Genesis of Anqing copper-iron deposit,Anhui Province:Constraints from mineral deposit geology and element geochemistry

The Anqing copper-iron deposit is a typical skarn-type deposit, which is located in the Anqing-Guichi ore cluster along the Middle and Lower Yangtze River Cu (Au)-Fe metallogenic belt. Ore bodies occur in the contact zone between marble layer belonging to the Nanlinghu Formation of Lower Triassic and Yueshan dioritic intrusion. Analyses of the typical skarn samples systematically collected from a cross section show that the ore bodies and surrounding rocks have a significant petrochemical zoning with variation of mineral assemblage in space. Namely, bodies of magnetite ore with dense block structure and skarn-type ore with crumb structure occur close to the marble zone, while that of skarn-type ore with disseminated structure exists far away from the marble zone. Diopside-riched skarn develops near the diorite. The content of Fe2O3T increases significantly from the marble zone to the magnetite ore body, and decreases gradually close to the diorite. However, there appears a varying trend of CaO contrary to that of Fe2O3T. REE patterns of minerals and whole-rocks show a magmatic origin of dense massive magnetite ore and crumb skarn-type ore, but a metasomatic origin of disseminated skarn-type ore. Comprehensive studies of orebody geology, mineragraphy and element geochemistry support division of the process of mineralization into two periods of ore pulp penetration and hydrothermal fluids metasomatism. The former includes oxide stage and sulfide-carbonate stage, while the latter consists of prograde stage, early retrograde stage, quartz-sulfide stage and late retrograde stage. The mentioned data combined with the research result of regional magmatic process indicate a complex origin of the Anqing copper-iron deposit by ore pulp penetration with contact metasomatism. Fe-riched ore pulp formed by immiscibility in shallow magma chamber should be the main source of ore-forming elements in the deposit.