Mount Porter hydrogeological recommendations

Abstract

This report contains a useful geological summary but is mainly concerned with hydrogeology. The dominate aquifer system within the proposed mining area is the metasediments of the Koolpin Formation. Permeability in this unit is largely secondary related to fractures, weathering and to a lesser extent, bedding. This unit, as evident from core descriptions, is moderately to highly fractured. Fracture density is believed to varying with depth (decreasing with depth) and proximity to fault zones (increasing closer to faults) across the site, suggesting yields may also decrease relative to these structures. The Zamu Dolerite sills form low flow boundaries throughout the site, with groundwater seeps (springs) occurring at the contact between these intrusive units and the Koolpin metasediments. Whilst these concordant sills have a low porosity they are not believed to be impermeable, with spring formation the result of a difference in hydraulic conductivity resulting in a proportion of groundwater flow discharging to the ground surface. Permeability within these intrusive igneous units would once again be secondary related to fractures and joint sets. Water levels across the proposed pit area have been measured in inclined diamond drillholes (MBDH247 and 243) drilled during previous exploration programs. These water levels have been corrected for drilling angle to obtain vertical depths to the water table. A water level measurement and water sample were also been taken from an unnamed bore to the south of the exploration lease. This hole is believed to be an old production bore used for water supply on previous drilling programs in the area. It is unregistered and limited information is known about its construction and geology. Comparisons of water level measurements between this unnamed bore and those within the proposed pit area show a 20m difference in water level over 265m (hydraulic gradient = 0.076m). This value is extremely high and indicates that the presence of a large fault system (F1) may act as the barrier to groundwater flow and connectivity between the sites. Water yields data for the site geology is limited, with no pumping tests or substantial airlifts during drilling having been recorded within the proposed pit area. Airlift measurements, however, are available for the unnamed bore with a discharge rate of 3 l/s, with a field pH of 6.26 and electronic conductivity of 341uS. This would equate to approximately 4 to 6 l/s if the bore was to be pumped, but no indication of sustainable long term yield is available. Although there is limited to no connectivity between the aquifer at the unnamed bore site and that within the pit, the geological setting is comparable, which may suggest yields encounter at this bore may be similar to those expected within the proposed pit. Groundwater yield and aquifer parameters associated with the rocks within the proposed mining area are limited. Therefore further investigations are warranted to enable the development of a realistic dewatering strategy. In order to acquire detailed information about the aquifer and to assist in dewater design several options are outlined below. The selection of a final option will be dependant on the level of development anticipated.