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) 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 D₂ 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) M₁ plagioclase-rich porphyry dykes, thus indicating their emplacement pre- to syn-D_2NC 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 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 M₂ 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 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-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 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.

新庆祝金矿床（ 〜 2万盎司金）位于西澳东部金矿区的Kalgoorlie地体内，Kalgoorlie东南35公里处的横地壳Boulder-Lefroy断层带的Boulder层断内。金沉积存在于超基性岩（科马提岩） ，分化辉长岩/辉绿岩及厚度不一的（ 0.5至5米，局部可达80米）侵入于的超镁铁质岩石接触的酸性斑岩脉。围岩经历了高绿片岩相程度的区域变质作用。新庆典矿记录了至少四个变形事件：（1） D1NC是垂直地层接触，是区域尺度直立褶皱的表征。 （2） D2NC变形的表现形式是一个发育程度高的北北西向的西南陡倾透入性面理剪切（ S2NC ）和平行面理的呈香肠构造的石英 - 碳酸盐岩脉（ V2NC ），代表着区域性D2Boulder断层带;在断层上的移动是左旋斜滑。 （ 3 ） D3NC变形导致北北东向延伸的、北西西向倾斜、走向长度短（ <50 米）断层，充填断层的石英 - 碳酸盐 - 绿帘石-绿泥石岩脉和间隔宽的S3NC叶理。D3NC断层面上近水平L3sNC 擦痕线理指示D3NC （无运动学指标被观察到）期间的走滑运动，而S3NC叶理中的S-C组构几何特征指示左旋运动。 （ 4 ） D4NC变形发育不佳，由横切S2NC叶理的西倾曲面断层而导致。D2NC变形作用期间形成的线理包括：（a ）中度至陡峭的南南东至南南西倾的L2mNC矿物拉伸线理， （b）S-和C -叶理面之间的西北向中至陡倾的L2iNC交叉线理，及（c ）中度至陡峭的南南东至南南西倾斜的L2sNC 擦痕线理。中度至陡峭的南南西下倾的石英L2mNC延伸线理，与S- C组构的取向相结合，将剪切带上的位移确定为左旋斜滑。该S2NC叶理还发育于薄的（ 1-5米宽） M1富斜长石斑岩脉之内，从而表明其侵位发生在D2NC变形作用之前或同时。第一代岩浆活动1 （ M1）的斑岩岩脉中的糜棱状组构与第二代岩浆活动2 （ M2 ）的石英 - 长石斑岩脉的̏新鲜̋的未变形火成岩组构不一样，后者易沿镁铁质 - 超镁铁岩接触面侵入。在M2斑岩脉中缺乏内部韧性变形组构和脆性结构的优势（如边缘的裂缝网）表明其在脆性变形环境侵位。新庆祝金沉积区的两次金事件在此得到解释。 （ 1 ）一个̏早̋糜棱岩式金事件，发生于中基性岩和M1斑岩脉中的高应力的石英 - 铁白云石 - 黑云母绢云母糜棱岩，被认为与D2NC变形作用同步。微小的黄金成分（ < 100毫米）存在于黄铁矿内，后者的排列平行于S2NC叶理面。基性糜棱岩中的金与铁白云石-绢云母±黑云母±黄铁矿平衡，在空间和时间上与黄金和非含金碲化物如碲金银矿，碲银矿，碲铅矿 ，碲镍矿和碲化铋相关联。（ 2） 一个̏迟̋金事件，包括接触充填和断裂充填式。接触式金成矿作用发生于S2NC叶理面之内的高镁玄武岩中，叶理面在D2NC变形作用晚期包裹M2斑岩脉。金与黄铁矿-绢云母-铁白云石平衡。断裂填充式金成矿作用发生在M2的斑岩脉边缘的脆性断裂网中。̏自由̋黄金和黄铁矿中的黄金包裹体与黄铁矿-绢云母2±铁白云石平衡。在接触充填或断裂填充式的矿化作用中还未观察到任何碲化物。该断裂填充式金矿成矿的确切时间是模棱两可的，它可能已经形成于D2NC后期或D3NC期间 或甚至在以后的变形事件中。新庆祝金矿床是位于第一阶横跨地壳断层系统的Boulder-Lefroy断层的造山型金矿系统的一个罕见的例子。其复杂断层构造和断层活动的长久性被解释为,与通常贫瘠的一级断裂带相比,至少部分是由于高金endowment。外蚀变带的磁铁矿和黑云母在空间上与M1岩脉相关，在空间和时间上碲化物限制于M1岩脉。这说明了造山活动成因的新庆祝金沉积物来源于岩浆流体,至少在矿床形成的早期阶段是这样的。

KEY WORDS:

• Archean
• Yilgarn
• New Celebration
• Boulder fault
• gold mineralisation
• structure
• tellurium

ACKNOWLEDGEMENTS

The findings presented in this paper are the results of a BSc Honours project completed by S. J. Nichols and funded by Harmony South Kal Mines P.L., a subsidiary of Harmony Gold (Australia). P.L. Harmony are thanked for their permission to publish the information presented here. Special thanks are extended to Peter Neumayr (CET, now Sipa Resources), the staff at South Kal Mines and the Centre for Microscopy and Microanalysis, University of Western Australia. The thorough and detailed reviews by Brett Davies, Trevor Beardsmore and particularly the guest editor Wally Witt were excellent and certainly made this paper a much better one.