The zircon-thorite mineral group in metasomatized granite, Ririwai, Nigeria 1. Geochemistry and metastable solid solution of thorite and coffinite

Autor: C. M. Pointer, R. A. Ixer, J. R. Ashworth
Rok vydání: 1988
Předmět:
Zdroj: Mineralogy and Petrology. 38:245-262
ISSN: 1438-1168
0930-0708
DOI: 10.1007/bf01167091
Popis: At Ririwai, Nigeria, a biotite granite has suffered extensive post-magmatic metasomatism to produce albitized, microclinized, and greisenized rocks; the latter two lithologies form the wallrocks adjacent to tin-zinc vein-style mineralization. All the rock-types carry accessory amounts of thorite, locally accompanied by trace amounts of paragenetically late coffinite and xenotime. Petrographical and electron-microprobe data are presented for these minerals. In the biotite granite, thorite and coffinite range to high Zr contents (25.1 wt% ZrO2 in thorite and 11.7 wt% in coffinite), exceeding the highest values previously reported in both minerals. In the lode wallrocks, yttrium contents range up to 13.9 wt% Y2O3 in thorite and 15.9 wt% in coffinite. From stoichiometric considerations, both minerals are inferred to contain OH. Petrographically, the thorites of the lode wallrocks are distinctive, and they are interpreted as having grown from the metasomatizing fluids. The albitized rocks and some of the lode wallrocks contain the thorites with the highest U contents, including one example of continuous zoning to coffinite. The unusual mineral compositions are interpreted as metastable solid-solution ranges formed from fluids that transported U, Th, Y and, over short distances, Zr.
Databáze: OpenAIRE
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