Trace and rare-earth element behaviors during alteration and mineralization in the Attepe iron deposits (Feke-Adana, southern Turkey)

摘要

Hydrothermal-metasomatic iron ores consisting mainly of siderite, ankerite and hematite are located in the Lower-Middle Cambrian limestone marbles of the Eastern Taurus Belt. The siderite, ankerite, hematite and host rock samples from the deposits have been investigated for major, trace, and rare-earth elements (REE) to evaluate the element mobility and mass transfer during fluid-rock interactions. Geochemical data show that the (Fe + Mn) concentrations regularly increase, and the (Ca + Mg + TC) contents progressively decrease from the host rock through ankerite, siderite, and hematite ores with increasing chemical index of metasomatism (GIM) values. The host limestone exhibits a pronounced positive Eu anomaly (2 to 3.45) and negative Ce anomalies (0.59 to 0.96). This limestone has much lower REE contents (3.92 ppm) than the Post-Archean Australian Shale (PAAS, 184.77 ppm). The elements of Cr, Y, Tb, Ho, and Lu are immobile and effectively conserved during hydrothermal processes. Compared with the host rock (precursor rock), the altered (ankerite) and mineralized rocks (siderite and hematite) were enriched in Fe, Mn, Ba and depleted in Ca, Mg, Si, Al, K, Na, Ti, and P during alteration and mineralization. The average REE contents of the ore samples (9.13 to 12.01 ppm, mean = 10.44 ppm) are higher than those of the host rock (3.92 ppm), but significantly lower than in the PAAS. Most of the REE were mobilized and fractionated during the hydrothermal processes that may have resulted mainly from low Eh and pH, plentiful complexing ligands, variable high temperature, changing ionic radius of REE and composition of the mineralizing fluids. The middle rare-earth elements (MREE) show diverse behaviors in different sample groups. The light rare-earth elements (LREE) were strongly leached out of the system, and the heavy rare-earth elements (HREE) were preferentially retained as REE-C1 and/or -SO4 complexes. The (La/Lu)_N ratios show a decreasing trend from the host rock to siderite, ankerite, and hematite with increasing HREE and decreasing LREE contents. The ore samples show pronounced positive Eu (1.61 to 8.86) anomalies and weak to moderate negative Ce anomalies (0.34 and 0.98). Three samples display slightly positive Ce (1.06-1.09) anomalies. In the ore samples, the high field strength elements (HFSE) such as Ti, Zr, and P were depleted, as were the light lithophile (LIL, e.g., K, Th, U, Pb, La, and Ce) and trans-transition (TRT, e.g., Cu, Ni, and Zn) elements. The Ba and Sr demonstrate dissimilar behavior during the fluid-rock interaction. Ba was enriched, while Sr was depleted.