Summary:
Deposit Type
The Concepcion Del Oro district represents a porphyry copper deposit that has been eroded down to a relatively deep level, as evidenced by the outcropping skarn alteration. Skarn alteration is barren unless the skarn is overprinted by the later porphyry style alteration marked by propylitic, phyllic and potassic alteration types. Trace metal geochemistry supports the classification of Concepcion del Oro as a porphyry deposit. Strongly anomalous bismuth, tellurium, arsenic and antimony are present and are typical trace metal assemblages associated with porphyry copper deposits. This trace metal assemblage is most anomalous in areas of phyllic and potassic alteration.
The deposit is a skarn/sediment replacement (manto).
Property Geology
The principal Aranzazu mineralized skarns are deposits formed by the contact between Eocene granodioritic intrusions and the sedimentary limestones of the Zuloaga, while La Caja formation was metamorphosed into a calc-silicate hornfels (pyroxene/biotite), and Taraises formation into Cabrestante–Glory Hole Hanging Wall skarn. The deposits are particularly rich in copper, gold, and silver.
In outcrops and shallow levels of the skarn bodies, an oxidation zone is consistent with some copper carbonates as malachite, azurite and brochantite. Decynes of meters below, the copper mineralization becomes more abundant in sulphides, majority in chalcopyrite >> bornite > chalcocite. All these sulphides (stage 4 retrograde) crosscut and replace preexisting prograde mineral such as garnets and pyroxenes (stage 2 prograde). Epidote, Vesuvianite, chlorite, iron oxides, quartz, and calcite are present everywhere as product low temperature alteration (stage 3 retrograde) in either exo or endo skarn zones.
The Aranzazu porphyry stock is adjacent to a non-mineralized granodiorite. The quartz monzonite porphyry is a potentially mineralized porphyry copper which covers an area of approximately 1 km2. Quartz-sericite veining is in part stockwork, but the stock margins are dominated by parallel veins indicating a strong flow vector of hydrothermal fluids into the skarn zones. The Aranzazu stock consists mainly of granodiorite to quartz monzonite and monzonites of porphyroblastic to equigranular textures. Biotite and hornblende typically make up 2.0% to 5.0% of the intrusive phases, and magnetite is commonly present in the 1.0% to 2.0% range. The borders of the stock exhibit other younger phases such as quartz-feldspar porphyries, medium grained biotite-rich granodiorites, and aphanitic dikes of andesitic composition. These younger phases could have a closer relationship to the magmatic chambers responsible for metasomatism and mineralization since they are observed in proximity to mineralized skarns and zones of alteration. In the intrusive border zone where the GHP-Cabrestante orebodies are located, pre-mineral quartz-feldspar porphyries occur as pre-mineral dikes between skarn deposits.
Quartz monzonite porphyries are intimately associated with the Aranzazu skarn deposit, in which they constitute numerous steep dykes, ranging from a few metres to >50 m wide. Many of the dykes parallel the bedding but others were emplaced along the eastern edge of the equigranular intrusions, abutting the western edge of the skarn deposit. Two porphyry phases are suspected to exist, the earlier displaying alteration and veining and the latter only weakly altered and poorly fractured. Both phases are characterised by rounded quartz phenocrysts, biotite ‘books’ and squarish plagioclase phenocrysts in a fine-grained to aphanitic quartz-Kfeldspar groundmass. The margins of the dykes are commonly converted to endoskarn (Sillitoe 2019).
Metre-scale dykes and centimetre-scale dykelets of pink-coloured aplogranite and aplite cut all the equigranular and porphyry phases described above, suggesting that these late differentiation products were emplaced in several stages (Sillitoe 2019).
Mineralization
The mineralization in Aranzazu has a strike length of 1.5 km, a width of up to 250 m, and depth extents up to 850 m. The Aura exploration and definition drilling shows that mineralization is closing off at BW, MX, and Arroyos Azules (AA) but open at depth in the GH zones. The deepest drilling conducted to date was carried out by Aura (M-20-0028) and indicated the presence of strong copper mineralization at an elevation of 1,272 m below sea level, or 850 m below surface.
Copper, gold and silver mineralization occurs in a porphyry alteration assemblage related to the intrusion of an igneous rock complex consisting of quartz monzonitic to granodioritic rocks. The igneous rocks have intruded a sequence of Cretaceous limestones and siltstones. The sedimentary rocks have undergone contact metasomatism and skarnification and now represent a group of rocks ranging from endoskarn and exoskarn, proximal to the intrusive, to marble, distal to the intrusive. Hornfels is present and reflects the contact metasomatic alteration of clastic rocks. Porphyry alteration consisting of propylitic, phyllic, and potassic alteration overprints the skarn, quartz monzonite and, in some areas, marble and hornfels. The porphyry alteration is the mineralizing event that deposited the suite of metals that comprise the mineral deposits of interest.
The skarn exhibits a typical zoning pattern: copper is concentrated near the endoskarn, molybdenum is found within the endoskarn zone, close to or within the causative intrusion, and silver, zinc, and lead are primarily located in the exoskarn, trending toward the distal zones. Manganese occurs at the edges of the skarn-marble contact, while gold distribution within the deposit is variable. The spatial distribution of the minerals is controlled by the tectonic structure and the degree of oxidation of the hydrothermal fluids.
The copper mineral species present in the different zones vary depending on the alteration style that is prevalent. In the BW zone, host to propylitic alteration, the copper mineralization is mostly chalcopyrite and covellite. Copper mineralization present in potassic alteration styles is chalcocite, copper sulphosalts, chalcopyrite, and bornite. The trace metal assemblages also vary depending on the copper minerals present. The BW zone contains moderate amounts of arsenic, but is relatively low in antimony, bismuth and tellurium. In areas of potassic alteration where multiple copper mineral species are present, the amounts of arsenic, antimony, bismuth and tellurium increase.
Other phases of mineralization are evident in the Concepción Del Oro area. Zinc-lead-silver deposits are noted as discrete, relatively small chimney and manto deposits. The zinc-leadsilver mineralization phase is present as both distal deposits relative to the Aranzazu copper mineralization and as late-stage mineralization that postdates and crosscuts the Aranzazu copper mineralization.
Some skarn and intrusive zones contain high concentrations of molybdenum. Often garnet skarn contains several percent of coarse-grained molybdenum, and it is also common in veins which cut skarn and intrusive zones. Molybdenum occurs within endoskarn and exoskarn as well altered granodiorite intrusive in GH zones, though it is not clear from geological relationships where the molybdenum mineralization occurs paragenetically relative to the copper and zinc-lead-silver pulses of mineralization.
Molybdenum content in mineralized zones varies. Average of molybdenum in GHFW is 429 ppm, in GHHW is 92 ppm, and in MX zone is 140 ppm.
Gold mineralization occurs throughout all of the alteration phases previously mentioned with the exception of prograde skarn alteration. Gold grades are generally higher in the phyllic and potassic alteration assemblages compared to propylitic altered rocks.