Abstract

In the last twenty years investigators have cast doubt on the metallic oxides being among the first minerals to crystallize in a cooling magma, in the case of the oxide ores in basic and ultra-basic rocks. They have considered them to be "late magmatic," late-phase injections, and even hydrothermal.Support to this interpretation is supplied by the microscopic study of some titaniferous magnetite ores occurring in a massive anorthosite body of variable facies, located in the San Gabriel Mountains, Los Angeles Co., California. These ores are thought to be deuteric (pneumotectic) in origin, for the following reasons : (1) They occur in an igneous rock type with which similar deposits are commonly associated, and therefore are not hydrothermal. (2) Their relationships, under the microscope, show that they are later than feldspar, pyroxene, apatite, mica, titanite, and in part, at least, later than hornblende, epidote and clinozoisite. (3) They are associated with deuteric phenomena, such as albitization and reaction rims and with late-forming minerals such as biotite and amphibole. (4) They are found in zones of granulation and alteration in the anorthosite.Not only are titaniferous magnetites considered to be deuteric, but the suggestion is made that the ore comprising titaniferous magnetite, pyroxene and apatite, may represent a pegmatitic phase of the anorthosite. The relationship of these minerals to one another, and the association of the apatite in specimens of altered anorthosite, together with long-recognized features of certain basic igneous rocks, all support this hypothesis.

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