The Ulu gold project in Nunavut, Canada, was the focus of advanced exploration that included extracting a bulk ore sample in 1996. The development included infrastructure pads comprising waste rock excavated from underground. The waste rock is predominantly PAG based on recent static testing. Seepage monitoring and rinse pH measurements have shown that drainage is circum-neutral pH, and the rock pads are predominantly weathering under circum-neutral pH conditions; however, acid generating conditions are present locally. The site currently runs both exploration and reclamation programs.
A method was developed to calculate delay to ARD using QEMSCAN mineralogical data from sieved <2mm samples that provided a record of the 24-year site weathering conditions of the waste rock. The mineralogical data represented in situ particle surface weathering and accounted for the duration that the waste rock was frozen during the winter. The current sulphide mineral content (predominantly pyrrhotite), and the current goethite content, were used to back calculate initial pyrrhotite content, and hence oxidation rate, for seven samples, assuming that all goethite had precipitated in situ from iron released during pyrrhotite oxidation. The relationship between initial pyrrhotite content and oxidation rate was used to calculate a rate constant for input into the following equation:
Where t = delay to ARD; (TIC/AP)current is the current TIC/AP (total inorganic carbon/acid potential) of the waste rock; k is the sulphide oxidation reaction rate constant (units of 1/time) calculated from site mineralogical data; and (TIC/AP)crit is the critical TIC/AP value indicating how effectively carbonate minerals neutralize the acid generated by sulphide oxidation.
The results are interpreted to give delay to initial onset of ARD, and delay to widespread ARD, for the infrastructure pads at Ulu. The method is applicable to other projects where PAG waste rock has undergone weathering at site under predominantly circum-neutral pH conditions, and iron oxyhydroxides have precipitated as a result of sulphide oxidation. At Ulu, delay to ARD predictions are being used to aid in reclamation planning and management of PAG rock.
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