Bao-Zhang Dai

Tourmaline as a recorder of magmatic–hydrothermal evolution: an in situ major and trace element analysis of tourmaline from the Qitianling batholith, South China

Four types of tourmaline in the Qitianling granitic batholith, South China, were identified in this study: euhedral disseminated tourmaline (DT type) in the early stage of granite consolidation; typical interstitial nodular tourmaline (NT type) formed late in the crystallization history; radial tourmaline veinlet (RT type) and vein tourmaline (VT type) of hydrothermal origin. We performed major and trace element analysis by in situ electron microprobe and laser ablation inductively coupled plasma mass spectrometry on these four tourmaline types. Compositionally, these tourmalines fall into the alkali group and schorl–dravite solid solution series. There is clear correlation between trace and major elements in VT-type tourmalines, likely related to co-variations of these elements in the hydrothermal fluid from which the tourmaline precipitated. Tourmaline from granites displays low REE abundances and negative LREE trends, positive Eu anomalies, and HREE below or close to their detection limits. The DT-type tourmalines show a positive Eu anomaly, which is probably due to a preferential incorporation of Eu2+ over Eu3+. REE depletion in NT-type tourmalines may reflect co-crystallization of REE-rich minerals. The positive Eu anomaly in NT-type tourmalines is due to late-stage release of Eu2+ caused by tourmaline replacement of early feldspar. Vein tourmalines (RT and VT type) display similar slopes from La to Nd, without an Eu anomaly. The chondrite-normalized REE patterns in tourmalines from veins display a concave upward-shaped, MREE-depleted pattern, with minimum values at Ho increasing steadily to Lu, and may reflect the REE characteristics of the precipitated medium. HREE enrichment in hydrothermal fluid can be attributed to the formation of REE-fluoride complexes during the internal fluid evolution.

Geochronology and Hf isotope study of pegmatite in the Xiaoqinling area of NW China: Implication for petrogenesis and regional metamorphism

In this study, we carried out petrography, zircon U-Pb geochronology and Hf isotopic analyses on a granitic pegmatite dyke in the Xiaoqinling area in southern margin of the North China Craton (NCC). Our study suggests that the pegmatite dyke likely crystallized from a volatile-rich pegmatitic magma. Different from most other pegmatite elsewhere, zircon from this pegmatite dyke does not contain unusually high U and Th concentrations and suffered no evident radioactive damage, therefore we successfully obtained a zircon U-Pb dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), which yields an average 207Pb/206Pb age of 1 814±6 Ma, representing the intrusive age of the granitic pegmatite dyke. Zircon ɛHf(t) values are between −8.3 and −3.0, corresponding to Hf depleted mantle model ages from 2 649 to 2 991 Ma with an average of 2 881 Ma. These data indicate that this granitic pegmatite dyke may have been derived from partial melting of Meso-Neoarchean metamorphic rocks from the Xiaoqinling basement. Granitic pegmatite magma may have emplaced within the Taihua Group wall rocks during the last stage of the middle to high grade metamorphism. Furthermore, according to the petrographic observation, the 1.81 Ga pegmatite dyke and the 1 800–1 750 Ma Xiong’er Group rocks were not undergone middle to high grade metamorphism, indicating 1.81 Ga as the termination of the latest regional metamorphism in the southern margin of the NCC.

Discrimination of Ore-Bearing and Barren Porphyries in the Yulong Porphyry Copper Ore Belt, Eastern Tibet

The Yulong porphyry copper ore belt is the largest belt of porphyry copper deposits in China. Detailed geological and geochemical comparison indicates that ore-bearing and barren porphyries in this belt were derived by progressive partial melting of veins of phlogopite-garnet clinopyroxenite in lherzolitic lithosphere. Ore-bearing porphyry represents the earliest melt derived by partial melting of accessory phases such as apatite, carbonate, and phlogopite, and thus contains high contents of volatiles F, Cl, and H2O. These volatiles promoted copper extraction and enrichment. Barren porphyry is the product of relatively high degrees of partial melting involving clinopyroxene and garnet, and thus is depleted in volatiles, hence barren of copper. This is the essential petrogenetic discrimination between ore-bearing and barren porphyries. Geochemical discrimination includes the following: Cu > 100 ppm, W > 5 ppm, F + Cl > 900 ppm, K2O/Na2O > 1.2, and Sm/Yb > 6.5 for bulk rock of the ore-bearing porphyries, and Mg/(Mg + FeT) > 0.57 and Fe3+/Fe2+ > 0.7 for biotite in the ore-bearing porphyries. In contrast, such values are all less for barren porphyries. These geochemical parameters have important significance for copper exploration in the Yulong porphyry copper belt.