ZONED HYDROTHERMAL ALTERATION AND GENESIS OF THE GOLD DEPOSIT AT LE-CHATELET (FRENCH MASSIF-CENTRAL)

Hydrothermal alteration envelopes in the walls of the Le Chatelet Au a rsenopyrite deposit are superimposed on a regional chlorite-phengite a ssemblage. The outer parts of the envelopes consist of a kaolinite-Li- bearing tosudite-siderite alteration facies, whereas the inner parts c omprise an illite-siderite and/or pyrite (pyrrhotite) alteration facie s accompanied by quartz near the feeder conduit. Transition between th e two facies is gradational. The economic gold-bearing arsenopyrite, a ssociated with rare pyrrhotite, lollingite, pyrite, and marcasite, is concentrated in siliceous veins and disseminated in illitized and sili cified wall rock. Analyses of fluid inclusions in quartz show two dist inct fluid generations: early (primary to secondary) volatile-rich (CO 2, CH4) fluids, mainly in metasomatic quartz and migmatite quartz incl uded in the mineralized veins, and late (secondary) aqueous and slight ly saline (<5.4 wt % NaCl equiv) solutions distributed widely througho ut 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 metasomati c quartz suggests that they were also involved at a later stage with a rsenopyrite deposition. The variable CO2/CH4 ratios of these early flu ids reflect possible effervescence and trapping at a temperature lower than or equal to 270-degrees-C. The stability of the alteration miner als and fluid inclusions indicates that the temperature of hydrotherma l activity at Le Chatelet ranged between 280-degrees and 180-degrees-C . The arsenopyrite association with pyrrhotite and lollingite or pyrit e indicates that oxygen fugacities during sulfide deposition were very low (log f(o)2 = -45 to -39 at 250-degrees-C), compatible with the es timate made from the CO2/CH4 ratio of the early inclusions (log f(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 par ticular, a rapid dissolution of plagioclase, due to temperature variat ions in the hydrothermal system and flow gradient. Comparison between the deduced age of mineralization (Stephanian A-B) and that of the reg ional silico-albitic to albitic and lithium-rich granite magmatism sug gests the latter as a possible heat and fluid source (indicated by abu ndant Li) at Le Chatelet. Nonequilibrium deposition of the Au arsenopy rite mineralization during an abrupt opening phase along brittle fault s 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 s uggest two possibilities for the hydrothermal activity at Le Chatelet: single-fluid evolution with a late overprint of the hydrothermal alte ration by aqueous fluids, or two-fluid evolution with introduction of the aqueous fluid during a phase of opening.