Hydrothermal alteration envelopes in the walls of the Le Chatelet Au arsenopyrite deposit are superimposed on a regional chlorite-phengite assemblage. The outer parts of the envelopes consist of a kaolinite-Li-bearing tosudite-siderite alteration facies, whereas the inner parts comprise an illite-siderite and/or pyrite (pyrrhotite) alteration facies accompanied by quartz near the feeder conduit. Transition between the two facies is gradational. The economic gold-bearing arsenopyrite, associated with rare pyrrhotite, loellingite, pyrite, and marcasite, is concentrated in siliceous veins and disseminated in illitized and silicified wall rock.Analyses of fluid inclusions in quartz show two distinct fluid generations: early (primary to secondary) volatile-rich (CO 2 , CH 4 ) fluids, mainly in metasomatic quartz and migmatite quartz included in the mineralized veins, and late (secondary) aqueous and slightly saline (<5.4 wt % NaCl equiv) solutions distributed widely throughout the hydrothermal facies. Spatial relationships between the volatile-rich fluids and the hydrothermal facies indicate that the fluids were contemporaneous with the first stages of hydrothermal activity; their presence as primary inclusions within arsenopyrite-bearing metasomatic quartz suggests that they were also involved at a later stage with arsenopyrite deposition. The variable CO 2 /CH 4 ratios of these early fluids reflect possible effervescence and trapping at a temperature lower than or equal to 270 degrees C.The stability of the alteration minerals and fluid inclusions indicates that the temperature of hydrothermal activity at Le Chatelet ranged between 280 degrees and 180 degrees C. The arsenopyrite association with pyrrhotite and loellingite or pyrite indicates that oxygen fugacities during sulfide deposition were very low (log fo 2 = -45 to -39 at 250 degrees C), compatible with the estimate made from the CO 2 /CH 4 ratio of the early inclusions (logf (sub O 2 ) = -38.5). The presence of illite reflects neutral to slightly alkaline pH conditions.Thermodynamic modeling carried out between 100 degrees and 250 degrees C shows that a single-stage zoned alteration in both the outer and inner parts of the envelopes (facies 2-O and 2-I) would result from differential dissolution of the protolith minerals, in particular, a rapid dissolution of plagioclase, due to temperature variations in the hydrothermal system and flow gradient. Comparison between the deduced age of mineralization (Stephanian A-B) and that of the regional silico-albitic to albitic and lithium-rich granite magmatism suggests the latter as a possible heat and fluid source (indicated by abundant Li) at Le Chatelet. Nonequilibrium deposition of the Au arsenopyrite mineralization during an abrupt opening phase along brittle faults is supported by well-developed breccia in conduits cemented by fine-grained arsenopyrite-bearing quartz and a well-developed patchy zoning of the Au arsenopyrite. The new data obtained from fluid inclusions suggest two possibilities for the hydrothermal activity at Le Chatelet: single-fluid evolution with a late overprint of the hydrothermal alteration by aqueous fluids, or two-fluid evolution with introduction of the aqueous fluid during a phase of opening.