Summary:
The Lost Sheep Fluorspar Property located in the Spor Mountain area. The Spor Mountain mining district has been an area of significant fluorite past production dating back to the 1940s, with the Lost Sheep Mine being the largest historical fluorite producer in Utah.
Spor Mountain fluorite belongs to the class of volcanic-epithermal fluorspar deposits, more specifically," sub-alkaline epithermal" type.
Property geology
The underlying geology of the Lost Sheep Property is a series of dolomite formations including the Bell Hill, Harrisite and Lost Sheep formations. Striking at approximately 040-045°, dipping 37-42° northwest, they are intruded by breccias and cut by at several faults that effectively control the distribution of fluoride mineralization.
Deposit geology
The Purple Pit Pipe has a vertical extent of 400 ft through the Lost Sheep and Harrisite Dolomite and terminates in Silurian Bell Hill Dolomite. The Purple Pit lies along the western edge of a rhyolite breccia plug measuring 1,150 ft in length and 400 ft in width. A normal fault that controls the location of the pipe strikes 055° and dips 38° southeast. Movement along this fault juxtaposes the Bell Hill Dolomite in the hanging wall against the top of the Swan Hill Quartzite in the footwall. For this reason, the Purple Pit Pipe was not expected to extend below the 400 ft level (Bullock, 1976).
The surface expression of the Purple Pit is a crescent-shape with slight eastward orientation over a length of 185 ft and a maximum width of about 75 ft. The pipe is relatively continuous down to the 150 ft level the pipe then splits and at the 250 ft level, there are three distinct mineralized zones, with the eastern, more vein-like in appearance, and measuring 4 ft in width and 20 ft length. At the 400 ft level, the main pipe is approximately 25 ft in diameter, and the western pipe is apparently terminated by the normal fault that dips beneath the pipe. Mineralization at this depth is reported as quite siliceous.
The Little Giant Pit is a more recent working on the Lost Sheep Property. On surface it is characterized by a "Y" shaped open cut which trends to the west with the two "arms" extending to the northwest and southwest. The pit extends 200 feet east-west and 220 feet north-south. The discovery point for this mineralization was a small pipe at the south end of southwest arm which was mined to a depth of about 60 feet. Discontinuous fluorspar bodies along a fault were followed for about 110 feet to the northeast where the main Little Giant pipe was encountered. This main pipe extends about 73 feet north-south and 49 feet east-west and has been mined down about 60 feet. It is reported to have widened with depth. A lobe of this pipe extends into the northwest arm of the pit. One bench about 25 feet above the floor of the pit has been mined about 90 feet to the northwest. Two near vertical veins, exposed in the pit wall, have been explored for a few feet on the north and east sides of the pit. The access road into the pit from the east is at the bottom of a long deep cut related to clay mining.
The pit faces clearly show extensive multi-stage epithermal mineralization, within and adjacent to semi-brittle or brittle faults. Host dolomites are west dipping and display preliminary vertical to steeply dipping faulting and localized brecciation, with argillic alteration, de-dolomitization, and banded fluorite-calcite-chalcedony/silica. Overprinting this is more intense fluorite mineralization, seen as a sub-vertical 'plume', with internal, almost concentric replacement and layering, characteristic of multiple mineralizing events within a permeable, low pressure and temperature setting.
At the Bell Hill Mine, all known fluorspar bodies are surrounded on surface by the Silurian Bell Hill dolomite, the thickest of the Silurian formations. Overall strike and dip for the strata is 015-055°/25-45° northwest. A dark grey, coarse clastic dolomite, with a finer, pale upper 45 ft, it forms prominent outcrops and steep hills. Marker beds are rare.
At Bell Hill, (breccia) pipes and veins were exploited, with the largest pipe described as H-shaped in outline by Bullock, (1976). The mineralization is bounded by two parallel faults striking 060º and with steep NW dip. Dimensions were a maximum length of 130 ft, and a width of 50 ft at the H-junction. Average plunge at the 168 ft level is 70° S, 81° E, with a hook-like cross section. The two main "veins" coalesce at the 69 ft level, but two separate zones were mined at the 168 ft level, with pipe dimensions of 30 ft width and 55 ft length, and 40 ft length and 27 ft width. Mineralized material at lower depths was more silicic, cherty. Irregular fine-grained bands of dark brown rhyolite tuff were exposed at the 87 ft level.
Mineralization was initially mined from surface, subsequently an adit was driven for 230 ft from the east, continuing in mineralization for 110 ft, starting at the 87 ft level. A raise with three levels was developed above the adit level and a 54° winze was sunk at the 87 ft or adit level.
Mineralization
Fluorite mineralization is usually hosted within breccia pipes, minor dyke-like breccias and replacement features oriented sub-parallel to stratigraphy generally plunging steeply east, possibly oriented in part due to block rotation. Lesser bodies include sub-strata parallel replacement bodies and metre to dm-scale vein-type fluorite. There is some potential for high-grade fluorspar deposits along the contact between dolomite and the porphyritic rhyolite in volcanic vents.
Bullock (1981) reports that Lost Sheep Property fluorspar mineralization consists of 65-95% fluorite, with montmorillonite, dolomite, quartz, chert, calcite, chalcedony and opal as impurities. The fluorspar closely resembles brown, white, or purple clay and forms either pulverulent masses or box works. With depth, the grade of the mineralization commonly decreases, and masses of montmorillonite, chert, or quartz and dolomite have been found in increasing abundance. In some deposits, the fluorspar mineralization contains 0.003-0.33% U, and uranium grade varies considerably from place to place.
Bullock (1981) describes five fluorspar mineralization types:
• Pulverulent mineralized material constitutes the majority of mineralization mined, it being friable, white to tan to brown or purplish, soft, almost clay-like, with significant extraction from Bell Hill, Blowout, LSM, and Fluorine Queen.
• Boxwork mineralization is intimately associated with the preceding, occurring in large, open space replacement zones within breccias.
• Aphanitic mineralization is hard, compact, fine, relatively dense, occurring as masses within more boxwork ore, 'lumps or veins. Banded mineralization is also noted, with these textures clearly representative of epithermal-type 'veins' or replacement.
• Sponge Mineralization is rarely noted, and defined by Bullock as rounded, hollow, tubular and columnar. Forming a very minor component of mineralization, Bullock surmised their origin to be related to relatively unrestricted hydrothermal fluid circulation and deposition within cavities and open channel ways.
• Crystalline Mineralization is represented by small, prismatic, 1-2 mm cubic F crystallisation, often seen as drusy "crusts" with carbonate and very rarely, idiomorphic topaz.