Abstract Orogenic gold deposits from the Reefton goldfield in New Zealand hosted in Ordovician metasediments of the Greenland group have produce 67 tons of gold before 1951. The Blackwater mine in Waiuta account for about 1/3 of the gold production at Reefton prior 1951. The ore system at Blackwater consist of NE trending steeply dipping gold-bearing quartz veins (Birthday reef) occurring in faulted, sheared and folded alternating sequence of sandstone-mudstone metamorphosed to low greenschist facies and hydrothermally altered proximal to distal from the quartz vein. Host rock and ore forming fluids interaction resulted in a systematic change in the mineralogy and geochemistry of the wallrock developing a distinctive footprint of the ore system. The mineral assemblage subsequent to hydrothermal alteration is formed by quartz veining, chlorite, carbonates (minor calcite-dolomite-ankerite-siderite), albite, K-Mica and composite quartz-carbonate veining, carbonates spotting, pyrite, arsenopyrite, chalcopyrite, cobaltite, galena and in a lesser extent ullmannite, gersdorffite, pentlandite, millerite and sphalerite. Mass balance calculation based on geochemical data obtained by ultra-low detection analysis with a detection limit in part per trillion of Au, As and Sb provide new insight in the geochemical footprint of the ore system at Blackwater. A clear zone of roughly 40 meters (20 meters above and below the birthday reef) is enriched in Au, As and Sb up to respectively 6806%, 605% and 891% compared to the un-mineralised protolith. Furthermore, mass changes in K2O and Na2O indicate a consistent decrease in Na and increase in K in the vicinity of the Birthday reef reflecting the replacement of albite by K-mica. This is corroborated by pattern of alkali alteration index Na/Al for albite and 3K/Al for K-Mica showing similar trend. Carbonation and de/-hydration index also exhibits peaks in samples adjacent to the Birthday reef, though carbonation index is also influenced by carbonates content in the protolith or late carbonation that may not related to gold mineralisation. Using the indicators above-mentioned it is possible to define the mineralogical and geochemical “footprint” for the ore system in the host rock allowing to use this footprint as a tool for mineral exploration for orogenic gold deposits similar to Blackwater. Given that the geochemical footprint of orogenic gold deposit such as Blackwater is significantly wider than the economically viable part of the deposit defining the footprint of the ore system offers the potential for vectoring from sub-economic mineralisation towards higher-grade ore that is economically viable.

This thesis presents an engineering geological and geotechnical investigation of the proposed Terrace Opencast Coalmine highwall in the Reddale Valley, Reefton. The proposed pit will target the 4-11m thick No. 4 Seam coal, which exists on the Valley floor beneath outwash gravels and Brunner Coal Measures (BCM) overburden dipping at 15-30° to the northwest. Rock coatings are providing friable sandstone units with protection from weathering in existing cut faces and may contribute to short term pit wall stability. The BCM core was divided into four geotechnical units for rock material testing purposes: unit 1 siltstone, unit 2 carbonaceous mudstone, unit 3 interbedded sandstone and carbonaceous mudstone and unit 4 loose sandstone. The average results for units 1-3 gave classifications within the medium to high porosity (9-13%) and dry density (2250-2470kg/m³) ranges, and medium to medium high slake-durability Id2 values (72-94% retained). Unit 4 (loose sandstone) recorded very low dry density (1694 kg/m³) and slake-durability Id2 (9%) average values. Strength testing confirmed that the units can be classed as weak rocks, with average UCS values of 12.8-13.7MPa for units 1-3, and for all four units Is(50) from point load testing of 0.26-0.62MPa with low cohesion values (0-6.2MPa) from triaxial testing. Friction angles from triaxial tests gave high values of 32-45°, while direct shear tests established 15° internal friction for bedding planes in carbonaceous mudstone and 37° for a high angle defect in interbedded sandstone/carbonaceous mudstone. The average Young's modulus values ranged from 0.82 to 10GPa, and Poisson's ratio between 0.39 and 0.50. Eight scanline defect surveys established that the major discontinuities in existing cut faces consist of high angle tension joints, shallow dipping bedding, and faults related to regional uplift. The defect orientations from the scanlines located in the southwest were significantly different from those in the northeast, and may be due to the faults that cross the Valley. In general the majority of defects displayed low persistence (less than 3m), were clean and tight, and had low joint roughness coefficients (JRC less than 6). Joint wall compressive strengths gave an average of 32MPa, but were affected by case-hardening on weathered faces. The results from the 8 drill holes analysed show that 37% of core was within the excellent rock quality designation class (RQD = 90-100%), while 29% was in the very poor quality rock (RQD = 0-25%). A semi-confined aquifer in the outwash gravels that will drain into the proposed pit was found to have a transmissivity of 58m²/day and hydraulic conductivity of 3.1 x 10⁻⁵ m/s. Kinematic feasibility assessment determined an optimum highwall orientation of 65° dip to 120° (dip direction), which is within at least 20° of the coal seam strike. The likelihood of planar, wedge or toppling failure depends on whether the structural conditions are similar to those encountered in the southwest or northeast scanlines, as well as the persistence of the defects present. The occurrence of small scale (less than 1m offset) 'step-up' normal faults, and the three larger faults that cross the valley, all of which are related to regional uplift, will also affect which failure mode will be kinematically feasible. Other crucial slope stability considerations include groundwater inflow from the saturated overburden and bedding parallel failures on the footwall dip slope of the pit. An investigation into case hardening on existing cut faces identified three interconnected rock coatings: iron films, lithobiontic (biological) and clay-dominated crusts. Jarosite was found at sites with abundant pyrite and the oxidation of iron may have been aided by microbial activity. A green algae inhabiting pore spaces approximately 1mm below the surface was noted beneath an iron film and it is suggested to be similar to that found in arid environments. Although lithobiontic and clay-dominated crusts did not provide the weathered surface with any additional strength, they were observed to form relatively quickly (from months to less than 5 years) and will aid short term stability by providing the batters with protection from weathering processes. This project concluded that the overburden material in the proposed highwall can be expected to behave like weak rock and in some cases (such as the loose sandstone) can be expected to have soil characteristics. Highwall stability is more likely to be affected by substantial inflows of groundwater than highly persistent joint sets. Establishment of the highwalls in their final position in the early mining stages will enable development of rock coatings that are expected to aid short and long term stability.

Fanny Creek drains from Island Block opencast coal mine, near Reefton on the West Coast of the South Island of New Zealand, and is impacted by acid mine drainage (AMD). The objectives of this study were to characterise drainage chemistry in Fanny Creek catchment, and to determine optimal passive treatment strategies for Fanny Creek AMD for future pilot or full-scale application. This was undertaken by monthly monitoring in Fanny Creek catchment between February 2008 and January 2009 to collect drainage chemistry and flow data. Laboratory trials of suitable passive AMD treatment systems were conducted and their treatment performance assessed to select and design optimal passive treatment strategies for Fanny Creek AMD. Oxidation of pyrite in Brunner Coal Measure sediments at Island Block mine generates AMD. Fanny Creek originates from a number of AMD seeps on the eastern waste rock slope of Island Block mine. Seeps have low pH (<3.23) and a single detailed metal analysis indicates drainage is enriched with aluminium and iron, and contains elevated concentrations of manganese, copper, nickel, zinc and cadmium relative to applicable water quality criteria such as ANZECC guidelines. Acidity and metal loadings of drainage in the catchment indicates AMD from the northern waste rock slope contributes most of the acidity (~70%) and metal (60%) in Fanny Creek, and acts to re-dissolve additional metals upon mixing with drainage from other slopes. The most suitable location for a passive AMD treatment system in Fanny Creek catchment is on the Waitahu Valley floor, near monitoring site R12, because this allows for sediment removal prior to a treatment system. Fanny Creek AMD at site R12 was characterized in detail because this data assists with selection and design of passive AMD treatment systems. Fanny Creek at site R12 contains on average 6.0 mg/L aluminium, 1.3 mg/L iron, 3.1 mg/L manganese, 0.49 mg/L zinc, 0.14 mg/L nickel, 0.0071 mg/L copper and 0.00048 mg/L cadmium. Average pH at site R12 was 3.95, calculated acidity averaged 42.7 mg CaCO₃/L, and flow rate ranged from 1.5 L/s to about 30 L/s. Acidity and metal generation from Island Block mine increases linearly with flow in the catchment, and therefore Fanny Creek drainage chemistry is not significantly affected by rainfall dilution. Natural attenuation of AMD occurs by addition of un-impacted alkaline drainage from Greenland Group basement rocks, wetland ecosystem processes, and geochemical reactions along Fanny Creek that decrease acidity and metal concentrations before AMD discharges into the Waitahu River. During low flow conditions (summer months), surface flow of AMD into the Waitahu River does not occur because of subsurface flow loss. Three suitable passive AMD treatment options for Fanny Creek AMD were selected and trialed at ‘bench top’ scale in a laboratory. These included a sulfate reducing bioreactor (SRBR), a limestone leaching bed (LLB), and an open limestone channel (OLC). The potential to mix Waitahu River water with Fanny Creek to neutralize AMD was also investigated. Fanny Creek AMD was employed for laboratory trials, and influent flow rates into SRBR, LLB and OLC systems were regulated to assess performance at different hydraulic retention times (HRT). Optimal HRTs for future treatment system designs were determined from effective AMD treatment thresholds, and include 51 hours, 5 hours and 15 hours for SRBR, LLB and OLC systems, respectively. To determine optimal treatment options for Fanny Creek AMD the effectiveness of each trial option was compared to applicable water quality criteria, and scale up implications of treatment options was assessed. The SRBR system had most effective AMD treatment, with water quality criteria achieved for metals, greatest alkalinity generation, and highest pH increase. However, a full scale SRBR system has significant size requirements, and long term treatment performance may be limited. The LLB system decreased metals to below, or just slightly above criteria for all metals, and has significantly smaller size requirements compared to a SRBR system. The OLC system was least effective, with effluent above water quality criteria for all metals except iron, and with lowest alkalinity generation. The Waitahu River is capable of neutralizing AMD because it is slightly alkaline. The flow volume of river water required for neutralization is between 65 L/s and 140L/s, which can be gravity fed to mix with Fanny Creek. These results indicate that either a LLB treatment system or the Waitahu River Mixing option are the optimal passive treatment strategies for Fanny Creek AMD. On site pilot scale testing of SRBR and LLB systems, and the Waitahu River Mixing option is recommended because of AMD treatment uncertainty, and to more accurately select and design full scale passive treatment strategies.

Changes brought about by chemical and physical weathering were investigated in a chronosequence of terrace soils near Reefton, New Zealand. The parent materials of the soil, which ranged in age from about 1000 to over 130,000 years were outwash gravels, sands and silts derived from granite (dominant) and indurated sandstone. Variations in pH, organic matter, particle size, cation exchange properties, total Mg, Al, Si, K, Ca, Fe and Ti, poorly-ordered and organic-complexed forms of Al and Fe, and mineralogy caused by increasing duration of weathering and by short range, short term variations in the intensity of the biotic factor were determined. It was concluded that the younger soils represented dynamic systems in which alternative weathering cycles could replace each other as the growth, death and eventual disappearance of individual red beech trees caused localised fluctuations in pH. It was further concluded that these processes would lead ultimately to the formation of gley podzols as are now found on the two oldest surfaces p and that podzolisation preceded gleying. Attempts were made to determine if minerals of the plumbogummite group were responsible for the high proportion of soil phosphate from these soils that, on fractionation, appeared in the residual P fraction. It was found that attempts to concentrate these minerals by prolonged digestion with HF resulted in their solution, and in precipitation of complex fluorides that yielded diffraction spacings that have been mistaken for minerals of the plumbogummite group.