Deposit Type
- Epithermal
- Vein / narrow vein
Summary:
The deposits at Diablillos, are high sulphidation epithermal silver-gold deposits, derived from activity of hydrothermal fluids in a relatively shallow environment, often associated with fumaroles and hot springs.
Principal economic minerals include native gold, acanthite, electrum, chalcocite, covellite, bornite, and enargite/luzonite, with accessory pyrite, chalcopyrite, sphalerite, tetrahedrite/tennantite, galena, marcasite, arsenopyrite, silver sulphosalts and tellurides. Dominant gangue minerals are quartz and pyrite, occasionally with barite. Alteration is characterized by lateral and vertical zonations of silicic, advanced argillic, argillic, sericitic, and phyllitic facies. Rocks typically have a bleached appearance owing to the acidity of the mineralizing solutions. These deposits can encompass a wide range of geometries from large lower-grade bulk-minable variants to smaller, higher-grade narrow vein types.
There are a number of mesothermal and epithermal precious and base metal occurrences situated along the trend of the Diablillos Cerro Galán fault zone within the northern and central Puna, including Diablillos, Incahuasi, Cóndor Yacu, Inca Viejo, and Centenario. Many of the mineral occurrences are spatially, and probably genetically, related to small Tertiary stocks and extrusive domes that are usually hydrothermally altered with disseminated and vein-hosted lead, zinc, silver, and gold (± tin, antimony, copper, and molybdenum) mineralization (Coira et al., 1993, quoted in Wardrop, 2009).
There are seven known mineralized zones on the Diablillos property, with the Oculto zone being the most important and best explored. These mineralized zones are:
1. Oculto including the Zorro and Cerro Bayo subzones
2. Fantasma
3. Laderas
4. Pedernales including the Pedernales Sur subzone (including Truchas and Saddle showings) and Pedernales Norte subzone (including Vicuna, Corderos Suri, and Guanaco showings)
5. Cerro del Medio
6. Cerro Viejo
7. Cerro Viejo Este
Oculto is the principal deposit on the property and is the locality of the bulk of the present Mineral Resource. It is a high-sulphidation epithermal silver-gold deposit derived from remnant hot springs activity following Tertiary-age local magmatic and volcanic activity. It is evidenced at surface by a broad zone of intense acid leaching located on the flank of Cerro Bayo, although the economic mineralization does not outcrop. Host rocks at surface are hornblende porphyritic andesite which has been intruded by a dacite porphyry body (or bodies) which are hypothesized to be the thermal driver(s) for the mineralization (Tate, 2018). The andesites overlie a basement assemblage of phyllites and granitic rocks. At the contact of the andesite with the basement, there is a paleo-surface occupied by a discontinuous conglomerate unit of widely ranging thickness. Recent review of drilling results suggests that this unit appears to thicken along a trend corresponding to one of the predominant controlling structures to mineralization and that this zone is coincident with broader lateral extent of the mineralization. Tate (2018) suggests that the conglomerate filled a paleo-trough related to that structure, which later reactivated and provided a conduit for ore-forming fluids.
The deposit is strongly oxidized down to depths in the order of 300 m to 400 m below surface. In the oxide zone, precious metal mineralization consists of native gold, chlorargyrite, comparatively less common iodargyrite, and locally common bismuthinite (Stein, 2001). These minerals occur as fine grained fracture-fillings and vug linings in association with quartz, jarosite, plumbojarosite, hematite, and goethite. Other accessory minerals include alunite, barite, native sulphur, and bismoclite.
Stein (2001) reported the occurrence of a high grade zone of native gold, native silver, and acanthite with accessory chlorargyrite, iodargyrite, and jalpäite in the southwest extremity of the deposit. Gangue minerals in this zone included quartz, alunite, jarosite, and iron oxides, along with intergrowths of barite.
Hypogene mineralization comprises vein- and breccia-hosted sulphides and sulphosalts underlying the oxide zones. Primary sulphide and sulphosalt minerals include pyrite, galena, enargite, chalcopyrite, sphalerite, tennantite, and matildite. Accessory minerals include barite and alunite. Incipient supergene enrichment was observed by Stein (2001), with covellite partially replacing chalcopyrite and polybasite replacing tennantite.
The precious metal mineralization throughout the deposit occurs as extremely fine grains along fractures and in breccias or coating the inside of vugs and weathered cavities. Mineral grains are very difficult to identity in core or hand specimen, and much of the identification of these minerals was done using electron microscope or microprobe.