Grassy Mountain Gold Project, Oregon, United States of America
The Grassy Mountain Gold project is an underground mine located in Malheur County, approximately 35km south of Vale city in Oregon, US. It is owned by Paramount Gold Nevada following the acquisition of Calico Resources, the former owner of the mine, in July 2016.
Calico completed the preliminary economic assessment (PEA) for the project in February 2015, and updated it in July 2015.
In August 2016, Paramount announced its plans to commence the pre-feasibility study (PFS) for the project, which is expected to be completed in 2017. Paramount will also commence the environmental impact statement (EIS) and the permitting process in early-2017.
The PEA estimates a mine life of ten years, with an average annual production of 53,000oz of gold and 82,000oz of silver from the mine.
Geology and mineralisation of the Grassy Mountain project
The Grassy Mountain formation contains granitic clast conglomerate, arkosic sandstone, clay-rich siltstone, fine grained sandstone, tuffaceous mudstone and olivine basalt flows. A sedimentary portion, 700ft-1,000ft thick, can also be found, along with other solidified sedimentary units.
Gold mineralisation at the mine is found in the sediments of interbedded siltstone and fine-grained sandstone, composed of thin quartz chalcedony-adularia veinlets and stockworks. Mineralisation also occurs in the epithermal hot springs deposition, along with the presence of several siliceous sinter terraces that are interbedded with the silicified clastic sediments.
Grassy Mountain gold mine reserves
The project contains both underground and open-pit resources. The measured and indicated underground resources are estimated to be 3.24 million tonne (Mt) containing 5.03 million ounce (Moz) of gold and 8.43Moz of silver, grading 0.155g/t Au and 0.260g/t Ag respectively.
The measured and indicated open-pit resources are estimated to be 65.44Mt, containing 11.49Moz of gold and 41.33Moz of silver grading 0.018g/t Au, and 0.063g/t Ag respectively. The inferred resources are estimated to be 2.21Mt, containing 15,000oz of gold and 22,000oz of silver grading 0.007g/t Au, and 0.010g/t Ag respectively.
Mining and processing at the gold mine
The proposed mining method for the project includes both underground and open-pit mining methods. Open-pit mining will be adopted upon completion of the underground operation.
The initial production fleet constructing the decline in the site will include a twin boom production drill, a load-haul-dump, three 33t haul trucks, a roof-bolter, and a shotcrete machine.
The mined ore will be processed using an initial gravity circuit followed by a gravity tails leach.
The mine site is accessible from the city of Vale through the twin springs road, which begins at US Highway 20.
Initial operations at the mine will be powered by on-site diesel generators as no electrical power is available at the site. A proposal is, however, in place to develop a new power line to connect to the Idaho Power network at a cost of $18.3m.
If approved, Idaho Power will construct a 69kV, 18 mile power line that will connect the existing network at the main connection near Vale to the mine site.
The PEA estimates that the mine will require between 150g/m and 300g/m of water for both mining and processing operations.
Geological modelling and engineering firm, Metal Mining Consultants, prepared both the PEA and amended PEA reports for the project. The resource and geologic information was provided by Hardrock Consulting.
Positive bench-scale metallurgical testing was performed by Resource Development.