PETROGRAPHY, SULFIDE MINERALOGY AND DISTRIBUTION, MASS TRANSFER, AND CHEMICAL EVOLUTION OF THE BABBITT COPPER-NICKEL DEPOSIT, DULUTH COMPLEX, MINNESOTA.

摘要

The Babbitt Cu-Ni deposit is one of several located within troctolitic and noritic rocks of the Duluth Complex. Sulfide mineralization is somewhat erratically distributed, but two major sulfide zones are identifiable at Babbitt, basal mineralization and cloud zone. Mineralization is typically dissiminated in form, with sulfide volume percentages ranging from less than 1 to 30. Massive mineralization is most common within a spacially restricted area of the basal zone. Sulfide assemblages consist of pyrrhotite, cubanite, chalcopyrite, pentlandite, and minor amounts of troilite, sphalerite, and bornite. Sulfide mineralization at Babbitt is copper-rich, with Cu/Ni ratios in the range 4 to 8:1.;Compositional characteristics of the Babbitt deposit may be explained by a combination of variables such as differences in magma metal content and the ratio of silicate to immiscible sulfide liquid, fractional crystallization, and local equilibration of sulfides and mafic silicates.;The igneous rocks in the Babbitt area are mainly composed of troctolites, norites, olivine-gabbro, augite troctolite, and anorthosite. The igneous mineralogy includes plagioclase, olivine, clinopyroxene, orthopyroxene, biotite, ilmenite, with minor amounts of magnetite, apatite, and graphite.;Contamination of troctolite by country rock material is reflected in the increase in orthopyroxene and biotite near the basal contact and around metasedimentary xenolith.;Mineralogical and textural features indicate that limited partial melting and dehydration had occurred in xenolith. Calculations of gains and losses of elements, indicate that the xenoliths have been depleted in Si, K, Na as well as volatiles such as S, C, and H(,2)O relative to unmetamorphosed protoliths. Increases in silica content of the melt due to transfer from xenoliths are thought to have been marked by homogenezation caused by convective movement and the influx of a more primitive silica-poor magma.;The distribution of sulfides and orthopyroxene, and lack of zones of Ni-depletion in the magma suggest that SiO(,2) addition has not been a primary control on sulfide saturation. Separation of immiscible sulfide droplets at depth within an auxiliary magma chamber is most consistent petrochemical data.