Exploration for REE and Gold in the Bear Lodge Tertiary Alkaline Complex, Cook County, Wyoming

Coauthors:  J.T. Ray and Dr. J. G. Clark
May 2015
New Concepts and Discoveries:
Geological Society of Nevada Symposium Proceedings, Sparks, Nevada

Please click here to review paper 

Geology and Mineralization

Geology

The Bear Lodge Mountains are part of the Black Hills Uplift, formed by the intrusion of a Tertiary-aged alkaline igneous complex during the waning stages of the Late Cretaceous-early Tertiary Laramide Orogeny. The uplift has a northwesterly orientation and extends from the western South Dakota - Nebraska border through northeastern Wyoming into south-eastern Montana. The exposed basement consists of Archean and Proterozoic schist, gneiss, and granite overlain by Paleozoic and Mesozoic clastic and carbonate sedimentary rocks that were subsequently eroded from higher elevations. The Paleozoic and Mesozoic rocks were subjected to large-scale monoclinal folding that encircles the Black Hills Uplift. Younger Oligocene, Miocene, and Pliocene sediments disconformably overlie the older sedimentary and igneous rocks at lower elevations of the uplift.

Project Location and Geological Setting of Bear Lodge Mountains

 Geologic Setting at Bear Lodge

Source: Ray and Clark, 2015 

The Bear Lodge mining district is in the Bear Lodge Mountains, near the western end of the northern Black Hills intrusive belt. The Bear Lodge Mountains expose and are underlain by multiple alkaline plugs, sills  and dikes intruded into Archean basement and Paleozoic and Mesozoic sediments approximately 38 to 51 million years ago, forming an elongate, northwest-trending dome.

Geologic Map of the Bear Lodge Mountains

Simplified Bear Lodge Geology

Source: Ray and Clark, 2015, after Staatz, 1983
 

The Bear Lodge REE deposits are sited near the axis of the elongate dome and are associated with carbonatite and silicocarbonatite dikes, veins, and stockwork hosted by the Bull Hill and Whitetail diatremes. The northwest alignment of diatremes extends from Bull Hill through Whitetail Ridge to Carbon Hill, and coincides with numerous north- to northwest-striking alkalic dikes and mineralized zones.

 

Carbonatite and silicocarbonatite dikes strike northwesterly to northerly and intrude diatremal heterolithic breccias and surrounding trachyte, phonolite and igneous intrusion breccias. Within the Bear Lodge deposit, the dikes are concentrated within the margins of the diatreme and in adjacent wall rocks along the northwest corridor, extending from the south and west flanks of Bull Hill through the Whitetail diatreme towards the Carbon Hill diatreme.  The carbonatite bodies range in size from veinlets to dikes that can reach more than 15 meters in width. A swarm of wide, steeply dipping carbonatite dikes cut across the southwestern flank of Bull Hill.  The dike swarm has been traced over 300 meters along strike and down dip, and it remains open in the down dip direction.  Rare earth-bearing minerals within the carbonatites are mainly ancylite and rare earth fluorocarbonates of the bastnaesite group.

Geologic Map of the Project Area 

Bear Lodge Geology 

Source: Ray and Clark, 2015

Resource

The Bear Lodge project has a Measured and Indicated (M&I) Mineral Resource of 18.0 million tons (16.3 million tonnes) averaging 3.05% TREO, and an Inferred resource of 31.8 million tons (28.9 million tonnes) averaging 2.58% TREO (see the Company’s news release dated August 26, 2014).  This resource was used in the preliminary feasibility study (PFS) which demonstrates the ability to support a 45-year mine life.  The entire tonnage is all within nearĀ­ surface oxide and oxide-carbonate (OxCa) mineralization zones.  The resource breakout is as follows:

Summary of Measured and Indicated Resource
Total Rare Earth Oxide (using a 1.5% Cutoff Grade)

 
Deposit Tons
(millions)
Grade TREO
%
Contained TREO lbs.
(millions)
Contained TREO Kg
(millions)
Bull Hill        
  Measured 3.0 3.77 226 102
  Indicated 10.7 3.09 661 300
    Total M&I 13.7 3.24 887 402
Whitetail Ridge        
  Measured  –  –  –
  Indicated 4.3 2.47 212 96
    Total M&I 4.3 2.47 212 96
Project Total        
  Measured 3.0 3.77 226 102
  Indicated 15.0 2.91 873 396
    Total M&I 18.0 3.05 1,099 498
The mineral resource estimate is classified as M&I resources, as defined by Canadian Institute of Mining, Metallurgy and Petroleum ("CIM") Definition Standards on Mineral Resources and Mineral Reserves, as of June 30, 2014 using a basket price of $24.60 and is inclusive of proven and probable reserves.  Mineral resources were estimated by Alan C. Nobel, P.E. of Ore Reserves Engineering (O.R.E.), an independent Qualified Person as defined by NI 43-101.  Readers are cautioned that mineral resources that are not mineral reserves do not have demonstrated economic viability.  The terms "Measured and Indicated Mineral Resource" are terms recognized and required by Canadian regulations but not by the United States Security and Exchange Commission (SEC).  U.S. investors are cautioned not to assume that any part or all of the mineral deposits in these categories will ever be converted into mineral reserves under SEC regulations. 

 

Both the Bull Hill and Whitetail Ridge deposits have a significant amount of Inferred Resource. While this portion of the resource has greater uncertainty than the M&I resource and is assigned no economic value in the PFS, it represents significant upside opportunity for the project. About one-third of the Inferred Resource is contained within the designed pit and could be recovered during the mining operations currently contemplated in the PFS. The Inferred resource at the project is summarized below.

Inferred Mineral Resource within the Bear Lodge Project
(Using a 1.5% cutoff grade)



 Deposit                         
 
Tons
(millions)
        Average Grade
TREO %
 
     Contained TREO lbs.
(millions)
   Contained TREO Kg
(millions)
  Bull Hill 24.0 2.54 1,212 550
   Whitetail Ridge    7.9 2.71 429 194
   Inferred Resource     31.9 2.58 1,641 744

 

The mine plan used in the PFS accesses areas of significantly higher grade within the M&I resource in years 1 – 9 of the Project’s life. This brings cash flows forward and results in an attractive payback period of 2.9 years. A breakout of the high-grade resource is shown in below.

Measured & Indicated Mineral Resource - High Grade Subset
(Using a 3.0% cutoff grade)

Contained High-grade Tons
(millions)
Average Grade
TREO %
Contained TREO lbs.
(millions)
Contained TREO Kg
(millions)
   Measured 1.7 4.92 167   76
   Indicated 4.3 4.45 383 174
     Total 6.0 4.58  550 250

These resources were identified using certain price and cost assumptions.  Please see the most current NI 43-101 Technical Report for further details. Mineral resources were estimated by Alan C. Nobel, P.E. of Ore Reserves Engineering (O.R.E.), and independent Qualified Person as defined by NI 43-101.