Economic Geology Vol. 85, 1990, pp. 1840-1856 Evolutionof Aqueous-Carbonic Fluids during ContactMetamorphism, Wall-Rock Alteration, and Molybdenite Deposition at Trout Lake, British Columbia* ROBERT L. LINNEN AND ANTHONY E. WILLIAMS-JONES Department of Geological Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7 Abstract The Trout Lake deposit is associated with a granodiorite-tonalite stock that intruded green- schist facies calcareous andpelitic metasedimentary rocks. It is typicalof the fluorine-deficient or granodiorite class of molybdenum deposits. The stockis surrounded by a contactmeta- morphicaureole in which the mineral assemblages record evidence of fluid infiltration.At the present erosional surface, temperatures were below400øC andXco= in the calcareous rockswas <0.01. The confining pressure is estimated to have been 2 kbars. Contact meta- morphism wasfollowedsuccessively by skarn andpotassic alteration. The latter developed as biotitehalos around quartz-albite veins in the metasedimentary rocks andas K feldspar halos aroundquartz veinsin the intrusive rocks. Fluid inclusions in veins{brmedduring potassic alteration are dominantly aqueous and have low to moderate salinities. The ensuing quartz- feldspar-muscovite and muscovite-ankerite alteration events were relatively pervasive. Aqueous-carbonic fluid inclusions are commonly observed in quartz-feldspar-muscovite-altered samples and are the dominant fluid inclusion type present in veins associated with muscovite- ankeritealteration. The aqueous fluid inclusions from all stages of alteration homogenize at temperatures between 170ø and 310øC, with a peak at 250øC. The aqueous-carbonic fluid inclusions homogenize to the H20 phase between 240 ø and 340øC. Oxygenisotope com- positions for muscovite-quartz pairs yieldtemperatures of 370øto 400øCfor quartz-feldspar- muscovite and muscovite-ankerite alteration. This indicates that the fluidswere trapped at approximately 1,400 to 1,700 bars, i.e., abovethe two-phase region in the H20-CO2-NaC1 system. Calculated tS•sO values for the fluid range from 8.0 to 8.4 per rail and, takenin conjunction with pressure and temperature estimates, suggest a magmatic and/or modified ground-water source. Phase relations of alteration minerals suggest that pH decreased during alteration. Mineralization is spatially andtemporally associated with quartz-feldspar-muscovite alteration; molybdenite is typicallyintergrown with muscovite that replaced alkali feldspar. This texturalrelationship is interpretedto indicate that molybdenite deposited in response to decreases of temperature and/orpH. The common occurrence of these intergrowths in other fluorine-deficient and-enriched deposits suggests that the controls of mineralization for these deposits were similar to those at Trout Lake. Introduction PORPHYRY-STYLE molybdenumdeposits have been subdivided into granodiorite and granite types by Mutschler et al. (1981 ), calc-alkaline andalkali-calcic to alkalic types by Westra andKeith (1981), andmore recentlyinto fluorine-deficient andfluorine-enriched typesby Theodore and Menzie (1984). The general characteristics of the fluids associated with these ma- jor subtypes havebeen described in several papers. Fluid inclusions in fluorine-deficient or granodiorite- type deposits are typically low to moderately saline and commonly contain COg (e.g., Blake et al., 1979; Burrows and Spooner, 1987). Fluorine-enriched de- posits generallycontainhypersaline fluid inclusions (e.g., Bloom, 1981; White et al., 1981); this fluid in- * Mineral Exploration ResearchInstitute contribution number A 9035. clusiontype is rare in fluorine-deficient deposits. However, the evolution of these different fluids and their relationship to mineralization are poorly un- derstood. In this paper we discuss the evolution of fluids of a fluorine-deficient granodiorite-type deposit and relate this evolutionto the processes of molyb- denitc mineralization. The Trout Lake depositis typical of its class; it is associated with a granodiorite-tonalite intrusion, lacks fluoriteor topaz, andhas weakly to moderately saline fluid inclusions that containCOg. A significant part of contact metamorphism, alteration, andmineraliza- tion occurred in pelitic andcalcareous metasedimen- tary rocks. Phaserelationships for mineral assem- blages in the metasedimentary rocks can thus be used to estimate fluid composition. We have employed these datain conjunction with fluidinclusion andsta- ble isotope datato interpretthe evolution of aqueous andaqueous-carbonic fluids in the deposit. This evo~ os61-012s/90/11,57/i s40-1753.oo 1840