Abstract

Samples of Lewisian granulite ranging from mafic to felsic in composition, average 0.54 ppb Au, well below the crustal abundance of about 4 ppb Au, but higher than granulite from the Bamble shear belt, Norway, with 0.22 ppb Au. Loss of gold during metamorphism requires modification of the primary sulphide phases, mainly pyrrhotite, that host gold in igneous rocks. Pyrite, formed in part by replacement of pyrrhotite, is the principal sulphide mineral of both the Lewisian and Bamble granulites. Conversion of pyrrhotite to pyrite in the Lewisian was by oxidized fluids during Inverian retrogression at temperatures below 625°C. Some grains show replacement of pyrrhotite by pyrite; ion microprobe analyses of such a grain shows an order of magnitude less gold in the pyrite than in the pyrrhotite. Scourie quartz dolerite dykes, which were intruded after Inverian retrogression, but while the Lewisian complex was still deep in the crust, retain pyrrhotite and average 1.47 ppb Au. Transformation of pyrrhotite to pyrite in the Bamble granulites also occurred well after peak metamorphism. Both groups of granulite are depleted in LILE (large ion lithophile elements) and in gold, but the Lewisian samples are more strongly depleted in LILE (median K/Rb of 1909 v. 1079 for Bamble granulite) and Bamble samples are more strongly depleted in gold. For both the Lewisian and Bamble rocks, LILE depletion was associated with prograde metamorphism, whereas loss of gold was a later and essentially unrelated event.

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