The programme has been completed with the aim of estimating the aquifer properties, including drainable porosity (specific yield) and hydraulic conductivity, for the upper section of the lake surface aquifer. This aquifer would be the target for initial production from a trench network. The trenching programme has covered the surficial aquifer extent within the core tenement E38/3065.
Trigg Mining Managing Director Keren Paterson said: “This is a another significant and encouraging milestone that provides further evidence that we have a very large and potentially company-making asset at Lake Throssell. The lake trenching and test pumping program has delivered highly encouraging results, providing invaluable data on the key characteristics of the surficial aquifer. Work is now underway on a maiden Indicated Resource, which represents a critical input to our ongoing Scoping Study work.”
The programme consisted of two trial trenches and seven test pits distributed across the current resource. The trenches were 100 m long and were surrounded on all sides by a number of monitoring pits.
The trial trenches were pumped until water levels in the majority of monitoring pits had stabilised – which was between 10 and 11 days in both cases. Test pits consisted of a small pumping trench between 6 and 9 m long with an adjacent monitoring pit. Test pits were de-watered and the brine level draw-down and recovery rates were monitored.
Throughout all testing brine levels, flow rate and brine quality were frequently monitored. Trenching was completed with a 15-t amphibious excavator that was able to excavate to depths of between 3 m and 4.5 m.
The geology observed in the excavations was consistent with that previously logged during the resource drilling programmes targeting the lake surface.
The lake surface to 2 m below the ground level is more heavily dominated by gypsum, with clayey horizons dominant to the base of the excavation, intermixed with silty zones. The large excavation provided an opportunity to see the layered nature of the system, the brine inflow horizons associated with gypsum dominated layers and the geotechnical stability of the walls.
The test pumping data has been analysed using local scale numerical models in groundwater modelling software FEFLOW. The models were calibrated to the brine level draw-down and recovery by changing the hydraulic conductivity (horizontal and vertical) and specific yield of the aquifer to obtain an acceptable fit between the measured data and the simulation.
The final aquifer property results are highly variable, confirming the highly heterogenous nature of the lake surface aquifer. Specific yield results varied between 0.01 (stiff clay) and 0.4 (the maximum possible related to gypsum evaporite sequences).
Horizontal hydraulic conductivity ranged between 0.2m/d (stiff clay) and 340m/d (gypsum dominated flow). These results will be used to upgrade the lake surface brine Resource and inform the Scoping Study of potential trench production rates, following further modelling.
Brine samples were obtained at the start and end of pumping of the test pits, while the trial trenches were sampled every day for field parameters of salinity, SG and pH, with samples retained for laboratory analysis on every second day.
The brine analysis results range between 4610 mg/L potassium (10.28 kg/m3 of equivalent SOP) and 6070 mg/L potassium (13.54 kg/m3 of equivalent SOP). These results align well with the Inferred Resource grade of the lake surface of 4867 mg/L potassium (10.85 kg/m3 of equivalent SOP).
Brine grade variations during pumping show a muted varied response with most test pits moderately reducing in grade by less than 3%, while the grade increased by up to 10% at LTTT01.
Following the completion of the on lake trenching programme, updated Mineral Resource modelling and Mineral Resource estimation work has commenced to calculate a JORC Indicated Mineral Resource Estimate for the surficial aquifer, which is expected to be completed before the end of this quarter.
This programme will also allow preliminary sizing of the trench network required to support the Scoping Study SOP production during initial years of the project.
Read the article online at: https://www.worldfertilizer.com/project-news/09072021/trigg-mining-completes-brine-pumping-trials-at-lake-throssell/