Clay mineralogy, illite crystallinity and polytypes in the Campana Mahuida porphyry Cu deposit, Neuquén, Argentina

AGNES IMPICCINI,; MARTA FRANCHINI; GEORGE GRATOFF; ABEL ISIDORO SCHALAMUK; LARRY MEINERT

A Clay Odyssey

Elsevier

Lugar: Amsterdam; Año: 2001; p. 101 - 108

Clay mineralogy, illite crystallinity, and polytypes were determined for 67 whole-rock samples from 24 drill cores of the Campana Mahuida porphyry copper deposit to develop a general exploration model for porphyry copper deposits. In the potassic core, between 10 m and 228 m depth, Mg-rich chlorite is the dominant clay-fraction mineral followed by illite ± smectite or smectite ± illite. Illite crystallinity indices decrease from 0.4 to 0.1° 2 with depth, and correspond to the high temperature, 2M1 polytype (> 200°C). The chlorite-rich zone may have resulted from the break down of secondary biotite at intermediate values of a(K+)/a(H+) as the system cooled. Clay-fraction minerals in the outermost propylitic halo are similar to the potassic zone except that chlorite is more abundant and consists of both Fe-rich and Mg-rich varieties. In the phyllic halo that surrounds the potassic core, clay abundance between 5 and 149 m depth outlines two zones: 1- an illite-rich zone (2M1 polytype) to the south, northeast-southwest and east of the potassic core, and 2- a less well-developed smectite-rich zone in the northwest section of the phyllic halo. Within the smectite (beidellite) zone, chlorite accompanies smectite in drill-holes near the potassic core, whereas farther northwest, illite is present with smectite. In both zones, the illite crystallinity index decreases with depth, except in a single drill hole that intercepts a zone of argillic alteration where smectite dominates in the hypogene ore zone and illite crystallinity indices remain low and constant. This zonation within the phyllic halo may reflect differences in cation removal due to differences in faulting intensity and water: rock interaction.