www.valenza-engineering.com ABN 81 147 114 743 Valenza Engineering Pty Ltd www.valenza-engineering.com ABN 81 147 114 743 [email protected] OceanaGold NZ Ltd Wharekirauponga Underground Mine Wharekirauponga Conceptual Mitigation Phase 1 Report June 2022 Report n°381_R_04_Rev0 This document has been produced for New Zealand consenting purposes only. Information contained herein must not be relied on for investment purposes Asia-Pacific Europe
www.valenza-engineering.com ABN 81 147 114 743 DOCUMENT INFORMATION Organisation OceanaGold NZ Ltd Project Name Wharekirauponga Conceptual Mitigation Phase 1 Report Job No 381 Completion Date 15 June 2022 Rev 0: Issued 17/06/2022 Print Date 17 June 2022 Status Phase 1 Version Rev 0 Distribution PDF emailed to the client Author Peter Gribbin Senior Principal Hydrogeologist Reviewed By Alexis Valenza Principal Hydrogeologist Pathname https://valenzaeng.sharepoint.com/sites/Projects/Shared Documents/Work/381 Oceana Gold - Wharekirauponga UG/Stage 1/4. Output/Letters Memos and Reports/Phase 1/Final Phase 1 Report/381_R_04_Rev0 OGNZ_WUG_Phase1_Conceptual_Mitigation.docx File Name 381_R_04_Rev0 OGNZ_WUG_Phase1_Conceptual_Mitigation.docx Cover Images and logos are © Valenza Engineering Commercial-in-confidence. This document and the information, ideas, concepts, methodologies, technologies and other material it contains remain the intellectual property of Valenza Engineering Pty Ltd (ABN 81 147 114 743). This report has been prepared for OGNZL following the terms and conditions of the appointment. Valenza Engineering cannot accept any responsibility for any use of or reliance on the contents of this report by any third party
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation DISCLAIMER This document has been produced for New Zealand consenting purposes only. Information contained herein must not be relied on for investment purposes. Scope of Services: Valenza Engineering has prepared this report in accordance with the scope of services set out in the contract, or as otherwise agreed, between the client and Valenza Engineering. Reliance on Data: In preparing the comments and edits to this report, Valenza Engineering has relied upon data, surveys, analyses, designs, plans, and other information provided by the client and other individuals and organisations, most of which are referred to in the report (the data). Except as otherwise stated, Valenza Engineering has not verified the accuracy or completeness of the data. To the extent that the statements, opinions, facts, information, conclusions, and/or recommendations in the report (conclusions) are based in whole or part on the data, those conclusions are contingent upon the accuracy and completeness of the data. Valenza Engineering will not be liable in relation to incorrect conclusions should any data, information, or condition be incorrect or have been concealed, withheld, misrepresented, or otherwise not fully disclosed to Valenza Engineering. Interpretations and Conclusions: In accordance with the scope of services, Valenza Engineering has relied upon the data provided in the preparation of comments and edits to the report. The nature and extent of monitoring and/or testing conducted and reviewed are described in this report. On all sites, varying degrees of non-uniformity of the vertical and horizontal soil, rock, and/or groundwater conditions are encountered. Hence no monitoring can eliminate the possibility that the data obtained are not representative of ground and/or groundwater conditions encountered. The interpretations and conclusions in the edits to this report are based upon the available data and are therefore merely indicative of the conditions from the available data at the time of preparing the report. Also, it should be recognised that the data reviewed are from a limited period and that site conditions can change with time. Within the limitations imposed by the scope of services, the analysis performed and the edits to the report have been undertaken and performed professionally, following generally accepted practices and using a degree of skill and care ordinarily exercised by reputable hydrogeological consultants under similar circumstances. No other warranty, expressed or implied, is made. Report for Benefit of Client Only: The edits to this report have been prepared for the benefit of the client, Valenza Engineering, and no other party. Valenza Engineering assumes no responsibility and will not be liable to any other person or organisation for or in relation to any matter dealt with or conclusions expressed in the report, or for any loss or damage suffered by any other person or organisation arising from matters dealt with or conclusions expressed in the report (including without limitation matters arising from any negligent act or omission of Valenza Engineering or for any loss or damage suffered by any other party relying upon the matters dealt with or conclusions expressed in the report). Parties other than the client and Valenza Engineering should not rely upon the report edits or the accuracy or completeness of any conclusions and should make their enquiries and obtain independent advice in relation to such matters. The Valenza Engineering report was prepared according to the scope of services set out in the contract, or as otherwise agreed, between the client and Valenza Engineering (scope of services).
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation TABLE OF CONTENTS ABBREVIATIONS ....................................................................................................................1 ABBREVIATIONS (Cont.) ........................................................................................................2 ABBREVIATIONS (Cont.) ........................................................................................................3 1. INTRODUCTION........................................................................................................1 2. BACKGROUND..........................................................................................................2 2.1. GEOLOGICAL SETTING AND MINERALISATION....................................................2 2.2. HYDROLOGY AND TOPOGRAPHY..........................................................................3 2.3. HYDROGEOLOGY....................................................................................................4 2.4. WHAREKIRAUPONGA UNDERGROUND PROJECT ...............................................5 3. PREVIOUS STUDIES ................................................................................................6 3.1. ACCESS TUNNELS ..................................................................................................6 3.2. POTENTIAL GROUNDWATER EFFECTS ................................................................8 3.3. POTENTIAL SURFACE WATER EFFECTS ............................................................11 4. CHARACTERISATION.............................................................................................13 4.1. SURFACE WATER SYSTEM ..................................................................................13 4.2. CHARACTERISATION OF SURFACE WATER - GROUNDWATER INTERACTIONS 15 5. MODELLING ASSESSMENTS.................................................................................16 5.1. PREVIOUS ASSESSMENTS AND MODELLING ....................................................16 5.2. GEOLOGICAL MODEL............................................................................................19 5.3. ENGINEERED MITIGATION MODELLING..............................................................19 5.4. CONCEPTUAL MITIGATION MODEL .....................................................................25 5.5. NUMERICAL MITIGATION MODEL ........................................................................28 6. GROUNDWATER MITIGATION MANAGEMENT.....................................................29 6.1. GROUNDWATER CONTROL STRATEGY..............................................................29 6.2. GENERAL MITIGATION AND CONCEPT BY ZONE...............................................32 6.3. GROUTING SEQUENCE AND SCENARIOS ..........................................................35 6.4. DEWATERING, DRAINAGE AND DEPRESSURISATION.......................................37 6.5. SEQUENCING AND MINE PLANNING ...................................................................37 6.6. EXAMPLE OF APPLICATIONS IN SIMILAR ENVIRONMENTS ..............................38 7. RESOURCE CONSENT PROCESS ........................................................................40 7.1. CONSENT CONDITIONS ........................................................................................40 7.2. GROUNWATER CONTROL STRATEGY DATA ACQUISITION ..............................41 7.3. ADDITIONAL FIELD PROGRAM.............................................................................42 8. MONITORING AND TRIGGER POINTS ..................................................................43 8.1. DEVELOPMENT OF MONITORING NETWORK.....................................................43 8.2. PRE-DEVELOPMENT .............................................................................................46 8.3. OPERATIONAL PHASE ..........................................................................................47 8.4. CLOSURE AND POST CLOSURE PHASE .............................................................47 9. CONCLUSIONS & RECOMMENDATIONS..............................................................48 10. REFERENCES.........................................................................................................50
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation LIST OF FIGURES Figure 2-1:WUG Vein System..................................................................................................3 Figure 3-1:WUG Proposed Tunnel Alignment ..........................................................................7 Figure 3-2:Tunnel Alignment Showing Hydrogeological Significant Features (Golder, 2021) ...8 Figure 4-1:Otahu Catchment (after GWS) ..............................................................................13 Figure 4-2: Spring Discharges and Radon Concentrations Indicative Groundwater Flow (after GWS) 14 Figure 4-3:Spring Locations, Groundwater Contours and T-Stream and EGV Veins (after GWS) .....................................................................................................................................15 Figure 5-1: Access Tunnel CHM for Waihou Groundwater Catchment (after CWS) ...............16 Figure 5-2: Access Tunnel CHM for Otahu Groundwater Catchment (after GWS) .................17 Figure 5-3: Schematic cross section of WKP-102 and WKP-04D (left) and WKP-101 and WKP-103 (right) (after FloSolutions) ......................................................................................22 Figure 5-4: Schematic cross section of WKP-104 (left) and WKP-03D (right) (after FloSolutions) ..........................................................................................................................23 Figure 5-5: Illustrated Tunnel Probing and Grouting Zones ....................................................26 Figure 5-6: Details of Tunnel Probing and Grouting Zones.....................................................26 Figure 5-7: Illustrated Grouting Zone from Surface Positions within the Mine Area ................27 Figure 5-8: Illustrated Grouting Zone from Underground Grouting Chambers ........................27 Figure 6-1: Groundwater modelling process (modified after Barnett et al. 2012) ....................31 Figure 6-2: Example of zones where operational (secondary and tertiary treatment) might be required..................................................................................................................................33 Figure 6-3: Combined Instrumentation and response grouting in a stopping configuration (adapted from MMG Dugald River trial stoping - Underground Design Methods 2015 – ACG) ..............................................................................................................................................34 Figure 6-4: Typical grouting pattern for tunnel sections (after Grouting Specialists) ...............35 Figure 6-5: Grouting and Instrumentation completed from Underground Chambers developed in conjunction with the Declines and Drives ...........................................................................35 Figure 6-6: Dewatering and Instrumentation completed from Underground Chambers developed in conjunction with the Declines and Drives (deep dewatering wells are shown in blue dotted lines)....................................................................................................................37 Figure 6-7: Example of active pore pressure monitoring within a UG block. ...........................39 Figure 6-8: Example of active temperature monitoring within a hot UG mine. ........................39 Figure 8-1: Proposed piezometer locations ............................................................................44
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation LIST OF TABLES Table 3-1:Aquifer Hydraulic Properties (GWS) .......................................................................10 Table 5-1:Scenarios simulated in AnAqSim (after FloSolutions April 2022, ref 17) .................18 Table 8-1: Proposed Piezometer details ................................................................................45 Table 8-2:Proposed EG Vein investigation Piezometers ........................................................45 LIST OF APPENDICES Appendix A Maps and Figures Appendix B Presentation on Conceptual Mitigations Appendix C Knowledge Gap and Consenting Timeline Appendix D Example of Grouting Applications Appendix E FloSolutions Proposed Monitoring Locations Appendix F Mitigation Measure Strategy and Baseline Data Acquisition (ref 381_M_05)
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation ABBREVIATIONS Ag Silver AGMG Australian Groundwater Modelling Guidelines AMA Aquifer Management Area AS/NS Australian/New Zealand Standard Au Gold BGL Below ground level BTOC Below the top of casing COPC Contaminants of Potential Concern CFP Coromandel Forest Park CMA Crown Minerals Act 1991 CHM Conceptual Hydrogeological Model CRF Cemented rock fill CSAMT controlled-source audio-frequency Magnetotellurics CSM Conceptual (Geological) Site Model CSV Coromandel Volcanic Zone DOC Department of Conservation DEM Digital elevation model DO Dissolved oxygen EC Electrical conductivity EDA Experimental data analysis
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation ABBREVIATIONS (Cont.) EGV East Graben Vein EP Exploration Permit ESP Electro Submersible Pump FHT Falling head test GL Gigalitres GLD Gigalitres per day GMMP Groundwater Monitoring and Management Plan GMO Groundwater Management Objective GOP Gladstone Open Pit Project g/t Grams per tonne ha Hectare HDC Hauraki District Council K Permeability (hydraulic Conductivity) m Metres Ma Million years masl Metres above (mean) sea level mg/L Milligrams per litre µg/L Micrograms per litre MOP Martha Open Pit Project MUG Martha Underground Mine mV Millivolts
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation ABBREVIATIONS (Cont.) NM Numerical model NZPAMNew Zealand Petroleum and Minerals OGNZLOceanaGold New Zealand ORP Oxidation-Reduction Potential OM Observational Method pH Potential Hydrogen PMDI Progressive modelling and mitigation measures design and implementation strategy RL Relative Level RMA Resource Management Act 1991 RQD Rock quality designation SWL Standing (or static) water level TARP Trigger Action Response Plan TDS Total dissolved solids TSF Tailing storage facility TVZ Taupo Volcanic Zone VWP Vibrating wire piezometer WNP Waihi North Project WRC Waikato Regional Council WRS Waste rock stack WUG Wharekirauponga Underground Mine
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 1 1. INTRODUCTION OGNZL, a subsidiary of OceanaGold Corporation, is developing the Wharekirauponga Underground Project (WUG) consisting of Wharekirauponga Underground Mine and associated access infrastructure. The WUG is located 10km north of Waihi township, 150km southeast of Auckland, beneath Crown Land administered by the Department of Conservation (DOC) as the Coromandel Forest Park (CFP). The project is currently operated under NZPAM CMA exploration permit EP 4059 which allows for exploration activities including surface drilling. Valenza Engineering Pty Ltd (VE) has been tasked by OGNZL to complete an assessment of underground engineered mitigation measures to avoid any impact on surface water systems. This scope includes the analysis of the information collated to date by OGNZL and other parties, the development of conceptual mitigation factors (pre-drilling strategy, monitoring, grouting patterns, advised mining methods, dewatering and groundwater control strategies) and provision of advice on the data requirements. Appropriate suggestions for further groundwater installation, monitoring, and testing programs for the immediate consenting process and for the operational consent compliance have been developed with the other consultants involved.
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 2 2. BACKGROUND 2.1. GEOLOGICAL SETTING AND MINERALISATION The regional geological setting of the Coromandel Peninsula is built up of Miocene to early Pliocene andesite-dacite-rhyolite and subaerial volcanic rocks of the Coromandel Volcanic Zone (CVZ) overlying a Mesozoic basement. The CVZ hosts gold and silver deposits that make up the Hauraki Goldfield. The regional geology is flanked to the west by the Hauraki Rift, a large graben filled with Quaternary and Tertiary sediments, and to the south by volcanics deposited by the presently active Taupo Volcanic Zone (TVZ). The volcanism that produced the CVZ formed during three main phases. Coromandel Group (18 – 3 Ma) andesites and dacites constitute the first phase; with Whitianga Group (9.1-6.0 Ma) rhyolitic units, the second phase; and the Mercury Bay Basalts (6.0-4.2 Ma), dominated by Strombolian volcanoes and dykes, the third. The final phase volcanic phase is responsible for the epithermal Au-Ag and Cu porphyry deposits and mineralised quartz veins, with hydrothermal alteration observed throughout the Coromandel and Whitianga Groups. These veins are steeply dipping persisting for a 200 to 2,000m strike and up to 700m dip, with vein widths commonly 1 to 5m and up to 30m. The entire area is overlain by postmineralisation andesites and pyroclastics. Early mining at Wharekirauponga was short-lived in the late 1800s but revived in the 2000s with prospect drilling of underground mineable veins, namely the Western Vein, Teawamoamutu-Stream Vein (TStream) and the East Graben (EG) Vein, the latter being continuous for a kilometre. Both the T-Stream and EG veins are resource targets for the WUG and have associated hanging wall and footwall stringer vein sets. These veins are described as extensional in relation to their structural controls and are hosted by Whitianga Group rhyolites, rhyolitic flow domes and intrusions within polymict lapilli tuffs. Mineralised sequences are overlain by andesitic flows, rhyolitic tuffs and recent ash deposits. Hydrothermal alteration often completely replaces the primary mineralogy of the rhyolites with quartz and adularia accompanied by illite-smectite clay alteration. Where this alteration is associated with veins and displacement, mineralised clay gouge is often present. As the East Graben footwall is approached, these veins become brecciated indicating the degree of fault control as opposed to a more extensional derivation. 2.1.1. LOCAL LITHOLOGICAL AND STRUCTURAL CONTROL A block model is constructed in Vulcan1 that contains the resource drilling data held in an acQuire2 drill database that defines lithological domains and provides a geological model primarily for grade estimation. Lithological controls are associated with hydrothermal alteration predominantly in the rhyolites. The Waiwawa sub-group (7.9-5.6 Ma) of the Coromandel Group is the most extensive geological unit in the area comprising andesite and dacite lava flows and tuff breccias, and dacitic ignimbrite, tuff and siltstone. The younger Omahine subgroup (6.7-6.6 Ma) andesites, dacites tuffs and breccias (which will 1 Maptek Vulcan software 3D geological modelling, mine design and production planning. 2 acQuire geoscientific information database management system
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 3 be encountered in the tunnel), are exposed by movement on the NNE structural alignment and subsequent erosion. Kaimai subgroup (5.6-3.9 Ma) intrusives and extrusives make up the covering dome, to the CVZ and are present to the east of the portal area. These comprise lava flows and domes, tuff and tuff breccias with intercalated volcaniclastic sediments and local welded dacitic ignimbrite. Older, faultemplaced rocks of the Coromandel Group are present, locally with extensive hydrothermal alteration. These lithic and pumice-rich ignimbrites, rhyolites and obsidian-rich pumice breccia deposits and tuff will be intercepted at the termination of the tunnel in the WUG. Tauranga Group sediments infill faulted and erosional depressions. These materials comprise pumiceous alluvial gravelly sand, silty clay and peat; estuarine silt and mud interbedded with ignimbrite; and tephra from the Taupo Volcanic Zone and are the host rocks of the deposit. Structural controls are associated with the East Graben fault sequence with quartz veins within the rhyolitic flows becoming brecciated. Vein dips become steeper and sub-vertical, with a striking change from the northeast to a more northerly orientation. Northeast-trending fault is inferred in the Waiharakeke valley with strong north to northeast-trending fault block at the tunnel termination. Less prominent faulting may occur along the other valleys and, if present, may be penetrated by the proposed tunnel. The orientation of the vein systems relative to the mine plan is shown in Figure 2-1. Figure 2-1:WUG Vein System 2.2. HYDROLOGY AND TOPOGRAPHY The topography around Waihi in the Ohinemuri River valley is flat within the alluvium with isolated volcanic outliers forming small hills to the north of the township. Notable is the area of Martha open pit directly north of the town centre. These outcrops become continuous moving north into the Coromandel Ranges which reach a height of 500masl. Within the Ranges and in the WUG area the topography is deeply incised by the northeast-trending valley system and densely bush covered. There are no established vehicle access routes into the Wharekirauponga area. The WUG lies beneath two surface water catchments, with the proposed access tunnel from the catchment divide in the Coromandel Ranges to the northwest of Willow Farm southward falling within the substantial Waihou River catchment, and the tunnel and WUG to the north of the divide falling within the Otahu River catchment. The Ohinemuri River drains to the south through Waihi township before joining
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 4 the Waihou River at Paeroa, which then discharges to the Firth of Thames. The northern catchment of the Otahu River drains the four upland sub-catchments with the Wharekirauponga sub-catchment overlying the WUG itself covering 14.5km2. These upland streams follow valleys aligned with the NNE structural trend. The Otahu River occupies a catchment area of around 70km2 draining to the northeast reaching the sea immediately to the south of the town of Whangamatā. The Wharekirauponga subcatchment is flashy following rainfall, with stream flows supported largely by surface runoff and interflow (unsaturated zone subsurface flow). 2.3. HYDROGEOLOGY The regional hydrogeology is typical of an upland volcanic setting with shallow perched aquifers occupying valley fill colluvium underlain by highly fracture-controlled groundwater systems within tight volcanic bedrock at depth. Groundwater flow in the surficial aquifers largely follows topography with short flow paths maintaining stream baseflows, where surface water is in connection with valley-fill deposits and displaying little opportunity for recharge to the deeper systems. Unsaturated zone interflow and surface runoff dominate the maintenance of stream flows as is evident from the flashy nature of the surface water systems. A degree of lithological control to the shallow groundwater is evident where hydrothermal alteration, weathering and volcanogenesis either increase permeability through silicification or decreases this with the presence of clay minerals and fine-grained or welded tuffs. Brecciated volcanics and pumiceous formations may also enhance permeability. The deep groundwater system is controlled by fracture flow in the post-formational veins and faults and follows the structural NNE trend with overall drainage following the topographic trend in a similar direction. Recharge is thought to occur in the upland areas where suitable lithologies allow a greater degree of infiltration (GWS). The structural control of recharge is likely to be dependent on whether these are related to extensional or compressional movement. Deep circulation of groundwater allows discharges to occur as warm and cold springs at intersections with surface water. There is a hydrochemical indication of a general positive hydraulic gradient that connects the warmer, more mineralised, deep groundwater system with the surface via the structural controls. Groundwater discharge to the Wharekirauponga Stream amounts to 68L/s and occurs primarily from the headwater springs that flow at rates between 1.5 and 10L/s (GWS). Underground mining experience elsewhere at Waihi suggests groundwater inflows outside the vein systems can be expected to be low where the mine and tunnel formations are unaltered (GWS).
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 5 2.4. WHAREKIRAUPONGA UNDERGROUND PROJECT The WUG is intended to have an eight-year operating life at 800ktpa following five years of development which includes the construction of a 6.8km access tunnel and other mine infrastructure. The WUG orebody is hosted in the EG and T-Stream veins, dips between 65 and 70⁰ and is between 3m and over 20m in width with a median of 4.2m. The vein-hosted nature of the deposit makes the Modified Avoca mining method used by OGNZL in its existing Waihi underground mines appropriate. The Modified Avoca method is a productive, low-cost mining method involving semi-selective ore extraction within declines and stopes suited for moderately dipping deposits of varying thickness. Mining would progress from the mid-level sill both up and down, with wider sections in the southern East Graben Vein (EGV) area having multiple parallel stopes being mined using cemented rock fill (CRF). Waste rock will be delivered from underground to a surface stockpile and then transported back underground as part of the AVOCA mining method. Ore will be delivered directly to the processing ROM. 2.4.1. WUG GEOTECHNICS, HYDROGEOLOGY AND HYDROLOGY Standard geotechnical parameters are determined from the resource drilling core logs, including RQD, fractures per metre and hardness, with samples taken for standard laboratory test work. The drillhole database is held in acQuire. A preliminary hydrogeological assessment has been completed by GWS consultants according to the current level of understanding which is based on limited hydrogeological information. The deposit lies beneath the Wharekirauponga Stream which has a catchment area of 15km2 and receives 2,170mm rainfall annually on average, producing a total catchment volume of around 90 GLD. The intrinsic permeability of the host volcanic extrusive formations (rhyolites, dacites and basalts) is generally low and is not expected to produce significant inflows. The silicic epithermal veins introduced during late stage vulcanism may be expected to produce large inflows as a result of their more porous nature. The EGV may be associated with spring flow to the headwaters of the surface water catchment and is expected to dewater in response to underground drainage. The hydraulic connection of the EGV with the surface aquifer and potential headwater springs is uncertain and needs to be proven. Engineered controls introduced during ore extraction may be required where there is potential for changes in discharge and gains in surface water flows. The management of catchment water balance changes in response to mine development, groundwater/surface water interaction, and operational dewatering have become critical elements from a resource consent perspective, with the need to demonstrate no measurable change in the natural flows or water quality of surface water systems and no measurable dewatering of the soil Regolith which could have an adverse effect on vegetation a key outcome to be achieved. Superficial deposits, including weathering alterations, regoliths and soils, are highly clay altered and may form groundwater flow barriers that are unlikely to drain because of mine dewatering.
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 6 3. PREVIOUS STUDIES Previous groundwater related studies during 2020 and 2021 have focused on several key matters required to allow the WUG mine development to proceed. These are notably: • Development of the twin access tunnel (exploration drive). Selection of alignment. Groundwater effect. Requirement for resource consent for the WUG to allow construction to proceed. • Effects of tunnel construction on surface water systems and their management such that there is no measurable change in the natural flows or water quality of surface water systems and no measurable dewatering of the soil Regolith which could have an adverse effect on vegetation. • Effects of mine development and mine operational dewatering on surface water systems and their management such that there is no measurable change in the natural flows or water quality of surface water systems and no measurable dewatering of the soil Regolith which could have an adverse effect on vegetation. • Groundwater and surface water testing and monitoring. Monitoring wells, water quality sampling, hydraulic tests (packer), streamflow gauging, and stream level logging. • Conceptual modelling. Geological model development. Numerical modelling (in progress). 3.1. ACCESS TUNNELS The access tunnel extends in two sections, the Willows Access Tunnel from the Willows Farm portal to the orebody, and the WUG Access Tunnel from the processing plant to the junction with the WUG access tunnel. Overall, the access tunnels extend northward from a portal close to the processing plant north of Waihi township for 6.8km, terminating in the WUG mine at the location of Ventilation Shaft 1. The tunnel is single-bore over the 1.3km WUG section northward from the processing plant portal up to the boundary of the CFP, and twin-bore for the remaining 5.5km from the point where it is joined by the Willows Farm extension. The Willows Farm access tunnel exits at a road head portal on the CFP land boundary. The tunnel alignment is shown in Figure 3-1 and in Appendix A Figure 1 (Site Overview). The portal is at an elevation of 180masl declining to -10masl at 1.5km chainage and then gently climbing to 50masl towards its termination in the WUG. These elevations place the tunnel invert level at between 150 and 275mbgl (at its maximum below the crest of the Coromandel Ranges). Alignment selection was based on OGNZL land ownership, geotechnical considerations, and minimising disturbance or nuisance to property, amongst other factors (Golder ref 3). GHD developed a Conceptual Geological Site Model (CSM) in late 2020 (GHD ref 2). Originally described as the WUG Exploration Tunnel, this was intended to allow additional mineral resources, geotechnical and hydrogeological drilling and testing from an underground position, where access from the surface was restricted by the current conditions of access to the conservation land.
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 7 Figure 3-1:WUG Proposed Tunnel Alignment The CSM was developed based on limited intrusive investigation and is intended to support further studies at a high level, including groundwater modelling and both physical and chemical surface water interaction. Significant faults and lineaments identified from surface mapping likely to control groundwater inflows to the tunnel were included together with lithological controls resulting from hydrothermal alteration (e.g., clay mineral development and silicification) and formation changes (e.g., andesite and rhyolitic ignimbrite). The tunnel alignment in relation to published geological mapping is provided in Figure 3 of Appendix A. The tunnel passes through Waipupu Formation Andesite (aw/Mci) consisting of andesitic flows, breccias, and tuffs some of which are hydrothermally altered. The tunnel then passes through the down-thrown Waihi Fault block of the younger Whitiroa Andesite (ah/Mco), consisting of andesitic flows, breccia and tuffs, before turning into Waipupu Formation Andesite. The tunnel crosses the regional Waihi Fault approximately 1.8km north of the plant portal, where ground conditions are expected to deteriorate in the presence of wide (10s to 100s of metres) brecciated zones and locally developed sheared clay gouge zones. Ignimbrites are likely to present locally poor ground conditions with low strength material and potential for high groundwater inflows. Rapid changes in tunnel face conditions can be expected with variable strength materials and groundwater inflow. The understanding of anticipated tunnelling conditions could be improved through a surface drilling campaign accompanied by geomechanical and hydrogeological testing and sampling. The tunnel zone around the Waihi Fault should be targeted. Geophysical investigations provide a non-invasive method for
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 8 informing likely tunnelling conditions when used in conjunction with surface drilling. Drilling ahead of face advancement is likely to be the only way to fully characterise ground conditions along the alignment and a remedial plan for ground stabilisation, grouting and dewatering will need to be in place to address this. The tunnel is designed to limit the potential groundwater inflow, with incidental seepage anticipated from the intact andesites managed through conventional mine water management, and larger potential inflows from fault zones grouted and sealed ahead of their interception in the tunnel face. 3.2. POTENTIAL GROUNDWATER EFFECTS The assessment of groundwater effects from both the access tunnel development and the WUG underground operations and the mitigation of these through engineered solutions is a prerequisite for the project to proceed. Groundwater interacts with the surface water system both maintaining springs feeding the headwaters of the Wharekirauponga Stream and summer baseflow along its course. Conversely, groundwater recharge is derived seasonally during high-stream flow conditions. Generally, springs are related to the intersection of veins at the surface that brings groundwater from depth. The Waihi Basin aquifer is designated an Aquifer Management Area (AMA) by Waikato Regional Council within the Waikato Regional Plan. The Waihi Basin AMA is sub-divided into the shallow aquifer system (0.5 to 30m depth) and the deep system (>30m). The proposed take for mine dewatering is from the Waihi Basin deep aquifer system. The granting of resource consent for the WUG operation is dependent on an assessment of groundwater and surface water effects. The impact of both access tunnelling, and mine construction and operation on spring flow, stream baseflow and groundwater recharge, will require management and effective avoidance through underground engineered mitigation. Uncontrolled tunnel inflows are likely to be significant where the alignment crosses the regional structures. The most significant of these is the Waihi Fault 1.8km north of the plant portal, containing wide brecciated and shear zones and weak tunnelling conditions (Figure 3-2). Figure 3-2:Tunnel Alignment Showing Hydrogeological Significant Features (Golder, 2021)
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 9 The conceptual hydrogeological model (CHM) produced by GWS (ref 11) was developed to assist in the assessment of effects on groundwater (summarised in Section 5.1.1). Groundwater inflow analyses completed by GWS indicate that between 1.1 and 5GL per year of groundwater will be removed from the Waihou groundwater catchment during the development of the WUG access tunnel and 4.6GL per year from the Otahu groundwater catchment (Waihi Basin deep aquifer). The Waihou groundwater catchment, which is sub-divided into the shallow and deep aquifer systems, is part of the Waihi Basin AMA. The Willows Farm access tunnel from the portal to the vent shaft lies within the Mataura Stream surface water sub-catchment which drains south to the Ohinemuri River. The upper reaches of this stream are steep, and flows are unlikely to be sustained by baseflow to any degree. A multi-level VWP installation in the vent shaft area suggests a vertical downward hydraulic gradient. In the flatter, lower reaches depths to groundwater are small and there is evidence from borehole hydraulic tests of a moderately permeable, perched aquifer in the pyroclastic materials forming the bedrock., with relatively shallow hydraulic gradients (0.02) towards the stream. Groundwater inflow analyses completed by GWS indicate drainage into the Willows Farm tunnel of 0.2GL per year. The twin tunnel section of the access tunnel from the junction of the two tunnels to the mine lies within the Otahu groundwater catchment beneath the Waiharakeke, Thompson and Wharekirauponga Streams. The upper reaches of these streams are steep with little baseflow contribution. Stream baseflow is expected to be mostly sourced from the shallow regolith soils, with low flows fed by bedrock discharge from rhyolite outcroppings. These areas are within the conservation estate. The acceptable effects envelope in these areas is particularly stringent, with the activity needing to be managed such that there is no measurable change in the natural flows or water quality of natural state surface water systems within conservation land and no measurable dewatering of the soil Regolith which could have an adverse effect on vegetation. Groundwater removed from the Otahu groundwater catchment is transferred via the access tunnel for treatment and discharge or use within the Waihou groundwater basin. 3.2.1. HYDROGEOLOGICAL UNDERSTANDING Groundwater catchments and resultant flow systems generally align with the surface water catchment and topographic divides that follow the ridgeline within the Coromandel Ranges to the north of Waihi. These general surface water and groundwater flow directions will be influenced locally by the structural controls that follow a similar NNE trend. To the south of the divide, in the first third of the tunnel alignment, relatively steep hydraulic gradients are anticipated reflecting the topography and the vertical recharge to the deeper groundwater. Towards Waihi, the groundwater system is present as a shallow unconfined aquifer in the surficial deposits of alluvium and younger volcanic materials with a low hydraulic gradient towards the south. Groundwater flow is controlled by paleochannels perched above the eroded basement andesites. In the area around Waihi township, hydraulic gradients steepen considerably and swing westwards towards the Martha open pit under the influence of mine drainage. The underlying andesites through which the immediate tunnel alignment passes from the portal form non-aquifers (they do not store or transmit any significant quantity of groundwater) with pore pressures also affected by the mining and dewatering of the Martha and Favona vein systems. The influence of these pore pressure reductions on the first section of tunnelling is likely to decrease rapidly advancing northwards. To the north of the divide, groundwater is present within valley-fill colluvial, perched aquifers that are topographically controlled, confined between bedrock outcrop and of limited lateral extent. Hydraulic gradients to the north and northeast mimic the topography. The depth of the tunnel alignment for the remaining two-thirds length from the divide to the WUG and the low hydraulic conductivity of the andesites
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 10 means there is little or no hydraulic connection between these perched aquifers and the deeper groundwater system except where structures are present. 3.2.2. HYDRAULIC PARAMETERS Lithologically generic groundwater hydraulic parameters have been established according to rock type from experience elsewhere at Waihi according to Table 3-1. Table 3-1:Aquifer Hydraulic Properties (GWS) Hydraulic Conductivity Storativity Material Max (m/s) Min (m/s) Geomean (m/s) Max Min Shallow Aquifers Ash / Alluvium 1 x 10-4 1 x 10-7 0.3 0.1 Ignimbrite 1 x 10-5 1 x 10-8 0.01 0.001 Rhyolitic Tephra 1 x 10-6 1 x 10-7 0.1 0.05 Deep Aquifer Andesite Surface 3 x 10-5 2 x 10-6 5 x 10-6 0.3 0.1 Andesite to 50 m Depth 7 x 10-9 6 x 10-9 0.01 0.005 Andesite to 100 m Depth 6 x 10-7 6 x 10-9 3 x 10-8 0.01 0.005 Andesite (>100 m depth) 1 x 10-5 1 x 10-8 0.05 0.001 Early geological and resource information was collected from fully cored inclined drillholes within the WUG by OGNZL, with four holes intersecting the EG Vein and one the T-Stream Vein used for hydrogeological characterisation. Hydrogeological observations during drilling were limited to the recording of voids and drilling fluid circulation losses. Packer testing was completed by Golder in July 2021 (ref 11) in all these drillholes across test intervals within, and on either side (hanging wall and footwall), of the veins to determine the range of hydraulic parameters. Within this work package, the objective was to determine the typical higher permeability of the veins compared to the expected low permeability of the host formations and to characterise the lower permeabilities expected in the weathered and altered rocks. Multi-level, three-sensor, vibrating wire piezometers (VWP) were installed in three of these boreholes (WUG-P03D, WUG-P04D and WUG-P05D). The packer test program completed 38 hydraulic tests, 35 of which are considered valid (12 constant rate injection, 23 Lugeon) in both single and double (straddle) packer arrays. The two FHTs completed in WUG-P03D did not produce a valid test result due to hole collapse during the test, however, it was possible to derive an approximation of the transmissivity of the T-Stream Vein at this location of 0.129m2/day. The packer test results overall indicate a moderate to high permeability (mean 1.3 x 105m/s) for both the rhyolite host rocks and the veins. The shallow weathered rhyolite, andesite cap, ignimbrite and tuff units indicate lower permeability (mean 3.0 x 10-7m/s) consistent with the presence of alteration and weathering products (clay minerals and silicification).
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 11 FloSolutions completed a robust re-evaluation of the packer test results to confirm the general distribution of permeability values across the test zones (Section 5.3.1). The VWP installations, consisting of 9 sensors in the three drillholes indicate a strong downward hydraulic gradient with a perched shallow system in the superficial geology. 3.3. POTENTIAL SURFACE WATER EFFECTS The access tunnel from the southern portal to the CFP boundary lies beneath the Waihi Basin surface water catchment with the main channel of the Ohinemuri River draining to the west and lying to the east of the proposed tunnel alignment. The tunnel does not pass beneath the Ohinemuri River but will be driven below tributaries to the river. Beyond the divide between the Otahu and Waihou catchments in the Coromandel Ranges, the tunnel alignment lies beneath the Wharekirauponga sub-catchment. 3.3.1. POTENTIAL TUNNEL EFFECTS GWS have completed both groundwater monitoring studies and assessments of effects from tunnelling and mining operations (ref 1). GWS conclude that the bulk of the formation through which the tunnel is constructed consists of low-permeability, low-storativity andesite with any groundwater being stored and transmitted in fractures. Dewatering is therefore largely limited to management of the groundwater contained in fault and vein systems. It is likely that some of these fractures connect with the surface water system where the impacts of uncontrolled dewatering and groundwater drainage within the tunnel could cause an impact. Control of underground groundwater inflow through an appropriate grouting method applied to these fractures as part of an engineered mitigation is therefore required, with dewatering at the surface needing to be avoided. The initial southern part of the WUG Access Tunnel decline is already dewatered from the existing Favona underground mining operations and for that reason, no further effects on the shallow groundwater system or surface waters beyond that which have already taken place are expected. Within the intact fresh andesite minimal tunnel inflows are expected, indicatively being 15m3/d (GWS). At two locations the tunnel alignment passes through fault or fracture zones (Section 3.1) which may be hydraulically connected to the Mataura Stream. Surface water losses in these situations (lost from diversion of flow paths in the andesite) amount to about 15m3/d (GWS) if uncontrolled from an underground position. According to GWS, this amount of stream water loss would be indiscernible in the context of the baseflow in the Mataura Stream. Assessment of the anticipated ground and groundwater conditions along the tunnel alignment is not achievable before construction commencement due to access restrictions. Geophysical investigations (CSAMT) are proposed as a non-intrusive method to aid in the assessment of tunnel conditions with the observational method (i.e., forward probing ahead of the advancing face) adding to the assessments presented in this report. The assessment of effects on groundwater includes the following: • Groundwater inflows to the tunnel elements. • Drawdown effects relate to the tunnel elements. • Potential for effects on aquifers. • Potential for effects on surface waters. • Potential for effects on other groundwater users.
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 12 • Potential for effects on the surface ecological system. Tunnel inflow measurement will be required to demonstrate the potential effects on surface waters or other groundwater users are consistent with the predictions made from modelling and assessment, and to demonstrate the effectiveness of the underground mitigation measures. GWS summarise that a minimal risk of effects exists to shallow groundwater, surface water, groundwater users and plant growth. 3.3.2. POTENTIAL MINE DEVELOPMENT EFFECTS Five vent shafts are proposed as part of the WUG project. The first of these lies within the Willows Farmland and outside of the CFP. Dewatering of the vent shafts during their construction is necessary and the reduction of any impacts from this activity is required through engineered mitigation. Further assessment suggests the potentially more significant interaction between the groundwater and surface water systems from the mine development associated with the high permeability sections of the EGV within the mine stopes.
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 13 4. CHARACTERISATION 4.1. SURFACE WATER SYSTEM The Wharekirauponga is one of four upland stream sub-catchments that flow into the Otahu River. The Otahu River occupies a catchment area of around 70km2 draining to the northeast reaching the sea immediately to the south of the town of Whangamatā (Figure 4-1). The Wharekirauponga sub-catchment is flashy following rainfall, with stream flows supported largely by surface runoff and interflow (unsaturated zone subsurface flow). Figure 4-1:Otahu Catchment (after GWS) Whangamatā Otahu Catchment N
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 14 Groundwater discharge occurs (low flow of 68L/s) primarily from the headwater springs that flow at rates between 1.5 and 10L/s (Figure 4-3). A small warm spring discharges at a rate of 3.5L/s from the EG vein where this intercepts the stream. A radon survey has been completed by GWS to determine the contribution of deep groundwater (high radon) with surface water and interflow (low radon) (Figure 4-2). Figure 4-2: Spring Discharges and Radon Concentrations Indicative Groundwater Flow (after GWS) 4.1.1. SURFACE WATER QUALITY Field parameters and laboratory analytical results of the surface water in the Wharekirauponga catchment have been monitored in parallel to surface water gauging events. Similar hydrochemical characteristics of sodium bicarbonate water type is reported between the surface water samples with the exception of the warm springs showing calcium bicarbonate composition GWS (ref 7).
www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 15 4.2. CHARACTERISATION OF SURFACE WATER - GROUNDWATER INTERACTIONS In general, warm springs are most likely to have a volcanic origin from deep groundwater with short travel times and it is reflected in the similar ionic characteristics observed between springs and deep groundwater (ref 7). However, similar shifts in water chemistry of surface water towards shallow groundwater have been noted during high flow events. These interactions have been re-assessed using naturally occurring tracer, radon gas. The estimated contributions of deep groundwater to surface water were found to be higher in the headwaters of T stream, Edmonds and Thompson sub-catchments, and Wharekirauponga catchment. Elevated radon levels in the warm spring suggest a deep underground origin. These radon results are more appropriate for applications to further understand possible mixing scenarios and end members based on simple linear mixing calculations than dating water samples as they have very short half-lives. Figure 4-3:Spring Locations, Groundwater Contours and T-Stream and EGV Veins (after GWS)
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