Mineralogy, stable isotopes (???′ 18 O and ???′ 34 S) and 40 Ar- 39 Ar geochronology studies on the hydrothermal carapace of the Igarap???? Manteiga W-Sn Deposit, Rond???′nia

摘要

ABSTRACT: The Igarap???? Manteiga W-Sn deposit is formed by a granite stock that intrudes in the Paleoproterozoic basement. The mineralization is encapsulated in an alumino-siliceous hydrothermal carapace formed by greisen, vein-veinlets and breccia, developed in the apex zone of a granite stock. At this site, wolframite, cassiterite and sulfides-minerals, as well as siderite, monazite, xenotime, hematite and others, are spread out or in massive clusters associated with quartz, topaz, zinnwaldite and fluorite. Wolframite is not zoned and relatively rich in Fe, and poor in Nb and Ta. Cassiterite exhibits growth-zones with light-yellow to dark-orange colors, and is rich in Ta, and poor in Ti, W, Mn and U. Oxygen and sulfur (???′ 18 O and ???′ 34 S) isotope data on the ore- and sulfide-minerals indicates that it is a magmatic source, with closing temperatures from 230 ???° C to 480 ???° C. The hydrothermal phase was cyclical and protractedly active, promoting greisenization and hydrofracturing. The lowering of temperature and the change in the composition of fluids (from oxidized to reduced) controlled the precipitation of the hydrothermal mineral assemblage. The 40 Ar- 39 Ar analyses reveal a plateau age of 988 Ma, interpreted as the closure time for the hydrothermal processes responsible for mineralization, which is linked to the final magmatic evolution of the Rond???′nia Intrusive Suite (995-991 Ma).