Summary:
The Altar copper-gold project is a cluster of several intermediate composition subvolcanic porphyries occurring within an area of approximately 5 km by 8 km and emplaced during Middle to Late Miocene times.
Deposit Type
The Altar porphyry cluster was deposited formed in a transitional environment that transitions from the basal roots of a high-sulphidation epithermal lithocap to a sub-volcanic deeper porphyry copper environment at depth. Some of the porphyry centres can have been described as telescoped because of the close spatial distance between the porphyry and the high-sulphidation systems; however the age of the porphyry copper mineralization is estimated to be Miocene, ranging from approximately 10 to 12 million years old (Maydagan et al. 2011), which suggests there has been superposition of different aged systems rather than telescoping of one single system.
There are three main mineralized zones within the main Altar area of the project that are referred to as Altar Central, Alter United, and the Altar East from west to east zones. Each zone is associated with a distinct porphyry intrusive center that spatially coalesce to form one large zone of mineralization. In addition, there is another copper porphyry centre called Altar North, with limited drilling to date, that is about 2 km north of Altar Central. Three and a half kilometers west of the Altar Central zone is a gold-silver deposit called the QDM, that is included in the 2024 statement of mineral resources. Just east and immediately adjacent at depth from QDM is a copper-gold mineralized porphyry discovered in 2017 called Radio Porphyry, which is currently not in the mineral resource.
Altar Central and East
Within the Altar Central and East areas, outcrops of the rhyolitic unit are widespread, and rhyolites constitute the most abundant country rocks to the intrusive porphyries related to the alteration and mineralization. Outcrops of andesitic units occur on the ridges, fringing the western side of the mineralizing system. The contacts between andesitic and rhyolitic units of the Pachón Formation are concordant and mostly transitional.
On the ridges to the north of Altar Central and East there are small outcrops of rheomorphic and parallel-laminated tuffs. These are highly welded tuffs characterized by flow foliations and containing crystals and crystal fragments of plagioclase, ferromagnesian silicates, and quartz, and fiamme in a partially devitrified groundmass.
Porphyry stocks cropping out at Altar Central (DIP5) and Altar East (DIPAE) show well developed quartz vein stockworks along with intense sericite alteration and associated strong mineralization. These have been given the field name “Early Quartz Diorite Porphyries”. Although the areas where these porphyries crop out are widely separated, they are petrographically indistinguishable and therefore it’s postulated that they are derived from a larger intrusive body at depth.
Altar North
The Altar North zone was defined during the 2011 field season, when float of a new porphyry stock was discovered about 1.2 km north of the Altar Central area. The distinctive subvolcanic intrusion has been given the field name Altar North Porphyry.
Three main stages of porphyry emplacement were defined in Altar North. An early-stage porphyry (NP4) that commonly displays the best grade copper-gold mineralization is associated with strongly developed quartz stockwork veining and intense k feldspar and biotite alteration. These rocks are cut by an inter-mineral stage porphyry (NP5), which crosscuts early quartz veins and mineralization, but is overprinted by a younger and locally less developed quartz stockwork and mineralization. A late stage of intrusions is represented by texturally distinctive, light gray color, quartz-diorite porphyries (NP6/LDP) characterized by a seriated population of plagioclase phenocrysts from 0.6 centimeter (cm) to 1 mm in size floating in a phaneritic, crowded groundmass, with amphiboles, biotite and magnetite as accessory minerals. These late, poorly mineralized, and in most cases barren units are widespread throughout the core and at surface, cannibalizing early stage mineralized rocks in the area. Interestingly, deep in hole ALD 195, this late porphyry unit (NP6) is cut by a much younger tourmaline breccia body with associated moderate copper-gold mineralization.
QDM
The QDM Dacite Porphyry (DAC) represents one of the oldest units from the Middle-Late Miocene Subvolcanic Porphyry Suite and is characterized by a matrix-supported porphyritic texture with plagioclase, biotite-books, minor amphiboles, and abundant, coarse grained quartz phenocrysts. The texture and chemistry of the Dacite Porphyry intrusion clearly differs from all the other intrusions described in the Altar project area and therefore is a very distinctive unit to recognize.
This dacitic porphyry is locally mineralized, hosting, near-surface, high-grade, epithermal-style gold mineralization referred to QDM Gold. This gold mineralization is most likely related to a deeper, underlying porphyry system, which was confirmed in 2019 with drill hole QDM-19-041, drilled below the QDM Gold area and that intersected porphyry-style copper-gold mineralization.
Radio Porphyry
Multiple diorite porphyries occur in the Radio area and consist of discrete dyke bodies and display remarkably similar textures and composition, characterized by feldspar-rich medium-coarse grained crowded textures with plagioclase, biotite, and amphibole phenocrysts (>30%), very rare quartz phenocrysts in a phaneritic micro-crystalline to very fine-grained groundmass.
The emplacement of these porphyry dykes occurred in several pulses. An early event is represented by quartz-diorite porphyries (RAD1), which display strong potassic alteration and, in some portions of the core, also display some spectacularly mineralized “A” type veins and “USTs” (Unidirectional Solidification Textures) indicative of quartz-sulphide-rich fluids directly exsolving from the magma. These textures and veins are associated with the deposition of high-grade copper-gold mineralization.
Breccias
Late events of brecciation occur in most of the zones in the project. These include locally overlapped pulses of rock-milled matrix breccias (RMB), quartz-tourmaline breccia veins (HBt), and anhydritequartz-sulphides matrix breccia bodies. These last associated with base metals and high-sulphidation epithermal fluids (HBbm, HBhs).
Different types of epithermal veins have been identified and classified based on their mineralogical characteristics.
Alteration and Mineralization
Main Cu-Au-Mo mineralized centres in the project are associated with porphyry style alteration mineral assemblages. These are, in some of the areas, locally overprinted due to telescoping by epithermal style assemblages. Subsequent weathering processes also produced supergene alteration assemblages which affected prior hypogene and primary minerals.
The copper mineralization associated with the potassic alteration, mainly porphyry style chalcopyrite– bornite mineralization, was reconstituted as hypogene assemblages of pyrite, chalcocite and bornite within the green sericite alteration zone. Magnetite originally present in the potassic alteration zone was pyritized during the high sulphidation overprint. Sulphide minerals found within sericite alteration include hypogene pyrite, chalcopyrite, chalcocite, bornite, and molybdenite along with supergene covellite and digenite. Latest stage pyrite-enargite veins related to a high sulphidation epithermal system cut through the Stage 1 and 2 mineralization but contribute a minor proportion of the copper mineralization.