Origin and mode of copper enrichment in biotite from rocks associated with porphyry copper deposits; a transmission electron microscopy investigation

Abstract

This paper presents results of a systematic transmission electron microscopy (TEM) study of biotites from a suite of core samples from the Cyprus Casa Grande porphyry copper deposit, Arizona. The primary purpose of this study was to determine the mode and timing of copper enrichment in biotite and to compare the results to TEM observations of biotites from other porphyry copper deposits. The core samples represent both oxidized and unoxidized rock. Biotites and chlorites from unoxidized protore contain less than 200 ppm copper as measured by electron microprobe and are free of copper-enriched expanded interlayers and the native copper inclusions described by Ilton and Veblen (1988); biotites and chlorites from oxidized ore are commonly copper enriched. The mode of copper incorporation in biotites and chlorites in oxidized rock is complex and varies with depth. Biotites from the goethite zone contain submicroscopic native copper inclusions and both copper-enriched expanded interlayers and submicroscopic iron oxide inclusions. Biotites and chlorites from the chrysocolla zone contain copper-enriched expanded interlayers, amorphous Al-Si intergrowths with dense arrays of copper- and iron-rich inclusions, and copper-rich inclusions that rapidly evaporate under the electron beam and hence have yet to be identified. It is possible that copper is incorporated into the brucite layer in chlorite. Biotites from the native copper zone contain native copper inclusions, as expected, but they also contain copper-rich expanded interlayers. Biotites associated with atacamite mineralization contain high concentrations of copper that are associated with expanded interlayers. In the chalcocite zone, biotites appear relatively unweathered and contain minor concentrations of copper that are associated with infrequent expanded interlayers.A comparison of observations from oxidized and unoxidized rock indicates that the occurrence of copper-rich biotites and both copper-enriched expanded interlayers and native copper inclusions in biotite are restricted to oxidized rock. We conclude that the native copper inclusions and copper-rich expanded interlayers in biotite from Cyprus Casa Grande formed during weathering and not during the hydrothermal event.Native copper inclusions and copper-rich expanded interlayers in biotites have been documented from a variety of porphyry copper deposits. Earlier studies of the same samples have suggested that copper in copper-rich biotites was representative of magmatic hydrothermal events and that the difference between mineralizing and barren intrusions was recorded by differences in copper concentrations in biotite. The present study, together with previous experimental work, suggests that the copper-rich biotites became copper enriched during weathering and that copper concentrations in copper-rich biotites do not reflect magmatic hydrothermal processes.