Summary:
Deposit Types
The Santo Tomás exploration programs have primarily focused on Cu-Mo-Au-Ag porphyry deposit types. Herein, exploration recommendations continue with this deposit-type target.
Santo Tomás Mineralization
Mineralization at Santo Tomás is developed along a 5 km strike length below a cap of Cretaceous limestone and later felsic volcanics. Erosion has exposed the mineralization along the Santo Tomás Ridge with the main area of surface exposure in the North Zone on the east flank of the ridge, above the Río Fuerte. Mineralization is mostly continuous along strike, but a lower-grade area separates the North Zone and South Zone and the Río Fuerte separate the North Zone from Brasiles area. Indications are that mineralisation extends down dip until it intersects the Granodiorite (GD) as either an intrusive contact or a faut bounded contact, as interpreted in the 3D geological model. The North Zone lies on the eastern flank of the prominent N-S trending Santo Tomás Ridge whereas the South Zone lies on western flank. The main mineralized zone varies in thickness between 100 to 400 m (locally 600 m) in true thickness and dips moderately to the west at 50° in the North Zone. Similar moderate angle dips are apparent in the southerly portion of the South Zone where mineralization dips sub-parallel, or slightly steeper than, the west-facing slope of the Santo Tomás Ridge. The moderate westerly dip of the mineralized zone is cut by later post mineralization faults faulting along the western fault zone.
Santo Tomás Cu (Mo-Au-Ag) mineralization is characterized by copper porphyry and skarn/replacement style mineralization linked to the Laramide orogeny (80-40 Ma age). Technical reports published by Bridge (2020) and the MRE (Ausenco, 2023) reported a K-Ar age of 57.2 ± 1.2 Ma for a quartz-monzonite. This age was originally reported by Damon et al., (1983), relating it to the age of the mineralization. This age is likely from a sample of an intrusive collected from a location not consistent with the Santo Tomás mineralization. To develop confidence in the mineralization age and genesis, Oroco undertook a comprehensive sampling program to develop a set of precise ages for the mineralization, and the intrusive and extrusive lithologies on the Property. Results from the Re-Os dating, define the Santo Tomás mineralization at between 76 and 72 Ma, coeval to the QMhbl and MZ/QM intrusions, which yielded U-Pb ages between 75.11 and 64.99 Ma, contemporaneous with the first generation (76?74 Ma) of copper porphyries within the southern North American Cordillera belt (Lazcano et al., 2023), and other associated Laramide- age deposits.
At Santo Tomás Laramide intrusions were emplaced into Jurassic-Cretaceous strata comprising metamorphosed andesite, limestone, and minor argillaceous and clastic units. Mineralization is strongly structurally controlled and associated with the Laramide-age Santo Tomás fault and fracture zone, interpreted as an early-stage structural zone which provided the pathway for the quartz-monzonite dyke swarm and related hydrothermal alteration, hydrothermal breccias, and sulphide mineralization. This early-stage structural zone can be mapped as a well-defined fracture zone (Mx) of sheeted quartz-monzonite dykes, and screens of highly fractured, hornfelsed andesite, and lesser limestone. Mineralization distributed along this zone forms a tabular NW dipping zone primarily defined by finely disseminated sulphides and fracture-fillings with subordinate sulphides hosted in stockwork quartz veinlets. Sulphides are dominated by pyrite-chalcopyrite–(molybdenite) and are distributed in altered quartz-monzonite and altered andesite. Minor mineralization is associated with skarn and replacement-style mineralization in the hanging wall limestone.
The mineralogy comprises chalcopyrite, pyrite and molybdenite with minor bornite, covellite, and chalcocite. Sulphides occur as fracture fillings, veinlets, and fine disseminations together with potassium feldspar, quartz, calcite, chlorite, and locally, tourmaline. Minor copper oxides occur near surface. Chalcopyrite is the main copper mineral associated with the altered monzonitic intrusives and andesites. Chalcopyrite occurs with pyrite both as fine-grained disseminations and in 1 mm to 2-3 mm microfractures associated with quartz and potassium feldspar, and locally tourmaline. Chlorite and magnetite may also present. Oroco developed a vein classification system to characterize the complex vein system and along with hydrothermal alteration, develop vectors to higher-grade mineralized zones, associated with potassic and phyllic (QSP) alteration. Mineralization is closely related to extant “white micas” and the zones of pervasive A and B-type veining with a sulphide assemblage of chalcopyrite-molybdenite and minor bornite. Increased vein thickness, up to 2.5 cm, is typically also accompanied by higher grades. B-type veinlets are the dominant Cu-Mo mineralization microstructures at Santo Tomás, hosted within altered quartz-monzonite and andesite. Proximal B-type veins are typified by quartz and chalcopyrite with molybdenite developed as a thin vein selvage or along the vein suture and a K-feldspar and illite alteration halo. Distal veins are typified by quartz with molybdenitechalcopyrite and pyrite and an illite and green sericitic alteration halo.