Dawson Property — 2017 Technical Report

This report describes the proposed processing methods and plant design for the Dawson gold project based on metallurgical testwork and in-house experience.

Report context

This technical report is dated March 2017 and presents the recovery methods and process plant design for the Dawson Property, located near Cañon City in Fremont County, Colorado. The process design is based on metallurgical test results from the 1991 Hazen and the 2015 Inspectorate test reports, along with interpretations derived from those programs. The proposed plant is designed for an annual throughput of 120,728 tn (109,500 t) of run-of-mine ore, operating 365 days per year at a nominal daily rate of 330.8 tn (300 t).

Processing route

Primary and secondary crushing

ROM ore from the open pit will be delivered by 20-ton trucks to a coarse ore dump hopper with a grizzly and vibrating grizzly feeder. Ore will feed a primary jaw crusher set to produce a nominal 3-inch product. Crushed ore will be conveyed to a vibratory screen with 2-inch top and 5/8-inch bottom deck openings. Screen undersize at -5/8-inch will fall into a 300-ton live capacity fine ore bin providing approximately 20 hours surge capacity ahead of grinding. Screen oversize will feed a secondary cone crusher set to deliver -5/8-inch product, which rejoins the jaw crusher discharge on the crushed ore conveyor in closed circuit with the screen. A magnet and metal detector will remove tramp iron. The crusher is scheduled to operate 16 hours per day or as required.

Grinding and gravity recovery

The grinding circuit consists of a single ball mill in closed circuit with a hydrocyclone. Fresh feed with an 80% passing size of 0.47 inches (12,000 µm) will be ground to a target primary grind 80% passing 200 mesh (74 µm). A portion equivalent to approximately 70% of fresh mill feed will be diverted from cyclone feed to the gravity circuit, which includes a safety screen, a centrifugal gravity concentrator, and a shaking table. Gravity gold concentrate from the shaking table may be processed on site to doré or mixed with flotation concentrate. The safety screen oversize and tailings from the gravity concentrator and table are recycled to the cyclone feed pump box.

Rougher/scavenger/cleaner flotation and regrind

Cyclone overflow from grinding will feed rougher/scavenger flotation cells. Reagents will include potassium amyl xanthate (PAX) as a sulphide mineral collector and methyl isobutyl carbinol (MIBC) as a frother, with potential use of a dithiophosphate secondary collector. Rougher and scavenger concentrates will be combined and reground ahead of cleaner flotation. The regrind mill will operate in closed circuit with a hydrocyclone to produce a target 80% passing regrind size of 400 mesh (37 µm). Rougher/scavenger flotation will provide 40 minutes residence time and cleaner flotation 38 minutes. The rougher/scavenger circuit is expected to produce approximately 75% to 80% of plant feed mass as tailings, cleaner tailings 15% to 20%, and final concentrate 5% to 10% of plant feed mass.

Concentrate dewatering

Final cleaner concentrate will be thickened to approximately 60% solids using flocculant, then filtered to a target 10% moisture content. Filter cake will be packaged into 2-ton bulk bags for shipment. An estimated 1.6 to 3.3 tons per day (5% to 10% of plant feed) will be recovered as flotation concentrate.

Tailings dewatering and disposal

A dry-stack tailings system is proposed. Combined rougher/scavenger and cleaner scavenger tailings will be thickened then filtered to a target 15% moisture content. Filtered tailings cake will be conveyed to a temporary stockpile then hauled intermittently by truck to the tailings management facility. The cost of this haulage is not covered in mill operating expenses.

Reagent preparation

PAX will be mixed in a tank with agitator then transferred to a head tank for gravity feed to metering pumps. MIBC will be fed neat. A vendor-supplied flocculant mix system will provide 0.25% strength flocculant. A secondary reagent may be added from an Isotainer if required.

Services and utilities

Fresh water for mill and mine will be supplied from drilled wells. Process water will be recycled through concentrate and tailings dewatering systems. Estimated fresh water requirement is 7.5 gpm (1.7 m³/h) for process and filtered water make-up. Air for flotation cells will be provided by a blower, with high pressure and instrument air from two compressors. A maintenance shop and assay lab will support operations.

Key reported parameters

Technical qualifications

The process design for the Dawson plant is based on metallurgical test results from the 1991 Hazen and the 2015 Inspectorate test reports, supplemented by in-house experience with processing similar ores. The plant description and flowsheet are prepared using these test programs and are considered preliminary. A complete list of major process and mechanical equipment providing preliminary sizing and power information has been prepared for the development of preliminary capital cost. The water balance was prepared by BOMENCO, with water requirements outside the plant provided by other consultants.

Source: NI 43-101 Technical Report, Dawson Property, Colorado, USA, March 2017, Sections 17.0–17.4.