Geology And Mineralization
During the Middle to Late Proterozoic, the area was underlain by uplifted basement rocks. These units were eroding into the Belt Basin, a large intra-cratonic basin located to the north. The tectonics are believed to have remained relatively stable until approximately 1,350Ma, when portions of the region were affected by compressional tectonics of the East Kootenay Orogeny. This orogeny was followed by rifting during the late Proterozoic-early Paleozoic when large portions of the Belt sediments were transported away and the western margin of North America was developed. Over the next several hundreds of millions of years, the area comprised the passive margin of the North American Craton.
The next major tectonic event occurred during the Cretaceous Sevier Orogeny. Early compressional tectonics dominated the area forming large-scale folds, reverse and thrust faults. Many of these structures were focused along the Salmon River Suture Zone (Figure 5-1), a wide, but linear, north-south trending zone of complexly deformed rocks of highly variable composition. It marks a major structural and crustal boundary approximately 95km to the west of the Project area. Rocks within the suture zone include volcanic and volcaniclastic rocks associated with the Blue Mountains Volcanic arc, sedimentary clastic rocks of the North American Continental Province, and the western margin of the Idaho Batholith. The suture zone is interpreted as the zone along which the oceanic Blue Mountains Volcanic-arc was accreted to the North American continent.
During the late Cretaceous, the Atlanta Lobe of the Idaho Batholith was emplaced throughout the area. The western margin of the Idaho Batholith is metamorphosed and foliated parallel to the suture zone, which indicates that it was emplaced while the suture zone was still tectonically active. The Atlanta Lobe consists of various types of igneous intrusive rocks, including early tonalite and foliated granodiorite (95 to 85Ma), porphyritic biotite granodiorite (88 to 82Ma), and late biotite-muscovite granite (78 to 72Ma),. The project area is located near the western margin of the Atlanta Lobe, which is composed of early tonalite and foliated granodiorite. The central part is composed of porphyritic biotite granodiorite and late granites are relatively common near its northern and eastern margins. Metamorphic roof pendants, inclusions, and hydrothermal mineral deposits also are relatively common near the northern and eastern margins of the Atlanta Lobe. Isotopic age determinations on late biotite-muscovite granite have about the same range (78 to 72Ma) as age determinations on sericite from batholith hosted veins and silica-sericite altered rocks (mostly 79 to 69 Ma). This suggests that most batholith-hosted mineralization is related to intrusions of late two-mica granite of the Idaho Batholith. That granite is peraluminous in composition and is interpreted here as an S-type granite, generated largely by partial melting of continental crustal metasedimentary rocks.
Eocene Intrusions related to the Challis Volcanics Field are common near the eastern margin of the Atlanta Lobe. These include dikes, dike swarms, and stocks. The intrusions generally are porphyritic in texture and intermediate to felsic in composition. They range in age from 51 to 39Ma. The Thunder Mountain Caldera Complex of the Challis Volcanic Field lies immediately east of the project area. It consists of predominantly felsic volcanic, pyroclastic and epiclastic rocks that were erupted and deposited in subaerial and lacustrine environments during Eocene time (51 to 43Ma). Several, hot-spring type gold-silver deposits occur in the Thunder Mountain district.
Pleistocene valley glaciers have carved U-shaped valleys with over-steepened, talus-covered sides, and hanging valley tributaries with cirques and tarns in their upper reaches. U-shaped valleys also have lateral, terminal and recessional moraines, moraine-dammed lakes, and glacial outwash deposits at their lower ends. Broadly glaciated areas have rounded hills with glacially scraped and scoured up-glacier slopes, ground-moraine covered down-glacier slopes. Early Pleistocene moraines contain thoroughly decomposed boulders, whereas boulders in Middle Pleistocene moraines have weathered rinds, and boulders of Late Pleistocene moraines are relatively un-weathered.
Bedrock in the District consists of granitic rocks of the Cretaceous-age Idaho Batholith and a roof pendant of meta-sedimentary rocks dominated by carbonates and quartzites. Much of the District's valley area is covered with unconsolidated alluvial and glacial deposits.
Widespread precious metal and antimony mineralization are related to igneous activity and mineralization is localized along north and northeast trending structures, which are believed to be related to the nearby Tertiary-age Thunder Mountain caldera complex.
Much of this mineralization has been extensively drilled by previous operators during early stage exploration and development as well as part of past mining operations focused on the previously exploited leachable oxide ores. Past work includes over 151,000 meters of percussion, rotary, reverse circulation and core drilling in over 2140 holes. Recent exploration in the district by Midas has discovered significant quantities of gold and silver mineralization.
Hangar Flats Deposit
Mineralization associated with the Hanger Flats Deposit has a primary structural control. High angle, brittle fault structures have formed conduits that provided access to receptive intrusive rocks for gold-bearing hydrothermal fluids. The Meadow Creek fault zone and adjacent quartz monzonite units are the primary hosts of the gold mineralization at the Hanger Flats Deposit. The Meadow Creek fault zone consists of two, major high angle faults, both striking north-south and dipping 80 to 85° west. The width of the fault zone, as measured between the eastern and western confining faults, varies from 50 to 150m. Several subsidiary structures extend northeastward from the Meadow Creek fault zone. These include the Sawyer fault, Leonard fault and related structures. The subsidiary structures strike along azimuth 45° and dip 20-35° northwest.
Gold mineralization occurs preferentially where the Meadow Creek Fault intersects the subsidiary faults. Quartz veins are closely associated with the faults and are indicative of higher-grade zones of mineralization. Overall, the resource area has a shallow level of oxidation preserved only in the elevated portions of the Meadow Creek fault zone.
West End Deposit
Mineralization associated with the West End Deposit has a primary structural control and a secondary lithologic control. High angle, brittle fault structures have formed conduits that provided access to receptive sedimentary rocks for gold-bearing hydrothermal fluids. The West End fault zone and adjacent calc-silicate units are the primary hosts of the gold mineralization at the West End Deposit. The West End fault zone consists of three, major high angle faults, all striking at azimuth 30° and dipping 50 to 75° southeast. The West End faults are referred to as the Hanging Wall, Middle and Footwall Faults. The width of the fault zone, as measured between the Footwall and the Hanging Wall faults, varies from 30-150m. Several subsidiary structures extend southeast from the West End fault zone. These include the Splay fault, Stibnite fault and related structures. The subsidiary structures strike along azimuth 60 to 90° and dip steeply north and south. Based on the relative offsets of the metasediments, the West End faults have experienced, right lateral and/or normal offset. The calc-silicate units are referred to as the Upper Calc-silicate and the Lower Calc-silicate. The units are nearly identical in composition and distinguished only by their relative stratigraphic positions. Gold mineralization occurs preferentially where either calc-silicate unit is cut by any of the West End or subsidiary faults. The hanging wall fault tends to exhibit relatively more dilatant and dispersed structures relative to the lower two faults and therefore has stronger mineralization. Open space fill quartz veins are closely associated with the faults and are indicative of higher grade zones of mineralization. Overall, the resource area has a moderately deep level of oxidation focused about the West End fault zone. The materials have been defined as an upper oxide zone and a lower sulfide zone.
Yellow Pine Deposit
Mineralization associated with the Yellow Pine deposit has a primary structural control. High angle, brittle fault structures have formed conduits that provided access to receptive intrusive rocks for gold-bearing hydrothermal fluids. The Meadow Creek fault zone and adjacent quartz monzonite units are the primary hosts of the gold mineralization at the Yellow Pine deposit. The Meadow Creek fault zone consists of two major high angle faults, both striking north-northeast and dipping 80 to 85° northwest on the southeast side and 80 to 85° southeast on the northwest side, creating a wedge shaped geometry. The width of the fault zone, as measured between the eastern and western confining faults, varies from 50 to 190 m.
Gold mineralization occurs preferentially where the MCFZ bends from north to northeast striking and, where north-striking faults have intersected the main structure, dilatant zones have provided conduits for movement of ascending mineralizing hydrothermal fluids. Quartz veins are closely associated with the faults and are indicative of higher-grade zones of mineralization. Overall, the mineral resource area has a shallow level of oxidation, which is preserved only in the elevated portions in the Homestake pit area. The bedrock is characterized as an upper oxide zone and a lower sulfide zone. The transition zone between the two is relatively thin and is not distinguished.