PACROFI VI - Electronic Program

Heavy-metal rich, highly saline fluid inclusions from the Cu-rich footwall contact, Little Stobie Ni-Cu-PGE Mine, Sudbury, Canada

Ferenc Molnar

(Department of Mineralogy, Eotvos L. University, Budapest, Hungary)

David H. Watkinson and John O. Everest

(Department of Earth Sciences, Carleton University, Ottawa, Canada)

The Little Stobie Mine (INCO Ltd.) consists of two Ni-Cu-PGE orebodies (Davis 1984); one is composed of pods and lenses of pyrrhotite-pentlandite-chalcopyrite in the Contact Sublayer between norite of the Sudbury Igneous Complex, and the footwall Murray granite and metabasalt of the Elsie Mountain Formation. Some Cu-enriched vein-like protrusions cut the footwall. Orebody 2 occurs in footwall units along a brecciated zone between the Murray granite and the greenstone. In the vicinity of orebody 2, greenstones are transected by quartz-rich veins. Stringers and veins are enriched in chalcopyrite and PGM (sperrylite, michenerite, moncheite) with galena and Au and Ag tellurides. Some sulfide assemblages coexist with fragmented quartz grains or are intergrown with quartz and tourmaline. The stringers and veins have selvages of quartz-epidote-chlorite-amphibole-K-feldspar. Barren veins with quartz and minor amounts of calcite, K-feldspar and chlorite cut granitic dikes in the Little Stobie Mine area; the dikes may result from remobilization of the Murray granite during emplacement of the Sudbury Igneous Complex. Quartz grains from sulfide-bearing veins contain at least four major types of fluid inclusions (Fig.1). Most fluid inclusions are secondary, occurring in fractures cutting grain boundaries; some isolated type III and CO2-rich inclusions also occur. CO2-rich inclusions occur in fractures with aqueous inclusions; variability in the volume of CO2-rich phase suggests inhomogenous trapping conditions.

SEM-EDS analysis of opened inclusions revealed abundant cubic or round halite, sylvite, acicular Fe,Mn hydroxychloride, and round or bipyramidal-rhombohedral Pb and Ba chlorides. Minor Ca chloride was also detected in analyses. Irregular grains of chalcopyrite as well as pyrrhotite or pyrite were also found as accidentally trapped solids.

Type IIA secondary inclusions homogenized to the liquid phase most commonly between 70 and 130oC and type IIB inclusions between 190 and 290oC. Their freezing behaviour with Te between -52 and -68oC, cotectic melting at about -45oC and Tm ice values between -22 and -35oC are compatible with NaCl-CaCl2-H2O type fluids with salinities between 21 and 27 CaCl2 wt%. Na/(Na+Ca) is less than 0.1.

During heating of type III inclusions solid phases usually melted at lower temperatures than did halite, but in a few inclusions the SHRI phase melted at a slightly higher temperature. Homogenization of the vapor phase occurred between 100 and 270oC. Th vapor versus Ts halite diagrams show significant differences between the samples from sulfide-bearing and barren veins.

Most type III inclusions failed to nucleate ice during undercooling; when freezing was achieved, eutectic temperatures were -64 - -78oC, ice melting occurred at -21 - -45oC, and metastable hydrohalite melting was above 0oC. The estimated apparent salinities are 39 - 49 NaCl+CaCl2 wt% equivalent. True concentration cannot be calculated due to metastability (lack of hydrohalite nucleation), the mode of homogenization (Ts halite>Th vapor) and the presence of Fe, Mn, Pb and Ba chlorides. In inclusions with SN phases, freezing occurred between -30 - -60oC and yellowish-greenish anisotropic assemblages appeared under crossed polars. These solids did not resorb the halite phase, but slight resorption of the SN phase was observed in a few runs. Melting began at about -35oC, but abundant liquid was developed only between -20 and -24oC. The anisotropic phases melted reproducibly between +10 and +35oC. Ice was not observed. Behaviour of these inclusions may be explained in the CaCl2-rich part of the NaCl-CaCl2-H2O system (Schiffries, 1990), but SEM-EDS data suggest that FeCl2*6H2O, FeCl2*4H2O or MnCl2*4H2O phases were formed instead of antarcticite.

Pure CO2 inclusions show different microthermometric data as a function of mode of occurrence. Tmelt CO2data slightly below -56.6oC imply the presence of other species, such as methane. The synchronously trapped pure CO2inclusions and CO2-free aqueous inclusions may represent the end-member fluids of the immiscible system. The minimum pressure for entrapment of type III inclusions was estimated by the pressure range corresponding to halite homogenization temperatures on the isochores of type IVA inclusions. This range is 1.8-2.2 kbars for single inclusions, 1.5-1.7 kbars and 0.7-1.2 kbars for fracture-hosted inclusions. Intersecting isochores for type IVA and type IIA inclusions indicate a pressure range between 0.2-1 kbars. This is compatible with entrapment during uplift.

Similar suites of highly saline fluid-inclusion types exist in quartz and sulfides at the contact of Ni-Cu-PGE-rich sulfides and their host Murray granite at the nearby Lindsley Mine (Watkinson, 1994). Hydrothermal veins mined for Cu, Ni and PGE in the North Range, Sudbury, have similar types of fluid inclusions, but lack the CO2-rich inclusion type (Farrow and Watkinson, 1992). Cu-Ni-PGE-rich vein and disseminated mineralization is accompanied by Cl-bearing hydrothermal alteration mineral assemblages; their occurrence is not a local phenomenon in the Sudbury Structure. The results suggest that high-temperature, high-pressure, very saline solutions, interacted with primary magmatic ores and host rocks, and redistributed metals into stringers and veins at the base of the Little Stobie orebody. The detection of heavy metal-rich, polyphase inclusions may be useful in exploration for enriched ores in footwall environments of other magmatic Ni-Cu-PGE deposits.