The Sisson deposit can be defined as an intrusion related, structurally controlled, bulk tonnage tungstenmolybdenum deposit. Deposits of this type have general hydrothermal similarities to porphyry copper deposits. These types of deposit form in convergent margin to collisional tectonic environments and are related to highly evolved granitic melts formed from continental crust.

A larger granitic pluton as a source for mineralizing hydrothermal fluids at Sisson has not been intersected by drilling thus far, but the numerous, albeit narrow, granitic dykes allow the presence of such a body to be confidently inferred at depth below the deposit.

Structures focused the ascent from depth of the intrusion derived hydrothermal fluids which formed the Sisson deposit. The physical relationship between QS and SX veins exposed in exploration Trench 1 in Zone I and the concentration of mineralization along the disrupted contact between gabbro and the Turnbull Mountain Formation suggest the possible nature of this control. Trench 1 is cut by several broadly north trending, approximately vertical QS veins that are between 0.3 m and 1.5 m in width. Kinematic indicators support long-lived to episodic, sinistral movement along the structures which host the QS veins. Sheeted arrays of northwest trending, steeply dipping, narrow SX veins, as well as fewer but larger, lensoidal quartz extension veins, form an envelope at approximately conjugate angles to the larger QS veins. Orientations obtained on 1,631 veins of all types in seven oriented geotechnical drill holes (Duncan, 2011) also show that most veins throughout the deposit form a sheeted array with northwest strike and steep southwest dip.

Mineralization in the Sisson deposit is related to several types of alteration and veins, each of which has a distinct relationship to metal introduction. The mineralization has an approximate strike length of 1,900 m, average width of 650 m and average depth of 350 m.

Early barren to weakly mineralized alteration was followed sequentially by an early stage of tungsten mineralization, an intermediate stage of molybdenum ± tungsten mineralization, and a late stage of polymetallic mineralization in which tungsten is accompanied by numerous base and trace elements. Most alteration and nearly all mineralization can be directly related to specific types of veins. The overall vein density at Sisson is only about 3% and pervasive alteration zones are spatially limited, which indicates that the hydrothermal system at Sisson had a low fluid flux.

Mineralization at Sisson occurs almost exclusively in quartz veins, fractures, and their alteration envelopes. Tungsten and molybdenum are the metals of principal economic interest, whereas several other metals, including copper, zinc, lead, arsenic, and bismuth, occur in geochemically anomalous but subeconomic concentrations.

Mineralization occurs in contiguous Zones I, II, and III and in the Ellipse Zone. Zones I and II trend approximately north from the northern end of Zone III. Zone I is hosted by the Turnbull Mountain Formation, whereas Zone II occurs in gabbro. Zones I and II are structurally controlled, up to tens of meters in width, and extend several hundred meters along strike. They are dominated by QS and SX veins and contain tungsten, copper, and associated trace elements, but lack significant molybdenum. Zone III contains the bulk of the tungsten and molybdenum resource at Sisson. It is an ovoid zone that obliquely spans the contact between gabbro and the Turnbull Mountain Formation and mineralization occurs in gabbro, volcanic, and sedimentary rocks. The highest grades of molybdenum occur in both volcanic and sedimentary rocks in the central part of Zone III, whereas the highest grades of tungsten occur on either side of the contact between gabbro and volcanic rocks in Zone III. The Ellipse Zone and the southern part of Zone III are very similar to each other and contain moderate grades of both tungsten and molybdenum. Host rocks in the southern part of Zone III are gabbro and volcanic rocks, whereas the Ellipse Zone is hosted by gabbro and quartz diorite. Mineralization diminishes very abruptly at the south end of Zone III.

The minerals of economic interest at Sisson are molybdenite, scheelite, and minor wolframite.

Mining operations for the Sisson Project are typical of open pit, truck-and-shovel mining methods for year round operations in Canadian climates.

The mining fleet consists of 136-t trucks, 16.5 m3 hydraulic shovels and support equipment. The mining fleet will excavate, on average, approximately 23.7 million tonnes per year (Mt/a) over the life of the Sisson Project, including 10.5 Mt/a of mill feed, nominal 30,000 tonnes per day (tpd), 10.6 Mt/a of barren open pit rock and mid-grade stockpile and 2.6 Mt/a of quarry material for construction of the tailings facility embankments. The mine life is estimated to be 27 years.

The principal economic minerals of the Sisson deposit are scheelite (CaWO4) and molybdenite (MoS2), the Sisson concentrator process flowsheet is based on the recovery of these two minerals into their respective concentrators.

The run of mine ore will be processed through an onsite concentrator that will produce a molybdenum flotation concentrate and a tungsten flotation concentrate. The molybdenum concentrate will be shipped offsite to a purchaser, most likely for roasting into molybdenum tri-oxide (MoO3), while the tungsten concentrate will be processed onsite to high purity ammonium paratungstate (APT) product.

The Sisson Concentrator includes the following major processing steps:
1. Three stage crushing.
2. Single stage, dual-line grinding and classification.
3. Molybdenum rougher-scavenger and bulk sulphide flotation.
4. Molybdenum regrind and four-stage cleaner flotation.
5. Molybdenum concentrate dewatering and packaging.
6. Tungsten rougher-scavenger flotation.
7. Tungsten three-stage cleaner flotation.
8. Reagent preparation and utilities.

The concentrator process was designed to treat 10.5 M t/a of run-of-mine (ROM) feed using conventional comminution and flotation techniques. A three-stage crushing and screening circuit followed by two parallel closed circuit ball mills will be utilized to produce a suitable feed for flotation as determined by metallurgical test work.

A molybdenite rougher concentrate is then floated, reground and cleaned in four stages. The final molybdenite concentrate is thickened, filtered, dried and bagged for markets. The molybdenite tailings stream enters an adjoining Bulk Sulfide Flotation (BSF) circuit. The BSF concentrate will contain pyrite and other sulfide minerals which are removed to mitigate their interference in the downstream scheelite flotation process. Furthermore, the BSF concentrate forms the potentially ARD generating molybdenum tailings stream and is sent to the tailings storage facility (TSF) for sub-aqueous disposal to prevent oxidation.

The BSF tailings stream is then conditioned in two stages with depressants and collectors for tungsten flotation. The conditioned pulp enters the tungsten rougher circuit followed by an adjoining scavenger flotation circuit. The rougher concentrate is cleaned three times, thickened, filtered, dried and then refined to APT. The scavenger concentrate is recycled to the scheelite conditioners while the tailings, containing low levels of sulphides, are disposed to the TSF as tungsten tailings stream from the plant.

The Sisson concentrator was designed to process 10.5 Mt of ROM feed materials per year, and operate 365 days per year at an average operating availability of 92%. Daily average operating throughput rate is 28,767 t and design operating rate will be 31,269 t or 1,303 t/h.

The process flowsheet for Sisson APT plant will include the following major processing steps:
• Feed Preparation
• Digestion and Residue Filtration
• Alkali Recovery and Solution Purification
• Conversion to Ammonium Tungstate
• Ammonium Paratungstate (APT) Crystallization
• APT drying and packaging
• Reagent preparation and utilities

The APT plant was designed to process Sisson tungsten concentrates at a maximum feed rate of 29 kt/a containing 881 k mtu WO3 per year at 92% operating availability. On average the APT plant will process 19 kt of concentrates per year containing 581 k mtu of WO3 per year to produce 555 k mtu per year of WO3 contained in high quality APT product. The APT plant recovery is predicted at 96.75%. Potential exists for additional tungsten concentrate processing through the APT plant when production from the Sisson concentrator is less than APT plant capacity stated above.

Tungsten concentrate slurry from the Sisson concentrator plant will be pumped to the APT plant feed preparation circuit which will employ a wet grinding mill to facilitate size reduction and further exposure of tungsten mineral grains.

Tungsten concentrate slurry from the Sisson concentrator plant will be pumped to the APT plant feed preparation circuit which will employ a wet grinding mill to facilitate size reduction and further exposure of tungsten mineral grains. The target particle size is less than 74 µm 95% passing. The mill will operate in closed circuit with a hydrocyclone and the finished product, cyclone overflow, is fed to a thickener for dewatering and density adjustment prior to filtering. The thickener overflow will return to concentrator process water tank as make up water while the filter cake discharge from the filter press is fed to a continuous dryer to further reduce moisture to less than 0.5%. The dryer discharge is screened on a sifter to ensure a minimum particle size necessary for efficient downstream digestion of the concentrate.

Oversize from the sifter is returned to the grinding mill for further grinding. The ground and dried concentrate is stored in a hopper for feed to the digesters. An emergency storage bin with a surge capacity equivalent to five days of dried concentrate production is provided between the feed preparation and the digestion circuit for unscheduled maintenance in the downstream circuits. This surge capacity may be extended by bagging the concentrates for external storage.

The digestion section consists of digesters, dilution tanks, filter presses, residue processing equipment, and storage tanks.

The digesters are designed to extract tungsten in the concentrate by digesting with alkali solution for a period of time which will allow maximum recovery of the tungsten at optimum conditions as concluded by the test results. This process is based on methods traditionally and widely used in the global tungsten industry and confirmed by testing at SGS Lakefield laboratories on Sisson tungsten concentrates. After digestion the digested slurry is transferred to filtration of the gangue from the sodium tungsten solution.

Filtering is designed to separate the sodium tungstate solution from the undigested gangue and wash the filter cake with water for maximum tungsten recovery.

Filter cake, the undigested residue, is hauled to the tailings storage facility for disposal.

The sodium tungstate solution is next processed through a purification process where undesirable impurities are removed from the solution.

The first step is an alkali recovery step where the products are alkali and concentrated sodium tungstate solution. This is accomplished by removing excess water and then separating the two streams. The alkali is reused in the digestion step and the recovered water is used where required in the process.

The sodium tungstate solution is further processed to remove impurities such as Al, As, Mo, P, and Si. This is accomplished by pH adjustment of the treated solution to various levels followed by filtering and absorbing the impurities from the solution.

The conversion of sodium tungstate to ammonium tungstate is accomplished by loading the WO4 anion on to an organic cation in a pH controlled process. The resulting aqueous solution is pumped to the tailings storage facility for disposal. The loaded organic is then washed and stripped with an ammonium hydroxide solution to form ammonium tungstate solution and a free radical organic. The free radical organic is regenerated for reuse in the process and the ammonium tungstate is sent to the next process.

The APT is crystallized in a continuous evaporator crystallizer. The ammonium tungstate is pumped from storage to the APT crystallizer. The crystals as formed are continuously removed from the crystallizer and separated from the mother liquor by use of a filter. Mother liquor is returned to the crystallizer. The crystals are washed and sent to a dryer.

Vapors from the crystallizer and other related sources in the plant are sent to a scrubber and stripping system for reclaim and re-use in the generation of ammonium tungstate.