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| | B 2143 Part I |
 | | The regional distributions of the Proterozoic Bayan Obo Group, the Archean metamorphic rocks of the Wutai Group, which unconformably underlies the Bayan Obo Group, and granitic rocks of Archean, Caledonian (early to late Paleozoic), and Hercynian (Permian) ages in the region are shown in figure 1. |
 | | The uppermost H9 unit of the Bayan Obo Group, which is the cap rock of the Bayan Obo orebodies, consists of dark-gray to fl shale, slate, biotite schist, and albitized biotite schist. |
 | | The key evidence for the epigenetic origin of the Bayan Obo ore deposit, in addition to the mineral textural relations, lies in the precise ages of episodes of mineralization of various generations of monazites and bastnaesites, and the minimum mineral ages of alkali amphiboles (fig. |
| pubs.usgs.gov /bul/b2143/part1.html (11966 words) |
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