Fluid-inclusion characteristics of hydrothermal Cu–Ni–PGE veins in granitic and metavolcanic rocks at the contact of the Little Stobie deposit, Sudbury, Canada

Abstract

Cu- and precious-metal-enriched massive ore and veins occur at contacts of two Fe–Ni–Cu sulfide orebodies of the Little Stobie Mine, Sudbury, with metavolcanic rocks of the Elsie Mountain Formation, and the Murray granite. Veins contain chalcopyrite, pyrrhotite, pentlandite, platinum-group minerals, quartz, carbonate, chlorite, amphibole and other minerals. A granitic dike, resulting from partial re-melting of the Murray Granite, cuts back into the Sudbury Igneous Complex near Little Stobie Mine, and contains barren veinlets with similar mineralogy but no sulfides. Quartz from ore veins contains fluid inclusions that were trapped during several stages. Early high-temperature fluids (at least 180–270°C in orebody 1 and 280–350°C in orebody 2) were extremely saline and occur as polyphase, isolated and fracture-controlled inclusions with halite, sylvite, Fe–Mn-, as well as Pb–Ba-chloride daughter minerals and other unknown solids. Late secondary aqueous inclusions are not chloride-saturated; they were trapped at a minimum of 80–150°C. Their microthermometric behavior may be modeled in the CaCl 2–NaCl–H 2O system with salinity of 21–27 CaCl 2 equiv. wt.%. Very late secondary inclusions have either Ca-rich saline, or very dilute (about 1 equiv. wt.% NaCl) compositions and homogenization temperatures of 200–300°C. With the exception of these very late secondary inclusions, the association of CO 2–(CH 4)-rich inclusions with aqueous ones was usually observed. The density of these carbonic fluids decreased from early to late stages. Microthermometric data from barren veins are fundamentally different from those of early inclusions from orebodies; this implies that these heavy-metal-rich fluids were responsible for ore deposition in veins. The minimum pressure of entrapment for early fluids was 1800–2200 bars. Late Ca-rich brines were trapped at lower minimum pressure (200–900 bars). High-pressure data are in agreement with the estimated minimum paleodepth of crystallization of South Range ores. Later fluids were probably trapped during the uplift of orebodies and their host rocks. Comparison of data to other Cu–PGE–Au-rich ore of the Sudbury Structure suggests that the presence of CO 2-rich fluids in South Range deposits and their absence in North Range deposits may be related to different metamorphic histories. The high-temperature hydrothermal fluids were driven by the heat of the Sudbury Igneous Complex; these very saline fluids interacted with primary magmatic ores, remobilized metals and redeposited them along convenient structures such as fracture zones and breccias in and along various units near the footwall contact. The identification of such highly saline fluid inclusions with high heavy-metal content may be useful in the exploration for Cu–PGE–Au-enriched, footwall vein-type ores in the Sudbury Structure.