Copper inclusions in sheet silicates from porphyry Cu deposits
Economic geologists have studied the Cu contents of biotite for at least a quarter of a century as potential guides for exploration and as clues to the genesis of porphyry copper deposits1–12. These earlier studies used bulk analytical techniques, and a common, although cautious, assumption was that Cu substituted for Fe in the octahedral sites of the biotite. Here we report data from high-resolution and X-ray analytical transmission electron microscopy which demonstrate that anomalous Cu in biotite is present primarily as submicrometre inclusions of pure native Cu in the interlayer regions of the mica. The inclusions occur with greatest density and size in mixed-layer sheet silicate domains which form by alteration of biotite and chlorite. Less abundant and smaller Cu inclusions are associated with biotites that show subtle alteration only on the high-resolution transmission electron microscopy scale. The association of Cu inclusions with alteration domains, coupled with mass-balance considerations, suggests that these sheet silicates act as Cu sinks, and not as sources for primary mineralization.
High-Resolution and Analytical Transmission Electron Microscopy of Mineral Disorder and Reactions
Crystal defects and chemical reactions occurring at scales beyond the resolution of light microscopes have major effects on the chemical and physical properties of rocks and minerals. High-resolution imaging, diffraction, and chemical analysis in the transmission electron microscope have become important methods for exploring mineral defect structures and reaction mechanisms and for studying the distribution of phases resulting from reactions. These techniques have shown that structural disorder is common in some rock-forming minerals but rare in others. They have also established mechanisms by which many reactions occur at the atomic cluster scale. These data thus provide an atomistic basis for understanding the kinetics of geological reactions. Furthermore, apparent major-element, minor-element, and trace-element chemistry of minerals can be influenced by submicroscopic inclusions or intergrowths, which commonly form as products of solid-state reactions.
An analysis of the cooling of intrusives by ground-water convection which includes boiling
A finite difference model of the cooling of an igneous intrusive of limited volume is developed and used to investigate the relation between igneous intrusion, the formation of liquid and vapor dominated geothermal systems, and the formation of porphyry-type ore deposits. The model takes into account the properties of pure water and accommodates the phenomena of boiling and condensation. Permeability, level of intrusion, and pluton volume are systematically varied. Pressure, temperature, and fluid velocity are computed as functions of time.It is found that a self-supported, vapor dominated steam zone is commonly (but briefly) formed above the intrusive. Condensed water bounds the steam zone above, and if the hydrothermal solutions are saline, a zone of boiling bounds the steam zone below. For pure water condensation is far more important than boiling--the solutions circulate around the critical point of water to become gaseous without boiling. Despite large temperature variations, convection causes fluid pressures throughout the whole uniform permeability system to be close to normal cold-water hydrostatic values. Thus, even in an active converting system with moderate permeability variations, fluid pressure will tend toward normal hydrostatic values. Fluid circulation appears easily sufficient to produce a typical porphyry copper ore shell, but base metal precipitation probably must be controlled by mechanisms other than simple temperature drop.