Summary:
The Silver Sand Property is located in the south section of the polymetallic silver-tin belt in the Eastern Cordillera of the Central Andes, Bolivia. The oldest rocks observed within the Property comprise Ordovician to Silurian marine, clastic sediments which have been intensely folded and faulted.
The Paleozoic basement is unconformably overlain by weakly deformed, lower Cretaceous continental sandstone, siltstone, and mudstone. These Mesozoic rocks form an open syncline that plunges gently to NNW and is bound to the SW and NE by NW trending faults. The unconformity between Mesozoic rocks and deformed Paleozoic basement is observed in the south-east part of the Property.
There is a thrust fault observed on the east side of the Property, north of the Snake Hole prospect, which faults Palaeozoic rocks over the Cretaceous sandstone. The Cretaceous sedimentary sequence within the Property is divided into the lower La Puerta Formation and the upper Tarapaya Formation.
The La Puerta Formation consists of a sequence of mixed aeolian and fluvial sandstones exhibiting distinct massive, bedded, cross-bedded, and bioturbated “streaky” units which unconformably overlies the Paleozoic basement. The Tarapaya Formation conformably overlies the La Puerta sandstones in the central part of the Property and comprises red siltstones and mudstones intercalated with minor sandstone.
Several Miocene aged subvolcanic porphyritic dacite intrusions occur within Cretaceous and Paleozoic sequences. A porphyritic dacite laccolith is exposed overlying the Cretaceous Tarapaya siltstones at the landmark San Cristobal Hill at Mascota located in the approximate centre of the Property. Porphyritic dacite dikes are also exposed in mine workings along the eastern Cretaceous Paleozoic thrust contact. Elongate stocks up to 5 km in length are recorded to the east of the Cretaceous sequence within Paleozoic basement.
A number of andesitic breccias with phreatic, crackle, and other breccia textures are recorded at the Property. A large, oval body of andesitic diatreme breccia cross cutting La Puerta Formation sandstone is seen in outcrop close to the west side of the major Silver Sand mineralization zone in the southern portion of the Property. Geological mapping has defined this zone over an area of approximately 300 m in length and 200 m in width along an NNE orientation. A separate ENE-striking sub-vertical diatreme breccia dike of about 13 m in width is seen in outcrop at Aullagas, central to the Property and about 500 m west of the diatreme outcrop. This unit has welded tuff and sandstone clasts and is cemented by abundant limonite.
Based on structural style (thrust assemblages) and geometric characteristics (listric faults and conjugate steeply dipping faults), the area has been divided into two blocks, North and South Block.
North Block
This block covers an area of approximately 0.6 km2 (0.69 x 0.86 km), characterized by thrust-type pre-mineralization structures and listric faults. Penetrative conjugate fault systems in X (crossing) and Y (abutting) are superimposed on these structures and are the main hosts of mineralization, with preferential attitudes of mineralized structures N12°W/77°SW (major) and N12°W/80°NE (minor).
South Block
This block covers an area of approximately 0.15 km2 (0.30 x 0.5 km), where the first order structure is constituted by the Machacamarca syncline, a cap of the Tarapaya formation (up to 30 m) and a system of penetrative conjugate fault system in X and Y being the main host of mineralization, with preferential attitudes of N20°W/74°SW (major) and N20°W/74°NE (minor).
Mineralization
A total of eleven mineralized prospects have been identified across the Property to date. These include the Silver Sand deposit and the El Fuerte, San Antonio, Aullagas, Snake Hole, Mascota, Esperanza, North Plain, Jisas, Jardan, El Bronce, occurrences. Silver Sand, Snake Hole, Jisas, and El Bronce have been tested by drilling. The other nine prospects were defined by rock chip and grab sampling of ancient and recent artisanal mine workings and dumps. Exploration results from surface outcrops and underground workings defined a silver mineralized belt 7.5 km long and 2 km wide.
Four mineralization styles have been recognized in the Property, and these in order of importance are: (1) sandstone-hosted silver mineralization, (2) porphyritic dacitic-hosted silver mineralization, (3) diatream breccia-hosted silver mineralization, and (4) manto-type tin and base metal mineralization.
Sandstone-hosted silver mineralization
The mineralization in the Silver Sand project comprises silver-containing sulphosalts and sulphides occurring within sheeted veins, stockworks, veinlets, breccia infill, and disseminated within host rocks. The most common silver-bearing minerals include freibergite [(Ag,Cu,Fe)12(Sb,As)4S13], miargyrite [AgSbS2], polybasite [(Ag,Cu)6(Sb,As)2S7] [Ag9CuS4], bournonite [PbCuSbS3] (some lattices of copper may be replaced by silver), andorite [PbAgSb3S6], and boulangerite [Pb5Sb4S11] (some lattices of lead may be replaced by silver). Most silver mineralization is hosted in La Puerta sandstone units with minor amounts in porphyritic dacite diatreme breccia.
The silver mineralization is the majority of mineralization occurring almost exclusively within the Cretaceous-aged red quartz sandstones of La Puerta formation, which demonstrate extensive sericitic alteration (bleaching). This style of mineralization is usually structurally controlled. The intensity of mineralization is dependent on the density of various mineralized vein structures developed in the brittle host rocks.
Sandstone-hosted silver mineralization is recognized at the core area of Silver Sand deposit and in all regional prospects.
At the Silver Sand deposit mineralization has been traced for more than 2,000 m along strike, to a maximum width of about 680 m and a dip extension of more than 250 m.
Porphyritic dacite-hosted silver mineralization
Silver mineralization within porphyritic dacite is observed at the Mascota and El Bronce prospects. These occurrences experienced extensive artisanal mining activities.
Systematic grab sampling on mining dumps has returned silver grade from 50 to 500 grams per tonne (g/t) Ag. The El Bronce zone has been traced with grab sampling results for more than 1,000 m along strike. The zone is defined by silver assays > 50 parts per million (ppm). In the Jardan area north of Jisas, tin mining is also conducted along north-east-trending veins in porphyritic intrusions at Chiaraque.
Drilling at El Bronce prospect intersected porphyritic dacite intervals with moderate oxidation (limonite-jarosite), weak to moderate sericitic alteration, and argillization patches. Moderate to strong pyrite dissemination with stringers of pyrite, unknown fine sulphides, and minor amount of chalcopyrite, sphalerite, boulangerite, and brecciated intervals of limonite and jarosite in oxidized zones also occur.
Diatreme breccia-hosted silver mineralization
Diatreme breccia hosted silver mineralization is observed in the Aullagas zone. Based on surface mapping, the Aullagas zone occurs within a north-east-trending dike-like breccia body of about 40 m in length and 13 m width, hosted by bleached sandstone. Breccia fragments consist of ignimbrite and sandstone clasts cemented with highly ferruginous material. Surface grab samples have returned silver grades from 50 to 298 g/t Ag. Further investigation is needed to define the size and potential of the diatreme breccia.
Manto-type tin mineralization
Manto-type tin mineralization on the Property occurs as metasomatic replacement of the calcareous horizons in the siltstone and mudstone at the base of the Tarapaya Formation. Very fine-grained cassiterite is accompanied by abundant pyrite and lesser ankerite, siderite, and barite in the stratiform manto.