Mg-rich mica and tourmaline in pink-colored “parelhas quartzite” intercalations in the equador formation, Borborema pegmatite province, NE-Brazil: record of evaporitic protolith?

Abstract

Light gray, green to pink-colored banded quartzite intercalations in the NeoproterozoicCEquador Formation (Seridó Group), Seridó Fold Belt, Northeast Brazil, were exploited during several decades as dimension stones under the fashion name “Parelhas Quartzite” (PQ). They can be easily sliced along intensively foliated mm-thick laminae with relatively high content of light green or pink-colored mica, in alternation with more massive, a few cm-thick bands with lower mica contents. The variable ratios between pink and light green micas in the partition laminae, define the color of the produced rock plates, used as outdoor floor or wall coating. Massive, quartzites, meta-arcoses and meta-conglomerates of the Equador Formation (EF) are the exclusive host rock of a few Li-bearing rare element class granitic pegmatites of the Borborema Pegmatite Province, mined for the prized gemstone known as “Paraíba Tourmaline”. The EF hosts also nearly 10% of the Be-Li-Ta-bearing pegmatites of the same province. Pink-colored tourmaline in the form of millimeter- to decimeter-long crystals occur as prominent accessory mineral in the pink-colored PQ and are empirically described by miners and field geologists as “rubellite” and the pink-colored mica as “lepidolite”. The PQ could therefore be considered as a potential source of Lithium. The tourmaline crystals occur isolated or as radial arrangements, always flattened and oriented in the foliation planes. Light green micas are dominant in the greenish-gray PQ and are always referred in the literature as common muscovite. Tourmaline with the same textural relations as the pink-colored counterpart may also occur in the gray PQ plates but in this case is always black. Electron microprobe analyses of the pink mica and tourmaline allowed establish that they are respectively Al-rich phlogopite and magnesiofoitite (this one preferentially occurring in the crystal cores) with transition to dravite and uvite (rims). Both, mica and tourmaline present insignificant lithium contents. The light green micas were identified as the Fe-Mg-rich variety of muscovite known as “phengite”). The PQ can therefore be discarded as possible source of Li for economic exploration. They can also be excluded as source for Li-enrichment in pegmatites by “in situ” anatexis of quartzite or metasomatic assimilation of ascending pegmatite forming melts. There is also no support for the hypothesis of the genesis of the pink-colored mica and tourmaline by pegmatite-related exomorphic alteration. The most probable origin of the B- and Mg-enrichment in the PQ is related to syn-sedimentary evaporitic or volcano-exhalative incursions during the deposition of the original sediments.