The Black Butte copper project is an underground copper mining development by Sandfire Resources in Montana, US.
A preliminary economic assessment (PEA) for the project was completed in August 2012 followed by an environmental impact statement (EIS) in December 2017. Sandfire Resources is currently in the final phase of a bankable feasibility study (BFS), which is expected to be completed by October 2020.
The company received the final approval for phase one development of the project from the Montana Department of Environmental Quality in August 2020. This permit enables the surface construction activities at the mine site.
The project is expected to require a construction period of 24 months while the mine life is expected to be up to 14 years.
Location and geology
The Black Butte copper project is located on privately-owned land, approximately 27km north of White Sulphur Springs in Meagher County, in south-central Montana, US. The project area comprises a mineral lease covering approximately 3,223 hectares (ha).
The project is based on the Johnny Lee copper deposit which was discovered by a joint venture between Broken Hill Propriety (BHP) and Cominco American (CAI) in 1985.
The Johnny Lee is a high-grade, massive sulphide copper deposit hosted by Mesoproterozoic sedimentary rocks of the Newland Formation of the Belt Supergroup. It comprises shale, carbonaceous shale, calcareous shale, dolomite, conglomerate, and sedimentary breccia.
Copper mineralisation and reserves
Two massive sulphide units host the copper mineralization at the Johnny Lee deposit that includes the Upper Sulphide Zone (USZ) and the Lower Sulphide Zone (LSZ).
The Black Butte copper project was estimated to hold 10.9Mt of measured and indicated resource grading 2.9% Cu, and approximately 2.7Mt of inferred resource grading 3% Cu, as of October 2019.
Mining and ore processing at Black Butte
The Black Butte copper project will be an underground mining operation utilising cut and fill technique with a small surface footprint of approximately 270 acres.
The Upper Sulphide Zone extends 3,000ft in a north-south direction with a width of 1500ft while the Lower Sulphide Zone extends 3,000ft in an east-west direction. The mineralised zones will be accessed via a 5,000ft-long access tunnel on the south side of the Sawmill Hill.
The run-of-the-mine (ROM) ore will undergo crushing and three-stage grinding, before passing through a froth flotation circuit to separate copper concentrate from tailings waste. A portion of the tailings will be used as cemented backfill in the underground mine.
The copper cleaner flotation concentrate will be thickened and dewatered prior to shipping for further processing at offsite facilities.
The Black Butte copper property can be accessed year-round via the US Highway 89 and 4km-long of the county maintained, gravel-surfaced, Sheep Creek road.
The electricity required for the current pre-construction activities at the site is being supplied from the local power grid by Fergus Electric Cooperative. The existing 100kV power line at Kings Hill Pass that stretches for approximately 25km along US Highway 89 is envisioned to be used as the main source of electricity supply for the project.
The project is expected to utilise groundwater which will be pumped to the Contact Water Pond (CWP) located near the mine portal. Water required for processing operations will be pumped from the CWP.
SRK Resources prepared the technical report published in December 2019 and is also currently engaged in the feasibility studies.
MK Weeden, a Montana-based company, has been awarded a construction contract for the commencement of surface earthworks, while the local company Menard Construction received the fencing contract for the Black Butte project site.