Petrogenesis of lamprophyre in Sawur, northern Xinjiang, China: Implication for volcanic hosted gold deposits

Abstract

The Sawur gold belt, northern Xinjiang, China, is the eastward extension of the Zarma-Sawur Au-Cu belt in Kazakhstan. Many gold deposits and other mineral occurrences are distributed in volcanic rocks of the Sawur area. Systematic studies are in need to learn how gold is associated with volcanic rocks. This study applied whole rock and mineral geochemical analysis on lamprophyres in Qia’erdunbasixi gold deposit. Clinopyroxene (Cpx) xenocryst in lamprophyre and Cpx megacryst core is Mg- and Cr-rich diopside, which is commonly eroded by salite with resorption boundary. These Cpx grains with low rare earth element (REE) abundance, light rare earth element (LREE) depletion ([La/Yb]N < 1) and without Eu anomaly are likely formed in the spinel-facies lithospheric mantle. Xenocryst in lamprophyre has undergone metasomatism, which is proved by the occurrence of amphibole and phlogopite along the rim of Cpx. In situ trace element analysis of amphibole and metasomatized clinopyroxene unravels the property of metasomatic melt with large LREE/HREE fractionation and strong depletion in Zr-Hf and Ti relative REE, with the occurrence of pargasite, phlogopite and apatite, which are typical characteristics of alkali carbonatite melt from upper mantle. Fe-rich salite with low Mg# (61 ∼ 64) and right-leaning REE pattern is different from any other clinopyroxene in Sawur lamprophyre, should be the subducted mélange, which is captured by upwelling melt together with xenocryst caught from spinel-facies mantle. Cpx phenocryst with low Mg#, Cr and high abundance of REE in lamprophyre has different compositions with xenocryst, should be formed in a shallow magma chamber. The lamprophyre is thought to be derived from deep mantle and therefore carry these various xenocrysts into surface. We suggest the mantle origin with alkali carbonatite metasomatism likely results in the gold fertility of Sawur volcanic rocks, which explains the common association with gold mineralization.