The Carr Boyd Rocks historic mine is located 80 km northeast of Kalgoorlie in Western Australia and was actively mined for its Ni-Cu resource until 1977. Mineralisation was described as hosted within ‘sulfide breccia pipes’ (Purvis et al. 1972), although their formation was poorly understood. The abandoned pit and surrounding prospects occur within a mafic-ultramafic layered intrusion informally known as the Carr Boyd intrusive complex (CBIC), with a surface expression of approx. 78 km2 and compositions ranging from peridotite to leucogabbro (Purvis et al. 1972, Hayden 2008). Recently, a massive sulfide accumulation was discovered at the contact between the intrusive complex and the country rocks at the T5 prospect. This project investigated the generation and emplacement of the CBIC and its contained sulfide mineralisation, the 3D geometry of the intrusive complex, identification of potential ore genesis processes at play and evaluated prospective areas within the complex. A detailed geochemical study was completed using company assays, resulting in a new lithogeochemical classification of the mafic-ultramafic rock types within the complex. 3D visualisation confirmed the company’s current interpretation of the geometry of the CBIC. Primary sub-horizontal igneous layering has been rotated and is now steeply dipping to the WSW. And the SW edge of the intrusive complex is interpreted as potentially representing the original floor of the CBIC (or basal contact). Representative samples of the two distinct sulfide occurrences, the Carr Boyd ‘sulfide breccia pipes’ and the T5 sulfide intersection both appear to have been shielded from deformation and display classic magmatic textures (pentlandite loop textures) and no clear evidence of sulfide remobilisation. Interestingly, sulfide compositions show clear differences with more elevated Ni tenors but lower Pd and Pt tenors for the Carr Boyd ‘sulfide breccia pipes,’ compared to T5 sulfides. The sulfides are interpreted as products of two distinct magma batches with varying metal compositions, suggesting two separate mineralising events within the CBIC. Olivine compositions (laser ablation ICP-MS) are clearly different between the Carr Boyd pipes and T5, which supports the interpretation of two separate magma pulses. Interestingly, a linear correlation between Co and Ni is observed in sulfide-bearing samples, which might represent a good indicator of sulfide presence. Within the CBIC spinel has Al-poor chemistry, interpreted as primary, and there appears to be a depletion of Ni content that can be correlated with the presence of sulfide. Finally, apatite from the Carr Boyd pipes yielded an age of 2694 ± 42 Ma and T5 apatite yielded an age of 2687 ± 56 Ma, both within uncertainty of each other. The CBIC mineralised complex is the first known example of a mineralised Archean intrusion-hosted nickel sulfide system in the Eastern Goldfields Superterrane of the Yilgarn Craton, opening a new search space. The CBIC is underexplored and still holds potential for both internal sulfide breccias resembling the Carr Boyd breccia pipes as well as basal contact massive sulfide occurrences like T5.

Reference(s)

Purvis AC. Nesbit RW and Hallberg JA (1972) The geology of part of the Carr Boyd Rocks complex and its associated nickel mineralisation, Western Australia: Economic geology, v. 67, p. 1093-1113. 

Hayden P. (2008), Carr Boyd Project, Annual Technical Report for the year ended March 2008: Brockman Resources limited: Geological Survey of Western Australia, Statutory mineral exploration report A078186, 21p