REE composition of primary and altered feldspar from the mineralized alteration zone of alkaline intrusive rocks, western Yunnan Province, China

The Yao'an gold deposit is situated northwest of Kunming in Yunnan Province, China, is associated spatially with Yao'an alkaline intrusion. The Yao'an gold mineralization may be divided into two ore stages: the stage I (sulfide stage) fine-grained pyrite-chaleopyrite-galena stringer veins and the stage II (oxide plus sulfide) tectonic breccias and banded ore. The stage I ores occur in the zones of pervasive potassic alteration. The laser ablation ICP-MS technique was used to analyse REE in primary perthites and altered K-feldspars of potassic alteration zone, related to mineralization in the Yao'an gold deposit. The primary perthites from Yao'an syenite porphyry all have similar REE features with light REE enrichment, large positive Eu anomalies and small but significant negative Cc anomalies. The REE patterns of altered K-feldspars have broadly similar features, but compared to the primary perthite have much higher REE concentrations and smaller positive Eu anomalies. These characteristics show that the hydrothermal fluids responsible for alteration were REE-rich, due to high levels of complex-forming anions, but had large negative Eu anomalies and other features best ascribed to magmatic crystal fractionation. These fluids were derived essentially from the magmatic system. (C) 2002 Elsevier Science B.V, All rights reserved.