Geology
Regional geology
The Blue Spec Shear gold project (as with Northwest’s regional gold projects) lies within the Mosquito Creek Formation, a thick sequence of siliclastic metasediments comprising dominantly quartz rich arenites and shales that has undergone multiple periods of deformation. Located within an extensive rhombic shaped basin in the southeastern extent of the Pilbara granite-greenstone terrain, the Mosquito Creek Formation is interpreted to be late Archaean in age.
Stratigraphy within the basin generally trends ENE with both NW and NS trending regional fault orientations exerting major influences on the architecture of the basin. Intrusive dykes comprising dolerite and hornblende-bearing tonalites are also observed within the basin while gabbroic sills are a prominent feature along the northern and southern margins of the basin.
Northwest has identified a significant large-scale topographic depression which the Blue Spec Shear gold project and the Camel Creek gold project transect. This feature is an important large-scale geological footprint and has given Northwest significant confidence in the area hosting large ore deposit settings.
Mineralised corridors
Initially, primary gold mineralisation within the Blue Spec Shear gold project was considered to be limited to the Blue Spec Shear zone, an ease-west trending shear zone which outcrops over 18 kms within Northwest’s project area.
However, Northwest has recently discovered and mapped a new shear, named the Red Ribbon Shear, which lies approximately 1km north of the Blue Spec Shear. The new shear trends parallel to the Blue Spec Shear over approximately 18kms. Detailed structural mapping, rock chip and soil sampling completed over the new Red Ribbon Shear has confirmed that the new shear has the same key characteristics as the Blue Spec Shear.
Mineralisation styles
High grade gold mineralisation along the Blue Spec Shear is associated with quartz-carbonate-pyrite veining and stibnite (the sulphide form of the metal antimony). The primarily shear vein-hosted mineralisation usually forms in steeply plunging lenticular shoots that display strong plunge continuity.
Some stockwork vein-hosted style mineralisation is also present at the eastern end of the Blue Spec Shear. This mineralisation is usually lower grade than the shear vein-hosted style.
No sheeted vein-hosted mineralisation styles have been recognised within the project although their presence is considered likely.
Controls on mineralisation
All primary deposit settings along the Blue Spec Shear appear to lie within structurally controlled fault settings, highlighting the importance for successful exploration of understanding the structural controls on mineralisation within the project.
Northwest has developed a detailed structural model for the Blue Spec Shear (which can also be applied to the Red Ribbon Shear) as a result of 1:2,500 scale structural and stratigraphic mapping over the length of the Blue Spec Shear and 1:1,000 scale mapping in areas of higher structural complexity. Structural mapping is usually undertaken on a 1:10,000 scale. The model also incorporates a detailed deformation history affecting the Blue Spec Shear zone. This model has allowed Northwest to determine the key structural criteria which control the introduction of gold bearing fluids into the localised deposit settings.
Northwest has also developed an alteration and fluid-chemical model to understand the nature of gold bearing fluids within the Blue Spec Shear corridor. The Blue Spec Shear fluid signature is unique and has allowed Northwest to develop specific pathfinder assemblages for down-hole and surface footprinting work. A key outcome from this work is an understanding of chemical mechanisms which control the efficiency of gold precipitation. This model provides an explanation the difference in grade between deposits located along the Blue Spec Shear corridor (high grade - efficient system) versus deposits along the Camel Creek corridor (moderate grade - less efficient system).
Northwest’s understanding of controls on mineralisation has provided the main criteria that are used within Northwest’s Predictive Targeting Model for the Blue Spec Shear gold project. Follow this link for more information on the Approach to Exploration .
