Figure 2 : PP51536 – Taranaki
The Taranaki project, PP51536 (Figure 2) holds potential for iron sand (ilmenite, titanomagnetite and/or magnetite) in offshore sand/mud deposits. The conceptual iron sand exploration target is a 500km2 muddy sand lens extending from New Plymouth southwards to Stony River and 8kms offshore. The terrigenous sediment is believed to be Pleistocene to Holocene in age and transported down the Stony and Oakura Rivers. P51536 (Figure 2) is predominantly covered by an inner continental she lf of sandy muds (10-50% sand) to muddy sands (50-90% sand) (Carter, 1980).
The sources of the sands and muds (including iron sands) were most likely from volcanic (andesitic) rocks of Cape Egmont and also further upstream from the flanks of Mt Taranaki during periods of Holocene transgression (Macorison, et al., 2003). From Cape Egmont up to Tirau Point a concentration of iron sand occurs to a depth of 40m and shallower, with concentrations up to 20% (Carter, 1980) (Figure 3).
Figure 3 : Percentage of ironsand from historic data points (from Figure 3 of Carter, 1980)
EXPLORATION RESULTS & POTENTIAL
In 2011, Ironsands Offshore Mining Limited (a subsidiary of CASS) conducted two aeromagnetic surveys, a shallow seismic (sub-bottom profiler) survey (Figure 4) and completed 123 vibrocore drill holes (Figure 5) in the northern section of PP51536 (Ironsands Offshore Mining Ltd, 2011).
Figure 4 : Aeromagnetic and seismic survey locations (from Figure 2 of August 2011 Progress Report)
The shallow seismic survey was used to determine the depth and thickness of Holocene mud (Figure 6) and the aeromagnetic survey aimed to detect possible concentrations of iron sand. High aeromagnetic anomalies and areas where mud thickness was less than 6.5m were selected as vibrocore drilling targets.
VIBROCORE DRILLING PROGRAM
CASS has completed 123 vibrocore drill holes (Figure 5) for a total of 400.19m drilled. The average drillhole depth achieved was 3.25m (target depth 6m) mainly due to a) poor recoveries (48% on average) or b) shallow rocky seafloor and reefs in places. Poor recoveries can be attributed to very fine clean sand escaping through the core catcher. Excess recovery can be due to bouncing of the corer on the sea floor and also “sediment suck-up” when the coring was caught on hard layers that prevented further penetration.
Only a small number of holes logged the presence of iron sand. Trace amounts were noted but no percentages were given, carbonaceous matter was also noted in same intervals as iron sand.
Magnetic susceptibility was measured at intervals of 0.05m along the core. The data was incorporated into the lithology logs. Currently results for 100 samples from randomly selected cores giving the per cent magnetite from Davis Tube analysis are available.
Figure 5 : Vibrocore locations (red circles)
Figure 6 : Mud thickness (SBP profile) (from Figure 4 of Sept 2013 Progress Report)
An inspection of the stored offshore Taranaki vibrocore cores was undertaken in early June 2014, at the core storage facility in Albany, Auckland. The cores are in very good condition and were inspected at random with some sub-sampled to confirm the presence of magnetite by hand panning to concentrate the heavy minerals.
The geological logs accurately record the cores as presented, and the magnetic susceptibility readings are a reliable indicator for the presence of elevated magnetite content. At present only 100 samples have undergone Davis Tube analysis to determine the magnetite content with results typically ranging up to 5% and only 2 samples containing >10% magnetite. No XRF has been carried out on the off shore samples at this stage. The remaining samples are scheduled for Davis Tube analysis in the coming months.
While there were no obvious heavy minerals concentrations (black sand layers), the hand panning has confirmed the presence of the silty to fine sand sized magnetite in roughly similar amounts to that recorded by the Davis Tube analyses. The magnetite was concentrated using a small hand magnet and is therefore only indicative, but the magnetite appears to be present in quantities of approximately 2%. Figure 7, below is a photograph of a hand panned sample with the magnetite concentrated by a small hand magnet under the pan. The magnetite content is estimated to be around 2-3% in this 1m sample. Davis tube analysis gave the result of 3.4% magnetite for this sample (number 1003, VC-080 200cm – 300cm) and a magnetic susceptibility reading of 12 -18 Kappa.
Figure 7 VC-080 200-300cm, hand panned sub sample with magnetite concentrated
The core recovery problems indicate that overall the vibrocores do not adequately sample the target iron sand and the analytical results can only be used for a general assessment. Forty cores had significant loss (>10% of the sample) and 23 cores had significant over sampling (>110% recovery).
The current exploration target tonnes and extent has been determined by a Fugro 3D inversion transform of 200 km2 of aeromagnetic data acquired in 2010 (Figure 4, dark blue outline) and has not been verified by drilling or assay test work.
The Exploration Target determined by CASS (at a 5% cut off and less than 60m water depth) consists of a volume of 11,662 million cubic metres, averaging 10.4% magnetite (CASS Offshore Minerals Ltd, pers comm, 2013). This is likely to be a “best case” scenario and requires systematic drilling and sampling to determine resource tonnages and grades.
The Taranaki Exploration Target is conceptual in nature and consequently the resource estimate produced by the 3D inversion survey is not reliable as:
- Drilling concluded that there is no correlation between the aeromagnetic anomalies and iron sand concentrations (Ironsands Offshore Mining Ltd, 2011).
- Preliminary review of lithology logs has not shown significant concentrations of iron
- At the time of this report, only 25% of vibrocore samples had been assayed for magnetite using the Davis Tube method. Two of 100 samples tested returned over 10% magnetite, with most containing up to 5%
FUTURE EXPLORATION PLANS
Further drilling and feasibility studies have been planned by the client to upgrade the exploration target to JORC Inferred status.
We note that Golders, when requested by Trans-Tasman Resources to carry out a JORC estimate on their exploration area south of Taranaki, did not consider the vibrocore results to be valid for use in the resource estimation, primarily due to the poor recovery, similar to that of the CASS vibrocores at the Taranaki project, and the restricted depth of this drilling method. We recommend an alternative drilling method to be used in future exploration.
Mapping of the 5km gravel apron around Cape Egmont and any reefs should occur to ensure it will not affect dredging operations.
A JORC Inferred Resource should be defined before any further scoping or feasibility studies are undertaken.