Skarn and porphyry copper mineralization at Mines Gaspe, Murdochville, Quebec

Abstract

Mines Gaspe extracts copper and molybdenum from skarn and porphyry copper orebodies at Murdochville in the Gaspe Peninsula, Quebec. Both types of orebodies occur in the zoned Copper Brook aureole, which formed in calcareous Lower Devonian strata around the Copper Mountain plug, one of a group of small rhyodacite porphyry sills, dikes, and plugs of Upper Devonian to Mississippian age. Progressive metamorphism is marked by the alteration of diagenetic pyrite to pyrrhotite and the successive appearance of phlogopite, tremolite, diopside, and grossularitic garnet toward the plug. Diopside formation was accompanied by destruction of calcite, pyrrhotite, and carbonaceous material to form a bleached inner aureole. Similar metamorphic zones surround the nearby Porphyry Mountain plug and occur (without garnet) in the Porphyry Brook aureole. The three aureoles appear to coalesce some 1,200 to 1,500 m below surface.Crackle veinlets in the strata domed by eraplacement of the Copper Mountain plug were healed by quartz, feldspars, diopside, and grossularitic garnet, the same minerals that were formed in the metamorphosed wall rocks. Wollastonite partially replaced diopside in a ring around the plug. Iron metasomatism near the plug then replaced diopside with hedenbergitic pyroxene and replaced pale grossularitic garnet and wollastonite with darker andraditic garnet in veinlets and concretions. Massive skarn formed in some metamorphosed limestone units. The Porphyry Mountain plug also domed the surrounding sedimentary rocks. No crackle veinlets or wollastonite formed but andraditic garnet and hedenbergitic pyroxene were deposited in adjacent joints and bedding fractures. Metamorphism and metasomatism were followed, in some parts of the Copper Brook aureole, by formation of scapolite and a second generation of tremolite. Two major periods of mineralization followed.Early mineralization is characterized by pyrrhotite, minor chalcopyrite, and traces of sphalerite throughout the three aureoles. It is associated with alteration of Ca-Mg-Fe silicates to tremolite or actinolite, chlorite, and epidote or clinozoisite and with deposition of albite, calcite, and quartz. More intense mineralization formed replacement orebodies at Needle Mountain and Needle East, some distance from the Copper Mountain plug. These orebodies were cut by late 1 quartz veins containing chalcopyrite, pyrrhotite, fluorite, and traces of scheelite.Porphyry copper mineralization occurs as disseminations and in four sets of veins centered on the Copper Mountain plug. Late 2 veins, which contain quartz + anhydrite + K-feldspar with traces of chalcopyrite and magnetite, may be coeval with biotization and K-feldspar alteration in the plug. Late 3 veins, which account for most of the copper and molybdenum mineralization, contain quartz + or - anhydrite with chalcopyrite, pyrite, and molybdenite. They are variously associated with kaolinization, sericitization, and minor anhydrite alteration in the plug and adjacent intrusions and with formation of tremolite, epidote, chlorite, and minor anhydrite in the metasedimentary rocks. Late 4 veins contain calcite or dolomite, quartz + or - anhydrite, and pyrite, with traces of K-feldspar, chalcopyrite, sphalerite, and galena. They are associated with calcite + sericite + pyrite alteration of the plug and calcite, dolomite, and phlogopite alteration of the metasedimentary rocks. Late 3 and late 4 veins often occupy almost vertical fractures parallel to the local north-northwest joint trend. Late 5 veins contain calcite, quartz, laumontite, and apophyllite, with traces of K-feldspar, chlorite, molybdenite, chalcopyrite, and pyrite or pyrrhotite. Traces of scheelite occur in late 1, late 3, and late 5 veins and in quartz + garnet veins which predate all sulfide mineralization.A model is proposed whereby emplacement of a pluton generated a convective flow of water which metamorphosed the overlying strata to form the aureoles. The Copper Mountain and Porphyry Mountain plugs, apophyses of the pluton, intruded the metasedimentary rocks and superimposed further metamorphic and metasomatic effects on them. Early mineralizaotion, like the prograde metamorphism, is widespread throughout the aureoles (although very weak in most areas). Late mineralization, by contrast, is closely associated with the Copper Mountain plug. The succession of widespread and spatially restricted events suggests that the postulated pluton and the visible plugs alternately controlled the sequence of metamorphism, iron metasomatism, and early and late mineralization. The difference in zoning patterns between Mines Gaspe and other porphyry copper-skarn complexes was probably due to the fact that the Copper Mountain orebody formed in previously decarbonated wall rocks.