Immobile elements as monitors of mass transfer in hydrothermal alteration; Phelps Dodge massive sulfide deposit, Matagami, Quebec

Abstract

Hydrothermal alteration associated with an Archean stratiform volcanogenic massive sulfide deposit has converted aphyric rhyodacite to chloritite. In addition to normal chlorite components (MgO, FeO, Al 2 O 3 , SiO 2 ), the chloritite is enriched in TiO 2 , Zr, Y, and Nb. These elements and Al 2 O 3 are shown to be immobile using binary plots of analyses which produce linear arrays, with high correlation coefficients, that go through bulk composition and origin. Al 2 O 3 is the most immobile, followed by Zr, Nb, TiO 2 , and Y.Immobility requires that these elements became enriched by in situ residual concentration. This was caused mostly by initial wholesale leaching of silica--mainly quartz--followed by leaching of SiO 2 and Fe 2 O (super [whitesunwithrays]) 3 (total iron as Fe 2 O 3 ) from chlorite. Alteration of rhyodacite proceeded from an initial stage of silicification and chloritization (addition of MgO and Fe 2 O (super [whitesunwithrays]) 3 ) of glass and feldspar (loss of Na 2 O and CaO) to a stage of leaching of quartz wherein over 50 percent of the mass of the unit was lost. In the final stage, leaching of Fe 12 Si 8 O 20 (OH) 16 accounted for a further 10 percent loss, leaving Mg- and Al-rich chlorite as essentially the sole component of the chloritite rock.