Regional evolution of hydrothermal fluids in the Noranda District, Quebec; evidence from delta 18 O values from volcanogenic massive sulfide deposits

Abstract

The results of oxygen isotope analysis of hydrothermally altered volcanic 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, Ansil, Norbec, Horne, and Mobrun) formed during temporally distinct events that span much of the volcanic stratigraphy in the district.The data show that the delta 18 O values of the altered rocks and the size (economic tonnage) of each deposit increase upward through the volcanic stratigraphy from low delta 18 O values at the Corbet (-2.2 to 4.8ppm) and Ansil (-0.8 to 5.0ppm) deposits, to intermediate delta 18 O values at the Amulet (3.6-6.7ppm) and Norbec (3.6-10.5ppm) deposits, to high delta 18 O values at the Horne (4.2-11.6ppm) and Mobrun (6.0-13.8ppm) deposits. A corresponding increase is indicated for the delta 18 O values of the discharging hydrothermal fluids, from -2 + or - 2 per mil at the Corbet deposit to +3.0 + or - 1.5 per mil at the Horne deposit. Alteration in most of the deposits is shown to have resulted from the flow of fluids having temperatures of 300 degrees + or - 50 degrees C, with the exception of the pyrite-rich Mobrun deposit where slightly lower temperatures (200 degrees + or - 50 degrees C) are indicated.These observations are interpreted to indicate that the duration of hydrothermal discharge (as indicated by deposit size) at a given site increased upward through the volcanic stratigraphy. Since the fluid, and thus the altered rock, delta 18 O values normally evolve from low to high with time in a volcanogenic massive sulfide-forming system (e.g., Cathles, 1983), the duration of fluid discharge is a primary factor in controlling the isotopic signature retained at a given deposit. In the Noranda district, the duration of fluid discharge depended strongly on the rate of volcanic accumulation, with short-lived systems (e.g., Corbet, Ansil) forming during periods of rapid accumulation of extrusive volcanic rocks, and longer-lived systems forming during more quiescent periods (e.g., Norbec), or at sites where the accumulation of extrusive material was limited by topography (e.g., Horne).