Summary:
The mineralization in the San Albino project area is best interpreted in the context of an “orogenic gold” deposit model based on the association of gold mineralization with metamorphic host rocks, and the textures and mineralogy of the San Albino veins, the wallrock alteration, and the “gold-only” character of mineralization.
The principal commodity exploited in the historical San Albino and nearby mines was gold, which was extracted from both placer workings and quartz-bearing lodes sporadically since the Spanish Colonial period (Roberts and Irving, 1957). Silver accompanies gold at the San Albino Deposit, but its economic significance is relatively low.
Lode gold deposits at the San Albino project are largely hosted in shallow- to moderate-dipping, northeast-striking 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.
Mining at the historical San Albino mine occurred on three separate vein systems: Arras, Naranjo, and San Albino. Subsequent exploration drilling 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. In general, veins are distinctively milky white but vary based on the degree of deformation observed, sulfide and carbonate contents, degree of banding, and gold contents. Many veins exhibit complex textures that indicate multiple stages of quartz deposition, internal deformation, and mineralization. Thus, an individual vein may possess characteristics of more than one vein type. In addition to diagnostic milky quartz, the San Albino 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. Visible gold and higher gold contents in assayed samples typically occur in zones containing greater sulfide contents. Sulfide banding may result from the concentration of late sulfides along quartz vein ‘ribbons’, which reflect early crackseal textures associated with vein growth. Such ribbon quartz is commonly defined by parallel bands of carbonaceous wall rock.
Wallrock hydrothermal alteration associated with gold-bearing veins at San Albino is subtle. In saprolite, removal of carbon from argillite reveals abundant muscovite and minor chlorite (English, 2009) together with pyrite. However, it is unclear to what degree this alteration reflects primary or diagenetic effects, metamorphism, or late hydrothermal fluids. Below the weathered zone, wall rocks immediately adjacent to gold-bearing veins at the San Albino Deposit have bleaching interpreted to be sericite alteration typical of many gold-bearing quartz vein-hosted systems in metamorphic rocks.
In summary, the following conclusions are drawn regarding the gold mineralization at the San Albino Deposit based upon descriptions of the mineralized material, vein geometries, textures, and ore minerals:
• Gold-bearing quartz veins were emplaced into shears (i.e., shears defined as brittle-ductile faults associated with metamorphism and contractional deformation);
• Shears and their enclosed veins have corrugated or anastomosing geometries, and typically veins are pinch-and-swell type, with thicker intervals in “flats”;
• Milky colored, massive-textured veins are characteristic, most of which are concordant with foliation in enclosing metamorphic rocks;
• “Ribbon quartz” is common and reflects shear-parallel crack-seal replacement processes wherein wallrock is incorporated during vein growth;
• Visible gold is present in late brittle fractures; and
• Gold occurs in association with generally low amounts (up to 3%) of pyrite, arsenopyrite, galena, and sphalerite in quartz veins.
The mean gold concentration (4,474 individual samples with >200ppb Au) from Mako’s San Albino verified drill holes at the San Albino Deposit is 7.16 Au g/t whereas the mean silver is 14.3 Ag g/t, yielding a relatively high Au:Ag ratio of 0.5. Copper in chalcopyrite was reported in the deeper workings of the historical San Albino mine, but no production records for copper are known (Roberts and Irving, 1957). The mean values of other metals from drilling at the San Albino Deposit from Mako’s drilling database (for Au>200ppb) are 95ppm Cu, 2,296ppm Pb, and 623ppm Zn. From composited drill samples wherein lead concentration was >500ppm, the average contents of metals and semi-metals are: 4,365ppm As, 0.8ppm Hg, 6.6ppm Sb, and 3.1ppm Tl. Gold and silver correlate moderately. Silver also correlates moderately with lead, and zinc correlates poorly with the other metals. These data support mineralogic information from drill hole core logging, surface sampling, and mining at the historical San Albino mine that indicate visible gold, probably as electrum, most commonly occurs in milky quartz veins in spatial association with pyrite, arsenopyrite, galena, and sphalerite. The moderate to poor inter-element correlations further suggest that although gold is spatially correlated with silver, lead, and zinc, the metals were partitioned differently among various ore minerals and therefore were deposited in differing amounts during more than one stage of mineralization.
The low- and moderate-angle faults control the distribution of gold-bearing quartz veins. At the San Albino Deposit, quartz vein-bearing shear zones up to several meters thick are stacked in subparallel fashion (e.g., San Albino, Naranjo, and Arras). The separation between shears and the enclosed veins averages about 90m. The shear-related veins and their enclosing faults have anastomosing, pinch-and-swell geometries.
High-angle faults that cut gold-bearing veins were summarized in English (2009) as having normal slip, with strikes that either are subparallel with, or nearly orthogonal to, earlier-formed low- to moderate-angle shear-hosted veins. The northeast-striking set of high-angle faults may also contain quartz veins and these possibly reflect extensional veins between shear strands.