Abstract

The Yeoval copper prospect in central New South Wales lies adjacent to the Molong rise, a belt of volcanic rocks considered to represent a fossil island arc of Siluro-Devonian age. The principal rocks of the prospect are equigranular granodiorite which has been intruded by porphyritic microgranodiorite. Porphyritic dacite dikes, up to 15 m wide, intrude both of these older intrusive rocks. Similarity in chemistry and mineralogy suggests that all three rock types are comagmatic.The widest dacite dike has a zoned mineral assemblage associated with the development of a 7 to 8 m-wide central zone of vesicles. The vesicles contain quartz, chlorite, sphene, calcite, and copper sulfides and vary in size from 1 mm in the outer parts of the zone to 4 mm in the center of the zone.It is proposed that vesiculation of the dike occurred after it intruded the granodiorite and after an abrupt fall in the confining pressure which may be related to pressure release in the parent stock. Retrograde boiling following this pressure release probably resulted in the formation of gas bubbles and in rapid cooling of the residual liquid. This boiling process accounts both for the occurrence and distribution of vesicles and for the texture observed within the dike.The zone of vesicle development was more permeable than the massive outer rind of the dike and allowed passage of solutions migrating upward from the parent stock. These solutions caused changes in mineralogy and produced chemical zonation. High copper values are associated with the depletion of K, Ca, Rb, and Sr and enrichment of Na, Si, Zn, and Mn. The assemblage quartz-chlorite-sphene-albite-epidote was produced in the most intense zone of alteration. This assemblage suggests a temperature range of 350 degrees to 400 degrees C. The pressures at the time of alteration may have been 1 kb or less since the dike is likely to have intruded to depths of approximately 2 km.

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