Metamorphic features in North American massive sulfide deposits

Abstract

Many massive sulfide deposits in North America show textures and structures that are deformational in origin. These features have been superimposed on original mineralogical ones to the extent that in many deposits the essential characteristics necessary to elucidate the conditions of primary genesis no longer exist. The massive sulfide deposits have been metamorphosed with their walls, thereby destroying much of the evidence of wall-rock alteration, if it existed at all.Thus, characteristically a metamorphosed ore shows a granoblastic texture; complex and paragenetically inconsistent grain-boundary relationships; the porphyroblastic growth of some minerals, and the re-equilibration of others under metamorphic conditions; and the migration of minerals like chalcopyrite in a stress-field.The least altered deposits studied by the writer are the Kam Kotia Mine, Ontario, and the Anacon Mine, N. B. Those showing the greatest effect of a metamorphic imprint are Heath Steele, N. B., but especially the Appalachian pyritic deposits extending from Maine to Ducktown. On the basis of a literature survey the writer proposes that the Horne Mine, Quebec, and the Thomson Lake nickel deposits in Manitoba also predate the last period of metamorphism.Concerning New Brunswick ores, the writer concludes from his work and the review of existing data that: (1) the ores were fine-grained originally; (2) one cannot deduce the sequential deposition of minerals, or introduction of metals, from a study of their textures; (3) the O 16 /O 18 equilibrium temperature (515 degrees C) on a quartz-magnetite pair from the country rock indicates the possibility of fairly high temperatures during the regional metamorphic episode; (4) the ores may be of Ordovician age; (5) the ores may have been deposited, or dumped, under slight lithostatic or hydrostatic load, forming replacements in consolidated or unconsolidated sediments.