Summary:
Regional Geology
The Coeur d’Alene Mining District is hosted in Precambrian (approximately 1.45 billion years old) metasedimentary rocks of the Belt Supergroup. For silver mineralization targeting, rocks of the Burke, Revett, and St. Regis Formations are prospective and belong to the Ravalli Group within the Belt Supergroup. These Middle Proterozoic rocks cover a large area of northern Idaho and western Montana with up to a 12.5-mile (20-km) thick layer of fine-grained siliciclastic strata. The Sunshine Mine and other Silver Valley deposits occur between the Osburn and Placer Creek faults that are significant regional-scale, east-to-west structures. The regional continuity of the Idaho Silver Belt mineralized system occurs along a strike length of over 20 miles.
Local and Property Geology
The Sunshine Mine is predominantly hosted in the St. Regis Formation, which is over 600 ft thick, and upper strata of the underlying Revett Formation. The lithostratigraphic boundary between these units is unclear. Rock types in the St. Regis are mainly argillite and siltite, which grade to siltite and quartzite in the Revett Formation. Both host units are intensely folded and faulted and metamorphosed to lowgrade, greenschist facies.
The Project area is bisected by several east-to-west faults, namely Polaris, Syndicate, C Fault, and, further south, the Alhambra Fault. Kinematics and rock fabric in the mine are reported to show dip-slip movement on the faults, even though the regional structural setting suggests that movement was strike-slip. Polaris is a normal fault, while the remainder have reverse displacement. The faults at Sunshine are variably mineralized. The C Fault is an example of a well-mineralized structure.
Dominant veins in the mine strike generally east-to-west between the faults and dip steeply (>60°) to the south. Subordinate veins are interpreted to crosscut between the major veins. The larger vein structures are quite extensive and can be traced over long strike distances and depths. Generally, mineralized veins vary between 1 to 5 ft thick with thicknesses pinching and swelling along strike. The strike length of individual veins has been tested up to 2.5 miles in length. Veins at Sunshine can continue from surface to over a mile deep.
Significant Mineralized Zones
Over 35 veins have been named and mined at the Sunshine Mine. Historically, mined grades are exceptionally high in some areas, with averages over 100 opt Ag. The Sunshine and Chester Veins are particularly well endowed, with each reported to have produced over 90 million (M) oz of silver to date. Mineralization is comprised of tetrahedrite, galena, and sphalerite, with typical gangue minerals of siderite, quartz, pyrite, and magnetite. Similar to other deposits in the Idaho Silver Valley, two main vein assemblages are distinguished, which tend to dominate certain areas of the mine: silver-copper veins and lead-zinc veins.
Deposit Type
Mineral Deposit
The deposits of the Coeur d’Alene District, including Sunshine, are classified as clastic metasedimenthosted, silver-lead-zinc mesothermal vein deposits. In addition to Coeur d’Alene, a world-class silver district, this deposit type includes several historical mining localities globally, including the Harz Mountains and Freiberg in Germany, Keno Hill and Kokanee Range in Canada, and Príbram in the Czech Republic. These deposits are typified by the following general characteristics:
• Deposits are hosted in thick sequences of fine- to medium-grained clastic sedimentary rocks transected by deep-seated regional-scale faulting.
• Sedimentary basins occur in a wide range of tectonic environments, but all have been subject to deformation, intrusion, and regional metamorphism, typically greenschist facies.
• Economic minerals are predominantly galena and sphalerite with minor accessory pyrite and a wide range of sulphosalt minerals, including tetrahedrite, pyrargyrite, stephanite, bournonite, acanthite, and native silver.
• Gangue minerals are comprised of siderite and quartz, with lesser amounts of dolomite or calcite.
• Temperature of sulfide mineral deposition is in the range of 250 degrees Celsius (°C) to 300°C.
It is generally accepted that the veins of the Coeur d’Alene District were formed during the Cretaceous to early Tertiary. Genesis of the orebodies may have been a result of regional-scale metamorphism and the development of hydrothermal systems associated with the emplacement of the Idaho Batholith pluton and concurrent deformation. Metamorphic hydrothermal fluids most likely scavenged syngenetic metals (silver, lead, zinc, and copper) from Proterozoic Belt Supergroup strata and emplaced these metals within pre-existing or concurrent structural features.
Geological Model
The signature for all economic deposits discovered within the Coeur d’Alene District is vein-like morphology hosted within the metasediments of the Belt Super Group. Within the Sunshine Mine, as well as other sub-districts in the Coeur d’Alene District, veins occur as branching fissures that crosscut the sedimentary host rocks. Previous studies have indicated the veins are of mesothermal origin.
The vein structures are known to branch or split, forming duplexing, and have anastomosing geometries. The majority of veins strike west, are steeply dipping, elongated down-dip, and can have strike lengths over 4,000 ft and dip lengths over 8,000 ft.
SRK incorporated Sunshine-provided geologic interpretations from the company’s internal experts, regarding the trends of vein domains and mineralization continuity. The QP for mineral resources considers the current geological model to be sufficient for conceptual exploration targeting, geological modeling, and resource estimation of the Sunshine deposit.