REGIONAL EVOLUTION OF HYDROTHERMAL FLUIDS IN THE NORANDA DISTRICT, QUEBEC - EVIDENCE FROM DELTA-O-18 VALUES FROM VOLCANOGENIC MASSIVE SULFIDE DEPOSITS

The results of oxygen isotope analysis of hydrothermally altered volca nic rocks are used to define the thermal and isotopic characteristics of the ore-forming fluids which produced several volcanogenic massive sulfide deposits in the Noranda district. These deposits (Corbet, Ansi l, Norbec, Horne, and Mobrun) formed during temporally distinct events that span much of the volcanic stratigraphy in the district. The data show that the deltaO-18 values of the altered rocks and the size (eco nomic tonnage) of each deposit increase upward through the volcanic st ratigraphy from low deltaO-18 values at the Corbet (-2.2 to 4.8 parts per thousand) and Ansil (-0.8 to 5.0 parts per thousand) deposits, to intermediate deltaO-18 values at the Amulet (3.6-6.7 parts per thousan d) and Norbec (3.6-10.5 parts per thousand) deposits, to high deltaO-1 8 values at the Horne (4.2-11.6 parts per thousand) and Mobrun (6.0-13 .8 parts per thousand) deposits. A corresponding increase is indicated for the deltaO-18 values of the discharging hydrothermal fluids, from -2 +/- 2 per mil at the Corbet deposit to +3.0 +/- 1.5 per mil at the Horne deposit. Alteration in most of the deposits is shown to have re sulted from the flow of fluids having temperatures of 300-degrees +/- 50-degrees-C, with the exception of the pyrite-rich Mobrun deposit whe re slightly lower temperatures (200-degrees +/-50-degrees-C) are indic ated. These observations are interpreted to indicate that the duration of hydrothermal discharge (as indicated by deposit size) at a given s ite increased upward through the volcanic stratigraphy. Since the flui d, and thus the altered rock, deltaO-18 values normally evolve from lo w to high with time in a volcanogenic massive sulfide-forming system ( e.g., Cathles, 1983), the duration of fluid discharge is a primary fac tor in controlling the isotopic signature retained at a given deposit. In the Noranda district, the duration of fluid discharge depended str ongly on the rate of volcanic accumulation, with short-lived systems ( e.g., Corbet, Ansil) forming during periods of rapid accumulation of e xtrusive volcanic rocks, and longer-lived systems forming during more quiescent periods (e.g., Norbec), or at sites where the accumulation o f extrusive material was limited by topography (e.g., Horne).