General Geology  and Mineral Resources of the Flathead Region

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Flathead Region Geology

General Geology - The Flathead region of Montana, Alberta and British Columbia is shown on the Thumbnail Image to the left.  The region is largely underlain by rocks of the extremely thick Proterozoic Belt Supergroup (called Purcell Supergroup in Canada).  The rocks of the Belt-Purcell Supergroup are cut by numerous thrust faults and normal faults.  A major feature of the region are the exposures of the Belt-Purcell Supergroup in the area of the Glacier-Waterton International Peace Park.  Both continental and deep valley glacial activity have been active in the region and much of the region is covered with various glacial deposits.  These deposits, in addition to common thick forest cover on the lower mountain slopes and hills, restricts outcrop in much of the area, complicating study of the regional geology.

Stratigraphy -  Stratigraphic units exposed in the region cover the broad expanse of time from the Proterozoic to the Recent as shown in the Thumbnail Image to the left.  The Belt Supergroup (called the Purcell Supergroup in Canada) has a thickness of about 16,000 feet (5,000 meters) in the area of the Waterton-Glacier International Peace Park and thickens to the west.  Behind the front of the Lewis Thrust, Paleozoic rocks are exposed only in the Whitefish Range, the Bob Marshall Wilderness Area east of Flathead Lake of Montana, and a small outcrop west of Flathead Lake while in British Columbia they are exposed throughout much of the Flathead region.  Mesozoic rocks have limited outcrop exposure behind the leading edge of the Lewis Thrust, except in the Fernie Basin of British Columbia, and are the only rocks exposed in front of the Lewis Thrust.  The general regional distribution of these basic stratigraphic units are shown in the regional geologic map in the first thumbnail, above.

Structure - The dominant structural elements in the area are the Lewis Thrust, with associated listric normal faults and additional thrusts carried on the plate of the Lewis Thrust.   The cross section shown in this Thumbnail Image is along the southwest to northwest section line shown on the geologic map above.  The origin of the Lewis Thrust was near the end of a major mountain building event  which started around 170 million years ago with the collision of crustal plates.  The compressive stresses of this event resulted in the detachment of lower Proterozoic rocks, probably overlying the underlying basement, in a region with a lateral extent of possibly as much as 250 miles (400 kilometers) east-west, and the lateral movement (over thrusting) of those lower Proterozoic rocks by a distance that may be as much, or more than, 60 miles (100 kilometers).  The end result, that we see today, is that Lower Proterozoic rocks (age of around 1,600 million years) are now laying on top of rocks of Upper Cretaceous age (less than 100 million years old).  The leading edge of the Lewis Thrust is well exposed in outcrop in the eastern part of the Waterton-Glacier International Peace Park.

Petroleum Exploration in the Flathead Region

Oil Seeps and Early Exploration - Exploration for oil and gas in the region began in 1892 with the formal report of the Canadian Geological Survey about oil seeps in the region.  Exploration for oil in the region has been nearly continuous from that time until the mid 1990's, with the bulk of the exploration taking place in Canada.  Early drilling was concentrated within close proximity to seeps but failed to achieve significant production.  Much of the early drilling was in sedimentary rocks of the Proterozoic Belt-Purcell Supergroup of the Lewis Plate from which the seeps were issuing.  Exploration in the Montana portion of the region ceased in 1910 with the establishment of Glacier National Park.  Exploration continued in Canada, along the front of the Lewis Thrust and near the prolific seeps at Sage Creek in British Columbia.

1980's Exploration - A significant amount of petroleum exploration during the early to mid 1980's was based on the idea that the Lewis Thrust carried Proterozoic rocks over younger rocks much further to the west than most students of the structure of the region.  Extensive seismic exploration was completed as far to the west as the Montana-Idaho border, over 150 miles from the front of the Lewis Thrust.  Present on many of these seismic lines were pronounced deep seismic reflectors that many in the petroleum industry felt were very similar to seismic reflectors in the Paleozoic-Mesozoic stratigraphic sequence east of the Lewis Thrust.

ARCO/Marathon No. 1 Paul Gibbs Well -   In 1983-84, the ARCO/Marathon No. 1 Paul Gibbs Well was drilled to a total depth of 17,774 feet (5,417 meters), about 30 miles (50 kilometers) due west of Kalispell to test the theory that Paleozoic and Mesozoic rocks were the cause of the widespread seismic reflectors in the subsurface.  The well was drilled on the Purcell Anticlinorium, a major regional anticlinal feature in the region.  Bill Boberg had an agreement with ARCO and Marathon to complete a detailed evaluation of the No. 1 Paul Gibbs well.  By this agreement he was allowed to sell the report and data from the well to any interested parties after the data which the companies had to provide to the State of Montana, would be made public.  The cross section shown in the Thumbnail Image to the left is the result of his study of the No. 1 Paul Gibbs well.  Two separate detailed reports were prepared from the data derived from the No. 1 Paul Gibbs well:

1. Geologic Evaluation of the ARCO No l. Paul Gibbs Well, Flathead County, Montana (including a Review of Geophysical Exploration and Petroleum Potential of the Northwest Montana Overthrust - Belt Basin)
2. Geologic Evaluation and Metals Analyses, ARCO No l. Paul Gibbs Well, Flathead County, Montana (including a Review of Petroleum Geophysical Exploration, Sullivan-type Sulfide Deposit Potential, and Petroleum Potential in Northwest Montana)

These studies of the No. 1 Paul Gibbs well demonstrated several things:

A key finding was that natural gas is trapped within zones of fracture porosity in these ancient rocks.  Bill Boberg has done follow up work on the organic chemistry of the Prichard Formation to demonstrate that organic carbon is indeed present within meta-sedimentary rocks that are in the range of 1,500 million years old.  These rocks have undergone greenschist facies metamorphism and porosity within the rock units is now essentially nil, except for zones of fracture porosity, commonly the fractured zones associated with thrust faulting, which is where gas shows were tested in the No. 1 Paul Gibbs well.  This suggests that the area of the Belt-Purcell Supergroup could be considered a potential gas province but only in areas where significant fracture porosity occurs

Mineral Exploration in the Flathead Region

Massive Sulfide Deposits - Near Kimberly, British Columbia is the world-class Sullivan Mine, a lead-zinc massive sulfide deposit.  It only recently closed after operating nearly continuously for nearly 100 years.  The Sullivan Mine deposit is contained in the Aldridge Formation (Prichard Formation equivalent in the US) of the Belt-Purcell Supergroup.  There appears to be a genetic relationship between the the Moyie Sills which underlay the Sullivan deposit, an intraformational chaotic breccia and the deposit itself.  Features of the Moyie (and Purcell sills in the US) suggest that they were seafloor lava flows, sometimes flowing at or near the sediment/water contact or sometimes below the sediment/water contact within the unconsolidated sediment.  The Sullivan deposit appears to have been formed as a hydrothermal synsedimentary deposit in a sub-basin on the sea floor near the end of the deposition of the Lower Aldridge Formation.  It is located immediately above crosscutting brecciated zones which appear to be the result of pore overpressure release along zones of crustal weakness.  These crosscutting brecciated zones became the conduits for boron-rich fluids which permeated the sediments around them while discharging onto the sea floor (as evidence by the tourmalinization of the zones).  With time the composition of the discharging fluid changed to become metal-bearing and sulfides began to be deposited on the sea floor.  Pyrrotite predominated during early sulfide deposition and later pyrrotite interlayered with galena and sphalerite became dominant over the western part of the deposit.  Deposition of sulfides from a brine pool collected in the sub-basin may have predominated after this time.  The result was the formation of a major massive sulfide deposit, stratiform and stratabound, of pyrrotite, galena and sphalerite.

The world-class Coeur d'Alene Mining District is also hosted in the rocks of the Belt-Purcell Supergroup.  It is localized within a major, generally east-west, broad structural zone generally referred to as the Lewis and Clark Line.  Genesis of these deposits is not as clearly understood as that of the Sullivan Deposit.  It appears very possible that the deposits of the Coeur d'Alene Mining District could have been formed in a similar manner to the Sullivan Deposit, as a hydrothermal synsedimentary deposit or series of deposits late in the deposition of the Prichard Formation during Proterozoic time, that were then later tectonically remobilized during Laramide structural deformation, and emplaced in structurally favorable zones along the Lewis and Clark Line.

Exploration for Sullivan-type deposits over the years has turned up a number of non-economic deposits but nothing minable, to date.  An indication from the petroleum exploration of the 1980's, with the Purcell-Moyie Sills being readily recognizable seismic reflectors, is that seismic data from that period could be useful in better defining the Prichard-Aldridge Formation in the subsurface, and could possibly help in defining brecciated zones where the favorability for Sullivan-type deposits would be greater.  Bill Boberg's report on the metal analyses of the well cuttings from the ARCO/Marathon No. 1 Paul Gibbs Well discusses the potential for Sullivan-type deposits in the region and the use of petroleum exploration derived data for mineral exploration.    

Stratiform-Stratabound Gold Deposits - Within the Prichard-Aldridge Formation are also stratigraphically controlled gold deposits in which the gold is present in banded quartz units  conformable to the clastic meta-sedimentary rocks.  These deposits appear to be similar to the major deposits of the Proterozic-hosted gold deposits of the Yellowknife region of the Northwest Territories, the Carriboo region of British Columbia, the Pilgrim's Rest and Sabie goldfields of South Africa and the Telfer Mine in Western Australia.  These banded quartz units are of probable sedimentary-exhalitive origin.  Within the Prichard-Aldridge Formation these stratigraphically definable deposits often exhibit an average grade of 0.5 to 1.5 ounces per ton (15 to 45 grams per tonne).  Individual banded gold-quartz beds range in thickness from about 6 inches (15 cm) to 10 feet ( 3 m) and are often stacked in multiples over intervals of 300 feet (100 m).  Initial research suggests that lower, but still significant, gold values also occur within the enclosing clastic rocks.

This Thumbnail Image is of a sample from the Murray District of northern Idaho which exhibits this deposit type as does the Elk City District of Idaho and the Perry Creek District of British Columbia.  In the Murray area, the USGS estimates that around 125,000 ounces of gold were produced from the lode deposits prior to 1934, with a total of around 320,000 ounces produced from the associated placer deposits and the lode deposits.  In this Thumbnail Image, the circles drawn on the rock slab are drawn around visible gold in the banded quartz.  Bill Boberg and Dick Huebschman, a specialist on the Prichard Formation, prepared a special study, in 1987, describing these deposits and cataloging the mineral occurrences in the region that match this deposit type.  The title of this report is "Proterozoic Stratiform-Stratabound Gold Deposits, Idaho, Montana and British Columbia - Compilation of Occurrences and Report" which is currently available for sale on this website.

Other Mineral Deposits - Rocks of the Belt-Purcell Supergroup are also host for a variety of mineral deposits in a number of areas in western Montana, northern Idaho and British Columbia.   Within the Greater Flathead region itself these deposits are the Hog Heaven Mining District, an historic silver producing district associated with Tertiary volcanics, west of Flathead Lake, and a relatively new discovery of gold in the Howell Creek area of British Columbia.  In the Howell Creek area, gold has been discovered in altered Paleozoic sedimentary rocks where they have been intruded by Cretaceous alkalic rocks in a zone of structural complexity.  These occurrences are currently undergoing exploration.

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Last modified: April 14, 2004