Metamorphism, structure and petrogenesis of the Linda volcanogenic massive sulphide deposit, Snow Lake, Manitoba, Canada

Abstract

The complex structure and mineralogy of the Linda volcanogenic massive sulphide deposit are the result of polyphase dynamometamorphism superimposed on primary sulphide mineralization and associated hydrothermal alteration. Outcrop mapping and drill-core logging elucidated the 3-dimensional configuration of the deposit. Microstructural and paragenetic relationships, mineral chemistry, and wholerock geochemistry constrain its petrogenesis and its significance to the structural and metamorphic history of the Snow Lake region. At the Linda deposit, two discrete zones of Fe-Mg alteration occur in felsic rocks of the stratigraphic footwall. The proximal alteration zone consists mainly of stratabound and stratiform mineralization resulting from the accumulation of chemical sediments in a topographic depression on the down-thrown side of a synvolcanic fault. The distal alteration zone is hosted bv felsic volcanic rocks lower in the stratigraphic sequence and may represent subsurface alteration. The Linda deposit lies in highly strained overturned rocks on one flank of the Anderson Bay structure, which marks a change in stratigraphic facing direction. The Anderson Bay structure is transected by S1 and S2 cleavages. Syn-D2 porphyroblasts at the Linda deposit record early sinistral S1/S2 cleavage vergence; phyllosilicate minerals in the matrix define dextral S1/S2 cleavage vergence resulting from late D2 (or D3) reactivation of the S1 cleavage. The reactivation has regional significance. Metamorphic parageneses at the Linda deposit apparently span three zones of regional metamorphism. The bulk-rock composition of the altered protolith allowed the crystallization of manganiferous garnet, zincian staurolite and F-rich biotite, thus expanding the stability fields of parageneses containing these minerals. A pressure-temperature-time trajectory is recorded by syn-D1 kyanite and by syn-D2 amphibotite-facies assemblages, which represent the culmination of metamorphism. Sphalerite geobarometry yields a pressure of about 5 kb; the maximum temperature is limited to about 550*C by the coexistence of margarite+quartz. Suiphide-silicate-oxide equilibria can be modelled in a petrogenetic grid in the SiO2- Al2O3-FeO-MgO-ZnO-K2O-H2O-S2-O2 system at fixed pressure and temperature. Metamorphic desulphida.tion of sphalerite released Zn to staurolite and gahnite. The largest massive sulphide body consists of pyrite and calcite, the latter introduced during post-tectonic synmetamorphic metasomatism.