Abstract

Mr. J. W. Pallister asked Dr. Mortimer if he would explain more fully the double zone of enrichment in the mineralized veins beneath the uppermost erosion surface

The Author replied: To explain this double zone of supergene enrichment the mineral profile in the Dulcinea Mine, investigated by Dr. R. H. Sillitoe, is described.

Dulcinea Mine lies immediately beneath the degraded Cumbre Surface in an inclined vein that outcrops on the mountain summit with massive chalcocite and many oxides to about 140 m depth. From 140 to 250 m there is no enriched ore in a zone of oxidized hypogene ore, but from 250 to 470 m massive chalcocite is again present with oxides. Below 470 m oxidized hypogene ore occurs to 720 m where the workings enter pure hypogene chalcopyrite ore and no more changes occur as far as, and beyond, the water table at 810 m.

For supergene sulphides to be present at the outcrop of the vein, solutions must have altered a mineral deposit originally above the present land surface and since removed by erosion. In no other way could the binary copper sulphides have achieved such a position since there is no evidence either regionally, or texturally, that the binary sulphides are of hypogene origin. Therefore at the time of enrichment the topography must have stood higher than it now does with a ground water level around the summit of the mountain. The higher zone with the massive chalcocite is nevertheless over 140 m thick and some

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