Star Kimberlite Geology And Mineralization
The Star Kimberlite was deposited within the Cretaceous sedimentary rocks of the lower Colorado and Mannville groups, which unconformably overlie Paleozoic limestones and dolomites. The glacial overburden thickness ranges from 90 to 130 m with an average of 92 m (Table 7-2). Portions of the Star Kimberlite have been emplaced contemporaneously with the deposition of the Mannville and lower Colorado sediments. However, the majority of the Star Kimberlite is interpreted to have erupted through the Mannville and into the early parts of the lower Colorado Group sediments (Joli Fou Formation time). The local lower Colorado and Mannville interface is situated approximately 170 m. The Mannville Group and Paleozoic interface lies approximately 340 m, as interpreted from the Company’s core drill holes.

The Star Kimberlite consists of two distinct types of kimberlite: dominant eruptive kimberlite and subordinate kimberlitic sediments. The eruptive kimberlite deposits at the Star Kimberlite are sub-divided into five main kimberlite phases emanating from a single vent, each with distinctive physical and chemical properties, which enable mapping and stratigraphic correlation of units.

Orion South Kimberlite Geology And Mineralization
Like the Star Kimberlite, the Orion South Kimberlite was deposited within the Cretaceous sedimentary rocks of the lower Colorado and Mannville groups, which unconformably overlie Paleozoic limestones and dolomites. The glacial overburden thickness ranges from 97 to 121 m with an average of 105 m (Table 7-2). Portions of the Orion South Kimberlite have been emplaced contemporaneously with the deposition of the Mannville and lower Colorado sediments as seen in Figure 7-7. However, the majority of the Orion South Kimberlite is interpreted to have erupted through the Mannville and into the early parts of the lower Colorado Group sediments (Joli Fou Formation time). The local lower Colorado and Mannville interface is situated approximately 191 m below surface. The Mannville Group and Paleozoic interface lies approximately 347 m, as interpreted from drill holes. The Orion South Kimberlite is comprised of multiple eruptive units (or phases), each of which is texturally, mineralogically, physically and chemically distinct. Within the kimberlite, the units have cross-cutting relationships near conduits, but are stacked vertically within the volcanic edifice and crater / extra-crater deposits. Several conduits, feeding different units, have been identified on Orion South.

During Cantuar (Mannville Group) deposition, thought to be a time of continental fluvial-deltaic deposition (Zonneveld et al., 2004), kimberlite was deposited and reworked. Drilling indicates that the Cantuar-aged kimberlite deposits are generally thin (< 30 m thick) sheets occurring at multiple horizons within the Cantuar sediments. The bulk of the kimberlite deposits are confined within the marginal marine to marine sedimentary strata (Zonneveld et al., 2004) of the Upper Mannville Group (Pense Formation) and the lower Colorado Group (Joli Fou Formation). These kimberlite deposits are associated with the main crater excavation and crater fill. Proximal to the conduits and in close proximity to the base of the Mannville Group sandstone, the conduits flare (Scott-Smith et al., 1994) at a steep angle giving way to shallow angles near the margin of the craters.

The Orion South Kimberlite consists of two distinct types of kimberlite: dominant eruptive kimberlite and subordinate kimberlitic sediments. The eruptive kimberlite deposits at the Orion South Kimberlite are subdivided into six main kimberlite phases, each with distinctive physical and chemical properties which enable mapping and stratigraphic correlation of units

Kimberlite Hosted Diamond Deposits
Primary diamond deposits such as kimberlites and lamproites have produced over 50 % of the world's diamonds, whereas the remaining 50 % are derived from recent to ancient placer deposits that have formed from the erosion of kimberlite and / or lamproite. Notably, it has been established by the scientific community that diamonds are not genetically related to kimberlite or lamproite but that kimberlite, lamproite and other deeply derived magmas serve as a transport mechanism for bringing diamonds to surface. The diamonds form at the same level as, or shallower than, the kimberlite magmas within the mantle and as the kimberlite magma ascends towards the surface they incorporate foreign fragments (termed mantle xenoliths) of the material they pass through. Those xenoliths commonly disaggregate into individual mineral constituents (termed xenocrysts). These xenocrysts include diamonds.

Clifford (1966) and Janse (1994) have stated that a majority of economic diamondiferous kimberlites occur in stable Archaean age cratonic rocks that have not undergone thermal events or deformation since 2.5 Ga. Such Archaean-aged cratons include the Kaapvaal, Congo and West African Cratons in Africa, Superior and Slave Provinces in Canada, East European Craton (Russia, Finland, etc.), and the West, North and South Australia Cratons. The only exceptions, to date, are the Argyle and Ellendale lamproite mines of Australia, which occur in Proterozoic aged remobilized cratonic zones.

To date, over 6,000 known kimberlite and lamproite occurrences have been discovered in the world, of which over 1,000 are diamondiferous. Economic diamond-bearing kimberlite and / or lamproite pipes range from less than 0.4 ha to 146 ha in footprint size, with the maximum size being greater than 200 ha (i.e. Catoca, Angola). Economic kimberlite diamond grades can range from 1.3 cpht to 600 cpht.

Kimberlite remains the principal source of primary diamond despite the discovery of high grade deposits in lamproite. Mineralogical and Nd-Sr isotopic studies have shown that two varieties of kimberlite exist (Mitchell, 1986):
• Group 1: or olivine-rich monticellite-serpentine-calcite kimberlites; and
• Group 2: or micaceous kimberlites (which predominantly occur in southern Africa).

With a few exceptions, such as the Finsch Kimberlite Mine in South Africa and the Dokolwayo Kimberlite Mine in Swaziland, most of the well known diamondiferous kimberlites in southern Africa and elsewhere are Group 1 kimberlites, including those in Canada and, in particular, FalC.

In contrast, Group 2 kimberlites are confined to southern Africa.

Currently, three textural-genetic groups of kimberlite are recognized in Group 1 kimberlites, each being associated with a particular style of magmatic activity (Mitchell, 1986). These are:
• crater facies
• diatreme facies
• hypabyssal facies.

Rocks belonging to each facies differ in their petrology and primary mineralogy, but may contain similar xenocrystal and megacrystal assemblages (Mitchell, 1986).

Mining is carried out using conventional open pit mining. The Orion South kimberlite is mined first, followed by the Star kimberlite.

Pre-Stripping
There is a considerable amount of unconsolidated overburden that is removed during a four-year prestripping period. The vast majority of this overburden is removed using three bucket-wheel excavators (“BWEs”).

Merchantable timber is harvested prior to stripping. A fleet of excavators and articulated trucks excavates a 20-30-metre-deep starting “key” or “slot” for a BWE. After that BWE has progressed for a period, enough space will be cleared behind it to allow a second, deeper key to be excavated – again using the excavator and articulated truck fleet. Then, the second BWE starts operating. The same procedure is used to establish the third BWE.

BWE spoil is removed from the pit on overland conveyors and delivered to a single, large spreader. This crawler-mounted spreader places the spoil on the waste pile.

Kimberlite Mining
Hydraulic shovels load the kimberlite into rigid-frame trucks. It is estimated that seventy percent of the kimberlite will require drilling and light blasting. The powder factor will be quite low to avoid diamond breakage.

The trucks transport the kimberlite to an in-pit mineral sizer, which crushes larger lumps before placing the kimberlite onto an overland conveyor for transport to the mill stockpile.

Backfilling
The Orion South kimberlite is mined first, followed by the Star kimberlite. After Orion South is mined-out, waste from Star and mill tailings are placed in the Orion South pit.

Pit Optimisation
The mine design is based on a sub-blocked version of 2015’s block model. The original block model used blocks of 50 m (x) x 50 m (y) by 25 metres (z), and each block incorporated several rock units. Grades were assigned to individual rock units using block factors.

The sub-blocked model contains varying block sizes to match the geology models such that each resource block is a single rock unit, thus easing optimization for the different diamond prices and different pit slope angles for different rock units.

Mine Development
Each pit is developed in three stages:
• Stage 1 comprises primary bucketwheel slot development, start-up and commissioning of each bucketwheel as the slots are developed, and establishment of the main bucketwheel waste conveyor.
• Stage 2 comprises full operation of the bucket-wheel excavators, with kimberlite and waste mining, with approximately five years of plant feed (designed to surpass the capital payback period). And,
• Stage 3 comprises completion of bucketwheel excavation and continued kimberlite production until the end of pit’s life.

Milling
There are two autogenous mills located in an insulated building. Each mill is fed separately from the stockpile. Dilution water is added to the mills either directly from the process water supply or as de-gritted effluent from the DMS, XRT and Recovery Plant circuits.

A single re-lining machine (including feed chute removal trolley, liner handling tool and Thunderbolts) is shared between the two mills.

Kimberlite is milled and exits to the classification circuit through a discharge grate with 50mm ports directly onto double deck sizing and de sliming screens. Large volumes of pulping water are added to improve washing and prevent clay build up on the screen decks.

The washed -50 +10mm material from the mill product screen top deck gravitates to a dry sizing screen where it is screened at 32mm. In order to maintain the graduated feed PSD, the +32mm material is returned to the mill feed by conveyor.

The screen undersize -32 +10mm is collected and conveyed to the XRT circuit feed surge bin in the adjacent building.

The bottom deck product -10 +1mm is collected and conveyed to the DMS feed surge bin in the adjacent building.

The sheer volume of solids and high slurry SG have eliminated the need for a traditional degrit and thickening circuit. The -1mm undersize from each of the mill product sizing screens is collected in the screen underpan and then pumped to the Pachuca’s from where it is pumped to the slimes impoundment facility.

The slurry reports to a cyclone cluster positioned on the slimes dam wall. The grits in the cyclone underflow are used to build the impoundment walls whilst the cyclone overflow reports to one of the dam paddocks and allowed to settle. No flocculants are used in this circuit.

No slimes dam wall building is undertaken during the winter months and the slurry is discharged directly into the slimes paddocks.

Introduction
For the Star – Orion South Diamond Project Preliminary Economic Assessment (PEA), DRA designed a diamond Process Plant that is best suited to treating the kimberlites at FalC. The FalC Kimberlites are generally categorized as “soft” and are amenable to autogenous grinding (AG) milling. The functions encompassed in the Process Plant are comminution, classifying of coarse kimberlite based on density and size, identifying and recovering diamonds and discharging processed kimberlite. The plant includes AG mills in the comminution section, 510 mm dense medium separation (DMS) cyclones in the DMS section, and in the recovery section the diamond sorting equipment includes X-Ray Transmission (XRT) sorting, XRay Fluorescence (XRF) based X-ray sorting and grease technology.

Plant Design
The Process Plant is designed to concentrate and recover diamonds in the size range from 32 mm to 1 mm.

Starting at the crushed (-400mm) ROM stockpile disc ........