The Salt Lick Creek intrusion presents as a well exposed oval shaped mafic/ultramafic body about 3km’s in diameter and 10km’s north of the Bow River intrusion, located approximately 5km’s east of the Great Northern Highway. Wilkinson et al. (1975) divided the body into two well defined zones. The Basal Zone, 360m thick, contains rhythmically layered olivine and plagioclase-olivine cumulates (troctolite, olivine gabbronorite, leucogabbro, plagioclase dunite) and the Main Zone 540m thick, is largely composed of plagioclase-orthopyroxene cumulates (norite, gabbronorite, leucogabbro) which grade upwards into plagioclase cumulates (anorthosite). Chromite is restricted to the olivine rich cumulates of the Basal Zone and sulphides (pentlandite, pyrrhotite, chalcopyrite, valleriite, bornite, cubanite and siegenite) form a trace component of most rocks (less than 1 %).
The ultramafic cumulates of the Basal Zone crop out along the southern rim of the intrusion and dip northwards at about 30°. The 11 basal contact is cut by granites possibly belonging to the Sally Downs super-suite (Sheppard et al. 1999). Hoatson et al. (2000) suggested depressions in the basal contact and possible feeder conduits should be high priorities for Ni-Cu-Co sulphide mineralisation.
Mineralisation and Exploration Models
The East Kimberley Halls Creek Orogen is widely regarded as having excellent potential for magmatic Ni-Cu-Co sulphide and PGE mineralisation, and Hoatson and Blake (2000) considers it one of the most extensively mineralized igneous associations in Australia.
The Savannah Intrusion (held by Panoramic Resources Ltd) hosts the largest Ni-Cu-Co sulphide resource discovered to date within the East Kimberley. Hoatson and Blake (2000) comment on the similar tectonic, stratigraphic and mineralisation features between Savannah and the world class Voisey’s Bay deposit in Labrador, Canada. Within the East Kimberley the Voisey’s Bay deposit provides a robust and realistic model to guide exploration strategies and targeting. Importantly the Voisey’s Bay and Savannah models indicate even small intrusive bodies can host giant nickel deposits and that mineralisation may not outcrop at surface.
Hoatson also recognized broad similarities between the HCO intrusions and the major mineralised layered intrusions at Sudbury, the Bushveld Complex and the Stillwater Complex. The HCO also has a number of similarities to the Tornio - Narankavaara (T-N) intrusive belt in northern Finland. This belt contains the Portimo and Penikat intrusive complexes that are known to host PGE mineralisation of potential economic grade and size. The mineralisation in the Penikat intrusive is analogous to the PGE mineralisation at the Panton and offers some similarity to the chromite layers within the Salt Lick Creek intrusive.
The vast majority of Ni-Cu sulphide deposits are magmatic in origin and are hosted or linked to igneous rocks that formed from magma ranging from ultramafic to mafic in composition. The vast majority of the world’s mafic and ultramafic rocks are not associated with any sulphide occurrences however, and special processes are required to form magmatic sulphides with nickel and copper and to concentrate them into economic deposits. Three processes are viewed as critical:
First is the generation of suitable sulphur under saturated magma from a mantle derived source.
Secondly the host magma has to have been contaminated by crustal material with resulting sulphur saturation and the formation of an immiscible sulphide melt scavenging copper and nickel from the magma.
Thirdly the presence of a suitable physical trap site is required to allow the dense sulphide melt to separate from the rest of the magma body and form an ore body.
Within the Halls Creek Orogen all three critical processes have occurred and the adjacent Savannah Ni-Cu deposit demonstrates these sulphide occurrences can be focused into economic deposits.