The alteration history of a series of shear zones, Mirage Islands, Yellowknife Bay, N.W.T

Relf, Carolyn Diane (1988) The alteration history of a series of shear zones, Mirage Islands, Yellowknife Bay, N.W.T. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Mirage Islands are part of a north-facing Archean supracrustal sequence which is dominated by felsic to intermediate volcaniclastic rocks interlayered with mafic flows, and cut by gabbroic dykes and sills. The western-most islands are intruded by granitoid veins, inferred to have been derived from an unexposed pluton centered south of the West Mirage Islands. -- Two metamorphic events have been recognized in the area. The first involved amphibolite facies metamorphism (M1), interpreted to have been caused by the intrusion of the inferred pluton. Temperatures approached 600°C and pressures were between approximately 2 and 5 kbar. The second metamorphic event (M2) entailed regional greenschist facies metamorphism, with temperatures near 450 to 500°C and pressures of about 2 to 4 kbar. M2 may have overlapped in time with M1, but it has overprinted (and therefore outlasted) M1 assemblages on the Mirage Islands. -- A series of steeply dipping east-west striking shear zones cut the rocks on the Mirage Islands, and are well exposed in metabasic rocks. Equilibration temperatures in the shear zones were calculated from the fractionation of 18₀ between quartz and chlorite. They have a bimodal distribution which corresponds to the temperatures determined for M1 and M2, and thus it is interpreted that the shear zones formed during M1 and that some of them have subsequently re-equilibrated during M2. -- The source of the hydrothermal fluids cannot be established unequivocally from isotope data, but it is suggested that the fluids which are present in the shear zones during M1 were of igneous origin, and that metamorphic and meteoric fluids moved through the shear zones as the temperature decreased. -- Quartz veins in the shear zones are host to fluid inclusions with a range of compositions and homogenization temperatures. High salinities determined by fluid inclusion analyses support the interpretation that the hydrothermal fluid was in part meteoric. No primary inclusions were identified, however a few inclusions had sufficiently high homogenization temperatures that an estimate of their maximum trapping pressure could be made using formation temperatures calculated from isotope data. These pressure estimates are all less than 0.75 kbar. -- Alteration in the shear zones has resulted in the formation of new mineral assemblages and changes in the bulk chemistry of the sheared rocks. The oxidation states of iron in sheared samples suggest that the hydrothermal fluids were ascending, reducing fluids. In general, the changes in bulk composition of the sheared rocks may be predicted from alteration reactions between sheared and unsheared mineral assemblages. Estimates of minimum fluid:rock volume ratios are on the order of 10:1, but may be as high as 100:1 or more locally. It is possible semiquantitatively to monitor the alteration progress in the shear zones, and it was found that the enrichment of depletion of some elements varies systematically with the extent of alteration. -- The shear zones are interpreted to have formed during the intrusion of the pluton south of the Mirage Islands. Alteration, which began in the presence of igneous fluids, continued variable into the regional greenschist facies event which followed plutonism. The calculated variations in equilibrium temperature, isotopic composition, bulk chemistry and fluid:rock ratio are attributed to a variable time of closure of each shear zone.

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