Blue Spec
The Blue Spec deposit is one of the highest-grade gold deposits in Australia with a current JORC resource of 156,000 oz gold grading 46.3g/t and 3,400 tonnes of contained antimony and is the most advanced resource within the Blue Spec Shear gold project.
Recent deep drilling results at the Blue Spec gold deposit have resulted in the deposit achieving key internal benchmarks related to resource continuity and growth potential. As a result, Northwest has now committed to accelerating the development schedule for the Blue Spec deposit, which aims to significantly expand the current JORC resource.
The company is targeting a revised JORC compliant resource estimate at the Blue Spec deposit of at least 300,000 oz gold and 7,000 tonnes of contained antimony to underpin a high-grade, low cost underground mining operation over an initial 5 year mine life.
Following the announcement of a revised JORC resource, expected by October 2007, Northwest will commence scoping studies which the company believes will demonstrate the robust project economics of the Blue Spec deposit which has already been indicated by preliminary economic benchmarking.
Follow this link for more information on Northwest’s Blue Spec Development .
History
The Blue Spec deposit was discovered in 1906 and named after the winner of the 1905 Melbourne Cup. Early mining to a depth of 180m was undertaken by following the vein system one level at a time. Stope developments were poorly designed for the style of orebody and drives often ended in ore grade material. As a result, only a small proportion of upper level ore was effectively mined as can be seen from the stoped areas in the Blue Spec deposit long section.
Anglo American acquired the mine in 1974, defining an ore reserve from the 180m to 320m vertical level, sinking a new triple compartment shaft and developing a decline. Anglo American utilized mechanized cut and fill mining methods with average stope widths of 2m. Stoping of the orebody to widths of less than 1.8m in the western extent of the Blue Spec orebody during the last three periods of production achieved lower dilution rates with production exceeding 36g/t gold and 3.5% antimony.
During production geotechnical stability and stress conditions were good. Locally sourced river gravel was successfully used for dry fill and when dampened with development drill water consolidated well under the pressure of the mucking loaders. The mine did not suffer from high water discharge.
Anglo American closed the mine in 1979 once the original reserve blocks had been extracted due to a combination of economic and management reasons:
- Anglo American experienced difficulties processing the auro-stibnite ore and achieved poor recoveries of gold.
The processing issues experienced by Anglo American are now well understood and modern process flows produce very good gold recoveries exceeding 90% for gold. Follow this link for more information on Metallurgy and Processing .
- Anglo American did not have an overall understanding of the morphology of the deposit evidenced by intermittent stope development along the strike of the orebody and ore mapping which continued to the termination of stope drives.
Northwest has invested significant resources collecting and digitizing the Blue Spec deposit's substantial historical development and drilling data in order to construct a detailed 3D geological model which enabled the deposit to be interpreted over its full extent for the first time. The model also highlighted key structural positions which control the deposit's plunge continuity enabling Northwest to predict with confidence significant extensions to the orebody below the lowest level of production
- Anglo American exited Australia very shortly after closing the mine. Blue Spec was Anglo’s first operating mine in Australia as part of a strategy to geographically diversify the company – a strategy that was subsequently changed.
The Blue Spec mine’s disjointed ownership, inadequate capital, and, most significantly, a lack of understanding of the style of mineralisation and its structural controls combined to prevent the mine from ever reaching its full potential. These factors resulted in the Blue Spec deposit never being properly developed or recoveries optimized and accounts for the relatively low recorded average production of around 85,000 ounces at 24g/t.
These factors present Northwest with a unique opportunity to develop a quality gold asset and establish Northwest as a future, sustainable, high-grade gold producer within the shortest possible timeframe.
Geology
The Blue Spec deposit is characterised by some of the highest gold grades in Australia. Significantly, gold grades have continued to increase with depth. Head grades increased significantly from around 16g/t Au in the upper levels to 36g/t Au at the lowest production levels. The current JORC resource has an undiluted grade of 46.3g/t.
Mineralization is hosted in quartz veins that average 1.5m to 3m 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. High grade gold mineralization is variously associated with the presence of stibnite (the sulphide form of the metal antimony).
The strike length of the Blue Spec ore body has varied from 120m to 160m. In the upper levels of development, the orebody exhibited a near vertical plunge. At the 450m vertical metre level, the plunge deviates from near vertical in orientation to north dipping. This is significant as such deviations have the potential to result in positive changes in the nature of the orebody.
The Blue Spec orebody is characterised by a well defined vein morphology, hosted within a localised structural setting along the Blue Spec Shear. The remarkable plunge continuity over its known depth extent is related to key connection points within the fault network that hosts the deposit. These same connection points have dictated the location of Blue Spec main high-grade shoot locations from surface to the bottom level of development as evidenced in detailed underground mapping. It is this strong depth continuity that has allowed accurate targeting of the extensions to the Blue Spec gold deposit.
Recently, a deep diamond drilling has intersected a parallel fault zone some 40m to the north of the Blue Spec deposit that has returned 4.0m grading 2.2g/t gold from a depth of 495m down-hole. A significant increase in the level of vein intensity and sulphide content in this zone encouraged Northwest to undertake assaying of samples over this zone. Analysis of Northwest’s other diamond holes that pass through this zone indicates that both deformation and alteration are intensifying with depth along this structure.
The presence of a second gold-bearing structure some 40m to the north of the Blue Spec orebody indicates the potential for the discovery of blind ore shoot positions within close proximity to Blue Spec.
JORC Resource
The existing Blue Spec JORC resource has been modeled over an extent of 150m from a vertical depth of 320m to 470m. The undiluted JORC resource is:
|
|
Tonnes |
Au g/t |
Sb % |
Au oz |
Sb t |
Au Eqv oz |
|
Indicated |
16,642 |
47.2 |
4.3 |
25,259 |
714 |
27,563 |
|
Inferred |
87,167 |
46.1 |
3.0 |
129,312 |
2,649 |
137,827 |
|
Total |
103,810 |
46.3 |
3.2 |
154,571 |
3,363 |
165,390 |
The diluted JORC resource, based on a 2m minimum mining width (the mining width achieved by Anglo American during previous production) is 152,000 tonnes @ 31.2 g/t Au and 2.2% antimony for 156,000 oz gold and 3,400 tonnes of contained antimony.
The current JORC resource has a metal profile exceeding 1,000 ounces of gold per vertical metre and 1,100 gold equivalent ounces per vertical metre (gold and antimony). This metal profile compares favourably with other Australian underground miners including Dominion Mining, Avoca Resources and Medusa Mining.
The deposit displays a sharp cut-off grade between ore and waste, mostly defined by material grading in excess of 20g/t. This feature in parallel with a diluted grade of 31.2g/t will underpin a very high conversion rate of resources to reserves.
Antimony (Sb) credits within the defined resource have a current market value exceeding US$7 million based on an antimony concentrate value of around US$1,800 per tonne (the current market price for antimony metal is over US$5,000 per tonne). The presence of antimony does not give rise to any significant metallurgical issues. Follow this link for more information on Antimony .
Page last updated: 09.08.2007
