Abstract
Two main periods of metamorphism have effected the Broken Hill base metal deposit. The first, at granulite grade, occurred at 1,700 m.y., the second, at lower amphibilite grade, occurred at 500 m.y. The earlier metamorphism correlates with two stages of intense regional folding; the latter occurs as narrow “shears” across the orebody. The prograde metamorphism caused intense brecciation, development of an ore “mush” with ore movement, formation of ore-bearing parapegmatites and boudins and much recrystallization of ore and gangue minerals. The orebody parallels an axial plane schistosity in the wall rocks with numerous ore piercement structures causing local discordancies. Ore in retrograde zones is again brecciated with galena further recrystallizing after destruction of prograde recrystallization. Gangue minerals remain essentially as brecciated fragments without further recrystallization. Secondary hydrothermal veins with rare silver minerals derived from the orebody transect the retrograde zones. Galena is plastically injected into fractures in the retrograde wall rock schists. Prograde ore shows co-recrystallization of various sulphides and gangue minerals yielding characteristic annealed textures. Quartz, garnet, hedenbergite, roepperite and apatite co-recrystallize with galena, sphalerite and chalcopyrite with balanced surface tensions. Retrograde ore shows mainly fragments of gangue and sphalerite set in a matrix of further recrystallized galena or schistose galena with a superimposed sub-grain structure. The significance of the ubiquitous sub-structures within galena is considered in terms of retrograde effects upon high grade metamorphic textures.
Zusammenfassung
Das Erzvorkommen in Broken Hill, N.S.W., Australien, unterlag zwei Hauptphasen regionaler Metamorphose. Die erste Phase, die eine Granulitfacies erreichte, fand vor 1700 Millionen Jahren statt; die zweite Phase, vor 500 Millionen Jahren, zeigt einen Amphibolit-Grad. Die frühere Metamorphose korreliert mit zwei Stufen intensiver Faltungen, die spätere trat in Form enger „Scherungen“ quer durch den Erzkörper auf. Die prograde Metamorphose verursachte intensive Breccienbildung, die Entwicklung eines „Erzbreies“ mit begleitender Erzbewegung, die Bildung erzführender Parapegmatite und Boudinagen sowie reichliche Rekristallisation des Erzes und der Gangmineralien. Der Erzkörper liegt parallel eine „axial-plane-schistocity“ im Nebengestein mit zahlreichen Erzdurchdringungen, die zu örtlicher Diskordanz führen. In retrograden Zonen wurde das Erz ein zweites Mal brecciert, was von einer weiteren Rekristallisation des Bleiglanzes unter Zerstörung der prograden Rekristallisation begleitet wurde. Die Gangmineralien bleiben in der Hauptsache im breccierten Zustand ohne weitere Rekristallisation. Sekundäre hydrothermale Erzgängchen mit seltenen Silbermineralien, die vom Erzkörper stammen, durchschneiden die retrograde Zone. Bleiglanz ist in plastischem Zustand in die Spalten des retrograden Nebengesteines (Schist) hineingedrückt worden. Progrades Erz zeigt Co-rekristallisation der verschiedenen Sulphide und Gangmineralien mit charakteristischen „Temperungs“ gefügen. Quarz, Granat, Hedenbergit, Roepperit und Apatit co-rekristallisieren mit Bleiglanz, Zinkblende und Kupferkies mit ausgeglichenen Oberflächenspannungen. Retrogrades Erz zeigt hauptsächlich zerstückelte Gangmineralien und Zinkblende in einer Grundmasse weiter rekristallisierten oder schiefrigen Bleiglanzes mit einer überprägten „Sub-grain“ Textur. Die Bedeutung der allgegenwärtigen „Sub“-Textur im Bleiglanz wird im Sinne eines retrograden Effektes auf hochgradige metamorphe Strukturen gedeutet.
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Lawrence, L.J. Polymetamorphism of the sulphide ores of Broken Hill, N. S. W., Australia. Mineral. Deposita 8, 211–236 (1973). https://doi.org/10.1007/BF00203205
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DOI: https://doi.org/10.1007/BF00203205