Madeira

The Madeira deposit, in the Pitinga mining district, Amazonas State, Brazil

Deposit type:

REE minerals: xenotime, gagarinite-(Y), fluocerite-(Ce)

Ore minerals: zircon

Gangue minerals: twinned cryolite, quartz, feldspar

Host rock types: granite

Average grade:

Status:

Company:

Resource estimates:

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 (Iricoumé Group, ~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.

References

  1. https://en.wikipedia.org

  2. https://hal-insu.archives-ouvertes.fr/insu-00805907

  3. http://www.canmin.org/content/47/6/1329.short