Significance of magma mingling and mixing during the formation of the host-rock successions of Archean massive sulfide deposits in the Noranda camp, Abitibi subprovince, Quebec, The

Abstract

The Noranda mining district contains some of the most important volcanic-hosted massive sulfide deposits of the Neoarchean Abitibi subprovince of northern Ontario and Quebec. With a historic production of 54.3 Mt of ore grading 2.2% Cu and 6.1 g/t Au, the Horne deposit was the largest base and precious metal producer within the district. The Quemont deposit, located just north of the Horne mine, was the second largest producer with a historic production of 13.9 Mt of ore grading 1.31% Cu and 5.38g/t Au. The Horne mine is hosted by a felsic-dominated volcanic succession that is crosscut by mafic dikes feeding into a conformably overlying mafic volcanic package. One of the best exposed and preserved sections of Horne stratigraphy is located in the Horne West area. In this area, a large flow-banded rhyolite cryptodome containing abundant mafic xenoliths is exposed. To characterize the occurrence of the mafic xenoliths, the shape, trend, size, and contact relationships were recorded for each xenolith. Volcanic facies analysis showed that tabular xenoliths are abundantly present in the lower portion of the cryptodome and become increasingly sparse, blocky, and irregularly distributed and shaped up-stratigraphy. Flow banding around the xenoliths is well developed in the lower portion of the rhyolite. The contacts between the mafic xenoliths and the surrounding rhyolite are commonly scalloped. The facies relationships suggest that incorporation of the mafic xenoliths into the rhyolite cryptodome is best explained by a process of magma mingling. Mingling presumably occurred through synchronous emplacement of the felsic feeder of the cryptodome and a mafic dike along a synvolcanic fault. The host rock succession of the Quemont deposit primarily comprises coherent rhyolite and felsic volcaniclastic units. The succession is cut by a dike-in-dike complex containing both mafic and felsic intrusions. Magma mingling and mixing are observed along the contacts between a mafic dike and a quartz- and feldspar-phyric rhyolite dike. The style of mingling and mixing is heterogeneous along the contact. Common textural relationships include the presence of elongate mafic xenoliths along the margin of the rhyolite dike, centimeter-sized inclusions of mafic material in a matrix of intermediate composition, centimeter-sized mafic inclusions and wispy xenoliths within a rhyolite matrix, and areas characterized by mixed rhyolite and basalt, resulting in an intrusion of andesitic composition. The intricate contact relationships observed suggest that the dike-in-dike complex at Quemont Hill formed through synchronous emplacement of felsic and mafic melts along a major synvolcanic structure. The results of the volcanic facies analysis lend support to models assuming that the Horne and Quemont deposits, which represent some of the largest synvolcanic gold enrichments world-wide, formed in a volcanic setting characterized by synvolcanic faulting caused by crustal extension. Widespread mingling and mixing of magmas of significantly different compositions likely resulted from the upwelling of mantle-derived melts into the crust, promoted by an extensional setting. As magma mingling and mixing indicate bimodal volcanism, a hallmark of crustal extension, it is proposed here that volcanic textures produced by these processes can be used to identify areas in volcanic belts that are favorable for volcanic-hosted massive sulfides.