The Power of Commitment i Executive Summary Oceana Gold NZ Ltd (OGNZL) is in the process of developing a new open pit and a new underground mining opportunity which is collectively referred to as the Waihi North Project (WNP). The project consists of several components including the Wharekirauponga Underground Mine (WUG) and the Gladstone Open Pit (GOP). Associated with the project are several new facilities including the Willows Access Tunnel, connecting tunnel to the processing plant area (WUG Access Tunnel), the Willows Rock Stack (WRS), the GOP TSF, TSF3 and the Northern Rock Stack (NRS) as well as associated collection ponds and infrastructure. This report comprises the following studies which inform the Waihi North Project: Water balance analysis – covering estimation of quantities and quality of water for the Life of Mine (LOM) requiring treatment including additional water from the development of the new areas associated with the Waihi North Project combined with the current operations, the MOP4 cutback and Martha Underground (MUG); Baseline flow and water quality characterisation for the Mataura Stream and Wharekirauponga Stream catchment areas; Investigation of the cumulative effect on the receiving water quality in the Ohinemuri River and other potentially impacted catchments. A water balance model (WBM) constructed in the software Goldsim has been utilised to assess how water gains change over the LOM and to inform the design of the proposed infrastructure for conveyance, storage and treatment. The WBM has been in use for some time at the site (since 2012) and as a result there is confidence that it does represent well the quantities of water generated from the different water sources that require treatment for both current and future operations. Additional inputs into the model are considered conservative and therefore the predictions outlined (in terms of both water volume and quality) are also a conservative estimate based on the Waihi North Project projections. Based on the water balance analysis completed, it is predicted that the Waihi North Project can be implemented using the existing WTP functionality and within the current consent discharge and receiving environment conditions subject to renewal of those consents for an appropriate term. The capacity of the current WTP facilities will require upgrading throughout the project’s lifetime to cope with the expected volume of water requiring treatment. The estimated volume of water from WUG has a significant impact on the total volume of water requiring treatment at OGNZLs Baxter Road facilities and there is a modelled risk to the inundation of deeper stopes in MUG during the LOM as WUG water treatment requirements increase. This could impact mine scheduling. Dewatering MUG in advance (before the critical period) could reduce the risk of this inundation. Assumptions relating to WUG groundwater inflow rates and water quality will be validated well in advance of the critical period. Due to discharge restrictions during low river flow periods, there is a modelled risk of excess water requiring treatment during summer dry periods once WUG is developed. These flows can be managed operationally by one or more of the following measures: Excess flows sent to the operational TSF Dewatering of MUG/WUG temporarily suspended Mine scheduling amendments / changes Potential future alternative discharge options The storage available within the collection ponds and TSFs at the Waihi operations is shown to be adequate to contain predicted water gains without overflow within their design criteria. Water quality predictions for the LOM are compliant with discharge and receiving environment consent conditions. The utilisation of the reverse osmosis (RO) plant to further reduce concentrations of elevated trace elements is a contingency that could be utilised in the event that predicted discharge
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