Mineralogy, stable isotopes (δ18O and δ34S) and 40Ar-39Ar geochronology studies on the hydrothermal carapace of the Igarapé Manteiga W-Sn Deposit, Rondônia

Thais Marcela Fernandes Do Nascimento,Valmir Da Silva Souza

doi:10.1590/2317-4889201720170068

Thais Marcela Fernandes Do Nascimento, Valmir Da Silva Souza

Open Access

https://doi.org/10.1590/2317-4889201720170068

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Abstract

ABSTRACT: The Igarapé 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 (δ18O and δ34S) 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 40Ar-39Ar 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 Rondônia Intrusive Suite (995-991 Ma).

Highlights

The State of Rondônia, located on the SW border of the Amazonian Craton in Brazil, hosts several Meso- to Neoproterozoic rapakivi granitic associations, which form the Rondônia Tin Province
The mineralized sites are distributed mostly in the north-central part of the Rondônia Tin Province and are associated with the Rondônia intrusive suite (Fig. 1). They occupy the apex zone of polyphasic magmatic systems that are linked to late- to post-magmatic hydrothermal phases, encapsulated in hydrothermal carapaces formed by greisen, veins, breccia and pegmatite bodies (Leite Jr. 2002, Souza 2003, Sparrenberger 2003, Bettencourt et al 2005a)
The present paper describes the textural relationships, the chemical and isotopic (δ18O and δ34S) composition, and the age of the 40Ar-39Ar on the mineralogical association found in the hydrothermal carapace zone of the Igarapé Manteiga W-Sn deposit

Summary

Introduction

The State of Rondônia, located on the SW border of the Amazonian Craton in Brazil, hosts several Meso- to Neoproterozoic rapakivi granitic associations, which form the Rondônia Tin Province. These granitic suites have A-type and within-plate to post-collisional geochemical signatures, which were emplaced during successive magmatic episodes that occurred between 1606 and 974 Ma (Kloosterman 1968, Priem et al 1966, 1971, Leal et al 1978, Isotta et al 1978, Bettencourt et al 1999, CPRM 2007). Important mineralized sites occur in paleoplacer deposits, which have been exploited through a type of artisanal mining known as “garimpo” in Brazil

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