| Description |
xii, 128 leaves : illustrations (some color) ; 29 cm |
| Summary |
"Drill core samples from the Boss-Bixby copper-iron-cobalt deposit were examined by reflected light microscopy and electron probe microchemical analysis (EPMA). These techniques were used to identify the metallic minerals and the paragenesis of the ores, and to study the elemental variations in specific minerals. Many of the minerals identified in this study were also identified in previous studies. These include magnetite, hematite, ilmenite, rutile, martite, garnet, quartz, molybdenite, cobaltian pyrite, carrollite, galena, sphalerite, chalcopyrite, bornite, chalcocite, covellite, epidote and fluorite, in addition to the minerals in the host rocks. Minerals identified for the first time at Boss-Bixby include pyrophanitic ilmenite, scheelite, anatase, clausthalite, cosalite, hessite, electrum and enargite, as well as several qualitatively characterized phases including a Pb-BiSe- Te mineral, a bismuth telluride, and a cerium-(lanthanum?) - phosphorous mineral (possibly monazite). Textural relationships indicate that two stages of Fe-Ti oxides formed in the deposit, prior to and following brecciation. The early stage consists of widely variable mixtures of hematite-ilmenite exsolution intergrowth, magnetite, and rutile disseminated in the rock fragments. This suite may have formed during magmatic crystallization products or potassic metasomatism. The second stage formed as open space fillings and replacements and is dominated by magnetite, with lesser amounts of hematite-ilmenite, orthoclase and rutile. Following the main period of oxide deposition, andraditic garnet, scheelite, quartz and molybdenite were formed. Sphenitization of hematite-ilmenite and the martitization of magnetite also formed after the period of oxide deposition and prior to sulfide deposition. The cobaltian sulfides, cobaltian pyrite and carrollite, were deposited next. The pyrite commonly is zoned with cobalt-rich centers and less rich rims, but oscillatory zoning was detected in some pyrite crystals. EPMA analysis of carrollite indicate that the carrollite is cobalt-rich and that cobalt content increases with depth. Since stoichiometric carrollite is not stable above 500°C, the EPMA data may indicate that temperatures were greater at depth than near the surface. The deposition of sphalerite, galena, cosalite, hessite, clausthalite, Pb-Bi-Se-Te, electrum, and enargite followed carrollite deposition, and preceded the introduction of chalcopyrite and bornite. Thermal stability data for cosalite indicate a maximum temperature of 450°C at the time of its crystallization. After the chalcopyrite and bornite precipitated they developed intergrowths resembling exsolution and replacement textures. It is believed that these textures are the result of true exsolution and possibly "oxidation exsolution". The minerals that subsequently formed indicate that slightly oxidizing conditions occurred during in the late stages of ore deposition. These minerals including epidote, bornite, hematite and anatase. Precambrian erosion and exposure of the ores resulted in oxidation of the iron-titanium minerals and leaching of the sulfides without producing a zone of secondary enrichment. EPMA analyses of magnetite from both oxide stages are nearly pure, containing less than one molecular percent ulvospinel. Monomineralic ilmenite crystals contain up to 50 molecular percent pyrophanite and thus could not be used for geothermometryjoxygen barometry determinations"--Abstract, leaves ii-iii. |
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