Hydrothermal alteration within the Coalstoun Porphyry Copper Prospect, Gayndah, Queensland

The Coalstoun porphyry copper prospect lies adjacent to the north-northwest trending Perry Fault system. The intrusives of the prospect comprise porphyritic biotite ± hornblende quartz diorite and tonalites, their porphyritic microcrystalline equivalents and associated dykes. The composite intrusion appears to have attained shallow crustal levels which has produced fracturing and brecciation of the overlying, and surrounding sediments. Pervasive hydrothermal alteration / sulphide mineralization accompanied the final states of crystallization and culminated in the formation of veins and fracture fillings. The zonal pattern of this alteration / mineralization resulted in a deep core low in both copper and total sulphides, a surrounding biotite zone within which all significant copper mineralization occurs, and a fringe of chloritized - pyritized intrusive and surrounding metasediments. These alterations are almost isochemical, with dominant variations in K, Ca, Rb, Sr and Cu. Petrographic observations and microprobe analyses of hydrothermal biotites showed that they are characteristically different in composition and form to those of primary magmatic biotites, including lower Ti, lower total Fe as FeO, lower Fe/ Mg and Si / A1 ratios, and possibly an increase in the Fe3+ / Fe2+ ratio. They are much finer grained and possess a greenish colour to their pleochroism. A model is suggested to explain the high level of the intrusion, its effects upon the surrounding sediments, the association of copper mineralization and biotitization, the characteristics of this biotite, the surrounding chlorite zone and the subsequent veining and later andesitic-dacitic dyke emplacement. The model shows similarities to those proposed by Guilbert and Lowell (1974), and Carson and Jambor (1974).