Proterozoic Terrains Research Articles

Regional geochemical and isotopic characteristics of high-grade metamorphics of the Prydz bay area: The extent of proterozoic reworking of Qrchaean continental crust in East Antarctica

Abstract Preliminary isotopic data for Late Proterozoic (∼ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (∼ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (∼ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons. As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K 2 O 2 , Rb, Pb, and Th, and decreases in CaO, Na 2 O 2 and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar.

Geochemistry of mafic igneous rocks of the northern Prince Charles Mountains, Antarctica

Abstract Rare mafic dykes, which intrude 1000 Ma high‐grade metamorphic rocks of the northern Prince Charles Mountains‐Mawson Coast area, are compositionally distinct from abundant early to middle Proterozoic tholeiite dykes, which are confined to Archaean or early Proterozoic terrains in the southern Prince Charles Mountains and elsewhere in East Antarctica, and which have therefore proved useful as stratigraphic markers. The younger dykes (and extrusive rocks) are a composition‐ally heterogeneous group with a wide range of ages (at least Cambrian to Eocene), although most are of K‐rich alkaline composition or have alkaline affinites. Their strong enrichment in highly incompatible elements (Rb, Ba, Th, Nb, K, Pb, Th and U) relative to less incompatible elements (La, Ce and P) suggests derivation by partial melting of more enriched mantle source regions than those of most of the Proterozoic tholeiite suites. However, unlike the latter, many incompatible element ratios have been significantly affected by f...

The nickel sulfide deposits of Western Australia in global perspective

Fifty-six nickel sulfide deposits in the Archean and lower Proterozoic terrains of Western Australia have been included in a global assessment of the geologic and economic characteristics of 145 Ni sulfide deposits. Essentially all deposits can be classified into one of two classes: (1) the dunite-peridotite class, which embraces deposits associated with host rocks of komatiitic affinities (usually Archean) and is subdivided into the intrusive dunite association and the volcanic peridotite association; and (2) the gabbroid class, which is subdivided by host rock characteristics into intrusive mafic-ultramafic complexes, large, layered intrusions, and the Sudbury Irruptive. Deposits in the gabbroid class are the most important source of Ni, even if Sudbury is excluded.Most original resources of sulfide Ni occur in stable platforms, but when the non-Sudbury component of ores with > or = 0.8 percent Ni is considered, Precambrian mobile belts are most important followed by Precambrian (mainly Archean) greenstone belts and Phanerozoic fold belts. The highest grades occur in the greenstone belts (volcanic peridotite-associated deposits) and the Phanerozoic fold belts (gabbroid intrusive mafic-ultramafic deposits of the Noril'sk region). The latter deposits are the most widespread and occur in all environments except the stable platforms. About 95 percent of the original resources in Western Australia occur in Archean greenstone belts and contribute about 11.4 percent of total original resources in ores with > or = 0.8 percent Ni. This contribution is split almost equally between intrusive dunite-associated and volcanic peridotite-associated deposits and has an advantageous grade distribution.Nickel sulfide deposits occur through geologic time, with most resources being lower Proterozoic in age. However, in the non-Sudbury component of ores with > or = 0.8 percent Ni, Archean deposits are the most important, followed by lower Proterozoic and Phanerozoic deposits. Gabbroid intrusive mafic-ultramafic complex deposits occur in each of these time intervals.The tonnage-grade distribution of original resources clearly shows the higher grade characteristics of volcanic peridotite-associated deposits (particularly those in Western Australia). Grade characteristics in gabbroid intrusive mafic-ultramafic complex deposits are improved considerably by allowance for their substantial Cu content, whereas recovery of their platinum-group element and Co content offers further scope for improvement. Intrusive dunite association deposits are characterized by a favorable size distribution, but commonly at low grades in contrast to the relatively small, high-grade volcanic peridotite-associated deposits.Mine production of sulfide Ni since 1900 has been largely derived from Sudbury, but the important contributions of intrusive dunite and volcanic peridotite-associated deposits since 1960, and major expansion of production from the Noril'sk region of the USSR has led to considerable diversification of ore sources. Gabbroid intrusive mafic-ultramafic complex deposits are now the principal source of sulfide Ni production. Current mine production from Western Australia represents about 12 percent of total mine production of sulfide Ni and is in balance with its 14.5 percent share of identified resources in ores with > or = 0.8 percent Ni.It is estimated that total identified resources of sulfide Ni in ores with > or = 0.8 percent Ni are adequate to maintain the existing rate of world mine production of sulfide Ni for about 40 years. The much larger component of identified resources in ores with < 0.8 percent Ni show a trend of increasing metric tons of Ni with decreasing grade, and it is likely that, in the long term, economic factors, rather than resource availability, will constrain production of sulfide Ni.