VOLCANIC LITHOGEOCHEMISTRY AND ALTERATION AT THE DELBRIDGE MASSIVE SULFIDE DEPOSIT, NORANDA, QUEBEC

The Delbridge orebody occurs within a thick sequence ( > 1 km) of porp hyritic to aphyric massive rhyolite and rhyolite breccia of the Archea n Blake River Group. The orebody produced almost-equal-to 370,000 tonn es grading 0.61% Cu, 9.6% Zn, 110 g/t Ag and 2.1 g/t Au (1969-71). The footwall consists of massive quartz porphyritic rhyolite mantled by p roximal rhyolite breccias. An irregular chloritic alteration pipe with mineralization is subvertical to the ore lens. The orebody occurs at a thick cherty horizon within rhyolite breccia, and is overlain by a s uccession of mafic debris flows, porphyritic to aphyric massive rhyoli te flows, and finally andesite. The main alteration assemblage in the rhyolite units is quartz-albite-sericite-chlorite-carbonate. Immobile element plots and rare-earth element data indicate that the footwall r hyolite flows and proximal breccias are tholeiitic to transitional (Zr /Y = 3.5-5.5; La(N)/Tb(N) = 1.7-2.6), whereas hangingwall rhyolite flo ws are mildly calc-alkaline (Zr/Y = 6.5-7.5; La(N)/Yb(N) = 2.8-3.8). T hese two rhyolite types also have separate alteration lines in Ti-Zr s pace and in various immobile element plots. The identification of chem ically different rhyolites above and below the orebody provides marker s that can be identified and traced even where strongly altered. An in trusive rhyolite mass in the footwall is chemically identical to the h angingwall aphyric rhyolite flows, and is interpreted as the feeder to these flows. Calculated mass changes in the footwall rhyolite commonl y are large, and result from major silica change (+/- 30%), significan t loss of Na2O+CaO, and important additions of K2O and FeO+MgO. The ma rgins of the pipe show net mass gain, whereas the interior of the pipe shows net mass loss. Hangingwall rhyolite shows mass changes that gen erally are much smaller than in the footwall. Felsic rocks in the sili ca-sericite alteration zone up to almost-equal-to 200 m from the orebo dy have high deltaO-18 values of 10-12 parts per thousand, reflecting low-temperature alteration. The orebody occurs near the contact betwee n a mainly tholeiitic rhyolite footwall and an overlying sequence of m ildly calc-alkaline rhyolite then andesite.