Structural and hydrothermal alteration evidence for two gold mineralisation events at the New Celebration gold deposits in Western Australia

Abstract

The New Celebration gold deposits (∼2 Moz Au) are located within the Boulder segment of the first-order, transcrustal Boulder-Lefroy fault zone about 35 km southeast of Kalgoorlie, in the Kalgoorlie Terrane of the Eastern Goldfields Province, Western Australia. The gold deposits are hosted by ultramafic rocks (komatiite), differentiated gabbro/dolerite and variably thick (0.5 to 5 m, locally up to 80 m) felsic porphyry dykes that have intruded the ultramafic–mafic rock contact. Host rocks have undergone regional metamorphism to upper greenschist facies.At least four deformation events are recorded at New Celebration: (1) D1NC is represented by vertical stratigraphic contacts, the expression of regional scale upright folds. (2) D2NC deformation is expressed as a well-developed, NNW-trending, steeply WSW-dipping penetrative shear foliation (S2NC) and foliation-parallel, boudinaged quartz–carbonate veins (V2NC), which are representative of the regional D2 Boulder fault zone; movement on the fault is sinistral oblique slip. (3) D3NC deformation resulted in NNE-trending, WNW-dipping, short strike length (<50 m) faults, quartz–carbonate–epidote–chlorite fault fill veins and widely spaced S3NC foliation. Subhorizontal L3sNC slickenline lineations on D3NC fault planes indicate strike-slip movement during D3NC (no kinematic indicators are observed), whereas the geometry of S–C fabrics in the S3NC foliation suggests sinistral movement. (4) D4NC deformation is poorly developed by west-dipping, curviplanar faults that cross cut the S2NC foliation. Lineations formed during D2NC deformation include: (a) moderate to steep, SSE to SSW-plunging L2mNC mineral elongation lineations, (b) moderate, NW-plunging L2iNC intersection lineations between S- and C-foliation planes, and (c) moderate to steep, SSE to SSW-plunging L2sNC slickenline lineations. Moderate to steep SSW-plunging quartz L2mNC elongation lineations, in conjunction with the orientation of S–C fabrics, constrain the sense of movement on the shear zone as sinistral oblique-slip. The S2NC foliation is also developed in thin (1–5 m width) M1 plagioclase-rich porphyry dykes, thus indicating their emplacement pre- to syn-D2NC deformation. The mylonitic fabric in the first-generation Magmatic 1 (M1) porphyry dykes contrasts with the ‘fresh,’ undeformed igneous textures of second generation Magmatic 2 (M2) quartz–feldspar porphyry dyke, which preferentially intruded along the mafic–ultramafic rock contacts. The lack of internal ductile deformation fabrics and a predominance of brittle structures (e.g. fracture network at the margins) in the M2 porphyry dyke indicate its emplacement into a brittle deformation environment.Two gold events are interpreted at the New Celebration deposit. (1) An ‘early’ Mylonite-style gold event in high-strain, quartz–ankerite–biotite–sericite mylonite in mafic rocks and M1 porphyry dykes, which is interpreted to be synchronous with the D2NC deformation. Minute inclusions of gold (<100 μm) are hosted within pyrite that is aligned parallel to the S2NC foliation planes. Gold in mafic mylonite is in equilibrium with ankerite–sericite ± biotite ± pyrite and is also spatially and temporarily associated with gold and non-gold-bearing tellurides such as calerverite, petzite, hessite, altaite, melonite and a bismuth telluride. (2) A ‘late’ gold event that consists of Contact and Fracture-infill styles. The Contact-style gold mineralisation is located in high-Mg basalt within S2NC foliation planes that wrap around the M2 porphyry dyke during late D2NC deformation. Gold is in equilibrium with pyrite–sericite–ankerite. TheFracture-infill-style gold mineralisation is located in brittle fracture networks formed at the margin of the M2 porphyry dyke. ‘Free’ gold and inclusions of gold in pyrite are in equilibrium with pyrite–sericite2 ± ankerite. No telluride species are observed in either Contact or Fracture-infill-style mineralisation. The exact timing of the Fracture-infill-style gold mineralisation is equivocal; it may have formed late-D2NC or during D3NC or even during a later deformation event.The New Celebration gold deposits are a rare example of an orogenic gold system located in a first-order, transcrustal fault system, the Boulder-Lefroy fault. Its complex fault zone architecture and long-lived nature of fault movement is interpreted to be at least in part responsible for the high gold endowment when compared with the typically barren first-order fault zones. Magnetite and biotite in the outer alteration zones are spatially related to M1 dykes, and the spatial and temporal restriction of tellurides to the M1 dyke suggest that the orogenic New Celebration gold deposits were sourced from magmatic fluids, at least during the early stages of the formation of the deposits.