Mining studies for the Watershed Project have been completed by Orelogy Group Pty Ltd (Orelogy), a specialist mining engineering firm. Mining studies for the Project have included:
- Scoping Studies;
- Mining Fleet recommendations;
- Mining Cost Modelling;
- Geotechnical drilling and modelling;
- Pit Designs, and
- Waste Rock Dump Designs.
Mine production is based on an optimised cut-off grade schedule developed to feed the processing plant at a rate of 2.5Mtpa.
The operation is planned to have a life of 10 years. Mining will be undertaken for a period of 9 years after which low grade stockpiles will be processed. There is significant potential that further exploration will be successful in identifying additional near mine resources (either surface of underground) and extending the mine life.
Primary rock breakage is planned to be achieved by conventional drill and blast techniques, with secondary breakage completed by either mobile rock breaker or secondary drill and blast (estimated to be less than 1% of ore moved).
Mining by 200 tonne hydraulic excavator and 100 tonne dump trucks was selected as it provided the lowest technical risk relative to other mining options. It also allowed for greater certainty in estimating mining capital and operating costs.
Besides the resource potential at depth, there is also significant potential along strike. Within a distance of 2 to 5km from Watershed, Utah discovered 3 prospects in 1982, which can are along-strike extensions of Watershed. Those prospects, Watershed South, Desailly North and Desailly, are very similar to Watershed with scheelite mineralisation being hosted by calc-silicate altered arenites. Utah carried out limited drilling on these prospect in 1983 and 1984 before exploration at Watershed ceased. Vital has recently continued exploration on these prospects, believing they hold the potential to extend the life of the Watershed processing plant.
Dempers & Seymour Pty Ltd (D&S) developed a Mining Rock Mass Model (MRMM) which was used to develop pit slope design parameters for the Project. In 2008 D&S geotechnically logged 131 exploration drill holes for 24,225m. D&S also designed and logged seven geotechnical holes to complete geotechnical coverage of the designed pit extents.
Based on the rigorous analyses of the MRMM, D&S has provided a range of recommend pit slope parameters for the pit design. Rock conditions at Watershed are generally good with the recommended pit inter-ramp slope angle being between 47 to 55 degrees for the weathered/transitional material and between 48 to 55 degrees for the fresh rock.
Batter heights and angles varied by material type and domain, the average height recommended for the weathered/transitional material was 10m with a 55 degree slope. In the fresh rock batter height were generally set as 20m.
Pit dewatering will be undertaken by in-pit sump pumping with inflow from pit walls and seasonal rainfall directed to these sumps for removal to storage/settling ponds. Prior to the wet season a portion of the pit will be advanced by at least one bench level below the active face with large capacity drainage sumps developed on this lower level.
The Waste Rock Dumps are designed for co-disposal of processing materials. As such the dumps will contain all waste materials generated by the mining and processing operations.
Conventional grade control will be completed with assaying at the onsite laboratory to facilitate quick turnaround times. As the ore is easily identified a high level of geological control will be maintained during mining. As can be seen in the images below, the ore is a creamy beige colour with the contained scheelite being strongly fluorescent and the waste is dark grey to black which will assist selective mining.