Multidisciplinary study of a complex magmatic system: The Savannah Ni-Cu-Co Camp, Western Australia

摘要

The Savannah (formerly Sally Malay) Ni-Cu-Co Camp is composed of a group of mineralised intrusions (Savannah and Savannah North) and petrologically similar mafic-ultramafic intrusions without known mineralisation (Sub-chamber D, Dave Hill, and Wilson's Creek). The results of a wide range of analytical techniques (geochemical, geochronological and geophysical) are integrated to gain a better understanding of the geometry of the Savannah mineralised magmatic intrusions and the timing of events. Magnetic anisotropy results suggest that Savannah and Savannah North have both been affected by similar deformation events, implying nearsynchronous intrusion. However, Savannah and Savannah North show evidence of fundamentally different styles of deformation (strong folding at Savannah, as opposed to gentle tilting at Savannah North). This discrepancy can be reconciled with a resolvable variation in age, with Savannah North being the younger by ca. 0.8-2.5 Ma, as well as difference in the geometry and associate rheological properties of the intrusions: the larger more equant Savannah North intrusion may have been more resistant to deformation than the thinner elongated Savannah body. Isotopic and geochemical data on the mafic-ultramafic intrusions of the Savannah Ni-Cu-Co Camp indicate that the entire intrusive suite, with the exception of the early mafic granulite dykes, was derived from a similar parent magma of basaltic composition with likely continental arc affinity. Results of sulfide tenor calculations combined with mineral chemistry and in-situ S isotope data provide insight into ore genetic processes: in both the Savannah and Savannah North intrusions tenors are low, with median values in semi-massive ores of 3.8% Ni and 0.9% Cu in 100% sulfide for Savannah and 2.7% Ni and 1.0% Cu for Savannah North. PGE tenors are similar and extremely low in both deposits, ranging from below 5 ppb to 200 ppb for Pt (median 6 ppb) and from 40 ppb to 480 ppb (median 110) for Pd, with Pd/Ir of 25-160 and Pd/Pt of 3-50 (10th and 90th percentiles). For both Savannah and Savannah North, S isotopic analyses indicate a limited amount of incorporation of crustal S, and the contaminant does not appear to have been the immediately adjacent Tickalara Metamorphics, implying either a deeper crustal contaminant, or a component of subduction-derived crustal S in the mantle source. Finally, olivine records a history of postcumulus re-equilibration with trapped silicate melt and cumulus sulfide liquid, giving rise to distinctive compositional trends. The Savannah Ni-Cu-Co Camp provides further evidence that arc and collisional environments are conducive to the formation of intrusion-hosted orthomagmatic Ni-Cu-Co deposits. The multidisciplinary approach that we describe here allowed us to unravel the petrological, structural, and mineralisation history of a cryptic but prospective region, and provided insights to help evaluate the prospectivity of neighbouring intrusions. This approach can be used in a wide range of settings to resolve critical factors such as S source, timing of intrusion and mineralisation, and structural controls.