Rare earth element geochemistry and fluid characteristics of scheelite in the Hutti gold deposit, Hutti-Maski schist belt, Raichur district, Karnataka, India

Abstract

Scheelite occurs as thin smears, isolated grains and veinlets in auriferous quartz reefs and sulfidized and auriferous biotite-chlorite schist in the Hutti deposit. Major host rocks at Hutti are mafic and sill-like felsic volcanic rocks and quartz reefs located at the shear contact of these rocks. Laser-Raman and fluid inclusion studies on scheelite show that H2O + CH4 ± CO2 + NaCl fluids, similar to gold mineralizing fluid are responsible for scheelite mineralization at Hutti. Rare Earth Element (REE) geochemistry of scheelite shows two types of chondrite normalized REE patterns which are classified as Type I (hump-shaped pattern), characteristic of REE incorporation by coupled substitution involving Na or K, and Type II (flat pattern) involving REE substitution with creation of a vacancy. The type I pattern is seen in scheelite within biotite schists whereas Type II pattern is associated with scheelite in felsic volcanic rocks and quartz mylonites. The Type II pattern is subdivided into Type IIa and Type IIb pattern based on Eu/Eu* values with Type IIa showing larger Eu/Eu* values than Type IIb. The lower ratio in Type IIb scheelite which occurs in altered felsic volcanic rocks is attributed to the overall enhancement of MREE, due to some degree of coupled substitution involving Na relative to Type IIa. The slope of the REE distribution pattern of scheelite is distinctly different from that of felsic volcanic rocks but similar to that of mafic volcanic rocks. It is inferred that REE fractionation between mineralizing fluids and greenschist–facies metamorphosed mafic rocks of the schist belt is not significant.