Characteristics of hydrothermal chlorite from the Niujuan Ag-Au-Pb-Zn deposit in the north margin of NCC and implications for exploration tools for ore deposits

Abstract

The Niujuan Ag-Au-Pb-Zn deposit is located in the northern Hebei Province of North China, which is hosted by Permian coarse-grained granite. The orebodies are controlled by a NE striking fault. Chloritization is the most extensive alteration with increasing alteration intensity from the distal to proximal end of the orebody. Chlorite and biotite from twenty-one altered samples from different depths of four drill holes were analyzed using electron microprobe (EMPA) and laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Empirical geothermometry shows that chlorite formed at 206–340 °C. Through comparing with compositions of unaltered biotite, two biotite converted to one chlorite (2Bt → 1Chl) is the main chloritization mechanism in weakly and moderately altered granites during the hydrothermal alteration in the Niujuan deposit. During the alteration, hydrothermal fluid mainly brought in Fe, Sr, Th, Pb and CO2 and took out Mg, Ti, Si, F, K, Li, Zn, Sc, Ga, Co from the biotite, which was accompanied by the formation of quartz, rutile, calcite and fluorite during the chloritization. Chlorite in quartz-chlorite veins was precipitated directly from the hydrothermal fluids. Veined chlorite in intensely altered granites formed by the dissolution-transport-precipitation mechanism. Under the high fluid/rock conditions, veined chlorite has nearly the same compositions as chlorite in quartz-chlorite veins. According to the compositions of chlorite from intensely altered granites and quartz-chlorite veins, we infer that the hydrothermal fluid had low HFSEs and transition elements, such as Ti, Ga, Sc and V, and low fO2. From weak to intense chloritization granites, FeO, Sr, Th and Pb contents of chlorite increase, while MgO, TiO2, Li, Zn, Sc, Ga, V, Co and Nb contents decrease. The composition variations of chlorite are controlled by the composition of biotite, the compositions of fluids and the intensity of fluid-rock interaction during alteration. This indicates that the chemical variations of chlorite could be as a tool for mineral exploration in the moderate-low temperature Ag-Au-Pb-Zn deposit. Comparison with chlorite of different genesis shows that compositions of chlorite mainly differ in Mg, Fe and AlIV, which are mainly controlled by the composition of host rocks and the formation temperature.