Figure 8 : PP52474 - Waihi
The Waihi project, PP52474, is considered to have potential for ilmenite-rich heavy mineral sands in offshore inner shelf deposits. Figure 8 shows the location of the tenement in the Bay of Plenty. The holder of the permit is Pacific Offshore Mining (POM), a subsidiary of CASS. Work carried out during the 1960s and 1970s focussed on the onshore dunes and beaches in the area and it was this work that first investigated the potential heavy mineral sand accumulations in beach strand lines.
The source of recent and Holocene heavy mineral strandlines at Waihi is predominantly from Quaternary volcanic ash deposits from the Taupo Volcanic Zone (TVZ) (Welch, 1993). Most tephra from the TVZ is rhyolitic or dacitic in composition with ilmenite a common accessory mineral. These were deposited directly on the continental shelf and also transported to the coast by local rivers. The cont ribution of Coromandel Volcanics is considered limited due to the small, short streams and highly embayed coastline typical of the Coromandel Peninsula (Bradshaw, 2004). Recent studies have demonstrated that although a net littoral drift towards the southeast exists, most Holocene sediment has been supplied to the near shore at Waihi via an onshore mass transfer of shelf sediments (Bradshaw, 2004).
Detailed work on the marine geology has only been carried out on the northern portion of the Prospecting Permit, focussing on an area off shore from Waihi Beach across to Mayor Island, within an area of 100km2. The inner shelf geology of this northern section has most recently been defined by Bradshaw (Bradshaw, 2004) using seismic data, to identify six discrete sedimentary units.
The conceptual model developed for the exploration target from the desk top study focusses on Unit 5 and Unit 6, Holocene incised-valley fill and transgressive sands respectively.
Figure 9 shows a schematic summary of the six seismic units.
- W is the Waihi Sequence from the Late Pleistocene and is dominated by regressive systems tract (RST, Unit 4). (NB: This sequence is not considered in the target as it is below the 10m thickness used).
- The Holocene to Recent Whangamata Sequence (Wh) comprises the target sediments. It is in early stages of development and is dominated by incised-valley fill sediments (IVF, Unit 5).
- Unit 6 has two components: 6a is transgressive systems tract (TST) and 6b a high-stand systems tract (HST).
- It is noted that the Bay of Plenty is currently sediment starved, so that reworking of the transgressive sands and incised-valleyfill is occurring.
Figure 9 : Schematic summary of the seismic units offshore Waihi (Bradshaw, 2004)
Unit 6, the uppermost unit, has a maximum thickness around 5m and comprises a sheet that thickens slightly shoreward. It includes Recent dunes oriented shore-normal as well as some relict transgressive sands (lag deposit). The sediments in the 30 to 50m zone are characteristically medium to coarse grained and moderately to well sorted. They are considered to be storm lag deposits which could include concentration of the heavy mineral fraction.
Unit 5, the low stand incised-valley fill, has a variable occurrence and thickness and is considered to be a mix of predominantly fluvial and estuarine-near shore sediments. Heavy mineral concentrations in these sediments are likely to be restricted to relatively small areas and localised (point bar accumulations etc.). The infill is thought to be a conduit for the delivery of the heavy mineral sands to be reworked on the shelf during the transgression and Recent.
The surface distribution of the different units is not defined at this stage. Isopach maps by Bradshaw (Bradshaw, 2004) indicate that Units 5 and 6 have a combined thickness of around 10m, offshore from Waihi (Figure 10). The best developed thickness of Unit 5 is in the north part of the Waihi area.
Figure 10 : Isopach maps for seismic units ( (Bradshaw, 2004))
The continental shelf extends nearly 30km off shore from Waihi Beach and, from about 300m offshore to 1.5km offshore, it has a typically gentle gradient of 1:200. From there to the shelf edge it is slightly convex. Some work has also been done on understanding the morphology of the sea floor, investigating the distribution of the sand dunes / mega ripples that form part of Unit 6. In water depths >15m, large sand ridges (submarine dunes) occur, oriented normal to the shoreline, with piggy -back mega ripples. The dunes have heights ranging from 0.2 to 2.5m, are spaced 250m to 1,300m apart and are up to 2,000m long. They are interpreted to be relict deposits from the initial stages of the transgression, which are currently being modified by storm waves and currents.
Figure 11 indicates that the dune / mega ripple zone starts approximately 4km offshore an d extends approximately to 50m water depth.
Figure 11 : Distribution of mega ripples off shore from Waihi (Bear, 2009)
Previous exploration work was completed by Zephyr Resources during the early 1990s, but was limited offshore to a maximum 25m water depth (Welch, 1993). Crown Minerals summarised the results of the Zephyr work (Drummond, 1995). Offshore from Waihi Beach, Zephyr Resources (Welch, 1993) list a “Unit 5 - Holocene transgressive sands”, equivalent to Bradshaw’s Unit It is generally 5m thick and is considered to be currently transported northwards by shoreline-parallel currents, creating the shore-normal dunes and mega ripples.
Sampling carried out prior to POM holding the permit in the Waihi area has so far consisted of approximately 50 grab samples and box cores. These are in six transects aligned from the beach to the 30m isobath and one transect continuing to the shelf edge. The six transects are 2 to 3km apart with sample spacing along transects of <1km in <20m water depth, but sample spacing is up to 2km apart in water deeper than 20m. Bradshaw (1991) examined these mineralogically; his work shows the sediments are very immature, including lithic fragments and volcanic glass.
Some of these grabs were sampled and analysed by Zephyr Resources. Ilmenite was found to dominate the opaque assemblage in the Waihi area. About 20 samples have reported heavy mineral percentages for the analysed 63micron to 0.25mm fraction, ranging from a few percent to a maximum of 68.3% in one sample (26m water depth, about 4km offshore). The heavy minerals are dominated by ferromagnesian minerals. Mineralogy data for four shelf samples within the POM holding (Bradshaw, 1991) (no mineralogy could be located for the remaining samples)show two have less than 1% opaques, one has around 3% and the highest opaque content was 14% of the sand fraction sampled. Of the five onshore beach samples from this area with data, three have 2% opaques, one reported trace and one reported 35% opaques in the sampled sand.
We consider it likely that the major impurity in the opaque fraction would be magnetite. Based on this extremely limited data it is not unreasonable to expect an offshore minable deposit to have an average grade of 5% to 15% ilmenite plus magnetite, dominantly ilmenite, and most likely be in the form of a relatively confined shore normal dune/mega-ripple, or other specific higher energy environment on the shelf. The thickness of any minable unit is likely to be a few metres, but could be up to 10m.
XRF analysis was done on ilmenite from Waihi Beach giving content of 44.46% TiO2 and 48.60% Fe3O4. The titanium content is low by world standards (Drummond, 1995) but this is partially offset by very low radioactive content (U + Th less than 10 ppm). The ilmenite has been identified as suitable for merchant pig iron (MPI) feedstock with some artificial rutile as a by-product. This grade of ilmenite is low in titanium compared with world traded ilmenite.
EXPLORATION RESULTS & POTENTIAL
The project is still in an early ‘greenfield’ stage, with only minimal field testing being reported. The work by POM has so far consisted of the desk top study and an aeromagnetic survey commissioned in 2011 covering the same area as previous historic work. Figure 12 shows the location of the aeromagnetic survey in 2011 and Figure 13 depicts the survey results, TMI image.
Figure 12 : Extent of the aeromagnetic survey offshore Waihi Beach, 2011
The aeromagnetic data shows the near surface magnetic intensity with the high intensity response (red to pink colours) dropping off at around the 50m isobath.
The target resource is considered to be located in the 20m to approximately 50m water depth, in the medium to coarse grained sediment, which corresponds to the high magnetic intensity zone in Figure 13. It also corresponds generally to the dune/ mega ripple zone in Figure 11.
Figure 13 : High pass filtered data, TMI, aeromagnetic survey results 2011 ( (Pacific Offshore Mining Ltd, 2011)
There is potential to locate heavy mineral sand bodies within the target area, but there is currently no information on grade deeper than 0.4m and an exploration target of 5 10m thick is speculative, and may include a number of different sedimentary systems (e.g. inner shelf for Unit 6, partially fluvial for Unit 5) with unknown heavy mineral distributions. Strand line deposits are the most obvious targets, and are typically long thin bodies in the order of 100m wide, a few metres thick and up to a few kilometres long. The dimensions of the submarine shore-normal dunes are comparable.
The project is following up on previous onshore work and limited offshore surf ace sampling results. The heavy mineral sands potential in the offshore environment is still at an early “first pass” stage. There is currently only very limited data available on the heavy mineral content of the shel f sediments.
The limited amount of surface sampling (with some good heavy mineral grades) combined with the detailed geological knowledge has led to the development of a conceptual target. Extensive surface sampling and coring will be required to confirm predictions and refine the model.
The geological model outlined by Bradshaw could be extrapolated to cover the remainder of the Prospecting Permit area south of Waihi Beach, but this requires further seismic work an d/or drilling for confirmation.
Environmental concerns will also have to be addressed as the stranding of the containership Rena on Astrolabe Reef in 2011 raised the profile of the area. Exploration by POM was suspended during late 2011 and 2012 as a consequence and an application for extension was lodged during October 2012.
There is very little information on the benthos of the shelf in the Bay of Plenty and the baseline grab sample program will be essential in defining the communities present. Depending on the information, there may be further requirements for study.
With the target resource located in water depths >20m, the work area is located a minimum of 4km offshore, which is outside a 2 nautical mile voluntary exclusion zone. This means that any new near shore exclusions to the Permit, such as recently declared Mt Manganui Mataitai (7km2), should not impact the future work and potential resources.
FUTURE EXPLORATION PLANS
The proposed exploration outlined by POM for the next 2 years, after grant of the new permit, includes (Exploration Phase Two – Survey and Sampling p.16 (CASS Offshore Minerals Ltd, 2013)):
- Staged vessel-based geophysical surveys using a “Chirp” (high frequency seismic reflection) profiler for sediment profiling, further magnetic surveys and / or IP
- Surface sampling by Van Veen grab (100 samples planned), for environmental sampling of the top 20cm of sediment, which will also assist to ground truth the Chirp
- Scout drilling, preferably not vibrocore drilling unless recovery problems can be alleviated, 100 locations, with a maximum core length of 12m, and 75cm Further in-fill drilling would be required for resource estimation.
These plans are, in our view, reasonable to test the desired targets. Stage 3 drilling should aim to test the full 10m depth used in the potential resource calculation. In most cases, vibrocore drilling, with a 6m core length, will fail to adequately sample the desired units. Alternative drilling methods, such as reverse circulation systems, should be investigated to ensure adequate depth penetration (ideally to 10 – 12m) and core recovery.