Summary:
The Meade Peak Member of the Phosphoria Formation contains the phosphate mineralization within the Conda Projects and is overlain by the Rex Chert Member and underlain by the Park City Formation. The Quaternary Alluvium is not very extensive and where it is present it is only about 5 feet to 20 feet thick.
The Meade Peak Member is broken into five mining zones throughout the Conda projects where the Upper Phosphate and Lower Phosphate Zones are the primary phosphate mineralized zones. The significant mineralized zones encountered on the property are shown below:
- Upper Overburden Zone (Hanging Wall mud).
- Upper Phosphate Zone – Low/medium to high grade phosphate zone. Interbedded phosphorite, mudstone, siltstone, limestone, and shale.
- Center Interburden Zone – Shale and mudstone.
- Lower Phosphate Zone – Low to high grade phosphate zone, interbedded phosphorite, mudstone, siltstone, limestone, and shale.
- Lower Underburden Zone (Footwall mud) – Reddish brown siltstone with black fossiliferous siltstone and some phosphorite.
RVM Structural Geology
The Snowdrift Anticline is the geologic structure that defines the Rasmussen Valley Mine (RVM) strata. The Snowdrift Anticline is a northwest-southeast trending anticline that plunges gently southeast. RVM is located on the southwest limb. The Snowdrift Anticline resulted in the geometry of the flat lying beds of the Phosphoria Formation to be modified by folding so that they now strike northwest/southeast and plunge southwest within the RVM. Both limbs of the anticline are very steep where the beds are near vertical or overturned. The strata of the Phosphoria Formation outcrop along the limbs of the anticline.
The Snowdrift Anticline is bound on the east by the Lanes Creek Fault, which dips at 83 degrees east with approximately a 400-foot normal displacement, and on the west by the Enoch Valley Fault, which is a normal fault that dips at 80 degrees west and can have up to 3,000 feet of displacement. The Rasmussen Fault strikes eastwest and intersects the Snowdrift Anticline axis north of the RVM area. The Rasmussen Fault has approximately 4,000 feet of left-lateral displacement and truncates the Phosphoria Formation in the RVM area.
North Dry Ridge (NDR) and Husky1 South Maybe Canyon (H1SMC) Structural Geology
The structural feature that dominates the NDR and H1SMC areas is the northwest trending North Dry Valley Anticline. NDR and H1SMC are located on the northeast limb of the anticline and as such, the strata of NDR and H1SMC dips very steeply to near vertical to the northeast.
Faulting in the northern portion of the NDR lease has forced the Meade Peak Member of the Phosphoria Formation to uplift to the overlying Dinwoody Formation. This has resulted in the absence of the Meade Peak Member north of the Blackfoot normal fault within the NDR property.
Additional folding and faulting are found in the southern portion of the H1SMC area, notably, the Stewart Anticline which trends northeast/southeast. The axes of the Stewart Anticline are within the southern portion of the H1SMC property and allow for a large outcrop area of the Meade Peak Member.
Depost Types
The phosphate mineralization is sedimentary in nature, occurring in a conformable sequence of alternating phosphatic and weakly to non-phosphatic shale, mudstone, carbonate, and chert beds within the Meade Peak Member of the Permian Phosphoria Formation. The Phosphoria Formation occurs within the Western Phosphate Field that occupies more than 135,000 square miles, spanning Eastern Idaho, Southern Montana, Western Wyoming, and the northern half of Utah (Sheldon, 1989).
The phosphate mineralization encountered in the Meade Peak Member is stratigraphic in nature, and the deposit type is considered a typical example of a marine sedimentary phosphate deposit. The phosphate mineralization occurred during the primary depositional processes and there are no known secondary phases of phosphate mineralization or enrichment identified in the deposits.
The beds of the Meade Peak Member were deposited within a marine sedimentary basin within the Phosphoria Sea that marked the western margin of the North American craton approximately 250 Ma. During the period that the Meade Peak Member was being deposited, access to the open ocean was intermittently restricted by barrier islands during cyclical periods of eustatic sea level change resulting from periods of glaciation and deglaciation (Sheldon 1984). This cyclical process resulted in the alternating beds of phosphatic shale and mudstone with layers of non-phosphatic shale, carbonate, and chert beds.
Low sea levels during periodsof glaciation gave rise to periods of intense upwelling currents of cold nutrient rich waters entering the basin; these nutrient rich waters would become confined within the basin by the barrier island structures and would result in algal blooms. Restricted access to the open sea limited recharge or mixing of the waters in the basin while the lower sea level and restricted access limited the impacts of both marine carbonate deposition as well as terrestrial sedimentation during development of phosphatic beds.
The phosphate mineralization within the Meade Peak Member consists of apatite pellets, oolites, and sand grains, some of which are further cemented into clusters of pellets and grains in an apatite cement; the apatite within the Meade Peak is entirely in the form of carbonate fluorapatite (Altschuler et. Al. 1958).
Individual beds of the Meade Peak Member are laterally continuous over significant distances, with some beds commonly found distributed over tens of thousands of square miles within the Western Phosphate Field (Sheldon 1989); however the thickness and geometry of the beds has been locally impacted on a deposit scale by both primary depositional variability as well as post-depositional structural modification due to both regional and deposit scale faulting and folding.