Summary:
Deposit Types
The Concepción 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. The Aranzazu skarn deposit is considered to have been generated by replacement of receptive limestone beds in proximity to the early porphyry intrusions, which were emplaced on the eastern edge of the Concepción del Oro stock. Skarn prograde alteration is barren unless the skarn is overprinted by the later porphyry style alteration marked by propylitic, phyllic, and potassic alteration types.
The deposit is a skarn/sediment replacement (manto).
The sheeted veinlet zone in the immediate footwall of the skarn suggests that porphyry emplacement was structurally controlled, perhaps by a reverse fault that breached the northwest-trending anticlinal axis (Sillitoe, 2019).
Weak potassic alteration and the contained EDM and A- and B-type quartz veinlets constitute porphyry copper mineralization in the early porphyry and flanking porphyritic quartz monzonite intrusions. They are the temporal equivalents of the prograde skarn, although the B-type veinlets also clearly cut garnet skarn in places.
Trace metal geochemistry supports the classification of Concepción 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.
In Aranzazu, geochemical zonation at the deposit scale shows the typical distribution of proximal copper and molybdenum in the deeper feeder zones, and zinc, lead and silver towards the distal peripheries, both longitudinally and laterally. Relatively high arsenic levels are consistent throughout the district and are particularly high in the peripheral zone like Cabrestante zones southeast of the Aranzazu Property.
The assays of Pb-Zn-Ag define a clear peripheral zonation surrounding the copper orebodies of the district. Southeast of the Cabrestante shaft there are numerous small workings that were developed in small pockets of high Ag-Pb-Zn which seem to reflect lateral zonation from the Cabrestante copper- rich zone. The north and south flanks of the Cabrestante-El Diamante-El Roble corridor northeast of Concepción del Oro town has numerous showings along small occurrences (mantos, veins, dike contacts) which exhibit gossan mineralization with high values of Zn-Pb-Ag-As. These small occurrences could reflect the periphery of possible copper deposits closer to the intrusive contacts under cover in the corridor.
Multi-elemental zoning in El Cobre mine is based only on the new sampling program carried out in 2013 (Albinson et al.). The results show that Ag, Pb, Zn, and as increase strongly in the peripheral narrow structures of the Conejos zone.
Zonation of zinc with respect to copper, is clearly evident in the Palomas deposits which represents the lateral zonation to the west of the copper-rich Mexicana and BW orebodies. It is also possible the zinc-rich Palomas zone could have an underlying copper-rich zone.
Molybdenum generally reports assays of interesting grade in the active Aranzazu Mine. The geochemical zonation of molybdenum in the BW to Cabrestante orebodies shows values more than 500 ppm in the intermediate and deep portions of the deep GHP-FW orebody and the northwest side of the Mexicana orebody. It is possible the deeper portions of the copper-rich orebodies average enough grade of molybdenum to justify economic recovery of this metal.
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 formation of the Aranzazu skarns follow a general worldwide paragenesis stages:
• Stage 1. Contact Metamorphism: The original limestones were transformed into hornfels and marble.
• Stage 2. Prograde Skarn: Minerals such as garnet and pyroxene formed (Endoskarn).
• Stage 3. Retrograde A: The precipitation of amphibole, magnetite, specular hematite, mushketovite, epidote, vesuvianite, and quartz.
• Stage 4. Retrograde B: Sulphides of copper, gold, and silver are formed, consolidating the economic value of the deposit. Also, chlorite, calcite and quartz.
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 hi