MINERAL CHEMISTRY OF CASSITERITES FROM PITINGA PROVINCE, AMAZONIAN CRATON, BRAZIL

Abstract

9. border"facies. ¿) indè) indicates tlzè area of occurrence of the greisens and episyenites, rcsPectively. siderite and Nb-bearing anatase. The cassiterite occurs mostly (1) as >1 mm large, idiomorphic to hypidiomorphic, reddish-brown to colorless clystals, associated to anatase, siderophyllite, sphalerite and quartz; (2) as fine, colorless to pale-red grains associated or included in topaz' quartz, sphalerite, chalcopyrite and partially chloritized siderophyllite. Both types show an irregular, speck-like zoning and a slight pleochroism, The GS2 is a massive, dark-to light-grayish green rock, composed of quartz, chlorite and phengite, with minol sphalerite, cassiterite, zircon, fluorite, belyl, topaz, pyrite, chalcopyrite and galena. In this greisen, cassiterite is more abundant than in the GS I. It occurs as (l) allotriomorphic to hypidiomorphic, corroded, dark red to cololless, irregularly zoned grains associated to fine aggregates of phengite and quartz, and (2) as hypidiomorphic to allotriomorphic, iocally corroded, colorless to pale red, irregularly zoned crystals associated to chlorite, quartz and pyrite. The sodic episyenites (NaEps) are medium, even-grained rocks composed essentially of albite. Other phases include quartz, cassiterite, fluor:ite, zitcon, adularia and opaque minerals (Costi et al. 1997).The cassiterite occurs as drusiform or irregular aggregates of hypidiomorphic to allotriomorphic, conoded crystals. They are reddish brown to colorless, slightly pleochroic, and show a strong and irregular , locally oscillatory, speck-like zoning. The cassiterite occurs in cavities, associated with (l) albite, phengite and chlorite, or (2) later secondary quartz, cloudy K-feldspar t albite. The lateritic profile developed over the albite granite is composed by a sequence of six horizons, which main part is a saprolitic horizon (Horbe & Costa 1997). The cassiterite found in the laterite shows inclusions of quartz, ilmenite, rutile, kaolin, hematite, xenotime and pyrochlore. The crystal size ranges from up to 5 rnm to less than I mm. the crystals are black, bipyramidal or prismatic and locally show corroded edges. ANALYTICAL METHODS Cassiterite grains of fresh albite granite, greisens and episyenites were analyzed on a Cameca SX-50 electron microprobe at the Instituto de Geociências of the Universida-de de Brasília, Brazil. Analyses were conducted at an accelerating voltage of I 5 Kv and a beam current of 25 nA for Ti, Sn and Fe, and 20 Kv and 40 nA for Nb, Ta, U and Y Standards included SnO, (SnL), TiO, (TiK), MnHO (FeK), UO, (UM), REE? (YL), metal Nb(L) and Ta (L). Each element was couñted for l0 séconds. ZAF corrections were applied.