PROJECTS – bald hill

Located near Wanilla, approximately 20–30km NNW of Port Lincoln and 10km from Greenpatch, the Bald Hill project comprises two exploration licenses, EL 3269 and EL 2905 with sparsely outcropping BIF targets and prospects. Two of the principal prospects, Charleton Gully and Bald Hill, are located along a series of long, moderately intense, NNE-trending aeromagnetic anomalies that extend for over 20 km. At their northern end they are truncated by an E-W structure that extends through Koppio.

BIF outcrop at Charleton Gully has a strike length of several hundred metres, is interbanded with amphibolite and contains up to 24% Fe. It correlates with an aeromagnetic anomaly that defines a complex synformal keel-shaped structure about 10km long, locally offset by NNW-trending faults and plunging shallowly N at its southern end. Although offset by a major late-stage ENE fault, the anomaly zone extends N into EL2905 and links with the Bald Hill prospect.

The Charleton Gully Synform is not a high priority magnetite target (Cowan, 2005a) but could be considered in conjunction with the Bald Hill prospect. Cowan notes that zones of reduced magnetic amplitude in the southern synformal keel structure may have hematite potential.

Fig. 7 :: Bald Hill :: Location of drillholes, rock samples and interpreted geology in relation to aeromagnetic imagery (Click to see image)

The Bald Hill prospect represents the northern extension of the Charleton Gully synform and is located on the southern end of a 9km long moderately intense aeromagnetic anomaly that, on ground magnetics undertaken by Dominion Mining Ltd and detailed aeromagnetics acquired by SAIOG, comprises 2–5 discrete anomalies that coalesce and bifurcate along their length. The Charleton Gully synform is interpreted to run along the length of the Bald Hill anomaly suggesting that this potential prospect is in the core of the fold.

There has been little if any previous iron ore exploration over the licence area. Numerous rock chip samples collected by Dominion Mining Ltd (for gold exploration) from scattered jaspilite outcrops in the Bald Hill area gave assays ranging up to 59% Fe. Dominion drilled several aircore holes across parts of the aeromagnetic anomaly but didn't assay for Fe. However, they recorded bottom-hole lithologies including ferruginous quartzite and jaspilitic chert. One hole assayed 92ppb Au.

Afmeco Pty Ltd drilled several RAB holes across the northern end of the Bald Hill anomaly, on the western limb of the synform, and also intersected BIF.

SAIOG Geophysics and Drilling
Exploration by SAIOG in 2002 included:

• F A low level aeromagnetic survey (519 line km; 40m line spacing; 20m agl sensor height);
• F 2 diamond drillholes for a total 323m of drilling;
• F 180 geochemical assays

This was focussed on the Bald Hill region and was followed by a detailed gravity survey for Centrex Metals in 2004 (4 sq km; 254 stations; 400m line spacing; 50m station spacing).

The highest amplitude eastern aeromagnetic anomaly in these surveys (at the southern end of the Bald Hill North survey; Fig.7) was modelled as a W-dipping BIF 60m thick with a high magnetic susceptibility. In nearby outcrop, BIF banding dips as shallowly as 50º W.

The western aeromagnetic anomaly is not as intense and was modelled as a 100m thick ESE-dipping BIF. Primary BIF banding in poor outcrop nearby is subvertical to steeply ESE-dipping.

Drilling of both the western and eastern limbs of the synform was undertaken by SAIOG in 2002. On the eastern limb, BADD001 intersected a sequence of interbanded BIF, amphibolite and schist with an aggregate thickness of magnetite BIF ~56m dipping about 40º W. A similar sequence was intersected on the western limb in drillhole BADD002 (estimated dip ca. 70º ESE).

Table 12: Aggregate DTS and XRF assays for BIF intercepts at Bald Hill meeting the cut-off grade of 20% DTS magnetic concentrate

Drillhole	From (m)	To (m)	Intercept (m)	DTS magnetic con (%)	Fe (%)	SiO2 (%)
BADD001	53.5	55.35	1.85	24.93	66.1	6.76
BADD001	104.1	114.9	10.8	25.4	65.1	7.6
BADD001	130.3	145	16.7	29.1	64.5	8.4
BADD001	157.95	159.56	1.61	22.34	67.7	4.91
Bald Hill East (weighted average)			30.96	27.2	65.0	7.8
BADD002	48	49.2	1.2	25.66	66.2	6.46
BADD002	94.83	96.9	2.07	26.26	68.4	4.32
BADD002	103.92	107.8	3.88	27.6	66.4	6.3
BADD002	111	113.32	2.32	34.54	66.6	5.84
BADD002	121.25	130.56	9.31	33.9	68.4	4.4
BADD002	167.5	178	10.5	22.7	68.2	4.2
BADD002	179.45	181.92	2.47	28.11	67.5	4.81
Bald Hill West (weighted average)			31.75	28.2	67.8	4.8

Relatively high silica / low iron DTS magnetite concentrate assays may be partly attributable to grind precision and poor or incomplete magnetite liberation at the nominal grind size (-75µm).

Recent work over Bald Hill has included a detailed gravity survey combined with detailed analysis of both aeromagnetic and gravity data (Hanneson, 2004; Cowan, 2005a). In addition to providing an interpretation of the density and susceptibility distribution at Bald Hill, Hanneson applied a phase/scatter diagram methodology to assess the various bodies of the model in terms of the magnetite and hematite content of the rocks that they represent.

In summary, magnetic depths appear to be largely in the 20-60m depth range, and the majority of magnetic sources are estimated to contain up to about 10% magnetite. In the most magnetic parts of the study area sources may contain 11-16% magnetite. The model also contains several dense non-magnetic bodies that may contain 40-50% hematite.

Magnetite-rich BIF diamond drill core from hole BAD0001 (Click to see image)

Based on magnetic modelling across the Bald Hill structure, Cowan suggested that the western limb is deeper and wider while the eastern limb is shallower and narrower. Based on this modelling a series of holes were recently drilled by Centrex (early 2006). Results of the shallow RAB/slimhole RC drilling are still being compiled and evaluated by company geologists and were not available at the time of writing this report.

Magnetite and Hematite Potential
SAIOG drilling confirmed potential for a large resource of magnetite BIF given the very long strike extent, but identified a high proportion of interbanded waste rocks (schist, amphibolite) and consequent high waste to ore ratios.

The exploration target at Bald Hill extends over a significant strike length because of the linear nature of the magnetic anomalies but there is only one drillhole in each of the two main anomalies that represent the individual limbs of the synform. Tenuous inferred resources have been estimated for the two fault blocks in which the drillholes are located (Table 13). Although these estimates should be treated with some caution, it is believed that they are representative due to the regional continuity of geology and magnetics.

	Bald Hill West	Bald Hill East
Average true thickness based on intercepts with >20% DTS magnetite Vertical extent of oxide zone below ground level	24.4m (BADD002) 23.4m (BADD002)	30m (BADD001) 33.8m (BADD001)
Drillhole declination	60º WNW	60º E
Assumed dip of BIF	70º ESE	40º W
Dip extent of magnetite BIF (excluding oxide zone) to 200m below ground level	186m	272m
Strike length (based on aeromagnetic interpretation)	600m	700m
Volume of magnetite BIF	2.7 million m3	5.7 million m3
Magnetite BIF density	3.4	3.4
Inferred magnetite BIF resource	9.3 Mt @ 28.2% DTS (67.8% Fe)	19.4 Mt @ 27.2% DTS (65% Fe)

There could potentially be significant hematite mineralisation above these magnetite resources based on surface outcrops grading up to 59% Fe, but there are no drillholes into the oxide zone so no hematite resources can be inferred at this time. However, as at Greenpatch, there is potential for moderately high grade oxidised BIF above the enriched magnetite.

Regional Potential
From the above observations and interpretation, it is concluded that Bald Hill represents a good grassroots exploration target for magnetite. It has the potential to host a number of moderate-sized exploration targets with ca. 40 Mt magnetite BIF per strike kilometre (based on the figures in Table 13 to 200m depth). However, these require an extensive drilling program.

Based on the magnetic-gravity modelling and interpretation (Hanneson, 2004), many bodies have a density consistent with that expected for the amount of magnetite inferred from the observed magnetic response. However, other bodies in the model seem to represent rocks that are much denser than what would be expected for the interpreted amount of magnetite, while others appear to be quite dense and essentially non-magnetic with potential for upwards of 50 percent hematite. Hanneson’s interpretation covered only a short length (2.5km strike length) of BIF so there is significant potential for high-grade hematite BIF(s) along the Charleton Gully-Bald Hill synform.

Cowan (2005a) noted evidence of a zone of reduced amplitude and increased source depth from 572560E, 6188280N to 572610E, 6189470N (GDA94).

A moderately intense aeromagnetic anomaly at Warunda, 30km NNW of Port Lincoln and 5km W of Bald Hill is a grassroots exploration target that has not had any significant previous work carried out on it. Dominion Mining drilled one line of holes across it and intersected a variety of gneisses, quartzite and ferruginous quartzite. The anomaly is interpreted to represent a similar sequence of rocks as at Bald Hill. It is ca. 4km long and has the form of another synformal keel-shaped structure.

A wide zone of NNE-trending moderate amplitude magnetic anomalies in the east of EL2905 is due to Lincoln Complex granitoids east of the Kalinjala Mylonite Zone. A series of magnetically less intense N-S anomalies in the centre of EL2905 are interpreted as Hutchison Group BIFs which continue north into EL2887. There are sporadic outcrops of BIF throughout this region but they are of secondary interest relative to the more intense magnetic anomalies at Bald Hill and Koppio.