Base metal exploration within the Jillawarra Sub-Basin was initiated during 1976 by Amoco Minerals Australia Company (Amoco), predominantly in areas of exposed prospective host units. The Abra Prospect was not tested until 1981 when Geopeko Limited, in joint venture with Amoco, targeted a single diamond drillhole into a discrete magnetic anomaly within the eastern, buried portion of the basin.
The first drilhole AB3 penetrated approximately 270 metres of barren cover sediment before intersecting a sequence of stratabound chlorite-iron oxide-barite altered rocks. Mineralisation included 27 metres averaging 6.1% Pb from 268 metres, 194 metres averaging 3.1% Pb from 318 metres and 19 metres averaging 3.68g/t Au and 1.1% Cu from 503 metres. Between 1981 and 1989, 13 drillholes were successfully completed at Abra, all intersecting a similar sequence of altered and mineralised rocks.
The most significant result was in AB16, which intersected 222 metres averaging 7.6% Pb, including 44 metres averaging 25.3% Pb (6% Pb cutoff). Three regional drillholes were also completed and a single drillhole at Genie, approximately 3.5km east of Abra, intersected similar but less intense alteration and lead mineralisation to that at Abra.
In 1995 Renison Goldfields Corporation (RGC) joint ventured into the project, then held 100% by North Limited (formerly Geopeko), and completed a single drillhole at Abra. AB22A was drilled to the south, opposite to all previous drilling, and intersected a sequence of chlorite-altered sediment largely in the footwall to the lead mineralisation.
The best lead mineralisation found was 89 metres averaging 3.0% Pb from 295 metres within the stratabound iron oxide-barite altered sequence, however, significant copper-gold mineralisation was intersected in the footwall of the lead domain, returning results of 11 metres averaging 3.3% Cu and 0.2g/t Au from 715 metres (1% Cu cutoff).
Oldcity Nominees Pty Ltd acquired the project in 2000 after the existing granted mining lease was relinquished by North Limited. Oldcity acquired much of the existing data and completed in-fill gravity surveying to better define the recognised association of Abra with a gravity anomaly. In 2005, the project was acquired by AML after its listing on the ASX.
Exploration completed by AML – 2005
AML completed regional gravity coverage over the project area at 200 metre by 400 metre spacing and acquired additional in-fill gravity at 50 metre spacing within the area of the Abra and Genie Prospects. The Genie Prospect is located 3.5km to the east of Abra and one diamond drillhole to date has intersected 28m@ 2.4% Pb.
A diamond drilling programme, which commenced in August 2005 comprised 10 drillholes at the Abra Prospect and two drillholes at the Bazaar Prospect, a prominent east-west gravity anomaly located approximately 750 metres south of Abra.
At Abra, alteration is dominated by iron oxide minerals (hematite with subordinate magnetite), over iron sulphide. Lead mineralisation dominantly occurs as galena associated with intense iron oxide-barite alteration within the upper red and black stratabound units and with intense silica veining overprinting chlorite-altered sediment breccia underlying these units.
A zone of copper-gold mineralisation overprints the lead zone, typically occurring as steeply dipping quartz-chalcopyrite-pyrite-magnetite±dolomite veins. The intensity of chlorite alteration and veining decreases into the footwall sequence of laminated lithic sandstone.
Each drillhole at Abra intersected thick zones of mineralisation consistent with those encountered in historic drilling, typically 170 metres to 260 metres thick. Mineralisation was more extensive than previously interpreted, with significant intersections up to 200 metres south of previous drilling (viz AB30, AB31 & AB32).
Although the two drilholes at the Bazaar Prospect were drilled to depths of approximately 400 metres, both failed to penetrate the cover sequence.
Exploration completed by AML – 2006
Induced Polarisation (IP) Electrical Geophysical Surveying
Due to the early success of magnetic anomaly targeting at the Abra deposit, historic exploration at Mulgul and throughout the Jillawarra Sub-basin was focused on targeting magnetic anomalies in conjunction with geochemical surveys and EM (electromagnetic) geophysical surveying was also used.
AML identified the absence of IP geophysical surveying in the province and set about applying the technique within the Mulgul project area. AML’s rationale reflected the fact that few of the major Proterozoic base metal systems worldwide (both SEDEX and IOCG) are expressed as EM anomalies but many give IP anomalies.
AML completed a wide (800 metre) spaced IP survey over seven kilometres of strike centred on the Abra deposit, which resulted in the definition of three significant anomalies. A broad IP anomaly, named Hyperion, was located two kilometres west from the Abra deposit and remained open to the west.
A major anomaly was detected in the northeast corner of the survey area. This anomaly, designated Rhea, appears to be relatively shallow and is of significantly stronger magnitude when compared with the anomaly over the Abra deposit.
An anomaly was detected to the south of Genie drillhole GD1, which was drilled to test a magnetic anomaly and intersected 28m averaging 2.4% Pb. This anomaly, designated Tethys, is comparable in magnitude and depth with that observed over the Abra deposit. It is noteworthy that drillhole GD1 was drilled to the north and has not tested the Tethys IP anomaly.
Drilling
Based on the initial scoping study results, AML set a goal of establishing a mineral resource with target size and grade of 75Mt to 100Mt at ≥4% Pb-equivalent.
A drilling campaign was conducted in mid-2006 with the aim of determining whether the style of mineralisation intersected at Abra would change, particularly in terms of metal association and/or in grade, or alternatively to find the boundaries of the Abra deposit.
The drilling was not intended to directly contribute to an upgrade in the mineral resource but to provide a framework for optimising future resource definition drilling. Five drillholes were completed representing step outs between 170 metres and 230 metres to the east, south and west of the deposit.
The five step-out drilholes at Abra all intersected mineralisation and alteration but largely with lesser intensity and of lower grade relative to Abra. It was thought likely that the drillholes represented proximity to the margins of the most intensely mineralised part of the system although the possibility that the system could improve beyond those drillholes remains valid. The variation in alteration intensity suggested that good mineralisation may continue directly south from Abra (towards Bazaar) and remained untested.
A single drillhole, HY1, was drilled 2 kilometres west of the Abra deposit to test the Hyperion IP anomaly at its furthest distance from Abra. This drillhole intersected stratabound mineralisation comprising 21.15 metres averaging 4.54% Pb and 23ppm Ag from 546.8 metres, including 6.05 metres @ 9.86% Pb and 42ppm Ag from 548.2 metres and 3 metres @ 0.6% Cu from 567 metres.
Geophysical modelling suggested that mineralisation similar in style to that in HY1 could be the cause of the Hyperion IP anomaly. This fact, and the similarity of the style of mineralisation with that in AB37, the westernmost drillhole at Abra, provides strong encouragement that the Hyperion IP anomaly is due to the presence of a broad sheet of stratiform sulphide mineralisation.
Exploration completed by AML – 2007
Drilling
In the 2007 reporting period, AML completed 12 effective drillholes (AB39 to AB45, AB47 to AB50 and AB53) to provide infill and to extend the known mineralisation to the north and south of the deposit. Results from this drilling were incorporated into the 2008 Mineral Resource and contributed to the successful increase in the size of the resource, the level of confidence in the resource as well as better definition of higher grade sections of the deposit.
Outstanding results were achieved during the period demonstrating that significant base metals mineralisation continues to the north of the known Abra deposit.
Induced Polarisation (IP) Surveys
Search Exploration undertook a pole-dipole induced polarisation survey over the eastern side of the Rhea prospect at the Mulgul project during the reporting period.
Three lines were undertaken to test the extent of the anomalous chargeability response present at Rhea with a total of 10.7km of survey being completed. The intention of the work was to close off the IP response at Rhea on line 665600E, that remained open to the east upon completion of surveys in 2006.
At the same time Search Exploration undertook a pole-dipole induced polarisation survey over the western side of the Hyperion prospect. The line (658000E) was intended to confirm the continuation of a near surface chargeable zone which is higher in the stratigraphy than mineralisation intersected at 450+m on line (658500E). The hole on line 658500E (HY1 – 2006) was drilled into a coincident magnetic gravity and IP response. Later logging with a down hole IP system identified a high chargeability zone in the upper 150m of the drill hole which is considered to be the source of the IP anomaly. The response from the new line was similar to that of previous years and is considered consistent with the near surface chargeability zone continuing. No evidence of a deeper source is present. The western line has a slightly lower resistivity than 658500E and this may represent the presence of slightly more recent transported cover as we go into a slight topographic low.
Unless otherwise indicated, all intercepts are for 1% lead cutoff allowing 10 metres of internal dilution. All intercepts are downhole depths and correspond to approximate true widths.
