Sudbury, Ontario Massive Nickel Ores Illustrating Magmatic Textures



Massive pyrrhotite (Po; tan and yellow) and pentlandite (Pn; white) Sudbury massive ores in reflected light under crossed polars. Pyrrhotite is strongly anisotropic and clearly shows its grain boundaries and variations in colors of anisotropism under crossed polars. These ores have a granular texture that illustrates the character of massive magmatic textures in which the grains are closely packed, crudely ovate, and with triangular junctions. Pentlandite grains are intergrown with pyrrhotite and constitute the main nickel mineral for which these ores are mined, although many by-product metals also are recovered during the metallurgical treatment of these ores. Sudbury nickel district, Ontario. Ore microscopy, reflected light, crossed polars.



Some of the ores from the Sudbury nickel district contain significant amounts of cubanite (CuFe2S3). Cubanite forms a complete solid solution series with chalcopyrite, and the single solid formed at high temperatures may exsolve with declining temperatures. Exsolution lamellae of cubanite in chalcopyrite are shown in the photomicrograph. Falconbridge deep copper ores, Sudbury nickel district, Ontario. Ore microscopy, reflected light.

Adirondack Stratiform Massive Volcanogenic Pyrrhotite-Pyrite Ores Illustrating Supergene Alteration Textures



The pyrrhotite-pyrite ores of the Adirondack Mountains are massive stratiform ores that probably formed as volcanic exhalations on the Precambrian sea floor, and they have subsequently been subjected to high-grade regional metamorphism (sillimanite zone, amphibolite facies). The ores exhibit a metamorphic granular texture (much like that of the Sudbury ores) with local development of euhedral pyrite metacrysts that are much larger than the dominant pyrrhotite grains. This photomicrograph has been selected to illustrate the effects of initial supergene alteration of those ores. Bird's eye texture is shown in which supergene pyrite-marcasite (yellow) has been deposited by replacement of pyrrhotite at the margin of a pyrite crystal. The bird's eyes tend to be shaped like a loaf of bread, and they typically exhibit colloform internal textures. They are comprised of very fine-grained mostly pyrite with some marcasite, and they represent an intermediate product in the oxidation of pyrrhotite to iron oxide-hydroxide, goethite. A single vein of geothite (Goe; dark grey) traverses both the pyrite crystal and the bird's eye. Although these iron sulfide ores contain minor amounts of copper, lead, and zinc, they were formerly mined for their iron sulfide contents for use in the paper industry of the Adirondack Mountains in New York. Adirondack stratiform massive pyrrhotite-pyrite district. Ore microscopy, reflected light.



More advanced supergene alteration of pyrrhotite (Po; yellow) to goethite (Goe; grey). Along the margins of the goethite veins, an intermediate product (IP) has developed from pyrrhotite before the formation of goethite. The intermediate product consists of supergene pyrite and marcasite. The goethite veins form a net texture in the pyrrhotite. Adirondack stratiform massive pyrrhotite-pyrite district. Ore microscopy, reflected light.

Pea Ridge, Missouri Precambrian Iron Ores Illustrating Hydrothermal Replacement Textures



The Precambrian iron ores at the Pea Ridge iron mine in Missouri are comprised mainly of magnetite and hematite. Magnetite was deposited as a magmatic injection and the ores exhibit a granular texture much like that of the Sudbury ores. Although the magnetite orebody contain minor amounts of early specular hematite, the principal development of hematite was the result of subsequent hydrothemal fluids. This photomicrograph has been selected to show the textural character of hydrothemal replacement of magnetite (Mt; brownish grey) by hematite (Hm; white). The initial hematite replacement occurs especially along the grain boundaries of the magnetite and to a lesser extent along the octahedral crystallographic planes of the magnetite. The black areas are holes in the surface of the polished section. Pea Ridge iron mine, Southeast Missouri Iron Province. Ore microscopy, reflected light.