Summary:
Lode gold deposits at the San Albino project are largely hosted in shallow- to moderate-dipping, northeaststriking quartz veins and thin vein margins concordant with the metamorphic fabric developed in carbonaceous schist (English, 2009). The gold-bearing quartz veins dip to the west and appear to be localized in zones that show greater degrees of strain than surrounding argillite, suggesting these dominant veins are shear-parallel veins (or, “shear veins”), although the similarity in footwall and hanging wall rocks suggests modest displacement.
The mineralization in the San Albino project area is best interpreted in the context of an “orogenic gold” deposit model (e.g., Goldfarb and Groves, 2015) based on the association of gold mineralization with metamorphic host rocks, and the textures and mineralogy of the San Albino and Las Conchitas veins, the wallrock alteration, and the “gold-only” character of mineralization. Orogenic gold deposits are fundamentally linked with orogenesis, or collisional plate tectonic settings. Unlike many other base- and precious-metal deposit types that formed in the shallow crust, orogenic gold deposits have formed over great intervals of depth. As such, orogenic gold deposits vary widely in characteristics that reflect variable formation depth and therefore, variable degree of metamorphism.
San Albino Deposit
Mining at the historical San Albino mine occurred on three separate vein systems: Arras, Naranjo, and San Albino. Subsequent exploration drilling and mine development by Mako confirmed that the three gold-bearing vein systems exhibit down dip continuity and comprise a stacked set of subparallel veins, with a regular spacing of about 90m between the veins. The shear-hosted quartz veins dip on average about 40° northwest, but dips range from nearly flat to about 60°. Veins pinch-and-swell (i.e., boudinage) in both their dip and strike directions but, commonly, thicker shear-hosted vein intervals and higher-grade gold in workings of the historical San Albino mine appear to be in “flats” flanking antiformal fold hinges or “crests” in the foliation.
Much thinner veins, commonly with steeper orientations that cut foliation, occur in the footwall and hanging wall of the principal shear-hosted veins. These gold-bearing veins are consistent with extensional veins. Extensional veins or veinlets commonly occur proximal to the shear-hosted veins, and they may also form late brittle quartz-filled fractures within the shear veins themselves. At San Albino, such extensional veins are developed mainly within a 1.0m rind adjacent to the primary shear-hosted veins; the extensional veins cut foliation and thereby exhibit greater degrees of folding.
The principal components of the San Albino deposit are the San Albino, Naranjo, and Arras quartz veins, as well as the smaller El Jobo vein. Quartz veins at the San Albino deposit have distinctive characteristics according to Gray (2019), who distinguished six vein types based on their mineralogy, texture, and geographic location. Observations made during the March 2023 review led to a re-assessment of conclusions reached in 2019 about vein styles and mineralization events. New observations confirm that contrasting textural, mineralogic, and geochemical characteristics are evidence of multiple vein forming events, of which two types at the San Albino deposit are gold mineralized and of significance to the mineral resource (Gray, 2023):
1. Quartz Sulfide (“QS”) veins: multi-stage crack-seal veins, gold and silver mineralized, late syndeformation, weakly deformed, concordant, banded, styolitic. San Albino is archetype and Arras and Naranjo belong to this vein type. These are early stage compact quartz veins described as mottled greywhite, with color variation sometimes defining weak banding; styolitic banding or ribbon vein texture is almost always present, most commonly at margins; visible gold is common as submillimeter scale blebs in quartz not associated with microfractures or sulfides. If not overprinted by the later Sulfide Veins and Stringers (“SVS”) event, the QS veins have relatively low sulfide content, Au:Ag ratio of approximately 1:1, with minor As. In general, the QS veins are milky white but vary based on the degree of deformation observed, sulfide and carbonate contents, degree of banding, and gold contents. In addition to diagnostic milky quartz, the QS veins commonly contain ankerite and/or siderite, possibly albite, and variable amounts of sulfide minerals up to 3% by volume. The sulfide grains are generally paragenetically late and consist of pyrite, arsenopyrite, galena, and sphalerite, which form weblike veinlets and clots as well as more regular bands within milky quartz.
2. Sulfide Veins and Stringer (“SVS”) comprises later Au-Ag-Pb-(Zn) mineralization. SVS occurs as sulfide bands, stringers, and impregnations in fractured, earlier-stage QS quartz veins; as sulfide impregnations in breccia and gouge zones in hanging wall (most often) and footwall (not common) in tectonized vein zones sometimes without appreciable quartz veining; and as sulfide bands or veins; relatively Ag rich, Au:Ag <1. Visible gold may be present as discrete blebs contained in quartz or less commonly in sulfides.
The supermajority of defined mineral resources at San Albino project are related to QS veins and SVS mineralization. A small but unquantified contribution of gold to the mineral inventory likely comes from local, discontinuous, discordant veinlets in the immediate margins of the QA and SVS vein zones.
Las Conchitas Deposit
Las Conchitas is situated between two past-producers, the San Albino Mine and the El Golfo Mine. It covers approximately 3.75 km² and is 2 km south of the San Albino Mine, and immediately to the north of the historical El Golfo Mine that is within the Company's El Jicaro Concession.
Las Conchitas contains numerous mineralized structures over a 1,700 m by 800 m area, which has been subdivided into three primary areas: Las Conchitas Norte, Las Conchitas Central and Las Conchitas Sur. Each area features multiple subparallel, northeast-southwest striking and gently dipping mineralized veins.
As with the San Albino gold deposit, the conceptual model for the Las Conchitas mineralization consists of multiple parallel quartz veins that dip gently to the northwest, associated with extensive shear and fault systems which represent possible feeders for mineralized fluids and a favorable environment for precious metal deposition. These characteristics are consistent with the model for orogenic gold-bearing veins, which can extend to depths in excess of a kilometer. Drilling at Las Conchitas has confirmed the down-dip continuity of highly mineralized zones; as demonstrated by drill results reported on July 29, 2024; gold mineralization is not restricted solely to quartz veins, but can also occur in the host rock (phyllite/schist) containing quartz vein.