In situ LA-ICP-MS study of garnets in the Makeng Fe skarn deposit, eastern China: Fluctuating fluid flow, ore-forming conditions and implication for mineral exploration

Abstract

Most garnets in Fe-Cu skarn deposits have simple petrographic textures, LREE-enriched geochemical patterns, are enriched in Fe, and are distinguished from garnets of typical W or W-Mo skarn deposits by their extremely low W, Sn, and Mo contents. However, little is known about the garnets in Fe skarn systems that have more complex textures and high concentrations of W, Sn, and Mo. Here we describe diverse textures and LA-ICP-MS data for garnets in the largest Fe skarn deposit (Makeng) in eastern China, and we discuss the dynamics of the mineralizing fluids and the significance of garnet for W-Mo exploration. Garnets from the Makeng deposit fall into four main types, corresponding to four-stage garnet growth based on their textural characteristics and trace element contents: Stage 1, Al-rich relatively homogeneous garnets (Adr51-55) with a portion (Adr17-48) and HREE-enriched patterns and weak negative Eu anomalies; Stage 2, Fe-rich garnet with LREE-enriched or flat patterns and weak negative or positive Eu anomalies, represented by oscillatory zoned rims (Adr50-90) with dodecahedral faces and oscillatory zones (cores Adr54-68, rims Adr65-80) with composite dodecahedral-trapezohedral faces; Stage 3, nearly pure andradite garnets with significantly LREE-enriched patterns and strong positive Eu anomalies, as represented by irregular zones (Adr70-99) in oscillatory zoned rims and relatively homogeneous garnets (Adr95-100) with extremely weak zoning; and Stage 4, Al-rich garnet with HREE-enriched patterns and weak negative or weak positive Eu anomalies, as represented by garnet veinlets (Adr45-77) with weak zoning. These four stages of garnet were formed by periodic fluctuations in the fluid flux which can be explained by a kinetic dispersion model: a low flux responsible for the Al-rich garnet, a high flux responsible for the Fe-rich garnets, the highest flux responsible for the nearly pure andradite garnet, and a low flux responsible for the veins of Al-rich garnet. Relative to garnet in Fe-Cu skarn deposits, all garnet types in the Makeng Fe skarn deposit are rich in trace elements such as W, Sn, and Mo, especially the pure andradite garnet in stage 3, which has the highest contents of W, Sn, and Mo. These elemental enrichments are closely correlated with Fe, LREE/HREE, and fO2 signatures. The W and Mo signatures are identical to those of garnets in typical W or W-Mo skarn deposits, and they are useful geochemical indicators during exploration for W-Mo ore deposits.