Abstract

Hydrothermal copper mineralization emplaced within Archean granulite facies metamorphic rocks are aligned along a northeast-trending line passing through the Artonvilla mine. The orebodies in the district include breccia pipes, fissure deposits, and disseminated replacement bodies, the latter being best developed in the Artonvilla mine, where four separate lodes are developed.The Emery Lode is a flattened ovoid (600 X 400 X 50 meters) dipping at 45 degrees with the stratigraphy. Elongated apophyses extend downdip, and the most westerly is known to extend to 700 m below surface.Mineralization, which is accompanied by intense hydrothermal alteration, is confined to a cordierite-biotite-garnet-gneiss and a pyroxene granulite. This alteration and mineralization, which does not extend to the present erosion surface (in the cases of the Emery and Reggie Lodes), is confined to restricted structural traps as described by Jacobsen (1974). The disseminated orebodies reveal a distinct concentric zonation of hydrothermal minerals from an outer zone of sericitization followed inward by zones of albitization, chloritization, and epidotization; concomitant with these changes the sulfides show a zonation from an outer pyrite, through chalcopyrite and bornite, to chalcocite and occasional native copper zones at the center.Chemically the most important changes are related to the increase in the K 2 O content of the sericite-bearing outer aureole and the sharp decline of this component as well as SiO 2 with increasing alteration. The abundance of Al 2 O 3 and Na 2 O increases with advancing states of alteration; and downward nepheline and acmite normative compositions are encountered in rocks of the conduit below the 500m level. The extent of oxidation of iron increases markedly with increasing alteration and accompanies the economically important copper sulfide mineralization.Indications are that the initial hydrothermal fluid entered the structural traps at elevated temperatures, probably in excess of 650 degrees C, and passed through the rocks down a temperature gradient, finally cooling to about 400 degrees C at the outer extremity. The remaining trapped solutions cooled inward in much the same fashion as pegmatites do, thereby creating the well-developed mineral zonation. The central parts of the orebody indicate mineral associations compatible with temperatures in the range of 140 degrees C to 280 degrees C.The extensive aluminum and sodium metasomatism suggest that the fluid which caused mineralization was rich in these components and was capable of albitizing quartz and dissolving silica in the deeper levels of the conduits, thereby producing nepheline normative compositions. The fluids were therefore not of granitic parentage.We suggest that these features are most easily reconciled with a concentrated magmatic fluid which did not reach surface but reacted to completion by processes akin to pegmatite-type differentiation.A single injection of ore fluid is therefore envisaged, which would evolve by reaction with the enclosing host rocks and cause the observed mineral zonation. Successive minor zones then represent increasingly lower equilibration temperatures.

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