The greenschist facies of an Archean assemblage near Wawa, Ontario

Studemeister, Paul A.

doi:10.1139/e83-127

Cited by 28 publications

(7 citation statements)

References 14 publications

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“…It is found that the increase of Fe 2+ /Fe 3+ ratio was accompanied by increasing the concentration of reducing agents such as H 2 and H 2 S in the fluids (Kerrich et al, 1977;Studemeister, 1983). The observed sulphides as main ore minerals in the Dungash alteration zones (more abundant in the carbonate-sericite and sericite zones) seem to reflect such phenomena.…”

Section: Discussionmentioning

confidence: 98%

Alteration Patterns Related to Hydrothermal Gold Mineralizaition in Meta‐andesites at Dungash Area, Eastern Desert, Egypt

2001

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: The hydrothermal alteration patterns associating with the gold prospect hosted by metavolcanics in the Dungash area, Eastern Desert of Egypt, were investigated in order to assign their relationship to mineralization. The metavolcanics of andesitic composition are generated by regional metamorphism of greenschist facies superimposed by hydrothermal activity. Epidote and chlorite are metamorphic minerals, whereas sericite, carbonates, and chlorite are hydrothermal alteration minerals. The auriferous quartz vein is of NEE‐SWW trend and cuts mainly the andesitic metavolcanics, but sometimes extends to the neighbouring metapyroclastics and metasediments. Quartz‐sericite, sericite, carbonate‐sericite, and chlorite‐sericite constitute four distinctive alteration zones which extend outwards from the mineralized quartz vein. The quartz‐sericite and sericite zones are characterized by high contents of SiO2, K2O, Rb, and As, the carbonate‐sericite zone is by high contents of CaO, Au, Cu, Cr, Ni, and Y, and the chlorite‐sericite zone is by high contents of MgO, Na2O, Zn, Ba, and Co. Gold and sulphide minerals are relatively more abundant in the carbonate‐sericite zone followed by the sericite one. The geochemistry of the alteration system was investigated using volume‐composition and mass balance calculations. The volume factors obtained for the different alteration zones, mentioned above (being 1.64, 1.19, 1.17, and 1.07, respectively), indicate that replacement had taken place with a volume gain. The mass balance calculations revealed addition of SiO2, K2O, As, Cu, Rb, Ba, Ni, and Y to the system as a whole and subtraction of Fe2O3 from the system. Initial high aK+ and aH+ for the invading fluids is suggested. As the fluids migrated into wallrocks, they became more concentrated in Mg, Ca, and Na with increasing activities of CO2 and S. The calculated loss‐gain data are in agreement with the microscopic observations. Breakdown of ferromagnesian minerals and feldspars in the quartz‐sericite, sericite, and chlorite‐sericite zones accompanied by loss in Mg, Fe, Ca, and Na under acidic conditions and low CO2/H2O ratio may obstruct the formation of carbonates and sulphides, and the precipitation of gold in these zones. The role of metamorphic fluids in the area is expected to be restricted to the liberation of Au and some associated elements from their hosts.

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Section: Discussionmentioning

confidence: 98%

Alteration Patterns Related to Hydrothermal Gold Mineralizaition in Meta‐andesites at Dungash Area, Eastern Desert, Egypt

2001

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“…Morrison, 1979;Studemeister, 1983 ). The assemblage (albite + ) actinolite + epidote + quartz + sphene is indicative ofgreenschist facies conditions with T >~350°C and P >2 kbar (Winkler, 1967 ).…”

Section: Significance Qf Amygdale Mineral Assemblagesmentioning

confidence: 99%

Vesicles, amygdales and similar structures in fault-generated pseudotachylytes

Maddock

Grocott

Nes

1987

Lithos

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LITHOSMaddock, R.H., Grocott, J. and Van Nes, M., 1987. Vesicles, amygdales and similar structures in faultgenerated pseudotachylytes. Lithos, 20: 419-432.Amygdales in fault-generated pseudotachylytes from the Outer Hebrides Thrust, Scotland, and the Ikert6q Shear Belt, West Greenland, contain mineral assemblages dominated by K-spar + sphene + epidote + quartz and carbonates respectively. These contrasting assemblages indicate that vesicle infilling took place under differing environmental conditions. Other 'filled cavities', resembling amygdales in form and mineralogy, are recognized as replacement pseudomorphs after porphyroclasts. Using published data relating vesicle abundance and solubility functions for H20 and CO2 to pressure, a palaeoseismic depth estimate (equivalent to the depth of vesiculation) of about 1.6 km is inferred for the Greenland pseudotachylytes described in this paper.

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“…All exhibit a metamorphic zonation similar to that observed in other granite-greenstone subprovinces, with greenschist-facies cores and amphibolite-facies zones developed marginal to large internal and external plutons (e.g., Ayres 1969, Studemeister 1983, McGill & Shrady 1986). In the Wawa area, Studemeister (1983Studemeister ( , 1985 suggested that pretectonic tonalite stocks produced greenschistfacies contact aureoles at 450-550°C and <2 kbar, with regional metamorphism affecting both the volcanic and plutonic rocks under conditions of 325-450°C and 2-3 kbar. In both the Shebandowan and Vermillion greenstone belts, regional metamorphism was associated with D 2 deformation, which affected Timiskaming-type metasedimentary and metavolcanic rocks.…”

Section: Quetico Subprovincementioning

confidence: 57%

Metamorphism of the Canadian Shield, Ontario, Canada. I. The Superior Province

Easton¹

2000

The Canadian Mineralogist

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This paper is an outgrowth of the compilation of metamorphic information for the Canadian Shield in Ontario, and complements the recently compiled Metamorphic Map of the Canadian Shield. The paper contains a summary of the Archean metamorphic history of the Superior Province in Ontario on a subprovince basis, with an emphasis on the limitations of the existing data and the reasoning applied in extrapolating metamorphic boundaries. Little of the early metamorphic history of the Superior Province (pre-2715 Ma) is preserved, but there is local evidence for events at 2870-2850, 2810, and 2730 Ma within some of the older blocks of crust. The present distribution of metamorphic grade and age of metamorphism largely reflects pan-Superior events in the interval 2710-2640 Ma that occurred subsequent to coalescence of a system of island arcs, back-arcs, oceanic plateaus and microcontinents between 2720 and 2690 Ma. The distribution of metamorphic events and facies is the result of three interrelated patterns. 1) There is a relationship between subprovince type, metamorphic grade and age, with granite-greenstone subprovinces generally preserving older greenschist-to lower-amphibolite-facies events, metasedimentary subprovinces preserving younger middle-amphibolite-to granulite-facies events, and high-grade gneiss subprovinces preserving the youngest events. 2) Discrete metamorphic episodes between 2720 and 2640 Ma are associated with periods of major plutonism. 3) There is a pattern of increasing complexity of metamorphic history with increasing metamorphic grade. The present distribution of metamorphic facies in the Superior Province also was influenced by uplift, tilting, and erosion during the Paleoproterozoic. The timing of lode-gold and rare-element-pegmatite mineralization within the Superior Province corresponds closely with metamorphic evolution; it is consistent with models whereby gold-bearing fluids and pegmatite-forming melts develop, in part, as a result of granulite-facies metamorphism of the lower crust.

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The greenschist facies of an Archean assemblage near Wawa, Ontario

Cited by 28 publications

References 14 publications

Alteration Patterns Related to Hydrothermal Gold Mineralizaition in Meta‐andesites at Dungash Area, Eastern Desert, Egypt

Alteration Patterns Related to Hydrothermal Gold Mineralizaition in Meta‐andesites at Dungash Area, Eastern Desert, Egypt

Vesicles, amygdales and similar structures in fault-generated pseudotachylytes

Metamorphism of the Canadian Shield, Ontario, Canada. I. The Superior Province

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