3 Economic Geology of the Rocky Mountain Region

Monday, 11 May 2009
8:00 AM - 12:00 PM

GEOLOGY OF THE BLACKFOOT BRIDGE PROPERTY

Paper number: 3-7
Program: Theme Sessions
Time: 10:00 AM – 10:20 AM

Authors

Abstract

The Blackfoot Bridge property is a phosphate mineral lease owned by the BLM and leased to P4 Production, LLC located approximately 12.2 mi (19.7 km) north east of Soda Springs, ID. The stratigraphic sequence of the deposit is the Pennsylvanian Age Wells Formation consisting of limestone and calcareous sandstones. Above the Wells Fm is the Permian Age dolomitic Grandeur Tongue Member of the Park City Fm followed by the Phosphoria Fm, containing interbedded mudstone, siltstone, shale, limestone, and chert, which in turn is overlain by the Triassic Age Dinwoody Fm containing calcareous siltstone, siltstone, and limestone. The Phosphoria is further divided into the Meade Peak, Rex Chert, and Cherty Shale Members. The two economic ore zones are found within the Meade Peak Member and are identified as the Upper and Lower Ore zones. Located in the central portion of the property are additional units of basalt, associated surface flows, and volcanic ash.

Primary geologic structures include north/south trending normal faults associated with post Laramide Basin and Range extension and east/west normal faults spaced approximately 1200 ft (2.7km) apart probably associated with the Yellowstone Hot Spot. In the center of the property, as identified by exploration drilling and surface mapping, is a massive slump approximately 2200 ft (677 m) by 3200 ft (985 m) in extent, that has disrupted stratigraphic units, including the ore zones from the crest of the ridge to approximately 400 feet (122m) down slope.

The ore has a strike length of 2.8 miles (4.5 km) and a proposed mine width of 2400 feet (730 m) over a length of 2000 feet (609 m), in the north, and 1400 feet (425 m) over the remainder of the strike. Exploration of this property has been ongoing since 1956 and consists of 231 core, reverse circulation, and various types of monitor wells totaling more than 52,000 feet (15, 837m).