Typomorphic feature of chromium sericite in granite hosted gold deposits in Jiaodong Peninsula, China

Abstract

Sericite, fine-grained muscovite, is one of the most common altered minerals from hydrothermal ore-forming processes. Due to the similar geochemical property of Au and Cr, chromium sericite usually occurred in gold mineralization. The typomorphic feature of crystal chemistry of chromium sericite from gold deposits are K content in the interlayer indicating degree of hydrothermal alteration, Si content in the tetrahedral sites indicating degree of silicification and Mg, Fe especially Cr contents in the octahedral sites indicating gold mineralization. There are systematic changes in chemical compositions and polymorphism of chromium sericite from the gold deposits hosted in granite in Jiaodong Peninsula. The SiO2 contents of the chromium sericite are relatively high from 44.92 to 52.22wt.%; and the K2O contents are relatively low from 9.44 to 11.55wt.%, with K cation in formula unit less than 1. These features reflect strong silicification and hydrothermal alteration. The Cr2O3 contents of the chromium sericite in gold deposits hosted in granite is below 1.92wt.%, much lower than those hosted in basic to ultrabasic volcanics like Helmo deposit in Canada. Different Cr background abundance and alteration degree are the reasons to explain these phenomenon. Chromium sericite occurring under low Cr background abundance means high alteration degree. Chromium sericite polymorphism includes 1M, 2M1 and 2M2. Different polytypes reflect multiple hydrothermal episodes, which are favorable for large-scale and strong alteration. The chromium sericite is a typomorphic mineral for mineralizing and prospecting of gold deposits in granite similar to Jiaodong Peninsula.