Melt-melt immiscibility and implications for the origin of Madeira albite-rich granite, Pitinga mine, Amazonas, Brazil: A melt inclusion study

Abstract

Presented here are the results of the first studies of silicate melt inclusions hosted in quartz crystals from the two late granitic facies of the Paleoproterozoic Madeira pluton in the Pitinga mine, State of Amazonas, Brazil. The study of these magmatic inclusions in the anorogenic granites of Pitinga furnished reliable results because the original composition of the trapped silicate liquids is well preserved. Petrographic, morphological, and chemical data show that both porphyritic hypersolvus alkali feldspar granite and albite-rich granite were formed by melt-melt immiscibility from a precursor magma. After phase separation, albite-rich granite evolution was characterized by strong enrichment in F, Na, Sn, Nb, Ta, Zr, Th, Y, Rb, and REE compared to the porphyritic granite, based on data obtained from analyses of both trace elements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and major elements by electron microprobe. These elements make up the polymetallic magmatic ore of the Pitinga mine. The contrasting evolutionary trends, peralkaline and peraluminous, stand out in the different geochemical signatures of the analyzed melt inclusions. The preliminary results of our studies on melt inclusions trapped in rare metal granite minerals from Sn-polymetallic deposits in Brazil suggest abundant possibilities for using this methodology in future petrological and metallogenic research.

KEYWORDS:
melt inclusions; melt-melt immiscibility; LA-ICP-MS; electron microprobe; peralkaline albite-rich granite; Pitinga mine