SIGNIFICANCE OF MAGMA MINGLING DURING THE FORMATION OF THE VOLCANIC HOST-ROCK SUCCESSION OF THE GIANT HORNE MASSIVE SULFIDE DEPOSIT AT ROUYN-NORANDA, QUEBEC

The Blake River Group in the Noranda mining district represents one of the most important hosts to massive sulfide deposits in the Archean Abitibi greenstone belt. With a historic production of 260 t of Au and 1.13 Mt of Cu, the Horne mine was the largest base and precious metal producer within the district. The host-rock succession of the Horne massive sulfide deposit is dominated by a proximal facies association comprised of coherent rhyolite and associated volcaniclastic rocks that formed by autobrecciation and quench fragmentation. Effusive and shallow intrusive volcanism occurred contemporaneously with the deposition of mass-flow derived volcanic debris containing pyroclasts generated by explosive eruptions of a felsic volcanic source.

One of the best exposed and preserved sections of Horne stratigraphy is located in the Horne West area. In this location, a large flow-banded rhyolite cryptodome that contains abundant mafic xenoliths is exposed at surface. To characterize the occurrence of the mafic xenoliths, the shape, orientation, size, and contact relationships were recorded for each xenolith. Several contour maps were constructed, displaying the outcrop distribution of xenolith characteristics. The contour maps indicate that there are several notable lateral and vertical trends, which include: 1) abundant tabular xenoliths present in the southernmost outcrops that become increasingly sparse, blocky, and irregular up-stratigraphy, 2) distinct flow banding in the southern outcrops showing E-W flow movement of the rhyolite around the xenoliths, 3) scalloped margins that are prominent on the southern and south-eastern xenolith contacts, and 4) planar contacts that are prominent to the north. The contact relationships between the flow-banded rhyolite and the mafic xenoliths are indicative of magma mingling, suggesting that the felsic and mafic melts were emplaced synchronously. Magma mingling associated with bimodal volcanism is a hallmark of extensional supra-subduction settings, which are interpreted to be favorable for the formation of volcanic-hosted massive sulfide deposits.