Gold mineralization, in the Flambeau Lake area, is found in and adjacent to quartz-carbonate-albite veins surrounded by halos of intense ankerite-albite alteration. These features are the result of the passage of hydrothermal fluid through tension fractures and shear zones developed 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 gradually removed and replaced by carbonates and albite; a zone of mainly ankerite and albite; and a rare marginal zone of mainly albite and quartz. A detailed analysis of potential mass balance indicators from a single 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 SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, and K 2 O were removed from the wall rock and that CaO, CO 2 , S, and Na 2 O were added. These changes are attributed to the breakdown of chlorite, sericite, calcite, and quartz, and their replacement by albite and ankerite. 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 SiO 2 from the rock (up to 30 wt %). The behavior of perfectly mobile elements indicates that the fluid/rock ratio increased toward the vein. A model is proposed in which the observed alteration is explained by the interaction of the wall rock with a quartz-undersaturated, alkaline, CO 2 -bearing 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 reaction paths can be investigated using simple mass balance calculations.