MASS-TRANSFER AND THE PATH OF METASOMATIC REACTIONS IN MESOTHERMAL COLD DEPOSITS - AN EXAMPLE FROM FLAMBEAU LAKE, ONTARIO

Gold mineralization, in the Flambeau Lake area, is found in and adjace nt to quartz-carbonate-albite veins surrounded by halos of intense auk erite-albite alteration. These features are the result of the passage of hydrothermal fluid through tension fractures and shear zones develo ped in response to regional dextral shearing. Wall-rock alteration is zoned, consisting of four distinct alteration assemblages: distal weak isochemical alteration; a transitional zone where chlorite was gradua lly removed and replaced by carbonates and albite; a zone of mainly an kerite and albite; and a rare marginal zone of mainly albite and quart z. A detailed analysis of potential mass balance indicators from a sin gle alteration halo in quartz diorite revealed that Zr, Ti, light REE, U, Th, and to some extent, Nb, remained immobile during alteration. A mass balance study of this halo shows that SiO2, Al2O3, Fe2O3, MgO, a nd K2O were removed from the wall rock and that CaO, CO2, S, and Na2O were added. These changes are attributed to the breakdown of chlorite, sericite, calcite, and quartz, and their replacement by albite and an kerite. Vein mineral textures show that the veins were open, allowing the passage of the substantial amounts of hydrothermal fluid required to produce the alteration and to remove large amounts of SiO2 from the rock (up to 30 wt %). The behavior of perfectly mobile elements indic ates that the fluid/rock ratio increased toward the vein. A model is p roposed in which the observed alteration is explained by the interacti on of the wall rock with a quartz-undersaturated, alkaline, CO2-bearin g fluid. The study emphasizes the importance of quartz undersaturation as a mechanism for the enhancement of hydrothermal alteration through porosity modification. In addition, it demonstrates how fluid-rock re action paths can be investigated using simple mass balance calculation s.