Sedimentation, economic enrichment and evaluation of heavy mineral concentrations on the southern Oregon continental margin Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pc289m70q

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  • Heavy minerals can contain potentially economic amounts of metals as both matrix and trace constituents. Such minerals appear as unconsolidated black sands on the continental shelf off southwest Oregon and along the Oregon coast. Two diverse energies are considered in this investigation. Environmental energy of the depositional regimen, Part I; energy involved in crystallization of transition metals from a magma, Part III. In Part II, an analytical scheme for the evaluation of opaque oxides is proposed, and an examination of the results as applied to two samples is presented. Part I The unconsolidated black sands on the Oregon continental margin have been profoundly affected by tectonic uplift aid by cyclic erosive transgression and regression. Progressive enrichment in heavy minerals from the Klamath Mountains has apparently occurred during each glacio-eustatic regression of the Pleistocene seas, each regression a period of intensified erosion and sediment transport. Subsequent erosive transgressions selectively sort and redistribute these heavy minerals into paralic beach and nearshore deposits. Uplift of the coast and shelf implies that the heavy minerals were reworked during the Holocene transgression into concentrations of greater extent and higher ore tenor than relict deposits of earlier transgressions in upraised Pleistocene terraces. Extrapolation of ore reserve values from the terraces by "Mirror Image" concepts might seriously underestimate the potential of offshore deposits. Offshore heavy mineral concentrations should be coincident with observed submarine terraces. Part II An analytic scheme was developed to investigate opaque oxides in two samples; one from the Pleistocene terraces; the other from near the present shelf edge. Analyses involving X-ray diffraction techniques, atomic absorption and neutron activation established the mineralogy and elemental distribution in magnetically separated diagnostic splits. Chrome spinel, ilmenite and magnetite comprise the opaque oxide fraction in both samples. Correlation studies of these analyses suggest: (1) Chromium is a matrix metal of chrome spinel and is diadochic into magnetite. (2) Iron appears in all opaque oxides and in increasing amounts with increasing magnetic susceptibility. (3) Titanium is a matrix metal in ilmenite, and diadochic into chrome spinel and magnetite. (4) Nickel and ruthenium are diadochic into and correlated to the spinel structure; i.e. to chrome spinel and magnetite. (5) Osmium appears to be correlated to chromium. (6) Zinc is limited to spinel in these samples. Part III Goldschmidt's and Ringwood's criteria for diadochy often fail to explain the distribution of the transition metals because crystal field effects are not considered. Favored d[superscript n] configurations, e.g. octahedrally coordinated, low spin d⁶ cations in the spinel minerals, result in shortened interatomic distance and significantly strengthened cation-ligand bonds, possibly affecting the distribution of such metal cations. The octahedral site preference energy parameter (OSPE) has been used to explain distributional behavior of the first (3d) transition series metals. OSPE calculations for four low spin d⁶ cations - Co(III), Ru(II), Rh(III), and Pt(IV) - give significantly high values for this parameter. High OSPE valued transition metal cations possibly form stable proto-mineral oxide complexes in the magma which persist through crystallization. These associations predetermine the enrichment of transition metals in oxide minerals and act as nuclei during cooling and solidification. Subduction of oxidized and hydrolyzed near-surface rocks down a Benioff zone provides progressively higher Eh in the magma, a variety of cation oxidation states, and water for sepentinization of ultramafic rocks. The distribution of the platinum metals in a strongly reducing magma environment should be different than in the oxidizing magma proposed for the Klamath ultramafics. The OSPE parameter offers an explanation for the observed distribution of platinum group metals in spinel minerals from this investigation, in chromites from Uralian dunitic massifs and the Stillwater complex; and of iridium from the Great Lake Doleritic Sheet, Tasmania. Chrome spinel from Oregon had twice the concentration of ruthenium, and one-third the amount of osmium as similar Uralian chromite deposits. The first significant concentration of ruthenium in magnetite is herein reported recommending continued research into the platinum metal distribution in southwest Oregon.
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