Systematics of chlorite alteration at the Phelps Dodge massive sulfide deposit, Matagami, Quebec

Abstract

Hydrothermal alteration at the Phelps Dodge Archcan volcanogenic massive sulfide deposit has converted rhyodacite and rhyolite footwall rocks into the following mineral assemblages: quartz-chlorite-albite-epidote + or - sericite, quartz-chlorite + or - sericite, chlorite + or - sericite, and chlorite-talc + or - stilpnomelane. Alteration at constant Al was initiated by hydration, addition of Fe + Mg, silicification, and leaching of Ca + Na which produced Al-saturated chlorites with a wide range of Fe/Fe + Mg ratios within the assemblage quartz-chlorite-albite-epidote + or - sericite. With continued addition of Fe and less Mg, combined with extensive leaching of quartz, the chlorite became Al-undersaturated (quartz-chlorite-talc assemblage), and ultimately formed a rock composed essentially of chlorite. The chlorite-talc + or - stilpnomelane assemblage is stable in late crosscutting veinlets and in the ore zone.Chlorites in the Si-Al-Mg-Fe system form a rectangular solid solution plane with limited substitution of Al for Si and extensive substitution of Fe for Mg. In Si-Al space the solid solution plane is outlined by Al saturation (presence of other Al-rich mineral) and Al undersaturation (presence of Al-poor or Al-free mineral) boundaries. Al iv substitution for Si in the chlorites increases from 2.3 to 3.01 formula units (based on 8 Si + Al iv ) with Fe/Fe + Mg, whereas Al iv occupancy decreases from 2.88 to 2.65. The valence charge balance is compensated by octahedral site vacancies or by the conversion of Fe (super +2) to Fe (super +3) . The Al iv content of chlorite also increases with temperature. Chlorites on the Al-saturated boundary of the solid solution field form a potentially useful geothermometer: T degrees C = 106 Al iv + 18; Al iv is corrected for changes with the Fe/Fe + Mg ratio as: Al iv corrected = Al iv + 0.7 Fe/Fe + Mg.The Fe/Fe + Mg ratios of bulk rock and contained chlorite correlate well. Birefringence colors of the chlorite change from green to brown, violet, and Berlin Blue as Fe/Fe + Mg changes from 0.18 to 0.64. The Fe/Fe + Mg variation apparently depends largely on the proportions of Mg-rich seawater and Fe-rich hydrothermal fluid in the altering brine.