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A Mineragraphic Study of the Pinnacles Lode Horizon, Broken Hill

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A Mineragraphic Study of the Pinnacles Lode Horizon, Broken Hill A MINERAGRAPHIC STUDY OF THE PINNACLES LODE HORIZON, BROKEN HILL A thesis submitted for the degree of Master of Science by S. VEDCHAKANCHANA School of Applied Geology University of New South Wales Sydney Supervisor : Professor L. J. Lawrence u.iiv:;i<iirv a? n.s.iv. 03538 -S.nAR.77 LIBRARY This work has not been submitted for a degree or similar award to any other University or Institution. iii ACKNOWLEDGEMENTS The author wishes to express his sincere gratitude to Professor L.J. Lawrence for his supervision, assistance and time spent discussing various aspects of the work throughout the project. Acknowledgement is also due to various people especially Dr. Ian Plimer for his assistance in visiting the deposit; Dr. M.B. Katz, Mrs. M. Krysko, Mr. R. Haren, Mr. J.. Chisholm and Mr. H. Le Couteur for their invaluable assistance with various aspects of the project. Mr. G.F. Small and Mr. J. Newbourne for their assistance in the preparation of photographic work and Mrs. C. Hutton for typing the thesis. The writer benefited from the reading of an M.Sc. Thesis by Dr. D.E. Ayres. Thanks go also to the Chulalongkorn-Amoco Fund for financial assistance to undertake the study. iv ABSTRACT The Pinnacles base-metal sulphide deposit occurs in the Willyama Complex which consists of intensely folded and moderate to highly metamorphosed sediments of lower Proterozoic Age. Despite the lack of unanimity as to the origin of the sulphide mineralization, recent investigations have given support to the view that the orebody has been metamorphosed contemporaneously with the enclosing rocks. The basis for the interpretation of textures of ore and gangue relationships in mineral assemblages has been the study of similar textures in deformed and annealed metals. The textures of some deformed sulphides are summarized with reference to recent studies of both natural occurrences and experimental work. In this investigation of the ore of the Pinnacles mine, the textures seen in the sulphides and in ore-gangue intergrowths have been interpreted as being due to the effects of prograde metamorphism at conditions approximating the amphibolite- granulite facies boundary, and during retrograde metamorphism at much lower pressure-temperature conditions. The major sulphide phases, and their textural relationships with each other and with gangue minerals, are described in detail and classified according to their formation. V INDEX TO FIGURES Page Figure 1 Regional map showing Pb-Ag-Zn Broken Hill- type mines in the Willyama Complex with three progressive metamorphic zone established by Binns (1964). 6 Figure 2 Regional geological map of the Pinnacles area (From Aust. Inst. Min. Met., 1968). 8 Figure 3a Quartz (Q) - Garnet (G) - Hedenbergite (Hd) granulite with medium to coarse-grained predominantly equigranular granoblastic microstructure defined by grains which possess straight-slightly curved boundaries. (Drawn from thin section, x 30 magnification) 13 Figure 3b Garnet (G) - Biotite (B) - gneiss with medium to coarse grained granoblastic elongate microstructure defined by xenoblastic quartz (Q) with curved boundaries and biotite with sutured grain boundaries, surrounding elongated grains of gahnite (Gh) and garnet. (Drawn from thin section, x 30 magnification.) 13 Figure 4 Geological map of the Pinnacles mine (after Burns, 1965.) 14 Figure 5a Dislocation model of grain boundaries with low angle of misfit (after Shewmon, 1966.) 21 Figure 5b Coincidence related type of structure of high angle grain boundaries (after Liicke et al., 1972). 21 Figure 6 Schematic diagram showing the balance of surface free energies (A ) and grain boundary free energy (Ag) (after Void and Glicksman, 1972). 22 Figure 7a Equilibrium existing in a single-phase system where 120° 23 Figure 7b Equilibrium existing in a two-phase system where £ Au = 2 A12 C0S 2 23 Figure 7c Equilibrium existing in a three-phase system where , , , A 2 3 _ A13 _ 12 sm 0^ sin 0^ sin 0^ 23 Figure 8 Diagram of polygons with curved sides, except a hexagon; meeting at 120u (after Smith, 1952). 26 vi Pa ge 9 Diagram of soap films in a regular tetrahedron wire-frame showing mutual angles of cosine 1/3 defined by the angle subtened by straight lines' joining corners of the tetrahedron to a point equidistant from them all (after Smith, 1952). 26 10 Some examples of typical grains in annealed Al-Sn alloy (after William, 1952). 28 11 A stress - strain curve showing: a - elastic strain region, b - plastic strain region, c - rupture region (after Honeycombe, 1968). 30 12 A creep curve showing: a - primary creep, b - secondary or steady-state creep, c - tertiary creep (after Honeycombe, 1968). 30 13a Schematic diagram of nucleation by sub-grain growth (polygonization) (after Cahn, 1966). 36 13b Schematic diagram of bulge-nucleation model (after Beck, 1954). 36 14a Diagram showing the movement of grain boundaries with the growth of a septagon and the shrinkage of a pentagon (after Nielsen, 1966). 38 14b Schematic diagram showing the adjustment of grain boundaries during grain growth (after Smith, 1952). 40 15 Schematic diagram showing shapes of a minor phase at triple-junction point corresponding to different dihedral angles (after Smith, 1948). 42 16 Banded ore (a hand specimen). 50 17 Banded ore - showing foliation defined by preferred orientation of platy mineral (biotite) and lenticular aggregates of garnet (G) (polished section x 60). 52 18 Banded ore - showing lineation defined by preferred orientation of prismatic brown hornblende (Hb) (polished section x 60). 52 19 Banded ore - showing a sulphide "spur" plastically injected across layering (polished section x 50). 53 20 Banded ore - showing flattened garnet (G) grains, and galena (Gn) plastically migrating along microfractures within garnet grains (polished section x 50). 53 vi i Page 21 Banded ore - pyrhotite (Po) showing polygonal pattern with smooth arcuate boundaries but abruptly .terminated at hedenbergite (Hd) grain faces (polished section x 150 oil). 55 22 Banded ore - sphalerite (Sp), galena (Gn) and chalcopyrite (Cpy) showing arcuate grain boundaries abruptly terminated against biotite (B) grains (polished section x 100). 55 23 Banded ore - discontinuous lenticles of chalcopyrite (Cpy) occurring at triple­ junction points along grain boundaries of sphalerite (Sp) grains (polished section x 400 oil). 56 24 Banded ore - triangular composite grains between spharerite (Sp) and gudmundite (Gd) or pyrrhotite (Po) and gudmundite (Gd) with small galena (Gn) lenticles occurring at triple-junction points and grain boundaries between garnets (G) or between garnet and hedenbergite (Hd) (polished section x 100 oil) . 56 25 Banded ore - the euhedral crystal of arsenopyrite (Asp) contains inclusions of galena (Gn) and pyrrhotite (Po) (polished section x 150 oil). 57 26 Banded ore - showing sulphide minerals wrapped around fragments of silicate gangues (biotite and quartz) and having smooth grain boundaries. Quartz sometimes shows internal polygonal patterns (thin section x 30 partial X-nicols). 57 27 Brecciated ore (a hand specimen). 59 28 Brecciated ore - showing fragments of quartz (black) with smooth grain-boundaries embedded in recrystallized sulphide matrix (polished section x 50) . 59 29 Brecciated ore - showing ilmenite (II) and other sulphide minerals occurring interstitially to rounded garnet grains with zcnally arranged inclusion (polished section x 60). Note also fractures trending top to bottom and possibly related to retrograde stresses. 60 30 Brecciated ore - fragments of single grain quartz showing smooth arcuate boundaries whilst garnet shows internal polygonal pattern (polished section x 50). 50 31 Brecciated ore - Quartz (Q) and Garnet (G) showing ,?disruptedn polygonal aggregate resulting from imperfect recrystallization or subsequent disruption (polished section x 80) . 62 vi i i Page 32 Brecciated ore - Galena (Gn), sphalerite (Sp) and pyrite (Py) showing mutual arcuate grain boundaries. Note pyrite appears to be an alteration product of pyrrhotite having, as it does, a typical pyrrhotite lenticular shape. (Polished section x 50). 62 53 Brecciated ore - showing sphalerite (Sp) grains demarcated by the arrangement of small triangular or globular bodies of chalcopyrite (Cpy) , galena (Gn) and pyrrhotite (Po). (Polished section x 100 oil) . 63 34 Brecciated ore - showing exsolution intergrowth between sphalerite (Sp) and chalcopyrite (Cpy) (polished section x 50). 63 35 Brecciated ore - showing graphic intergrowTth between galena (Gn) and tetrahedrite (Tt) (polished section x 60 partial x-nicols). 64 36 Brecciated ore - showing sphalerite (Sp) plastically injected into biotite (B) flakes wrapping around rounded fragments of garnet and recrystallized quarts (polished section x 50). 64 37 Brecciated ore - showing sphalerite (Sp) metamorphically corroded by silicate gangue (polished section x 50). 66 38 Brecciated ore - etching of galena brings out a fine polygonal recrystallized pattern (polished section x 100). 66 39 Brecciated ore - etching of sphalerite brings out a polygonal pattern. Grains possess recrystallization twinning. Numerous lenticular bodies of chalcopyrite occur along twin-boundaries, grain boundaries, and at triple junctions (polished section x 100) . 67 40 Brecciated ore - etching of sphalerite brings out polygonal patterns with grains possessing recrystallization twinning superimposed by fine-subgrains (polished section x 125).’ 67 41 Brecciated ore - etched sphalerite showing branched deformational twinning (A) cutting across an earlier
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