Blue Spec Shear
The Blue Spec Shear is an east-west trending shear zone which outcrops over 16kms within the Nullagine Gold & Antimony Project area. The shear zone is typically a narrow, sub-vertical band of strongly sheared wall rock and quartz reefs, which broadens in a number of places. A number of splay faults have been identified along the shear and these have been shown to be important for focusing high-grade mineralisation.
High-grade gold mineralisation along the shear is associated with quartz-carbonate-pyrite veining and stibnite (the sulphide form of antimony). The primarily shear vein-hosted mineralisation usually forms in steeply plunging lenticular shoots that display strong plunge continuity and increasing grade with depth.
Within the project area, the shear currently hosts four deposits:
- Blue Spec - 254,000 oz @ 24.3 g/t Au and 5,500t Sb @ 1.72% (including 122,000 oz @ 42.5 g/t Au)
- Golden Spec - 75,000 oz @ 8.5 g/t Au and 2,300t Sb @ 0.84% (including 49,000 oz @ 18 g/t Au)
- Red Spec - 18,000 oz @ 1.9 g/t Au and 290t Sb @ 0.01%
- Green Spec - 10,000 oz @ 3.2 g/t Au and 1,100t Sb @ 1.10%
Whilst Northwest has completed over 21,000m of RC drilling and 22,000m of diamond drilling along the Blue Spec Shear this has been mainly focused on the 5km long strike between Golden Spec in the west and Green Spec in the east. There remains excellent potential to discover further high-grade deposits along the extent of the Blue Spec Shear.
Current Development
Northwest is focused on developing its core Blue Spec Shear high grade gold-antimony deposits into one of the highest grade gold underground mining centres in Australia.
Please click here for more information on the current development of the core Blue Spec Shear high grade gold-antimony deposits.
Blue Spec
Resource: 254,000 oz @ 24.3 g/t Au and 5,550t Sb @ 1.72%
The Blue Spec deposit was discovered in 1906 and named after the winner of the 1905 Melbourne Cup. The mine suffered from disjointed ownership and inadequate capital until Anglo-American developed a new shaft and decline and mined down to the 320m vertical level between 1976 and 1978.
Mineralization is hosted in quartz veins that average 1.5m to 3.0m in width but which are known to blow out to over 8m in certain zones. It is expected that these widths will allow mechanized mining of the deposit and result in comparatively less dilution. The deposit has a near vertical plunge and demonstrates excellent continuity.
The strike length of the Blue Spec ore body varies from 120 to 160m but has not been closed off to the east or west. Recent drilling at Blue Spec has returned intersections of 4m @ 4.49 g/t Au (incl. 1m @ 8.74 g/t Au) from 121m and 2m @ 3.7 g/t Au (incl. 1m @ 5.13 g/t Au) from 42m located approximately 50m to the west of the modelled position of the remnant mineralisation in the historical upper levels. The position of these intersections suggest that the interpreted strike of the Blue Spec deposit at depth may be significantly extended by further drilling with positive implications for potential underground development.
The Blue Spec resource is divided into the Upper Zone and the Lower Zone based on contained metal and drilling density.
- Upper Zone (320m to 440m) - 122,000 oz @ 42.5 g/t Au and 2,300 t Sb @ 2.61%
- Lower Zone (440m to 850m) - 132,000 oz @17.5 g/t Au and 3,200 t Sb @ 1.38%
The Upper Zone contains greater than 1,000 ounces per vertical metre (opvm) of gold over 120m immediately below the existing underground development at the 320m level. Approximately 66% of this Upper Zone exceeds 1,100 opvm of gold.
The current grade estimation within the Lower Zone has been adversely affected by a combination of lower grade significant intercepts as well as a relatively low number of drill holes compared to the Upper Zone. The amount of drilling Northwest has been able to direct at the Lower Zone has been constrained by the significant time required to complete deep diamond drill intersections from surface down to the 440-850m levels and accordingly the gold grade estimation in the Lower Zone may not reflect the true potential of the Blue Spec ore body at depth.
Northwest has identified several areas within the Lower Zone which it predicts could have potential for higher grade mineralisation if subjected to additional drill testing. Northwest plans to carry out further drilling from underground platforms during production to delineate further resources in the Lower Zone. Extensional diamond drilling of the Lower Zone has recorded some very significant intersections, including 2.5m @ 156.2 g/t Au & 14.75% Sb from 715m down-hole.
Northwest has also identified a significant amount of remnant mineralisation potential within the Remnant Zone (the historical upper levels of the Blue Spec underground). This is thought to reflect a combination of poor development in the historical upper levels of the underground and a poor understanding of the ore body morphology with depth.
A target mineralization of approximately 86,000-100,000 tonnes grading 14 g/t – 18 g/t for 45,000 - 50,000 oz Au has been estimated for the Remnant Zone.
Golden Spec
Resource: 75,000 oz @ 8.5 g/t Au and 2,300t Sb @ 0.84%
The Golden Spec deposit lies 950m west of Blue Spec and was discovered in 1982. It was mined by a consortium including Chase Minerals and Minproc in the late 1980’s to a depth of 120m. The mine was closed with Chase Minerals went into receivership.
The deposit comprises two steeply dipping lodes (East and West) within an east-west trending shear zone. The two lodes are each around 50-150m in strike length and are around 2m in width. These widths will also support mechanized mining of the deposit.
The recently increased JORC reported Mineral Resource includes significant near surface resources which will have a positive economic impact upon the proposed underground development of the combined Golden Spec - Blue Spec deposits. It is important to note that within the current Mineral Resource there are high grade zones estimated to contain 49,000 ounces of gold grading 18 g/t and 1,750 tonnes of antimony grading 2.1%.
Northwest believes that there is significant potential to further increase the size of Golden Spec through further drilling as the deposit is open to the east and at depth (the adjacent Blue Spec deposit has been defined to a depth of 850m).
Red Spec
Resource: 18,000 oz @ 1.9 g/t Au and 290t Sb @ 0.01%
The Red Spec deposit lies approximately 150m east of the Blue Spec deposit. The mineralisation at Red Spec is controlled by a sub-vertical brittle 2 to 3m wide ductile shear zone with lateral displacement and secondary tensile splay structures.
The Golden Spec and Red Spec deposits display different types of mineralisation along the same host structure. The Golden Spec deposit is located on a section of the Blue Spec Shear that is dominated by one major structure, with little evidence of splay structures. By comparison, the Red Spec deposit is located on a section of the Blue Spec Shear that has splayed into several structures. Due to structural and chemical controls, the Golden Spec deposit has been more endowed with the high-grade gold and antimony mineralisation hosted in a single quartz reef, whilst the Red Spec deposit has been endowed with lower grade gold mineralisation in stacked vein sets.
Green Spec
Resource: 10,000 oz @ 3.2 g/t Au and 1,100t Sb @ 1.10%
The Green Spec deposit was discovered by Northwest in 2006 and lies 3.5kms east of the Blue Spec deposit.
The Green Spec deposit comprises two lodes, an east-west trending gold lode along the main Blue Spec Shear, and an east-north-east trending antimony lode which has been interpreted as a splay structure similar to that seen at Blue Spec. The separation of gold and antimony into the two lodes suggest that the deposit is structurally immature compared to Blue Spec and Gold Spec, which has had more time to develop high-grade mineralisation.
The deposit has a strike length of 130m, and has been extended from surface to almost 200m vertical depth. The deposit remains open at depth and along strike.
The Green Spec resource is a lower grade and higher tonnage deposit relative to Blue Spec and is amenable to open pit mining.
Orange Spec
The Orange Spec prospect was discovered by Northwest in 2010 and is a new zone of high-grade gold mineralisation located 300m west of Green Spec. Drilling has returned outstanding results including:
- 2m @ 26.30 g/t Au from 44m
- 4m @ 6.0 g/t Au from 72m (incl. 1m @ 18.40 g/t Au)
- 2m @ 9.30 g/t Au from 40m (incl. 1m @ 17.40 g/t Au)
- 7m @ 2.81g/t Au from 33m (incl. 1m @ 10.50 g/t Au)
The mineralisation is associated with quartz-carbonate veining over a strike length of 100m and at vertical depths of 40-60m. The high-grade results are not reflected in shallow drilling vertically above the mineralized zone.
The discovery of Green Spec, Red Spec and Orange Spec are very significant as they demonstrate the potential of the Blue Spec Shear to host additional medium to high grade gold-antimony deposits along its 16km extent.