Doctoral Dissertations

Abstract

"Field observations, petrography, electron microprobe analysis (EPMA), geothermometry, and geochemical analyses (ICP-MS and XRF) of the fluorspar deposit associated with the Late Cretaceous alkaline igneous-carbonatite complex at Okorusu (Namibia) have identified two previously unrecognized types of carbonatites, namely pyroxene and pegmatitic carbonatites. These carbonatites exhibit interesting textural characteristics, with the pyroxene carbonatite exhibiting occasional bands of diopside alternating with coarse-grained calcite-rich bands, and pegmatitic carbonatite having the same mineralogy but much coarser texture. Both types of carbonatites are closely spatially associated.

Ülvospinel exsolution lamellae were recognized in magnetite crystals within the pyroxene and pegmatitic carbonatite. Stable isotope determinations for calcite crystals separated from pegmatitic carbonatites, pyroxene carbonatites, and marbles indicate that the carbonatites are primary in origin.

Cathodoluminescence microscopy (CL) and emission spectrography of the carbonatites indicated that the carbonate mineral is almost entirely calcite rather than dolomite, and there are at least two generations of calcite. CL study of fenites, which are metasomatised Precambrian metasedimentary rocks, intruded by carbonatites reveal that fenitization is mostly incipient, marked by the introduction of Fe3+ activated feldspars. Geothermometric determination from EPMA of apatite and biotite in pyroxene carbonatite provided a range of 537-409⁰C except in one sample which clearly indicated later hydrothermal alteration. The temperature range is similar to a previous titaniferous magnetite-ilmenite temperature determination and is interpreted to represent magmatic crystallization. Trace element patterns of carbonatites are largely consistent with the results of previous studies, although phosphorus values are anomalously high because of apatite.

As one of only two producing carbonatite-related fluorspar deposits in the world, the host rock-ore relationships of the Okorusu fluorspar deposit were identified and characterized. The recognition of certain fluorite ore textures, together with replacement remnants of carbonatite in fluorite ores, has shown that the fluorite ore bodies have formed largely by the replacement of the pyroxene and pegmatitic carbonatite. Fluid inclusion geothermometry of purple and green vug-filling shows that fluorite crystallized over a temperature range of 168-144 ⁰C (uncorrected for pressure) and from fluids with a salinity of 1.57-4.96 wt % NaCl eq. Beneficiation problems, such as the presence of phosphate, rare earth elements and silica in the final fluorspar concentrate, have resulted from those replacements"--Abstract, pages iii-iv.

Advisor(s)

Hagni, Richard D.

Committee Member(s)

Gregg, Jay M.
Wronkiewicz, David J.
Hogan, John Patrick
Miller, F. Scott, 1956-
Mariano, Anthony N.

Department(s)

Geosciences and Geological and Petroleum Engineering

Degree Name

Ph. D. in Geology and Geophysics

Sponsor(s)

National Science Foundation (U.S.)
Society of Economic Geologists (U.S.)

Publisher

University of Missouri--Rolla

Publication Date

Spring 2003

Pagination

xvi, 213 pages

Note about bibliography

Includes bibliographical references (pages 199-212).

Geographic Coverage

Namibia

Time Period

Precambrian

Rights

© 2003 Purnima Ashok Shivdasan, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type

text

Language

English

Subject Headings

Geology -- NamibiaFluorspar -- NamibiaPyroxene -- NamibiaPegmatites -- NamibiaCarbonatites -- Namibia

Thesis Number

T 8263

Print OCLC #

53966534

Electronic OCLC #

936337431

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