Economic Geology Vol.91, 1995, pp. 302-321 Mass Transfer and the Path of Metasomatic Reactions in Mesothermal Gold Deposits: An Example fromFlambeau Lake, Ontario BRUCE W. MOUNTAIN s ANDANTHONY E. WILLIAMS-JONES Fluid-Rock Interaction Laboratory, Department of EarthandPlanetary Sciences, McGill University, 3450 Uuiversity Street, Montrdal, Qudbec, Canada H3A 2A7 Abstract Gold mineralization, in theFlambeau 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 fluidthrough tension fractures andshear zones developed in response to regional dextral shearing. Wall-rock alteration is zoned, consisting offour distinct alteration assemblages: distal weak isoehemi- cal alteration; a transitional zone •vhere ehlorite was gradually removed and replaced bycarbonates 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, andto some extent, Nb, remained immobile during alteration. A mass balance study of this halo shows thatSiO.2, A1.203, Fe•203, MgO,and K•20 were removed from thewallrock and that CaO, CO2, S, andNa20were added. These changes areattributed to the breakdown of ehlorite, serieite, calcite, and quartz, and theirreplacement by albite and ankerite. Veinmineral textures show thattheveins wereopen, allowing the passage ofthesubstantial amounts of hydrothermal fluid required toproduce thealteration and to remove large amounts of SiO2 from therock (upto 30 vet %). Thebehavior of perfectly mobile elements indicates thatthe fluid/rock ratio increased toward the vein. A model is proposed in xvhieh the observed alteration isexplained bytheinteraction of thewall rock witha quartz-undersaturated, alkaline, COs-bearing fluid. Thestudy emphasizes the importance of quartz undersaturation as a mechanism fortheenhancement of hydrothermal alteration through porosity modification. In addition, it demonstrates how fluid-rock reaction paths can beinvestigated using simple mass balance calculations. Introduction THENATURE of alteration associated withmesothermal gold deposits has been welldocumented in numerous studies. As well, many ofthese studies have established the temperature, pressure, andcompositional parameters under which these deposits were formed (Boyle, 1979; Kerrieh and Fyfe, 1981; Neall and Phillips, 1987; Perring et al., 1987; Robert and Kelly,1987; Spooner et al., 1987; Weir andKerriek, 1987; Colvine et al., 1988; Groves et al., 1989; Ho et al., 1990; Kesler, 1990; Groves and Foster, 1991; Guha et al., 1991; de Ronde et al., 1992; MeCuaig andKerrieh, 1994). In many eases, estimates havealso been made of the amounts of mate- rial transferred between fluid and rock. However, to our knowledge, there has been only one attempt (B6hlke, 1989) to use estimates of mass transfers to investigate the path of alteration, i.e., toreconstruct the relative spatial and temporal distribution of changes that occur in a rockas a result of progressive reaction with fluid during the formation of a mesothermal gold deposit. In this paper we report on a study of wall-rock alteration associated with gold-bearing, quartz-earbonate-albite veins at Flambeau Lake, Ontario. A single alteration halo, considered representative of those in a quartz diorite unit, was sampled to generate detailed mineralogieal and geochemical profiles. Mass balance calculations were done to estimate the mass changes during alteration across the halo, andthese results were used to interpret a possible reaction path between the fluid and thewallrock. This approach differs from thatem- ployed in most alteration studies, which aretypically based on a fewwidely spaced samples taken from relatively large alteration halos. Our study demonstrates that, if sampled in sufficient detail, alteration zones can beused todecipher theprocess offluid- rock interaction. The mineralogieal information and mass bal- ance calculations presented here have permitted usto show thatalteration in the quartz diorite consisted of aninterplay ofearbonatization, desilieation 1, albitization, and pyritizati-on, which were the result of the passage of a quartz-undersatu- rated, alkaline, CO.•-bearing fluid through open fractures. The distribution of highly mobile elements indicates that therock reacted with progressively greater amounts offluid the closer it was tothe vein. The presence ofthe most intense alteration in quartz diorite isinterpreted tobetheresult of a combina- tion of structural controls and creation of wall-rock porosity due to desilieation bythe quartz-undersaturated fluid. Geologic Setting TheFlambeau Lake showing islocated in theEagle-Wabi- goon Lake section ofthe Wabigoon greenstone belt, Superior province. It ishosted bythemarie to felsie Lower Wabigoon voleanies and their associated intrusiverocks, all of which are composed of greenschist facies mineral assemblages. In the immediate vicinity of the shoxving, these rocks have been subdivided into five units: marie voleanies, intermediate and felsie voleanies-voleanielasties intermediate to felsie subvol- canic intrusive rocks (including quartz diorite), quartz-feld- spar porphyry, and diabase (Fig.1). * Present address: Institut fiir Mineralogle und Petrographie, ETH Zen- • The term "desilication" isused rather than "desilicification" as the latter trum, Sonneggstrasse 5, 8092 Ztirich, S•vitzerland. term implies thereinoval ofpreviously added silica. 0361-0128/96/1816/302-2055.00 302