TWO STAGES OF WHITE MICA ALTERATION IN THE GABY PORPHYRY Cu DEPOSIT, NORTHERN CHILE

In porphyry systems, white mica alteration can be spatially associated with economic Cu-

Mo(-Au) mineralization. White mica alteration is fine-grained and mainly occurs as pervasive or in

halos, commonly paragenetically associated with chlorite and chalcopyrite, as well as sericitic

alteration halos along porphyry D-veins. Locally, coarse-grained infill of open-space veins with quartz

can be found in apical to root zones of porphyry systems. Current classifications of white mica

alteration are primarily based on textural characteristics, alteration mineralogy and color, the latter

being largely influenced by the presence of fine intergrowth textures with chlorite.

In the Gaby porphyry copper deposit in northern Chile, white mica alteration is well-

developed in a suite of Eocene porphyries intruding a Carboniferous–Permian volcano-sedimentary

sequence and a Permo-Triassic plutonic complex, its intensity generally correlating with relatively

high Cu grades. Based on a combination of short-wave infrared data, automated mineralogy

(QEMSCAN), whole rock geochemistry, in-situ analyses and elemental mapping, we identify two

stages of white mica alteration. The early stage is associated with relatively high Cu grades, it is

characterized by low Mg/Fe ratios and is subdivided into five substages: E1 and E2 are developed as

coarse-grained white mica in the apical zone of the porphyry marked by the presence of USTs, as

pervasive alteration, with high Fe content, and as infill of quartz-mica veins with high W and Sn

content, respectively; E3 is intergrown with chlorite and chalcopyrite as disseminations or early halo

veins, with high Li. White mica alteration in volcanic rocks denotes influence of the host rock and is

characterized by high Ti, Sc, V, Cr and Ni contents. Late white mica alteration has lower Cu grades,

high Mg/Fe ratios and is divided into two substages: L1 forming thin veinlets with chlorite is rich in

Li, Fe, Mg; L2 in D-veins with pyrite has low Mg and Fe content. Chlorite is present in both, early

Cu-rich and late Cu-poor white mica alteration, but in different proportions. Comparative study of

Mg/Fe molar ratio in white mica of porphyry Cu systems around the world indicates the frequent

presence of two white mica stages in many deposits, suggesting that the Mg/Fe ratio is the most

reliable chemical indicator for distinguishing between early and late white mica.