Metamorphic textures in Archean copper-zinc massive sulphide deposits

CIM Bulletin, Vol. 73, No. 816, 1980

C.J. ROCKINGHAM and R.W. HUTCHINSON, University of Western Ontario, London, Ontario

A comparative study of sulphide textures has been carried out on samples from four Archean copper-zinc massive base metal sulphide deposits: Kidd Creek, Ontario; Lake Dufault -Norbec, Quebec; Hood River-41, N. W. T.; and Geco, Ontario. These deposits have undergone metamorphic recrystal-lization under a range of conditions, and results of this study indicate that certain sulphide textures may be used as qualitative indicators of metamorphic grade, whereas other cannot.The sulphide textures of most value in this regard are sulphide grain sizes, which increase with increasing metamorphic grade, and the presence or absence of sedimentary-/diagenetic textures, which are destroyed at the lower am-phibolite fades of metamorphism. Chalcopyrite and pyr-rhotite intergrowths in sphalerite increase in size, in degree of coalescence and in the ratio of unmixed pyrrhotite to chalcopyrite inclusions with increasing metamorphic rank. With increased metamorphic grade, the pyrrhotite intergrowths change from rounded inclusions on grain boundaries to rod-like shapes along sphalerite twin planes. There is also a general increase in the relative proportion of pyrrhotite in the deposits. Textures that appear to have little value as indicators of metamorphic grade are sulphide deformation textures, and triple junction point angles and their standard deviations, which show little variation from the lower greenschist to the upper amphibolite fades of metamorphism. The presence or absence of myrmekite-like magnetite also has little value in determining the relative metamorphic history of these deposits, as it is a product of local contact metamorphism.

Keywords: Geology, Ore deposits, Metamorphic textures, Archean rocks, Copper deposits, Zinc deposits, Sulphide deposits, Massive sulphide deposits, Kidd Creek, Lake Dufault, Norbec, Hood River, Geco.