends for a distance of at least 1 km in a north-south orientation and has been drilled to a thickness of 280 m.

Three dominant alteration types are recognised being potassic, intermediate argillic and advanced argillic. Potassic alteration is overprinted by intermediate argillic and both these are affected by the advanced argillic alteration assemblage. Massive silica and vuggy residual silica ridges crop out in the south and north of the Caspiche Porphyry prospect.

Siliceous ridges outcrop on the northern and southern peripheries of the Caspiche Porphyry prospect and are the surficial expression of high sulphidation epithermal style mineralization that extends to depths up to 200 m.

Oxidation effects are notable in the upper 100 m to 150 m and occasionally down to 200 m. The contact between oxide and sulphide material is sharp.

Immediately to the west, underneath a west dipping barren breccia a new zone of mineralisation named the “MacNeill zone” was identified. This appears to be a separate later pulse of gold rich, copper poor mineralisation. Its shape appears to be lenticular and thickest underneath the shallower portions of the overlying breccia.

 

Alteration and Mineralisation

Mineralisation at the Caspiche Porphyry is described as a stockwork-hosted, gold-copper porphyry. This large tonnage, relatively low grade style of deposit is host to some of the world’s largest ore bodies and are often mined in a bulk tonnage scenario.

The bulk of the mineralisation is hosted in a dense stockwork of grey veins. Potassic alteration is the earliest alteration phase and is more pronounced at deeper levels.

Intermediate argillic alteration overprints much of the potassic alteration. The main effect of this overprint are to convert magnetite to hematite (martitization). Due to this the porphyry mineralization is non magnetic to depths of around 400-500 m.

There is a “telescoped high-sulphidation epithermal” gold system overprinting the upper portion of the porphyry system, which provides some local upgrading of the gold from this upper zone.

Copper has been almost totally leached from the relatively flat lying oxide zone making it suitable for heap leaching. Secondary copper is rarely observed on the oxide-sulphide contact, but there is no significant copper enrichment. Visually the contact is easy to pick and is geochemically sharp with depletion in copper in the oxide zone.

Dominant ore minerals at Caspiche include chalcopyrite, covelite with bornite becoming the dominant mineral species with depth. Advanced argillic altered zones contain a high-sulphidation sulphide assemblage in which pyrite is accompanied by typically fine-grained intergrowths of enargite, tennantite, chalcopyrite, chalcocite, covellite and minor bornite.

Gold is associated with sulphides within the veins and locally disseminated in the wallrocks outside the intrusions but still spatially associated with vein material.

The MacNeill zone is dominated by pyrite as the primary sulphide with subordinate chalcopyrite.