PROJECTS – carrow

The Carrow prospect is a 4km long, 2km wide, intense NE-SW aeromagnetic anomaly midway between Port Lincoln and Whyalla 6km NNW of Port Neill.
Fig. 10 Carrow Iron Ore Prospect: location of drillholes and geological interpretation in relation to aeromagnetic imagery.

Fig. 10 :: Carrow Iron Ore Prospect :: Location of drillholes and geological interpretation in relation to aeromagnetic imagery (Click to see image)

There is no basement outcrop but laterite float and a low topographic rise suggest that the depth to basement is shallow. The anomaly is immediately adjacent to the Kalinjala Mylonite Zone and is likely fault and shear zone bound particularly along the eastern side.

Several SADM drillholes across the main Carrow magnetic anomaly suggest that the main magnetite-rich zone is 60m wide and dips NW. Previously reported assays in CD1 include 11m @ 28.6% Fe (acid soluble) and 25.6m @ 27.8% Fe. For CD2, assays include 17m @ 26.2% Fe and 52.4m @ 23% Fe.

Existing SADM drill core was resampled by SAIOG and assayed over selected iron-rich intervals. The analytical results were quite varied but gave locally quite high-grade intercepts grading up to ~36% Fe. Some of the better intervals in CD-1 included 9.6m @ 32.97% Fe (60.96-70.46m) and 7.62m @ 32.66% Fe (124.66-132.28m). In CD-2, the best intercept was 32m @ 29.43% Fe (38.71-70.71m) and in CD-3, the best intercept was 22.86m @ 29.29% Fe (103.02-125.88m). These results were better than the original acid-soluble assays (45m @ 28% Fe from 61-106m in CD-1) inferring that some iron is tied up in silicate minerals (amphibole and diopside).

Petrological work on drillcore by Whitehead (1978) describes a suite of diopside-magnetite-quartz (apatite, calcite, hypersthene, olivine, serpentine, amphibole) rocks similar to Greenpatch. Only a minute trace of sulphide was noted. The grainsize of magnetite is variable from 0.05-3mm: it is 0.2-1mm with pyroxene, 0.8-2mm in calcite-pyroxene layers, and 1-2mm in monomineralic magnetite bands.

A gravity and ground magnetic survey was undertaken across the anomaly by SADM in the late 1950’s (Whitten, 1964c) and defines a central high amplitude anomaly with lower order anomalies on the SE side. The main anomaly is due to an 80m thick very strongly magnetic unit.

High-resolution aeromagnetic data was acquired by SAIOG in January 2002 (428 line km at 20m agl and 40m line spacing with some 20m line spaced infill over the central anomaly). The aeromagnetic data defines a series of intense magnetic anomalies with an aggregate strike length of 2km and a long wavelength 'tail' to the NW suggesting down-dip continuation of the BIFs at depth (Fig. 10). The magnetic source was modelled as a complexly folded antiformal structure striking NNE and plunging steeply to the N or NNE at the northern end but broken into a series of blocks by NW-trending faults and truncated by a fault at the southern end. Cowan’s (2005a) 3D Euler depth estimates show shallow source depths for the southern anomaly and variable depths for the main central anomaly. For the northern anomaly zone, the eastern BIF is shallow whereas the western BIF is deeper.

SAIOG also commissioned a detailed gravity survey over Carrow. A well-defined 3-milligal anomaly coincides closely with the position of magnetite BIF as determined from magnetics and drilling. Anomalous gravity data is centred over the northern part of the prospect and suggests that the bulk of high-density rocks are in the northern lobe, which comprises 1.2km of the total strike length.

SAIOG Drilling
SAIOG undertook two drilling programs along the length of the aeromagnetic anomaly for a total of 19 drillholes. Basement rocks have been subjected to deep lateritic weathering and are concealed by 2-40m of Tertiary sand and minor clay with thinner Tertiary over BIF subcrop that generally constitutes basement highs. Saprolite depth varies according to lithology with deeper saprolite developed over granite (50m), shallower saprolite over marble (10 – 30m) and shallowest over iron formation (1 – 14m).

Drilling has confirmed that the banded quartz magnetite BIF forms a fairly thick coherent series of pods with interbands of magnetite-poor BIF and variable dilution by granite sills / dykes. Individual BIF ‘pods’ conform closely to the magnetic anomalies. Drilling and gravity data confirm that the break in magnetics between CADD006 and CADD007 is caused by a sizeable granite intrusion.

Oriented structural data confirms that the BIF and enclosing carbonate package dips moderately to steeply 50-70º NW. Magnetite bands are significantly thickened in the axial region of minor folds. This process, which probably occurs at all scales from microscopic to macroscopic, together with coarse metamorphic recrystallisation, is thought to have been important in thickening and upgrading the magnetite BIF at Carrow.

Diamond drilling to date shows that the magnetite BIF at Carrow is between 10m (CADD006) and 133m (CADD007) thick. On the basis a DTS magnetic concentrate cut off value of 20% in diamond drillholes, the average drilled thickness of BIF is 51m (Table 19).

Table 19: XRF assays on DTS magnetic concentrates aggregated for intercepts with >20% DTS

Drillhole	From (m)	To (m)	Intercept (m)	DTS Magnetic Con (%)	Fe (%)	Al2O3 (%)	MnO (%)	P2O5 (%)	SiO2 (%)
CD1*	124.66	132.28	7.62	27.7	61.68	0.54	0.08	0.11	10.9
CD2*	38.71	70.71	32	28.75	67.25	0.38	0.31	0.05	5.28
CD2*	79.86	84.43	4.57	39.1	67.82	0.33	0.28	0.10	3.3
CD3*	103.02	125.88	22.86	32.96	66.80	0.41	0.24	0.07	5.59
CD3*	135.03	141.12	6.09	35.6	68.45	0.31	0.21	0.06	3.1
CADD003	104	133	29	25.13	52.13	0.63	0.26	0.12	20.35
CADD003	133	181.59	48.59	35.69	68.36	0.25	0.30	0.04	4.84
CADD006	145	155.8	10.8	34.97	69.90	0.28	0.18	0.02	1.47
CADD007	92.14	101	8.86	36.29	70.71	0.06	0.06	0.00	1.34
CADD007	104.8	133	28.2	31.53	69.62	0.58	0.07	0.00	1.85
CADD007	138.4	143.9	5.5	40.52	70.47	0.12	0.23	-0.01	1.48
CADD007	156	202	46	33.39	70.19	0.27	0.32	0.01	1.3
CADD007	205.5	214.31	8.81	39.83	70.25	0.16	0.19	-0.01	1.55
CADD007	219.1	226	6.9	29.95	69.36	0.51	0.07	0.02	2.09
CADD008	109	133.48	24.48	31.72	69.77	0.35	0.17	0.01	1.71
CADD008	140.76	194	53.24	31.98	70.42	0.11	0.20	0.00	1.41
CARC009	45	51	6	25.74	70.47	0.12	0.19	-0.01	1.53
CARC009	63	75	12	27.87	70.47	0.31	0.12	-0.01	1.63
CARC010	33	71	37	33.09	70.09	0.34	0.15	-0.01	1.57
CARC010	102	159	54	30.73	69.35	0.33	0.35	0.02	1.69
CARC011	57	85	27	29.47	70.28	0.25	0.44	0.00	1.34
CARC012	36	51	15	31.14	69.13	0.39	0.31	0.02	1.97
CARC012	84	90	6	21.95	66.42	0.78	0.39	-0.01	4.52
CARC013	40	72	32	31.26	66.23	0.35	0.33	0.05	3.93
CARC016	63	66	3	20.42	71.17	0.19	0.20	-0.01	1.17
CARC016	69	72	3	26.18	69.77	0.32	0.14	0.01	1.79
CARC017	57	90	33	21.3	68.05	0.43	0.07	0.00	4.03
CARC019	45	51	6	23.23	71.2				1.15
CARC019	57	70	13	24.41	70.72				1.18
CARC019	75	87	12	37.8	70.75			1.08
Weighted Average on >20% DTS concentrates				31.21	69.12	0.32	0.24	0.02	2.51

*Early SADM diamond drillholes

Initial DTS determinations on drillholes CADD003 and CD1 – CD3 were carried out with an inappropriate grind of 90% passing 100µm. It can be seen that concentrate grades for these drillholes contain relatively high SiO2 and low Fe values. Subsequent grinding to 100% passing 75µm achieved generally improved concentrate grades.

Recrystallisation of magnetite during high-grade metamorphism is believed to account for the very low component (in magnetic concentrates) of deleterious elements particularly P2O5. The latter occurs as fine-grained apatite that is easily separated during Davis Tube separation.

Magnetite Resources
Whitten originally estimated magnetite BIF at Carrow to comprise 6 Mt per 30m depth (900m strike length) @ 23-28.6% Fe (acid soluble). However, the strike length of strongly magnetic BIF is double Whitten's original estimate and the depth extent is >250m.

Resources are inferred from drilling (19 SAIOG DHs and 3 SADM DHs) and interpretation of high-resolution aeromagnetics. The two main fault blocks are treated separately for the purpose of calculating volumes (Table 20) but the ore grades are those averaged over the whole prospect (see table 19 above).

Table 20: Inferred magnetite BIF resource for Carrow

	Northern Deposit	Southern Deposit
Average true thickness based on intercepts with >20% DTS magnetite	37.2m (CADD003**)	63.7m (CADD008)
Vertical extent of oxide zone below ground level	39.9m	43.0m
Assumed dip of BIF	65º NW	65º WNW
Drillhole declination	65º SE	60º ESE
Dip extent of magnetite BIF (excluding oxide zone) to 200m below ground level	176.6m	173.2m
Strike length (based on aeromagnetic interpretation)	1150m	700m
Volume of magnetite BIF	7.6 million m3	7.7 million m3
Magnetite BIF density	3.4	3.4
Inferred magnetite BIF resource	25.7 Mt @ 31.2% DTS (69.1% Fe)	26.2 Mt @ 31.2% DTS (69.1% Fe)

**CADD003 intercept 104-133m has not been included even though DTS >20% because Fe < 60%.

The true thickness of mineralisation in the northern deposit could be 56.5m if the drill intercept from 104-133m in CADD003 is included. This would increase the resource estimate for the northern deposit by ca. 50% but is dependent on verification of increased Fe grades with finer grinding so is not included in the above inferred resource.

Hematite Resources
In addition to magnetite BIF, there is potential for both direct-shipping hematite ore and hematite BIF or taconite at Carrow. The northerly extension to the main gravity lobe of the prospect is an exploration target for either direct-shipping hematite or high-grade hematite BIF since it does not coincide with a strong magnetic anomaly yet indicates a dense body of rock concealed below sand dunes. It has not been tested by drilling.

Oxidation of magnetite BIF at Carrow has produced a variably siliceous, goethite-hematite rich rock with whole-rock iron values up to 40% Fe (Table 21).

Magnetite-rich BIF in drill hole CADD008 :: Carrow Deposit (Click to see image)

Table 21: Selected drillhole intercepts and assays for oxidised BIF at Carrow

Drillhole	From (m)	To (m)	Intercept (m)	Al2O3 (%)	Fe (%)	MnO (%)	P2O5 (%)	SiO2 (%)
Northern Deposit
CARC011	24	36	12	3.79	33.02	0.27	0.38	38.73
CARC012	18	33	15	3.70	34.48	0.67	0.53	39.64
CARC013	30	36	6	9.14	33.53	0.68	0.35	27.95
CARC019	9	30	21	8.55	33.90	0.07	0.51	29.06
Weighted Average				6.21	33.82	0.35	0.47	34.02
Southern Deposit
CARC009	15	48	33	2.80	30.97	0.14	0.36	45.18
CARC010	9	33	24	4.09	32.36	0.10	0.33	40.55
CARC017	14	27	13	4.20	36.52	0.11	0.36	34.48
CARC017	36	48	12	0.57	37.32	1.92	0.99	30.35
Weighted Average				3.07	33.19	0.38	0.44	39.96
Other
CARC016	24	36	12	7.10	26.69	1.09	0.25	38.53

Based on SAIOG RC drillholes, the thickness of sand, gravel, saprolite and laterite overlying the actual BIF is 12-15m thick and oxidation extends 30-60m (average 40-43m) below ground level. Drillholes are too widely spaced to give a complete section across any of the identified high-grade magnetite bodies. CARC011 and CARC019 combined give a high-grade hematite BIF intercept of 33m along a section through CADD003 so this has been used to estimate an inferred hematite BIF resource over the northern deposit (Table 22). The best intercept on the southern deposit is also 33m.

Table 22: Inferred magnetite BIF resource for Carrow

	Northern Deposit	Southern Deposit
Average true thickness based on aggregate intercepts with >30% Fe	23.3m (CARC011 and CARC019)	23.3m (CARC009)
Vertical extent of sand and saprolite cover sediments	12.5m	14.6m
Vertical extent of oxide zone below ground level	39.9m	43.0m
Assumed dip of BIF	65ºWNW	65ºWNW
Drillhole declination	65ºSE	60ºESE
Dip extent of oxidised BIF (excluding overlying sand)	30.3m	30.3m
Strike length (based on aeromagnetic interpretation)	1150m	700m
Volume of hematite BIF	0.8 million m3	0.5 million m3
Hematite BIF density	3.0	3.0
Inferred hematite BIF resource	2.4 Mt @ 33.8% Fe	1.5 Mt @ 33.2% Fe

Given that the high-grade magnetite BIF intercepts are up to 85m thick (aggregate true thickness in CADD007), there is potential for >12 Mt of hematite BIF.

Regional Potential
13km NE of Carrow is a similar but smaller intense aeromagnetic anomaly that has never been drill tested (Dutton Bay ELA). The anomaly, which has only become clear on recent PIRSA aeromagnetic enhancements, is just as intense as Carrow and approximately 1.5km long. Like Carrow, it is on the immediate western edge of the Kalinjala Mylonite Zone. Although smaller than Carrow, it outlines a ca. 25 Mt magnetite BIF exploration target with associated hematite BIF as at Carrow. It is a high priority target and detailed low-level aeromagnetics, gravity and drilling are warranted.

The Neill prospect is located on a major linear aeromagnetic feature that extends ca. 65km in length from about 15km SW to 50km NE of Port Neill. The anomaly is broad (up to 2km wide) and enclosed within the Kalinjala Mylonite Zone. Drilling by BHP Ltd in 1977 intersected quartz-feldspar-magnetite gneiss in drillholes EEP13 and EEP14. Both holes intersected thin cover (<10m) over weathered basement (saprolite) to ca. 40m and were completed at, respectively, 38 and 42m, in quartz-plagioclase-biotite-magnetite (diopside, muscovite) gneiss with ca. 5% magnetite. They are not considered economic at this stage.