Geochemistry of hydrothermal chlorite replacing igneous biotite

ical compositions ofcoexisting biotite and chlorite determined by electron microprobe and wet chemical methods were used to evaluate chemical mass transfer during the alteration process, The mole ratio Mg/ (Mg + Fe) varies from 0.52 to 0.65 in the chlorite and from 0.51 to 0.60 in the parent biotite. The Mg content of the chlorite decreases systematically with increase in the volume percent replacement ofbiotite. Ho-mogenization temperatures of fluid inclusions in nearby quartz microveinlets indicate that the chloritic alteration took place at approximately 200~ Textural relationships suggest tha the alteration of biotite to chlorite is isovolumetric, but a comparison of mineral compositions and mineral assemblages with phase diagrams in which A1 or volume are conserved among solid phases suggests that he chlorite cq_mpositions are bes ~ explained:as a f!anciio~_~f~reaction progress in an Al-conservative s~stem. The chlorite composition changes in r ponse to changes in solution ~omposition produced by the dissolution of successive small amounts ofbiotite. Representative mass bal-ance for the alteration of all of the biotite to chlorite in 1 m 3 of rock containing 336 moles of biotite indicates that 74 moles of Mg, 35 moles of Fe 3+, 420 moles of H +, and 2 moles ofMn are added to the rock and that 311 moles of K, 54 moles of Fe 2+, 76 moles of Ti, 53 moles ofF, and 6 moles ofC1 are lost to solution. The mass transfer for partially altered biotite is ll to 188 moles of K, 2 to 46 moles ofTi, 2 to 44 moles ofF, and 0.3 to 6 moles of C1 removed per cubic meter of rock and 1 to 26 moles of Fe 3 § and 20 to 347 moles of H + added. The mass transfer of Mg varies from 12 moles added to32 moles removed per cubic meter of rock depending on mineral composition and extent of replacement. Key Words--Aluminum conservative alteration, Biotite, Chlorite, El ctron probe analysis, Fluid inclu-sions, Hydr0thermal alteration, Is volumetric alteration