Major, trace and REE characteristics of the high grade gold geochemical reference material from Middle Reef, Hutti gold mines, Karnataka

The geological, petrological, mineralogical and geochemical characteristics of the Hutti high-grade gold reference material representing the composite sample of sheared gold-sulphide bearing smoky-grey quartz vein hosted in felsic volcanics (Middle Reef) and hydrothermal altered wall rocks are investigated in the present work. Petrological studies on these rocks reveal the presence of deformed quartz ribbons, plagioclase, chlorite, biotite, stipnomelane, actinolite and abundant native gold, arsenopyrite, pyrite and minor scheelite, monazite and xenotime. The major, trace and REE geochemistry of the host barren rhyolites are interpreted as low Yb adakitic variety which show high Ba, Cr, V and Ni concentrations. REE patterns of the gold-sulphide bearing quartz reef is characterized by a pronounced positive Eu anomaly, LREE depletion and nearly flat HREE with a low total REE which indicate possible contribution from deep-seated fluids to the ore-forming process. These patterns also suggest a source which is enriched in Eu and depleted in LREE either due to crystal fractionation or the formation of scheelite. The associated hydrothermally altered wall rocks from proximal zone, exhibit flat REE patterns coupled with high concentrations of Cu, Zn, Cr, V, Sc and Ni indicating their derivation from secondary epigenetic fluids responsible for metasomatic alteration. The abundant barren schistose amphibolites from distal zone represent enriched type of island arc basalts (IAB). The trace, REE-HFSE data on the rhyolite (adakite) and IAB of Middle Reef in the Hutti schist belt substantiates the inference that Archaean subduction processes in an island arc/convergent margin setting are responsible for adakitic magmatism.