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Diamonds in an Archean greenstone belt : a study of diamonds and host meta-conglomerate from Wawa (northern Ontario) Bruce, Loryn Frances


I studied a diamondiferous Archean meta-conglomerate from Wawa (North Ontario), part of the Michipicoten Greenstone Belt (MGB) of the Superior Craton. Field observations determined the 170 m thick meta-conglomerate displays poorly sorted, matrix to clast supported and massive to bedded textures. Petrographic and SEM analyses determined it was metamorphosed in the greenschist facies. Twenty-four clast types were identified and classified into groups: igneous with subophitic texture; coarse-grained felsic; vesicular igneous; porphyritic mafic; porphyritic felsic; untextured volcanic; chert-like, unidentifiable clasts rich in chlorite, and opaques. The pebble - cobble sized clasts are derived from nearby meta-volcanic and meta-sedimentary rocks. Predominance of local volcanic clasts confirms the meta-conglomerate formed as a Timiskaming type deposit, between 2700.4±1.0 Ma and 2697.2±1.8 Ma. 383 diamonds (<2 mm) extracted from the meta-conglomerate were characterized using morphology, Fourier Transform Infrared (FTIR), and luminescence analyses. These diamonds show octahedral, cuboid, cubo-octahedral, and twinned growth habits; most are unresorbed. These diamonds are classified as Type IaA, Type IaAB, Type II, and Type IaB based on nitrogen contents and aggregation. Their calculated mantle residence temperature is 1000-1200˚C. Characterization of meta-conglomerate diamonds indicates they are cratonic in nature. A primary source is still unknown, speculated to be of mafic to ultramafic composition. Proximal diamondiferous meta-breccia may not be the source of meta-conglomerate diamonds, based on comparisons of morphological properties, nitrogen content and aggregation, and host rock mineralogy for meta-breccia and meta-conglomerate diamonds. Meta-conglomerate diamonds display a coarser size distribution, more diamond colours, and larger proportion of unresorbed crystals. Both suites are comparable in terms of growth habits and calculated mantle residence temperatures. Despite contrasting traits, meta-breccia and meta-conglomerate diamonds both display green, yellow, orange, pink, and red-orange cathodoluminescence (CL) colours with emittance at 520, 576 nm, and 586 - 664 nm, dissimilar to diamonds in unmetamorphosed rocks that show blue CL colour with emittance at 415 - 440 and 480 - 490 nm. This shift of CL results from presence of optical centers with zero-phonon lines at 575 and 637 nm detected via photoluminescence (PL) spectroscopy. The centers may have formed due to irradiation of diamonds in the upper crust and annealing.

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