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.

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