Studies have been undertaken to characterise site hydrogeological conditions in the context of the proposed development of a new mining project in a high rainfall environment.
The site is a copper-gold porphyry deposit that occurs within a north-east to north-north-east trending corridor of epithermal intrusives. The surrounding host rocks are of volcanic origin which have a massive, rather than bedded, structure. Both the intrusives and host rocks are of a fine-grained nature.
Core logging and packer testing has been undertaken in boreholes to evaluate hydrogeological conditions and estimate permeabilities at the site. The results of this testing indicate hydraulic conductivities across the various geological domains and at depths of more than 40-50 m were relatively consistent and generally low, ranging from 8 x 10-10 to 5 x 10-8 m/s. Airlift testing from open boreholes and wells indicate much higher (orders-of-magnitude higher) hydraulic conductivities in the shallower (less than 40-50 m depth) hydrogeological units. These units and local faults will provide the key preferential pathways for groundwater flows, as hydraulic gradients change during mine dewatering.
The local conceptual hydrogeological model comprises a shallow “active” groundwater system of weathered / altered rocks (Unit 1) that experiences high rates of rainfall recharge and is directly connected, and feeds, local river systems. Unit 1 is underlain by a deeper, less permeable and less active groundwater system (Unit 2). Both Units 1 and 2 comprise the same intrusive and volcanic host rock lithologies but are differentiated by the degree of alteration and fracturing, which are significantly higher in Unit 1. The results of hydrogeological borehole investigations, and observations of flowing (artesian) boreholes, suggest that deep, highly permeable faults are present and will likely provide key pathways for groundwater inflows to the pit once open pit mining progresses.
The results of baseflow analyses of local river flows and groundwater (numerical) model development and model calibrations indicate that groundwater recharge rates are high, and that the local groundwater system is highly connected to local rivers. Long term constant rate tests results in wells undertaken near pit boundaries and adjacent to rivers also reach the same conclusion, i.e. that the groundwater system and local rivers have high connectivity and will influence rates of pit inflow.
The study concludes that hydrogeological conditions at the proposed copper mine will be controlled by alteration and faulting, with connectivity to local rivers significantly influencing mine inflows.