Details

Autor(en) / Beteiligte

• Nichols, S. J.
• Hagemann, S. G.

Titel

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

Ist Teil von

• Australian journal of earth sciences, 2014, Vol.61 (1), p.113-141

Ort / Verlag

Taylor & Francis

Erscheinungsjahr

2014

Quelle

Taylor & Francis Current Content Access

Beschreibungen/Notizen

• 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) D 1NC is represented by vertical stratigraphic contacts, the expression of regional scale upright folds. (2) D 2NC deformation is expressed as a well-developed, NNW-trending, steeply WSW-dipping penetrative shear foliation (S 2NC ) and foliation-parallel, boudinaged quartz-carbonate veins (V 2NC ), which are representative of the regional D 2 Boulder fault zone; movement on the fault is sinistral oblique slip. (3) D 3NC deformation resulted in NNE-trending, WNW-dipping, short strike length (<50 m) faults, quartz-carbonate-epidote-chlorite fault fill veins and widely spaced S 3NC foliation. Subhorizontal L 3sNC slickenline lineations on D 3NC fault planes indicate strike-slip movement during D 3NC (no kinematic indicators are observed), whereas the geometry of S-C fabrics in the S 3NC foliation suggests sinistral movement. (4) D 4NC deformation is poorly developed by west-dipping, curviplanar faults that cross cut the S 2NC foliation. Lineations formed during D 2NC deformation include: (a) moderate to steep, SSE to SSW-plunging L 2mNC mineral elongation lineations, (b) moderate, NW-plunging L 2iNC intersection lineations between S- and C-foliation planes, and (c) moderate to steep, SSE to SSW-plunging L 2sNC slickenline lineations. Moderate to steep SSW-plunging quartz L 2mNC 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 S 2NC foliation is also developed in thin (1-5 m width) M 1 plagioclase-rich porphyry dykes, thus indicating their emplacement pre- to syn-D 2NC deformation. The mylonitic fabric in the first-generation Magmatic 1 (M 1 ) porphyry dykes contrasts with the 'fresh,' undeformed igneous textures of second generation Magmatic 2 (M 2 ) 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 M 2 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 M 1 porphyry dykes, which is interpreted to be synchronous with the D 2NC deformation. Minute inclusions of gold (<100 μm) are hosted within pyrite that is aligned parallel to the S 2NC 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 S 2NC foliation planes that wrap around the M 2 porphyry dyke during late D 2NC 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 M 2 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-D 2NC or during D 3NC 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 M 1 dykes, and the spatial and temporal restriction of tellurides to the M 1 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.