Summary:
In the most general sense, deposits in the District are orogenic, polymetallic veins with lesser disseminated mineralization emanating from the principal veins.
The Bunker Hill Mine workings extend 8,600 feet along strike of the overturned beds of the Upper Revett formation that host the mineralization, extending 7,000 feet downdip parallel to the axial plane of the plunging anticline, covering 5,200 vertical feet from ~3,500 ft msl to -1,700 ft msl. More than 30 individually named deposits were mined historically in separate stopes, with two distinct types of deposits exploited: tabular Bluebird (BB) zones that parallel bedding and are associated with the fold structures, and later Galena-Quartz (GQ) Veins cutting through bedding with sharp walls. The Bluebird Deposits, such as the March, have been mined for up to 1,400 ft along strike, 4,000 ft downdip, covering 2,400 ft in elevation, with thicknesses of the generally tabular zones up to 150 ft. GalenaQuartz Veins were historically mined along strike lengths of up to 800 ft, and downdip up to 3,700 ft, with mined thicknesses from 5-15 ft.
Mineralization at Bunker Hill falls in four categories, described below from oldest to youngest events:
- Bluebird Veins (“BB”): W--NW striking, SW-dipping, variable ratio of sphalerite-pyrite-siderite mineralization. Thick, tabular cores with gradational margins bleeding out along bedding and fractures;
- Stringer/Disseminated Zones: Disseminated, fracture controlled and bedding controlled blebs and stringer mineralization associated with Bluebird Structures, commonly as halos to vein-like bodies or as isolated areas where brecciated quartzite beds are intersected by the W-NW structure and fold fabrics;
- Galena-Quartz Veins (“GQ”): E to NE striking, S to SE dipping, quartz-argentiferous galena +/-siderite-sphalerite-chalcopyrite-tertahedrite veins, sinuous-planar with sharp margins, cross-cut Bluebird Veins;
- Hybrid Zones: Formed at intersections where GQ veins cut BB veins, with open space deposition of sulfides and quartz in the vein refraction in quartzite beds, and replacement of siderite in the BB vein structure by argentiferous galena from the GQ Vein.
The four types of mineral zones listed above are truly only two separate structural events: the NW trending Bluebird Veins and the E-NE trending Galena-Quartz Veining. Initial 3D modeling (Rangefront Technical Services 2020) and structural + mineral zonation analysis (Juras and Duff, 2020) has indicated the various vein segments are likely post- mineral offsets of two vein systems that initially comprised four distinct Bluebird Veins and three to five Galena- Quartz Veins.
Although the mineralogy of the two veins types is distinct, and there are significant differences in vein textures and structures that are not germane to this Technical Report, the physical mechanism of both types of mineralization is sulfide minerals filling open spaces (Duff, personal communication, 2020). The creation of intra-bed open space by differential movement of a folded rock package leading to a structurally prepared host rock, is one of the main theories regarding the origins of mineralization along these structures (Juras and Duff, 2020).
Quartzite is the primary host to mineralization in all vein types, deposited in open-space caused by refraction of the vein structure as it passes from softer siltite argillite packages into quartzite units. The vein deflects to cross the quartzite unit more orthogonally, bending to normal with the bedding plane, in essence decreasing the length of quartzite that needs to fracture to continue propagation. Mineralizing fluids ascending the vein structure deposited sulfides in the open-spaces and pressure shadow created by these refractions. Although the veins are commonly mineralized to some degree along their entire length, economic ore shoots in historic mining operations were largely hosted in these dilated zones in quartzite beds, with the shoot plunging up and down at an orientation defined by the intersection between the vein and bedding (Juras and Duff, 2020).
Both vein sets at Bunker Hill exhibit textures typical of orogenic veins, with no boiling textures or sharp textural differences from pressure-temperature changes, nor any significant wallrock alteration other than disseminations of the vein minerals. The huge vertical extent (3000-600ft+) of mineralization typical of all the vein types in the District strongly indicates that all mineralization was emplaced at moderate to deep crustal levels. Juras and Duff note examples of open-space-filling textures in sulfide minerals in veins in their 2020 report, and classify all of the veins at Bunker Hill as open space fissure veins. If all of these observations hold true, an active fold system is one of the few ways to geologically explain the spaces and pressure shadows necessary to form those open-space cavityfill textures under the pressures and temperatures present at the time of vein emplacement.