Abstract

A high tonnage-low grade gold deposit at Mount Rawdon in southeastern Queensland is hosted by a diatreme (1.5 x 1.3 km in dimension) emplaced into metasediments of the Carboniferous Curtis Island Group and bodies of Briggs Creek Granodiorite immediately north of an east-northeast-trending fault. Andesites, trachytes and rhyolites of the Late Triassic Arabanga Volcanic Group outcrop less than 500 m west of the diatreme. Swarms of trachyte and rhyolite dikes crosscut the diatreme, one forming a ring dike along the contact of the diatreme and host metasediments. The diatreme is filled by polymict, moderately to poorly sorted, matrix-supported fragmental units dominated- by clasts of variably altered dacite porphyry, basement metasediments, and granodiorite clasts. The fragmental units are gradational to marginal breccia which forms a steeply dipping pipelike structure generated by emplacement of a body of dacite porphyry. The diatreme is intruded by a central dacitic plug and numerous smaller dacite plugs and dikes. Premineralization alteration within the diatreme is associated with intrusions and consists of phyllic and propylitic assemblages generated by dacite porphyries, crosscut by trachyandesites with propylitic alteration and trachytes with weak illite-smectite alteration. Sericitic alteration is centered on a sparsely porphyritic dacite plug forming an apophysis to the central dacite and overprints propylitic alteration in it and in the fragmental units. Sericitic alteration is overprinted by silica-pyrite and chlorite alterations. The chloritic alteration is intimately associated with mineralization. Mineralization postdates dacitic and trachyandesitic intrusions and occurs as disseminations in the matrix of fragmental units, as replacement patches and stringers. Three stages of mineralization are recognized: stage 1, which is volumetrically the most significant, contains pyrite, arsenopyrite, sphalerite, galena, native...

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