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Geologische Rundschau

, Volume 79, Issue 2, pp 345–353 | Cite as

The scale of ore-depositional systems: an important restraint on epigenetic vs remobilized syngenetic origins for Archaean mesothermal gold deposits

  • David I. Groves
  • Julian R. Vearncombe
Article

Abstract

There is general consensus that the majority of gold deposits in the important group hosted by volcanic or intrusive host rocks within Archaean granitoid-greenstone terrains have an epigenetic origin. However, there is controversy surrounding those stratabound deposits hosted by Fe-rich sedimentary rocks, such as banded iron formation (BIF), for which epigenetic, syngenetic and remobilized syngenetic (lateral secretion) origins have all been proposed.

In Western Australia, such controversial gold deposits in Fe-rich sedimentary units have similar ore mineralogies, ore-element ratios and, in places, alteration assemblages and, on the craton scale, show very similar structural controls to the other deposits: equivalent relationships are recorded for similar deposits in other cratons. All Western Australian deposits occur along kilometre-scale shear or fault zones linked to trans-craton, obliqueto strike-slip shear zones that were a focus for: i) carbonation with a mantle-like isotopic signature, and ii) emplacement of high-level A- and I-type granitoids, felsic porphyries and/or calc-alkaline lamprophyres. The scale of the mineralizing systems and the broad contemporaneity of mineralization, as indicated by radiogenic isotope studies, is much larger than that envisaged in conceptual secretion models which involve volumetrically small, locally-enriched source rocks. Instead, Archaean gold mineralization in Western Australia, at least, is probably the result of high fluid-flux during deformation, hightemperature metamorphism and magmatism which may be related to tectonism at convergent plate margins, as in modern examples. There is strong evidence for similar tectonic and mineralizing processes from the Canadian Shield.

Keywords

Source Rock Gold Deposit Band Iron Formation Convergent Plate Margin Mesothermal Gold Deposit 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Zusammenfassung

Goldlagerstätten in vulkanischen oder intrusiven Gesteinskörpern innerhalb von Granitoid-Grünstein Regionen archaischen Alters gelten im allgemeinen als epigenetisch. Unklarheit besteht dagegen bei Lagerstätten, die an eisenreiche Sedimentgesteine wie gebänderte Roteisensteine (banded iron formation (BIF) gebunden sind und für die epigenetische, syngenetische sowie remobilisierte syngenetische (laterale Sekretion) Entstehungstheorien diskutiert werden.

In West-Australien gibt es diese kontroversen Goldvorkommen in eisenreichen Sedimentgesteinen. Sie haben ähnliche Lagerstättenmineralogien, Elementverteilungen und z. T. auch ähnliche Umwandlungsprodukte wie die zuerst genannten, eindeutig epigenetischen Goldvorkommen, außerdem erkennt man vergleichbare strukturelle Kontrollen der Lagerstätte und Ähnlichkeiten, wenn sie mit Vorkommen in anderen Kratonen verglichen werden. Alle westaustralischen Lagerstätten befinden sich in trans-kratonischen Scher- und Störungszonen mit kilometergroßen Versatzbeträgen. An diesen Störungszonen fand Inkohlung mit einer den Mantel ähnlichen Isotopensignatur und der Aufstieg von A- und I-Typ Granitoiden, felsischen Porphyren und/oder kalk-alkalischen Lamprophyren statt. Das Größenverhältnis des mineralisierenden Systems und die weitreichende, zeitgleiche Mineralisation, auf die Untersuchungen an radiogenen Isotopen hindeuten, ist viel größer als sie durch Sekretions-Modelle, die auf kleinen Volumina und nur lokal angereichertem Gold im Gestein basieren, prognostiziert werden können.

In der Zeit des Archaikums resultiert die Goldanreicherung in West-Australien wahrscheinlich aus einem enormen Lösungsdurchfluß während der Deformation, aus Hoch-Temperatur-Metamorphose und Magmatismus, was in möglichem Zusammenhang mit tektonischen Vorgängen an konvergierenden Plattenrändern, ähnlich modernen Beispielen, steht. Es gibt zahlreiche Hinweise auf entsprechende tektonische und mineralogische Prozesse des Kanadischen Schildes.

Résumé

Il existe un consensus général quant à l'origine épigénétique des dépôts d'or contenus dans les ensemble volcaniques ou intrusifs des formations de granitoïdes — greenstones d'âge archéen. Par contre, une controverse subsiste à propos des dépôts stratiformes contenus dans les roches sédimentaires riches en Fe, telle que la «banded iron formation» (BIF), pour lesquels ont été proposées des origines épigénétique, syngénétique et syngénétique de remobilisation (sécrétion latérale).

Il existe en Australie occidentale de tels dépôts d'or situés au sein d'unités sédimentaires riches en Fe. Ces dépots présentent, par rapport aux dépôts épigénétiques du premier type, un certain nombre de similitudes: leur minéralogie, les rapports entre leurs éléments, en certains endroits leurs produits d'altération et, à l'échelle du craton, leurs relations avec la structure. Des similitudes analogues peuvent être relevées à propos de gisements de même type situés dans d'autres cratons. Tous les gisements ouest-autraliens sont localisés le long de zones de failles ou de cisaillement d'échelle kilométrique liées à des shear zones transcratoniques à rejet oblique ou horizontal. Ces zones sont marquées par: 1) une carbonisation avec signature isotopique mantélique; 2) la mise en place de granitoïdes peu profonds de type A et I, de porphyres felsitiques et/ou de lamprophyres calco-alcalins. L'échelle du système minéralisant et la simultanéité des minéralisations révélée par la géochronologie isotopique sont bien supérieures à ce qu'impliqueraient les modèles par sécrétion, qui mettent en jeu des sources de faible volume à enrichissement local.

En conclusion, les minéralisations de l'Australie de l'ouest ont probablement pour origine l'action d'un flux important de fluide minéralisant au cours de la déformation, du métamorphisme de haute température et du magmatisme, phénomènes que l'on peut mettre en relation avec une convergence de plaques par analogie avec des exemples récents. Il existe de nombreux indices d'une tectonique et d'une minéralisation analogues dans le bouclier canadien.

Краткое содержание

Месторождения золот а в вулканических, или интрузивных телах в гранитоидо-зе ленокаменных регион ах архейского возраста считают обычно эпиге нетического происхождения. Но име ется неясность относ ительно происхождения место рождений, связанных с регионами осадочных пород, бога тых железом, как напр.: полосчатый красный ж елезняк (banded iron formation (BIF)); здесь идут споры меж ду теориями эпигенет ического и сингенетического п роисхождения, а также сингенетического пр оисхождения в резуль тате повторной мобилизации.

В западной Австралии находят такие, вызыва ющие споры месторождения золота в богатых желе зом осадочных породах. Они обладают минерализацией, расп ределением элементов и даже част ично продуктами преобразования, сход ными с месторождения ми, явно эпигенетического пр оисхождения, описанн ыми выше. Но при сравнении с дру гими кратонами броса ется в глаза сходство влиян ия структур тектонич еских систем. Все месторожд ения западной Австра лии расположены в транс-кратонически х зонах скола и наруше ний, размерности которых представлены киломе трами. В этих зонах нарушени я установлены углефи кации с распределением изо топов, подобным таков ому мантии, а также поднят ие гранитоидов типа А и I, фельзических порфир ов и/или известковоще лочных лампорфиров. Соотнош ение величин минерал изующей системы и широко расп ространенных минера лизации одного и того же возра ста, который определи ли по радиоактивным изо топам, дает значения н а много выше тех, прогноз кото рых основан на секрец ионных моделях, где объемы зо лота не велики, а обога щение им породы имеет тольк о локальное значение.

Во времяна архея обог ащение золотом в запа дной Австралии имело мест о, вероятно, в результа те прохождения гигантских потоков р астворов, появившихс я в период деформаций, в ысокотемпературног о метаморфизма и магма тизма, которые, возмож но, стояли в связи с текто ническими процессам и на конверрующих краях платформы; прим еры такого рода мы наблюдаем и в насто ящее время. Имеются многочисленные указания на то, что соо тветствующие тектонические и мине ралогические процес сы имели место на Канадском щи те.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1990

Authors and Affiliations

  • David I. Groves
    • 1
  • Julian R. Vearncombe
    • 1
  1. 1.The Key Centre for Strategic Mineral Deposits, Department of GeologyThe University of Western AustraliaNedlandsAustralia

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