BACKGROUND With the advantage of high spatial resolution, low detection limit, and multi-element surface analysis, the mapping technique of laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) provides a new method for mineralogy research, which can display the element distribution characteristics in minerals, and constrain the evolution process of the ore-genesis fluid and ore genesis.　　The wall-rocks of magnetite-rich ore from the No.2 mining area of the Gongchangling iron deposit suffered obvious alteration, and the scale of magnetite-rich ore is roughly proportional to intensity of alteration. However, regarding the hydrothermal nature, it is argued for metamorphic or magmatic hydrothermal fluid. The garnet widely occurs in the altered wall-rock, which is closely related to the genesis of the magnetite-rich ore. Thus, by LA-ICP-MS mapping of garnet in the altered wall-rock of Gongchangling magnetite-rich ore, the element composition and distribution characteristics can be used to constrain the evolution process of hydrothermal fluid and the genesis of the magnetite-rich ore.

OBJECTIVES To study the composition and distribution characteristics of major and trace elements in garnet by LA-ICP-MS mapping, and to constrain the evolution process of the ore-forming fluid and the genesis of magnetite-rich ore.

METHODS The LA-ICP-MS mapping technique was applied to garnets from the Gongchangling No.2 mining area by simultaneously using Agilent 7700X inductively coupled plasma-mass spectrometry (ICP-MS) and Analyte Excite 193nm laser ablation system at the laboratory of mineralization and dynamics in Chang’an University, with laser frequencies of 10-20Hz, laser ablation spot sizes of 20-150μm square, laser ablation speeds of 20-150μm/s and laser ablation energy of 5.9J/cm², within 4 hours. Fifty-one elements (from ⁷Li to ²³⁸U) were chosen for ICP-MS analysis and the dwell time of each element was 6ms. This method adopted an external standard (NIST610) as the calibration standard without an internal standard. The result was semi-quantitative and the color brightness was used to represent the elemental content. The Iolite software can be used to generate multi-elemental pictures and elemental ratio mappings, to facilitate data analysis and interpretation for geologists.

RESULTS  (1) LA-ICP-MS mapping indicates that the Si, Al, Fe, La, Ce, Pr and Nd compositions of the centimeter-scale garnet (Grt-1, particle size of 1.5cm×1.5cm) from the altered wall-rock are homogeneous, while the Mg, Mn, Ca, Li, Sc, V, heavy rare earth elements (HREEs) and Y retain the original compositional zonation. Most elements in the smaller garnet (Grt-2, particle size of 0.6cm×0.7cm) are mainly homogenized without zonation.　　(2) The two garnets from the altered wall-rock of the Gongchangling iron deposit show different elemental distribution. The centimeter-scale garnets (Grt-1) are more likely to retain the original compositional zonation when the metamorphic temperature is below 600℃. The results of LA-ICP-MS mapping of the centimeter-scale garnet (Grt-1) reveal the element correlations, to better understand the geochemical process in minerals.　　(3) The Mg content gradually increases and Mn content gradually decreases from the core to the rim of the garnet, indicating that the formation of the Gongchangling garnet is controlled by equilibrium growth and the formation temperature of the garnet gradually increases from the core to the rim. The Ca content of the garnet increases firstly and then decreases from the core to the rim, indicating that the pressure increases firstly and then decreases, which is consistent with the garnet formed during prograde metamorphism. The δEu anomalies of the garnet decreases firstly and then increases from the core to the rim, indicating that the oxygen fugacity of the metamorphic hydrothermal fluid decreases firstly and then increases. Since the characteristics of HREEs and Y in garnet are consistent with the characteristics of Ca, it is inferred that the distribution of the HREEs and Y is also mainly controlled by pressure.

CONCLUSIONS The centimeter-scale garnet from the Gongchangling altered wall-rock retains the original compositional zonation, and the LA-ICP-MS elemental mapping of the centimeter-scale garnet can be completed within 4 hours. The element distribution in the garnet indicates that the temperature gradually increases, the pressure increases firstly and then decreases, and the oxygen fugacity decreases firstly and then increases in the evolution process of the metamorphic hydrothermal fluid. Thus, it is inferred that the garnet in the altered wall-rock of the Gongchangling magnetite-rich ore was formed in the stage of prograde metamorphism associated with the Jiao—Liao—Ji Belt, and the magnetite-rich ore was derived from the reformation of BIF (low-grade iron ore) by metamorphic hydrothermal fluid.