Massive sulfide deposits of the Noranda area, Quebec. IV. The Mobrun Mine.

Barrett, T.J., Cattalani, S., Hoy, L., Riopel, J. and Lafleur, P.-J. 1992. Canadian Journal of Earth Sciences, v. 29, 1349-1374.
The Mobrun polymetallic deposit near Rouyn-Noranda comprises two complexes of massive sulfide lenses within mainly felsic volcanic rocks of the Archean Blake River Group. The Main lens has 3.37 million tonnes of massive sulfides, with 1989 reserves of 0.95 Mt at 0.81% Cu, 2.44% Zn, 30.3 g/t Ag and 2.2 g/t Au. The 1100 complex, located 250 m to the southeast of the Main complex, contains estimated 1989 reserves of 10.4 million tonnes at 0.76% Cu, 5.43% Zn, 37.4 g/t Ag and 1.35 g/t Au.

Host volcanic rocks of the Main complex are mostly massive, brecciated and tuffaceous rhyolites. The rhyolites are commonly strongly sheared parallel to lithological contacts, which are locally displaced by high-angle faults. Immobile element plots such as Y-Zr and Nb-Zr show a separation of rhyolite data into two distinct alteration trends that generally correspond to massive and in-situ-brecciated rhyolite of the footwall, and tuffaceous rhyolite of the hangingwall. The hangingwall has tholeiitic Zr/Y ratios (3-5), whereas the footwall has mildly calc-alkaline Zr/Y ratios (7-9). The immobile element trends indicate that there was a subtle but clear change in rhyolite composition near the time of ore deposition. Identification of chemically distinct footwall and hangingwall rhyolites allows these units to be recognized and traced along strike, even where alteration is strong. Sericitization and silicification extend at least 100 m from the orebody, with local chloritic zones in the upper footwall. Calculated mass changes indicate that the footwall generally has lost silica mass relative to the hangingwall. Alteration zones associated with mineralization have mass gains in FeO+MgO and K2O gains, but mass loss in silica.

The 1100 complex, located stratigraphically below the Main complex, is hosted by rhyolite, with one main andesite-dacite unit in the footwall. The footwall contains three chemically distinct rhyolite types, all tholeiitic. Hangingwall rhyolites are, however, mildly calc-alkaline, and thus are chemically comparable to, and correlated with, the footwall of the Main complex. Rhyolites within 100 m stratigraphically of the Main and 1100 complexes commonly have positively shifted d18O whole-rock values of 11-13. These high values are interpreted as the result of an initial, widespread phase of low-temperature hydrothermal alteration that increased d18O values by 3-5 relative to unaltered rhyolites. Some footwall rhyolites, however, are relatively depleted in 18O, strongly Ca-Na-depleted, and depleted in Eu+2. Rhyolites with these chemical features have been overprinted by higher temperature alteration, presumably in localized feeder zones. All four rhyolite types near the 1100 complex are chemically recognizable despite contrasting alteration.

The orebodies are interpreted as synvolcanic, based on their occurrence along distinctive volcanic contacts, and the presence of primary sulfide textures where deformation is minor. The chemostratigraphic framework defined for the host rhyolite sequence can be used to trace critical volcanic contacts in lithologically monotonous, strongly altered and faulted terrains.

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