Summary:
From 2010-2022 Southern Silver completed programs of geological mapping, surface geochemical sampling and airborne and ground geophysical surveys in support of 93,494 m of core drilling in 218 holes resulting in the delineation of seven mineral deposits: the Blind; El Sol, Las Victorias, Skarn Front, South Skarn, North Felsite and Mina La Bocona deposits on the Cerro Las Minitas Property.
The Blind, El Sol and Las Victorias deposits comprise multiple sub-vertical northwest-southeast trending zones of semimassive to massive sulphide mineralization. Mineralization is hosted in the skarnoid- and hornfels-altered margins of monzonite and felsite dykes and may be localized along through-going structures or occur as replacements within stratigraphic units. The mineralized zones can be traced for up to 1000 m along strike and up to 800 m down dip.
Sulphide mineralization in the Skarn Front deposit is localized at the outer boundary of the skarnoid alteration zone surrounding the Central Monzonite Intrusion at or near the transition to the recrystallized/marbleized carbonate sediments (marmorized zone). The Skarn Front deposit can be traced for up to 1,100 m along strike and up to 1,000 metres depth. Geological modelling suggests that intersections between the sub-vertical, northwest-trending Blind and El Sol mineralized zones and the generally more shallowly dipping Skarn Front may localize higher-grade shoots of mineralization.
Drilling in 2020-22 confirmed laterally extensive skarnoid-style mineralization within the altered halo around the central intrusion in the South Skarn, La Bocona and North Felsite deposits. Mineralization occurs adjacent to the central intrusion and features similar replacement styles as is observed in the Skarn Front deposit. The skarnoid-style mineralization in the La Bocona and South Skarn deposits show a similar variability in metal assemblages as is identified in the Skarn Front deposit, but tends to be more galena biased and is generally associated with elevated silver values when compared to the Skarn Front mineralization. Mineralization in the North Felsite also show a similar variability in metal assemblage, but drilling to date has not been sufficient to define the mineral zonation accurately.
Drilling also identified manto-styled mineralization within the La Bocona deposit which occurs as replacements in the hanging wall of the skarnoid mineralized zone within variably altered marble-skarn-hornfels. The mineralization is strongly silver enriched with elevated lead, arsenic, and gold values. The upper portion of the mineralized zone is strongly oxidized and makes up in part the small oxide resource identified in the current mineral resource update.
Mineralization discovered to date on the Cerro Las Minitas project can be broadly classified into two genetically related deposit types; Skarn and Carbonate Replacement Deposits.
Mineralization
To date, mineralization at Cerro Las Minitas has been classified into four types based on surface and underground field observations and the examination of drill core. Although production records from the area are incomplete, sufficient sampling of core dumps, underground exposures and drill core has been completed to estimate typical grades in each of the four deposit types: skarnoid, chimney, manto, and dyke margin.
Skarnoid
Skarnoid is a descriptive term for calc-silicate rocks which are relatively fine-grained, iron-poor, and which reflect, at least in part, the compositional control of the protolith (Korzkinskii, 1948; Zharikov, 1970). Genetically, skarnoid is intermediate between a purely metamorphic hornfels and a purely metasomatic, coarse-grained skarn.
At Cerro Las Minitas, contact metasomatic gold, silver, zinc, lead and copper mineralization formed within the altered sediments adjacent to contacts with the central intrusive complex or larger dykes. These deposits are characterized by substantial pyrite content, higher copper content, zinc levels that are greater than lead levels, and sphalerite with high iron content. The deposits have been exploited mainly for silver, zinc, lead, and copper by artisanal miners at Cerro Las Minitas, especially in the Santo Niño-Puro Corazón area. The deposits occur as massive replacements of remnant carbonate bodies and disseminated calcite present in the garnet-wollastonite-pyroxene-epidote skarnoid assemblage. The mined bodies were variable in form and distribution. Typical grades in the skarnoid mineralization were 80–300 g/t silver, 2–8% zinc, 2–4% lead, and 0.5–2% copper. Characteristics of this style of mineralization suggest that it is properly classified as zinc skarn (Megaw, 1998).
Chimney
Pipe-like bodies of massive to semi-massive zinc, copper, and lead sulfides, often with high silver values, that have been found in and near the intersection of high-angle mineralized structures and the more moderately dipping skarnoid zone. These produced the richer mineralization in the Santo Niño-Puro Corazón area. Mineralogically, these deposits show characteristics of both the skarnoid and manto styles of mineralization and are believed to have been formed by multiple mineralizing events. The mineralization consisted mainly of massive to semi-massive aggregates of pyrite, sphalerite, galena, chalcopyrite and bornite replacing recrystallized calcite or filling open spaces. Typical grades in the chimneys were 200–400 g/t silver, 2–10% zinc, 2–6% lead, and 0.5–1.5% copper.
Portions of the newly identified hangingwall lens of the South Skarn deposit has similarity with the chimney-style deposits. Mineralization occurs at or just outboard of the lower Skarnoid alteration zone and is characterized by both replacement textures and mineralized hydrothermal breccias.
Manto
Manto-style silver, lead, zinc, and copper deposits form as replacements of carbonate strata peripheral to or outside of the skarnoid aureole. The deposits are typically restricted to selected carbonate strata (favourable beds) that have been replaced by massive to semi-massive lead and zinc sulfides with accessory pyrite, and small amounts of copper sulfides. Drilling in 2020-21 identified manto-styled mineralization within an area of the La Bocona deposit known as the Muralla Chimney. Mineralization occurs as replacements in the hanging wall of the skarnoid mineralized zone within variably altered marble-skarn-hornfels. The thickest zone of sulphide mineralization occurs in the footwall of a thick monzonite dyke. The mineralization is strongly silver-enriched with elevated lead, arsenic and gold values. The upper portion of the mineralized zone is strongly oxidized and makes up in part the small oxide resource identified in the current mineral resource update.
Dyke Margin
Replacement mineralization located alongside dykes of various compositions outside the skarnoid aureole of the central intrusive complex. Massive to disseminated sulfides of lead, zinc, and copper are seen replacing carbonate and carbonate-bearing rocks, with or without associated skarnoid alteration. This is a dominant style of mineralization occurring with the Blind, El Sol and Las Victorias deposits.
Of these four deposit types, the skarnoid and chimney deposits have been reported to have produced the bulk of mineralization exploited in the district and such observation appears to be born out in the most recent drilling and resource modelling by Southern Silver.