Mineralogically, the Savannah North mineralisation is observed to be similar to the Savannah ore, i.e. pyrrhotite-dominant sulphides with sub-ordinate pentlandite and chalcopyrite.
Flotation testwork was conducted by site-based metallurgists using composite samples from the Savannah North Upper and Lower Zones. Standard Savannah sample preparation, grinding, flotation and reagent test procedures were conducted to provide a direct comparison of the metallurgical performance between Savannah and Savannah North mineralisation. These tests were conducted across three grind sizes being 100um, 110um and 120um, which are typically achieved at Savannah. Results of the flotation tests showed both the Savannah North Upper and Lower Zones performed within the range of typical Savannah ore performance.
Processing records per shift for the twelve year period from commencement of the Savannah Project in 2004 up to placing the operation on care and maintenance in May 2016, were reviewed as part of the Savannah FS. These records demonstrate a consistent relationship between nickel recovery, head grade and concentrate grade. A best-fit recovery equation was developed from the data for the range of expected nickel head grades and targeted concentrate grade of 8% Ni.
The existing Savannah processing plant consists of conventional crushing, grinding, flotation and concentrate handling (Figure 1). The Savannah plant operated continuously from August 2004 to May 2016, with continuous improvements in recoveries and throughput achieved from changes in operating practices and minor circuit modifications. The nominal throughput capacity of the Savannah plant is approximately 1.0Mtpa.
Modelling assumes all ore is milled as it is mined, with little or no ROM stockpiling. Life-of-mine mill throughput averages 0.8Mtpa, with higher throughput, averaging 870ktpa, in the first five years of full production before decreasing to average 750ktpa in later years. Life-of-mine nickel head grade averages 1.38% Ni, with quarterly averages varying from 1.2% Ni to 1.6% Ni. Lower grades are processed in the first year of production, associated with the remnant Ore Reserves at Savannah. The annual milling profile is shown in Figure 2.
The product will be a bulk Ni-Cu-Co concentrate targeting a concentrate grade of 8% Ni. The Company notes that for the month of May 2016, prior to going into care and maintenance, the Savannah plant averaged 9.0% Ni in concentrate. Processing recoveries at the target concentrate grade over life of mine are expected to average 87% Ni, 96% Cu and 90% Co, based on historic Savannah plant performance.
Metal in concentrate production is anticipated to average 9,700t Ni, 5,000t Cu and 670t Co per year, with nickel in concentrate production in the first five years averaging 10,900t Ni and peaking at 12,300t Ni. Over life of mine, approximately 99,200t Ni, 51,500t Cu and 6,900t Co in concentrate is expected to be produced. Annual metal in concentrate production is shown in Figure 3.
Figure 1 – Savannah Processing Plant
Figure 2 – Savannah FS – Annual production and grade profile
The existing processing plant and accommodation camp are sufficient for the proposed future operations.
All of the infrastructure required for a re-start of operations remains on site and is being maintained by the care and maintenance crews. Major fixed plant and mobile equipment is either ready to be recommissioned or can be at short notice, with refurbishments fully costed in the financial model. Underground decline and pumping systems have been maintained and operated so that minimal rehabilitation will be required to recommence underground operations.
The major infrastructure works required on a re-start of operations include: Savannah North decline access development, Savannah North FAR and TSF1 tailings dam lift. A new tailing storage facility (TSF2) will be required within three years of commencing production. In October 2014, Coffey Mining Pty Ltd undertook an options study for a new tailings storage facility (TSF2) and a preferred option has been selected, designed and costed. The capital cost estimate for TSF2 was updated by Coffey in January 2017.