Keywords:
Yilgarn Craton
Greenstone belt
Dike
Hornblende
Diorite
The lower stratigraphy of Agnew-Wiluna greenstone belt is composed of two main elements; a mafic/komatiite domain and a felsic/komatiite/basalt domain. Previous stratigraphic models show the mafic domain overlying the felsic domain. Komatiites in the latter host the vast majority of the nickel sulphide endowment of the belt (>20 significant deposits) whereas those in the mafic domain contain 3 and 4 relatively small deposits. Recently published geochemical data from well-preserved mafic domain rocks exposed in the Agnew area opens up the possibility to match these units with mafic rocks within the more structurally disrupted felsic domain. Analytical data from basalts at the Cliffs and Mount Keith Ni deposits and from the Wiluna Au mine sequence show that these can be matched to the basalt sequence stratigraphically below the Agnew Komatiite at Agnew and also show that basalts previously thought to occupy different stratigraphic positions (Centenary Bore and MacFarlanes Basalts) are laterally equivalent but structurally displaced. The revised stratigraphic model together with available age dates show that komatiites in both domains, Mount Keith and Cliffs/Agnew Komatiites, are laterally equivalent and part of the 2705 Ma Kalgoorlie-Kurnalpi komatiite LIP. This greatly enhances the Ni prospectivity of komatiites within the mafic domain which, previously being thought younger, were historically considered less prospective. The footwall to the komatiite is composed of basalt (Never Can Tell Basalt, in the mafic domain) and felsic sequences (Mount Keith Dacite in the felsic domain) that are laterally separated but occupy the same stratigraphic position and together with the komatiite correlate with the Kambalda Sequence in the south of the Kalgoorlie Terrane. The oldest crystallisation ages from the Mount Keith Dacite are 2719–2725 Ma but whether these rocks belong to the Kalgoorlie or Youanmi Terrane is currently unknown. The Kalgoorlie-aged sequence has an unconformable contact with underlying Youanmi-aged sequence (the latter including dates of 2724–2729, 2734, 2749 Ma) composed of basalt, komatiitic basalt, komatiite and minor felsic volcanic (in decreasing stratigraphic order; felsic volcanics, Songvang Basalt, Hickies Bore Basalt, Donegal Komatiite, Butchers Well Basalt). The Youanmi sequence is exposed throughout the AWB, is present in the Leonora area to the immediate south and extends eastward to other areas within the northern part of the Kalgoorlie-Kurnalpi Terranes.
Felsic
Greenstone belt
Yilgarn Craton
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Citations (5)
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1968)
Concluding the previous papers of the authors, adding some new data especially on the mode of occurrence and anorthite contents of a number of plagioclases in each plutonic and metamorphic rocks, the authors made it clear that the complexed sequences of plutonism and metamorphism of the Tanzawa mountainland is genetically related as successive phenomena of three stages of plutonism and metamorphism. Thev are as follows. First phase; activity of gabbro series of plutonic rocks which accompanies metamorphism leading to the formation of amphibolite and greenschist. Second phase; formation of main rock types of diorite series from earlier gabbroic rocks. This is accompanied by earlier diaphthoresis and later hornfelsization. Third phase; the intrusion of trondhjemite dikes and the formation of pools of the last diorite series, which seem to be broadly synchronous with the cleavage formation and zeolite facies metamorphism. Such sequence of plutonism and metamorphism in the deeper zone of the Tanzawa mountainland is tectonically related with the deposition of each stratum in the upper zone. Examining the pebbles of plutonic and metamorphic rocks from various strata, with previous data of the anthers on the areal extension of each phase of metamorphism on each stratum, the following correlation was possible, i.e., the first phase metamorphism predates the deposition of the upper part of the Misaka series, the second phase metamorphism predates the deposition of Ashigara bed and postdate the upheaval and denudation of a part of the lower part of the Misaka series, and the third phase metamorphism synchronizes or postdates the deposition of Ashigara bed and postdates the upheaval and denudation of a main part of the lower part of the Misaka series.
Plutonism
Diorite
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Citations (8)
The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is a narrow package of complexly deformed Archean supracrustal rocks that hosts two of the world's largest komatiite-hosted nickel sulfide deposits, the Mt Keith and Perseverance deposits. These deposits and several others in the belt are centred on thick lenses of adcumulate-textured komatiite interpreted to represent areas of channelised magma flow. The large nickel sulfide deposits are located in parts of the belt associated with ca 2720 to 2700 Ma felsic volcanism (e.g. the Leinster and Mt Keith nickel camps). In these areas, felsic to intermediate volcanic rocks are intercalated with syn-volcanic massive sulfides of inferred exhalative origin. While these primary magmatic features are clearly first-order controls on the distribution of Ni sulfide deposits in the belt, several regional-scale deformation events have significantly complicated the interpretation of primary stratigraphic relationships. The earliest recorded deformation events (D1,2,3) resulted in an east–west trending greenstone belt with recumbent isoclinal folds and ductile shear zones. Subsequent west-southwest–east-southeast shortening during the D4 event at ca 2666 Ma involved the refolding of the tectono-stratigraphy to produce belt-scale, north- to north-northwest-trending upright folds, a pervasive axial planar schistosity in all rocks, and the present-day steeply dipping, overturned supracrustal sequences, and emplacement of granitoids in major antiformal fold hinges. Polyphase folding of supracrustal rocks produced Type 2 fold interference patterns with multiple facing reversals at various scales across the belt. West-southwest–east-southeast extension during the D5 event at ca 2665 Ma triggered the development of terrestrial basins (i.e. Scotty Creek and Jones Creek) in areas flanking major antiforms, resulting in the deposition of the Jones Creek Conglomerate. Subsequent west-southwest–east-southeast shortening during the D6 event resulted in the folding of the Jones Creek Conglomerate and formation of gold-bearing veins in the Agnew gold camp. Belt-wide relaxation in east–west shortening during the D7 event caused open, recumbent F7 folding of the steeply dipping stratigraphy. Broadly east–west shortening during the D8 to D10 events resulted in the tightening of existing fold hinges, the dismemberment and displacement of panels of supracrustal rocks by sinistral (e.g. Perseverance shear zone) and then dextral (Waroonga) shear zones. The Agnew–Wiluna belt displays (para)autochthonous associations within the belt, with district-scale heterogeneities caused by primary volcano-sedimentary facies changes combined with polyphase deformation. Importantly, nickel sulfide-bearing sequences identified in nickel camps can potentially be traced to different parts of the belt by unravelling the effects of polyphase deformation.
Greenstone belt
Yilgarn Craton
Felsic
Volcanic belt
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Citations (30)
Komatiites occur in many Archean greenstone belts and host significant Ni sulfide ore deposits. Establishing the stratigraphy and the control that stratigraphy has on the emplacement and morphology of ultramafic magmatism is crucial in understanding Archean geodynamic environments and in targeting for Ni sulfide mineralization within these environments. The ~2.9 Ga Lake Johnston greenstone belt, in the southern portion of the Youanmi Terrane of Western Australia, contains komatiite flows and related subvolcanic intrusions, mafic volcanic rocks, felsic volcanic rocks, banded iron formation, and sedimentary rocks. The stratigraphic sequence is intact, preserving original sedimentary and igneous textures and contact relationships, despite being overturned and variably deformed. This study proposes that the lithostratigraphic succession and ultramafic intrusions identified within the Lake Johnston greenstone belt record a transition from arc- to plume-dominated magmatism, accompanied by the establishment of a banded iron formation-dominated sedimentary basin. It is proposed that the rheological contrast between the felsic volcanic unit and overlying banded iron formation acted as a stratigraphic barrier, trapping ascending ultramafic magmas. The stratigraphic barrier inhibited the upward ascent of ultramafic magma causing the development of a subvolcanic magma chamber. Magma trapped beneath the banded iron formation progressively inflated and spread out along the contact, until overpressuring breached the banded iron formation and magma escaped, forming the overlying extrusive komatiites. Both the geodynamic and lithologic transitions gave rise to favorable substrate rock units and an ideal tectonic setting for formation of komatiite-hosted Ni sulfide ores.
Yilgarn Craton
Greenstone belt
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Citations (19)
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1983)
K-Ar dating on hornblende from biotite-hornblende quartz diorite in northeastern part of the Kanto Mountains was carried out. The age of the quartz diorite is 251±8 Ma. The result, together with geological facts, supports the idea that island arc composed of granitic plutons was present between the Sanbagawa and the Ryoke metamorphic belts.
Diorite
Hornblende
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Citations (12)
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1963)
Hornblende gabbro, diorite and gabbro-diorite occur in the sheared zone of biotite gneiss and basic hornblende plagioclase rocks. Careful observations on the occurrence of these rocks and the comparison of the component minerals between the coarse igneous textured rocks and the country metamorphic rocks have revealed that the basification and the acidification of plagioclase occur more intensively in igneous textured rocks than in metamorphic rocks, and moreover quality of the country metamorphic rocks provide for that of gabbro and diorite and the regulation is not vice versa. Consequentry, these igneous textured rocks in these districts are the metablastic rock originated from the country rocks, from which it is produced by metamorphic differentiation.
Diorite
Hornblende
Country rock
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