Quebrada
Phase: Exploration
Tags: andesite azurite chalcanthite dacite porphyry malachite neotosite sericite sphalerite stock stockwork tourmaline
The Quebrada de la Mina (“QDM”) Au-Cu prospect is located on the Altar property approximately 2 kilometers to the west of the Altar Central porphyry Cu system.
Regional geology: Andean Cordillera
The Andean Cordillera extends for about 5,000 km along the western coast of South America, attaining a maximum width of about 700 km in the Central Andes of Bolivia.
Tectonism in the Cordillera varies both along strike and across the range; along-strike variations reflect changing plate geometry along the Pacific margin, whereas across-strike variations generally assigned to four sub-domains reflect the generally eastward migration of Andean arc magmatism and deformation through time.
In general terms, there are three units within each sub-domain, from west to east: a fore-arc zone, a magmatic arc, and a back-arc region. In the southern flat-slab sub-domain of the Central Andes (from 28°S to 33°30’S), the forearc zone is a steady rise to the crest of the Andes, which is formed by an inactive magmatic arc and thrust belt (Frontal Cordillera and Cordillera Principal). The Triassic magmatic (rift) arc has a general northwest–southeast trend. The foreland consists of an active, thin-skinned fold-thrust belt (Pre-cordillera) and zone of basement uplifts (Sierras Pampeanas, with altitudes ranging from 2,000 to 6,000 m).
The Altar Project is located in the Cordillera Principal. Basement rocks in the Altar region have been assigned to the Choiyoi Group, of Permo-Triassic age; the Choiyoi …
Deposit geology:
QDM is underlain by the same andesitic volcanic sequence that forms the country rock sequence at Altar. The volcanics are intruded by a circular and funnel shaped Dacite Porphyry stock, approximately 700m in diameter. The Dacite Porphyry belongs to the Middle-Late Miocene Subvolcanic Porphyry Suite and is characterized by is matrix-supported porphyric texture with placioglase, biotite-books, minor amphibols and the characteristic and abundant quartz phenocrysts. Texture and chemistry of the Dacite Porphyry clearly differs from all the other intrusions described at Altar Central, Altar East and Altar North.
Mineralization:
The Quebrada del la Mina (QDM) deposit is primarily a gold deposit with minor associated copper. The Pachon Andesite volcanics were intruded by a circular dacite porphyry stock approximately 700 m in diameter and host a large alteration footprint centered on the porphyry stock. Surface rock exposures at QDM are characterized by pervasive quartzsericite-tourmaline alteration with disseminations and veinlet stockworks of jarosite after pyrite and less-abundant fine quartz veinlets. The area affected by the alteration is coincident with the center of an induced polarization (“IP”) geophysical anomaly that measures 300 meters by 900 meters, as defined by the 20 millivolt per volt chargeability contour on the 3,500 meter elevation level plan.
Visible oxide copper mineralization occurs in the alteration zone at QDM and includes malachite, chalcanthite, neotosite and azurite impregnating fractures. Sphalerite mineralization of up to a few percent in volume was observed in several surface exposures in the northern and eastern part of the alteration footprint. Well-defined hydrothermal breccias occur at the eastern contact between the dacite intrusion and the andesitic host rocks. Three campaigns of geochemical rock chip sampling have consistently returned gold grades ≥0.5 g/t along with low copper grades reflecting the fact that the rocks at surface are leached of more mobile copper while leaving behind the immobile gold. The 2011 and 2012 exploration drilling at QDM confirmed significant Au mineralization in the leached capping and the underlying sulfide zone, where Au mineralization is associated with abundant pyrite dissemination.