Magmatic-hydrothermal processes and controls on rare-metal enrichment of the Baerzhe peralkaline granitic pluton, inner Mongolia, northeastern China

Abstract

Although it is well known that many peralkaline granite plutons show enrichment of the rare earth elements (REE) and other high field strength elements (HFSE), the magmatic-hydrothermal processes that concentrate these elements are still poorly understood. The Early Cretaceous Baerzhe pluton in northeastern China provides an ideal example of super-enrichment in REE, Zr, Nb and Be in peralkaline granites. Three kinds of granites are identified, namely, the barren, the moderately-mineralized and highly-mineralized granites. The barren granite includes hypersolvus porphyritic arfvedsonite granite, transsolvus fine-grained granite, as well as transsolvus pegmatite and aplite; the moderately-mineralized granite includes transsolvus spherulitic granite and coarse-grained granite; and the highly-mineralized granite is subsovlus and contains spherulitic granite and medium-grained granite. Replacements of arfvedsonite by aegirine and alkaline feldspar by albite occurred in the moderately-mineralized granite, whereas an extensive alteration assemblage of Fe-Ti oxides and albite laths developed in the highly-mineralized granite. The barren porphyritic arfvedsonite granite samples show moderate SiO2 (63.20–68.00 wt%), high total alkali (Na2O + K2O = 9.94–10.23 wt%), and variable and high total FeO contents (7.56–13.45 wt%). The barren fine-grained granite, as well as moderately-mineralized spherulitic granite and coarse-grained granite, are characterized by high SiO2 (73.21–75.92 wt%), high total FeO (1.04–5.44 wt%) and total alkali (Na2O + K2O = 8.72–9.85 wt%) concentrations with A/CNK = 0.80–0.99 and NK/A = 1.00–1.24, indicative of peralkaline affinity. In comparison, the highly-mineralized spherulitic granite and medium-grained granite have more variable SiO2 (72.10–79.50 wt%) and lower total alkali (K2O + Na2O = 4.40–8.70 wt%) contents. All these samples have positive and uniform εNd(t) varying from + 2.25 to + 3.28, along with restricted 206Pb/204Pb (18.107–18.464) and 207Pb/204Pb ratios (15.497–15.549), showing that they were probably derived from a common juvenile melt. When compared with the barren fine-grained granite, the spherulitic granite contains more microcline and albite but less perthite, suggesting the spherulitic granite formed in a more evolved magma. The amounts of spherulites are in direct proportion to the intensity of hydrothermal alteration and mineralization. It is suggested that the spherulite structure was a product of low nucleation rates due to the high volatile content of the late-stage evolved magma. Extensive magma differentiation leads to the exsolution of F- and Cl-rich hydrothermal fluids which are enriched in large amount of REE and HFSE elements. Furthermore, intense fluid-rock interaction occurred in the most advantage stage of magmatic-hydrothermal evolution and led to extensive alteration as well as the formation of associated REE-Zr-Nb-Be mineralization in the large Baerzhe deposit.