Related Granitoids Research Articles

Porphyry intrusions and albitites in the Bardoc–Kalgoorlie area, Western Australia, and their role in Archean epigenetic gold mineralization

Minor intrusions in the Menzies – Kambalda greenstone belt of the Archean Eastern Goldfields Province, Western Australia, range from quartz–feldspar porphyry to plagioclase–hornblende porphyry. The porphyries display enrichment of mobile and incompatible elements (K to Zr) and depletion of relatively compatible elements, with negative Nb, P, and Ti anomalies, on mid-ocean-ridge basalt-normalized spidergrams. The composition and timing of emplacement of the porphyries are consistent with a genetic relationship with spatially related granitoids. Porphyries occur in 30% of gold mines in the Menzies–Kambalda belt. The association appears to be largely structural, since both the intrusions and the mineralizing fluids exploit zones of weaknesses, such as lithological contacts and shear zones. Porphyries have been modified to varying degrees by hydrothermal alteration, especially pervasive albitization. Textural evidence indicates that secondary albite and associated sodic amphibole formed late in the deformation history of the greenstones and were broadly contemporaneous with secondary phyllosilicate, carbonate and sulphide minerals related to gold mineralization. Recent studies in the Alleghany district of California suggest the initial rock composition may critically influence the nature of alteration associated with gold mineralization. Therefore, albitization of porphyries may be caused by the same hydrothermal fluids that deposit gold and produce potassic alteration in mafic rocks.

Late cretaceous to early tertiary structural elements of west Kalimantan

Recent geological and geophysical mapping in West Kalimantan has allowed the definition of three domains with distinct geological and geophysical characteristics which were imprinted on the region by Early Cretaceous to Eocene convergent tectonic events. Subduction related granitoids intruded low-grade regional metamorphic rocks during the Early Cretaceous forming the Schwaner Mountains domain extending east-west across the southern and central part of West Kalimantan. This is flanked in the northwest by a complex sequence of sedimentary and volcanic rocks ranging in age from Late Carboniferous to Cretaceous which comprise the Northwest Kalimantan domain. Deformed Cretaceous flysch deposits east of the Northwest Kalimantan domain form an accretionary complex representing the southern continuation of the extensive, dominantly Tertiary accretionary wedge in Sarawak. The boundary between the Cretaceous accretionary domain and rocks of the Northwest Kalimantan domain is a fault which we interpret as a transform that marked the western limit of Late Cretaceous southward subduction beneath West Kalimantan. Growth of the accretionary complex resulted in uplift of a ridge of melange and flysch deposits, on which a half graben extensional basin formed and accumulated thick lacustrine deposits. A sedimentary basin developed between continental rocks to the south and the emergent ridge comprising accretionary complex rocks and outer shelf deposits to the north, in the position of a forearc basin. As accretion proceeded, the locus of underthrusting migrated northward and a second melange ridge and sedimentary basin developed farther to the north. The evidence for accretionary processes and the association of these with sedimentary basin formation, supports the hypothesis that an ancient, south-dipping subduction zone existed in the eastern part of West Kalimantan during Late Cretaceous to Early Tertiary times.

Dynamothermal transition zone between Archaean greenstone and granitoid gneiss at Lake Dundas, Western Australia

A 500 m wide shear zone occurs between the base of an Archaean greenstone sequence and adjacent granitoid gneiss complex on the shores of Lake Dundas, Western Australia. The dynamothermal margin remains distinguishable due to the preservation of upper amphibolite facies assemblages, related granitoid anatectites and mylonitic, schistose and gneissose fabrics developed parallel to the contact, which itself lies subparallel to the greenstone bedding surface. The margin contrasts with less deformed greenschist to low amphibolite facies assemblages which characterize lithologies within the greenstone belt, many of which retain igneous textures and relict primary phases. Structural, petrological and textural evidence indicates that the dynamothermal contact originally evolved as a subhorizontal ductile shear zone during juxtaposition of the greenstone pile with granitoid gneiss and that its formation preceded regional folding, greenschist facies overprinting and granitoid intrusion which occurred at about 2700 Ma. The amount of heat generated within the transition zone during thrusting was limited to maximum temperatures of c. 650°C due to the buffering effect of granitoid anatexis.

Structure and evolution of the Himalaya–Tibet orogenic belt

The 1981 French–Chinese expedition to Tibet focused on the Lhasa block, extending earlier coverage 400 km north of the Tsangpo suture. The Lhasa block stood between 10 and 15° N latitude over most of the Upper Cretaceous and Eocene and, if Gondwanian in origin, had detached from Gondwana by early Permian. Seismic profiles reveal a complex Moho topography resulting both from multiple continental thrusting and large-scale strike-slip faulting. Subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated. This study emphasizes the role of smaller blocks in the accretion of the continental mosaic.

Metallogeny and tectonics of porphyry copper–molybdenum deposits in British Columbia

Metallogenic studies in British Columbia are based on data for over 1200 deposits that include metals and ore minerals recorded, age and lithology of sedimentary and volcanic host rocks, and the nature of any related granitoid intrusions. Analysis of these data defines a bimodal source of metals in porphyry deposits. Cu and Mo are derived from the primary granitoid melt, and the Cu/Mo ratio reflects the composition of that melt. Pb, Zn, Ag, and Au occur in veins peripheral to porphyry intrusions, and the metal content of these veins is controlled by the lithology of their wall rocks.Petrogenetic constraints imply that granitoids are predominantly anatectic crustal melts; thus we use the distribution of porphyries with varying metal ratios to define a map of the lower crust, which then constrains a tectonic model. Two major volcanic–plutonic arcs (Coastal: Jurassic – early Tertiary; Takla: Triassic) were generated above two subduction zones. Anatexis of the associated underplates produced Cu porphyries. The Cu–Mo porphyries of the Intermontane Belt were produced by anatexis of mixed volcanic, volcaniclastic, and clastic sedimentary rocks deposited on a fragment of oceanic crust trapped between the two magmatic arcs in the Jurassic.