Ore genesis of the Zhuxi supergiant W-Cu skarn polymetallic deposit, South China: Evidence from scheelite geochemistry

Abstract

Scheelite is the main ore mineral in skarn-type tungsten deposits, which can be used to track the ore-forming processes and sources. The Zhuxi W-Cu deposit (3.44 Mt WO3 at 0.54% and 0.10 Mt Cu at 0.57%), located in the northern part of the Jiangnan orogen, is one of the largest tungsten deposits in the world. Skarn and orebodies occur mainly within the contact zone between the Mesozoic highly fractionated intrusions and Carboniferous carbonate sediments (Huanglong and Chuanshan formations). Our data reveal that the REEs of scheelite are mainly controlled by the substitution mechanism 3Ca2+ = 2REE3+ + □Ca. Scheelites from the Zhuxi deposit show negative Eu anomalies in the prograde skarn stage, but positive Eu anomalies in the retrograde skarn stage in the chondrite-normalized REE patterns. Variation of Eu anomalies recorded the ore-forming processes. The negative Eu anomalies were inherited from the Mesozoic granites and the positive ones formed under more reduced redox conditions. The decrease of Mo concentrations, along with increase of δEu and δCe of scheelite from prograde to retrograde skarn stage, support a temporal decrease in oxygen fugacity of the mineralization fluids. The scheelites from the Zhuxi deposit are characterized by low εNd(t) values (−10.1 to −7.0) and high initial 87Sr/86Sr ratios (0.71231–0.72371), which are different from the scheelites hosted in vein-type Au-W deposits contributed mainly by mantle. The Nd-Sr isotopes of scheelites are consistent with the results of the three Mesozoic granites, indicating their genetic relationship. However, the scheelites show variable Nb/Ta compositions, which straddle between the values of Mesozoic granites and the country rock of Shuangqiaoshan Group meta-sedimentary rocks, indicating the Shuangqiaoshan Group also has contributed to mineralization. This is different from a commonly accepted model that the ore-forming fluids and metals were exsolved exclusively from the granite plutons.