The world-class Sn, Nb, Ta, F (Y, REE, Li) deposit and the massive cryolite associated with the albite-enriched facies of the Madeira A-type granite, Pitinga mining district, Amazonas State, Brazil

• Published in: Canadian mineralogist Vol. 47; no. 6; pp. 1329 - 1357
• Main Authors: Bastos Neto, Artur C, Pereira, Vitor P, Ronchi, Luiz H, de Lima, Evandro F, Frantz, José C
• Format: Journal Article
• Language: English
• Published: Mineralogical Association of Canada 01-12-2009
• Subjects: A-type granites; albite; Amazonas Brazil; Arctic region; Brazil; cassiterite; cryolite; Economic geology; feldspar group; fluid phase; fluocerite; fluorides; framework silicates; gagarinite; granites; Greenland; halides; igneous and metamorphic rocks; igneous rocks; immiscibility; Ivigtut Complex; liquid phase; lithium ores; Madeira Deposit; Madeira Granite; magmas; metal ores; metals; mineral deposits, genesis; nesosilicates; niobium ores; orthosilicates; oxides; pegmatite; Petrology; Pitinga mining district; plagioclase; plutonic rocks; pyrochlore; rare earth deposits; rare earths; silicates; South America; tantalum ores; tin ores; yttrium; zircon; zircon group
• ISSN: 0008-4476; 1499-1276
• DOI: 10.3749/canmin.47.6.1329

The Madeira deposit, in the Pitinga mining district, Amazonas State, Brazil, is associated with the A-type Madeira granite (∼1820 Ma), which is hosted by A-type volcanic rocks (Iricoume Group, approximately 1880 Ma) formed in a caldera complex. The deposit is characterized by an association of Sn with cryolite, Nb, Ta (Y, REE, Li, Zr, U, and Th) in the same albite-enriched granite that hosts a massive cryolite deposit. At the magmatic stage, the high F content in the melt was buffered by crystallization of cryolite. Tin (cassiterite) and Nb (uranoan plumbopyrochlore) were homogeneously dispersed throughout the deposit, transported by F-bearing complexes in the melt. Zircon crystallization, inhibited at the early magmatic stage by high-F activity, intensified at the late magmatic stage owing to a decrease in alkalinity associated to riebeckite crystallization, forming concentrations, together with xenotime and polythionite, in pegmatitic zones. Yttrium and REE mineralization in the lower portion of the deposit is represented by gagarinite-(Y), with fluocerite-(Ce) inclusions formed by exsolution of the early gagarinite. There is no evidence for silicate-fluoride liquid immiscibility, nor for a continuous transition from volatile-rich silicate melt to solute-rich fluids. The abrupt magmatic-hydrothermal transition triggered three processes: (1) albitization accompanied by the crystallization of hydrothermal cryolite in the rock matrix, (2) conversion of pyrochlore to a columbite-group mineral, characterized by gradual loss of Pb and enrichment in U and Nb, and (3) formation of the massive cryolite deposit, made up of crystals of twinned cryolite (87%) plus quartz, feldspar, and zircon from an aqueous saline (1.7 to 22.4 wt% equiv. NaCl) hydrothermal fluid, starting at a minimum temperature of 400°C and continuing down-temperature. The evolution of parameters (REEtotal, La/Lu, LREE/HREE, Y) in cryolite is continuous from the magmatic stage until the low-temperature hydrothermal stage. The models of mineralization invoked for Ivigtut, in Greenland, are not considered applicable. The evolution of the A-type magmatism at Pitinga is briefly discussed.