PROJECTS – WIlgerup

Wilgerup EL 3317 is located on farmland in central Eyre Peninsula just SE of Lock and only 16–18 km E of the railway from Port Lincoln. There is no outcrop in the immediate vicinity, the whole area being overlain by sand dunes and unconsolidated Tertiary sand (total ca. 20m) with saline groundwater at the base. The Polda Basin, a narrow E-W intracratonic graben containing Carboniferous to Jurassic sediments cuts across the northern part of Wilgerup EL 3317.

In the early 1990’s, WMC Ltd and subsequently Rio Tinto Exploration Ltd (RTE) and Dampier Mining Ltd identified a small medium-grade hematite deposit at Wilgerup (or Kartanya as previously known). Hematite occurs in two distinct lenses 3 km apart (Figs 3–4) and a report by CSIRO for Hamersley Iron Pty Ltd (Ramanaidou, 1998; in Fielding, 2003) estimated the following inferred resources based on 2m intervals meeting an external cutoff grade of 55% Fe and internal cut-off grade of 50% Fe:

• 6.08 Mt of high phosphorous(>0.1% P), low silica (3.16% SiO2) hematite @ 59.41% Fe in the northern lens;
• 1.77 Mt of low phosphorous (0.05% P), high silica (12.01% SiO2) hematite @ 61% Fe in the southern lens; and
• an additional exploration target of 2.35 Mt based on interpretation of cross sections.

Fig. 3 :: Wilgerup :: location of drillholes and hematite deposits in relation to regional aeromagnetics (Click to see image)

A semi-detailed aeromagnetic survey was flown by Stockdale in 1988 and WMC (1991-93) undertook gravity surveys, ground EM and dipole-dipole IP over selected targets principally in the search for base metals. That was followed by mainly reconnaissance percussion, diamond and RC drilling leading to the identification of the two hematite resources. Acacia Resources Ltd (1994–95) unsuccessfully followed up weak base metal anomalies then Rio Tinto Exploration Ltd (RTE) explored for magnetite and hematite iron ore from 1995–99.

RTE undertook a detailed magnetic and gravity survey over part of the area, drilled one diamond drill hole and re-evaluated the Wilgerup hematite resources. The diamond-cored hole (DD98KAT1) on an overlapping gravity and magnetic anomaly intersected magnetite BIF with up to 32% whole rock Fe.

Coarse-grained magnetite-rich BIF intersected by the RTE Kartanya drillhole was resampled by SAIOG but returned Davis Tube separations (DTS) of less than 20% magnetic concentrate and concentrate assays of less than 60% Fe. Given the relatively low amplitude of the magnetic anomaly and the low tenor of these results no further exploration for magnetite was undertaken.

Recent work by Centrex and SAIOG has focussed on reviewing all data and re-interpreting gravity-magnetic data over the area to define strategic exploration and development models and targets. A detailed geophysical report (Hanneson, 2002a) concluded that known hematite mineralisation is associated with gravity anomalies. A number of untested gravity anomalies may reflect further hematite mineralisation (Fig. 4).

Fig. 4 :: Wilgerup :: Location of hematite deposits and exploration
targets in relation to Bouguer gravity anomalies (Click to see image)

More recent detailed reprocessing, enhancement and interpretation of a 1996 aeromagnetic survey (Cowan, 2005a & b) showed levelling problems in the gridded data and recommended that the tenement be reflown at 100m flight line spacing and 30m nominal terrain clearance. Nevertheless, based on reprocessing, enhancing and modelling the existing data, Cowan made a preliminary interpretation in which he identified several targets for follow-up drilling.

Whilst an inferred hematite resource totalling nearly 8Mt in two deposits has been defined, Robinson (2004) expressed concerns regarding the validity of the assay data that supports the grade estimates. These uncertainties in relation to drilling, sampling and assay accuracy are such that the estimated grades and resources cannot be relied upon without further work.

Northern Lens
The northern lens is defined by two lines of drill holes 400 m apart and is open to the north. Mineralisation is reflected by a coincident gravity-magnetic anomaly that appears to terminate 300m north of the northern section line while to the south mineralisation appears to grade into lower grade mineralisation seen in a third line of drill holes 400m south.

On this basis the mineralised lens is ca. 500–900 m long and 10–30m thick dipping moderately to the east.

Mineralisation occurs in the form of martite (hematite replacing former magnetite), micro-platy hematite, earthy hematite and minor goethite in varying proportions with accessory apatite, minor carbonate and trace chlorite/clay. Unaltered BIF is composed of meso- and micro-layered, fine to medium grained (0.02-0.2mm but locally up to 0.5mm) magnetite (up to 35%), amphibole (cummingtonite/grunerite), and minor to trace chlorite, carbonate and apatite.

Typical assays from un-enriched BIF comprise 20-40% Fe but enriched altered BIF/iron ore typically contains 56–66% Fe with variable but relatively low SiO2 (1.4–3.7%), Al2O3 (0.7–2.3%), CaO (0.1–5.4%) and Mn (0.2–1.9%) based on composite drill hole samples. Alkalis (Na2O + K2O) are high but uncertainties in regard to data reliability prevent resolution of this issue. Saline groundwater is the obvious source of alkalis but further analysis of potential ore is required to ensure that average values are <0.2% Na2O + K2O. Phosphorous assays were very erratic in the northern lens but selective resampling by both RTE and Centrex has shown that assays are typically >0.1% P (Table 2).

Table 2: Comparison between Centrex and WMC assays for the
northern lens.

DH	FROM	TO		Fe (%)	LOI (%)	SiO2 (%)	Al2O3 (%)	P (%)
SJPC21	50	64	WMC	59.7	4.15	3.61	1.49	>0.1
			Centrex	63.5	2.56	0.8	0.5	0.55
SJPC23	26	60	WMC	59.19	5.51	3.47	2.58	0.55
			Centrex	62.9	4.67	1.35	1.05	0.35
SJPC107	64	74	WMC	0.35	—	2.66	0.85	0.16
			Centrex	63.2	63.2	1.42	0.64	0.185

Petrography suggests phosphorous from the northern lens occurs as fine grained (5μm) apatite forming micro-mosaic as well as discontinuous laminae up to 0.15mm all intricately intergrown with hematite.

As limited sample material remains available at PIRSA Core Library for testing, a detailed close-spaced drilling program is required to resolve the outstanding issues in respect of ore chemistry, resource definition and metallurgical characterization. To obtain a high-grade commercial product from the northern lens, the resources may need to be finely ground and separated either magnetically or by flotation. However, there is potential for at least some of the ore (ore with low P) to be sold as direct-shipping ore or mixed with ore from the southern lens.

Southern Lens
The southern lens is defined by one line of drill holes, is open to the south but grades into low-grade jaspilitic BIF 400m north. Mineralisation occurs as a shoulder on the flank of a regional gravity anomaly.

Hematite mineralisation occurs in the form of hematite schist ca. 10–20m thick apparently enclosed within mica schist and dipping ca. 30ºE.

The chemical composition of the southern lens is more favourable with respect to phosphorous but silica and alumina assays are inconsistent (Table 3).

Table 3: Comparison between Centrex and WMC assays for the southern lens.

DH	FROM	TO		Fe (%)	LOI (%)	SiO2 (%)	Al2O3 (%)	P (%)
SJPC126	32	38	WMC	55.43	—	8.52	2.47	<0.06
			Centrex	59.0	7.77	1.08	0.84	0.07
SJPC128	80	88	WMC	62.43	—	10.04	3.45	0.04
			Centrex	63.7	2.86	3.74	0.97	0.035

Containing much less phosphorous than the northern lens, the resources from the southern lens may be suitable for direct shipping depending on the combined silica-alumina-alkali content. The resource is open to the south for several hundred metres but the gravity interpretation by Hanneson (2002) does not support a large orebody. A detailed drilling program is required to define ore chemistry, indicated resources and metallurgical characteristics.

Regional Potential
The detailed gravity-magnetic interpretation by Hanneson defines several prospective hematite targets within the Wilgerup licence area (Fig. 4). There is clearly potential for several small but significant hematite orebodies and a reconnaissance drilling program combined with infill gravity surveys is warranted. Calcrete sampling for gold and base metals by Peninsula Minerals Ltd (2004) did not identify any significant coherent anomalies but one isolated anomaly of 4ppb Au and a couple at 3ppb Au were noted.

Preliminary interpretation of the reprocessed 1996 Tumby Bay aeromagnetic survey by Cowan (2005b) confirmed the complex structural pattern of approximately N-S trending, steeply dipping, high amplitude linear and curvilinear anomalies intensely refolded and faulted during high grade metamorphism.

Cowan suggests that the high degree of faulting and folding is favourable for mineralisation and that the northern and southern lenses of hematite mineralisation appear to be associated with NNE-trending shears. There are a number of magnetic anomaly zones similar to the northern Lens and Cowan identified two as possible drilling targets including a coincident magnetic/gravity high along a NNE structure just south of the northern lens.

Cowan also identified a few hematite targets associated with magnetite destruction along major N-S faults. A major N-S fault which passes through the southern lens provides scope for finding more hematite resources either along or adjacent to the fault.

While RTE (supported by SAIOG DTS results) considered the Fe grades of magnetite BIF to be too low compared with their ‘benchmark’ magnetite ore (a Brockman Iron Formation source in the Hamersley Range of WA) if Wilgerup hematite deposits prove viable, serious consideration should be given to re-examination of the Wilgerup magnetite potential. Cowan considered that there is some potential for coarse-grained magnetite suitable for beneficiation. Cowan identified three potential magnetite targets for follow-up drilling, one including the RTE DD98KAT1 drillhole that appears to have been drilled largely down dip (Gaughan, 2005). Calculated depths to the tops of magnetite targets is generally 100–150m over much of the area but results show a progressive increase in source depth from S to N.

30–70km NNE of Wilgerup in the Warramboo-Kopi-Hambidge area there are a series of intense linear aeromagnetic anomalies that were the focus of exploration by SADM for iron ore in the early 1960’s (Whitten, 1965; Davies, 2000) and, more recently, Adelaide Resources Ltd. Drilling by SADM and others intersected a variety of quartz-feldspar-biotitemagnetite (garnet-cordierite-sillimanite) gneisses with head grades up to 24% Fe. The gneisses are of Archaean age and contain coarse-grained magnetite with good beneficiation characteristics. The very southernmost tip of these anomalies extends onto Centrex ELA 33/05 near Lock. Other weaker anomalies on this ELA are possibly similar magnetite-rich gneisses although on the eastern margin the anomalies may be due to the same BIF unit as at Wilgerup.