Zhu Jinchu

Partitioning of F between aqueous fluids and albite granite melt and its petrogenetic and metallogenetic significance

The fluid/melt partitioning experiments on fluorine were carried out in the system albite-H2O-HF atP = 100 MPa, 770°C ≤T≤800°C: and wt = 2% −6% conditions. The concentrations of fluorine in quenched glasses (melt) were determined by electron microprobe and those of fluorine in the coexisting aqueous fluid were calculated by the method of mass balance. The result shows that the fluorine was concentrated in granitic melt relative to the coexisting fluid. The partition coefficient DF(wtFF1/wtFMt) ranges from 0.35 to 0.89. It increases with increasing fluorine content in the system. This means that there is not just one single value of partition coefficient for fluorine in the granitic melt-fluid system. The partitioning behavior of fluorine in this system depends critically on fluorine and proton (H+) concentrations. Our data suggest that F-rich granitic melts exist in nature and that fluorine may not be an important complexing agent of metal elements in F-bearing fluids.

The Hercynian-Indonesian collision type granites of west Yunnan and their tectonic significance

Abstract The Linchang Composite Granite Batholith (LCGB) of west Yunnan is an important constituent of the collision-related granite belt of a Permian-Triassic suture zone in this area. It consists of Pinhejie granite in the north and Linchang granite (LG) in the south, which are connected with each other at depth. The Pinhejie granite is N.N.W.-trending, 70 km long, 3–9 km wide and 450 km 2 in area. Its eastern part is Pinhejie gneissic granite (PGG), with an Rb-Sr isotopic age of 309 ± 27 Ma and an initial Sr ratio of 0.7153 ± 3. Its western part is Pinhejie massive granite (PMG), with an Rb-Sr isotopic age of 294 ± 6 Ma and an initial Sr ratio of 0.7205 ± 3. The LG is an extremely large batholith, with a total length of 370 km, a width of 18–36 km and a total area of some 10000 km 2 . The Rb-Sr age of Rb-Sr isochron line for LG and PMG could be obtained with an age of 292 ± 15 Ma and an initial Sr ratio of 0.7216 ± 7. These data show that emplacement of LCGB took place approximately in the range of 292-275 Ma, i.e. in the Permian period. The modelling composition of source materials for LG is equivalent to a mixture consisting of 80% upper crustal Sr plus 20% lower crustal Sr. It suggests that the collision-related LCGB is a typical peraluminous S type granite in genesis.

Skarn-type tungsten mineralization associated with the Caledonian (Silurian) Niutangjie granite, northern Guangxi, China

The Niutangjie tungsten deposit is a bedded skarn-type scheelite deposit and is located at the junction between Ziyuan and Xingan counties in the north of Guangxi, China. The deposit is genetically related to a fine-grained two-mica granite within the orefield. Zircon LA-ICP-MS U-Pb dating of the granite yielded a Silurian (Caledonian) age of 421.8±2.4 Ma, which is contemporaneous with the adjacent Yuechengling batholith. Mineralization within the skarn is associated with a quartz, garnet, and diopside gangue, and scheelite is present in a number of different mineral assemblages, such as quartz-scheelite and quartz-sulfide-scheelite; these assemblages correspond to oxide and sulfide stages of mineralization. Sm-Nd isotope analysis of scheelite yielded an isochron age of 421±24 Ma. Although the uncertainty on this date is high, this age suggests that the scheelite mineralization formed during the Late Caledonian, at a similar time to the emplacement of the Niutangjie granite. Zircons within the granite have ɛHf(t) values and Hf two-stage model ages of −6.5 to −11.6, and 1.79 to 2.11 Ga, respectively. These data suggest that the magma that formed the granite was derived from Mesoproterozoic crustal materials. Scheelite ɛNd(t) values range from −13.06 to −13.26, also indicative of derivation from ancient crustal materials. Recent research has identified Caledonian magmatism in the western Nanling Range, indicating that this magmatism may be the source of contemporaneous tungsten mineralization.

Ore-forming fluid constraints on illite crystallinity (IC) at Dexing porphyry copper deposit, Jiangxi Province

Illite, a distinctive kind of clay minerals of potassium alteration within the hydrothermal alteration zone, frequently occurs at the Tongchang porphyry copper deposit ore field. The illite crystallinity (IC) value and expandability are mainly affected by water/rock ratio or fluid flux. It was formed by illitization of plagioclase and micas during hydrothermal fluid-rock interaction within the porphyry body and near the contact zone with wall rocks. Moreover, the negative correlation between illite index (IC) and copper grade indicates that within the alteration zone, the smaller the illite crystallinity value, the higher the alteration degree, and the higher the copper grade due to higher water/rock ratio. At lower levels of the porphyry body, however, the illite crystallinity (IC) values are mainly controlled by temperature and time duration.

Time-Space Distribution of Tin/Tungsten Deposits in South China and Controlling Factors of Mineralization

The abundant tin and tungsten deposits in South China were formed in different geologic ages. They began to appear in the late Protereozoic, and reached their paramount development during the Yenshanian period. In regional distribution the W deposits tend to occur in the eastern part of this region, mostly in post-Caledonian uplifts, while the Sn deposits tend to occur in the western part, mostly in Hercynian-Indosinian depressions. Both of them are found either within the middle-late Pro-terozoic and lower Paleozoic basemental structure, or adjacent to the basement rocks. Genetically, they are mainly associated with continental crustal anatectic granitoids. The following factors are considered to be most important in controlling the time-space distribution of Sn/W deposits: regional existence of ore source beds of middle-late Proterozoic and early Paleozoic ages; crustal mechanism in the generation of multi-cyclic granitoids and their evolution; activities of ore-bearing hydrothermal solutions; and localization of Sn/W mineralization in close relation to structural environment and wall rock properties.

Petrogenesis of the concealed Daqiling intrusion in Guangxi and its tectonic significance: Constraints from geochemistry, zircon U-Pb dating and Nd-Hf isotopic compositions

The samples from the hidden Daqiling muscovite monzonite granite, which has recently been recognized within the Limu Sn-polymetallic ore field, have been analyzed for zircon U-Pb ages and whole rock geochemical and Nd-Hf isotopic compositions to discuss its genesis, source, and tectonic setting. LA-ICP-MS zircon U-Pb dating indicates that the granite crystallized in the late Indosinian (224.8±1.6 Ma). The granite is enriched in SiO2 and K2O and low in CaO and Na2O. It is strongly peraluminous with the A/CNK values of 1.09–1.20 and 1.4 vol%–2.7 vol% normal corundum. Chondrite-normalized REE patterns show slightly right-dipping shape with strongly negative Eu anomalies (δEu =0.08–0.17). All samples show enrichment of LILEs (Cs, Rb and K) and HFSEs (U, Pb, Ce and Hf), but have relatively low contents of Ba, Sr and Ti. The zircon saturation temperatures (Tzr) are from 711 to 740°C, which are slightly lower than the average value of typical S-type granite (764°C). The granite has negative ɛNd(t) and ɛHf(t) values, which change from −9.1 to −10.1 with the peak values of −9.2 to −9.0 and from −3.7 to −12.6 with the peak values of −6 to −5, respectively. The TDMC (Nd) and TDMC (Hf) values are 1.74–1.82 Ga with the peak values of 1.73–1.75 Ga and 1.49–2.04 Ga with the peak values of 1.5–1.6 Ga, respectively. These characteristics reveal that the source region of the granite is dominantly late Paleoproterozoic to early Mesoproterozoic crustal materials. Seven inherited magmatic zircons are dated at the age of 248.6±4.3 Ma, which suggests the existence of the early Indosinian granite in Limu area. These zircons have the ɛHf(t) values of −6.7–−2.3, similar to those of the Daqiling granite, implying the involvement of the early Indosinian granite during the formation of the Daqiling granite. Inherited zircon of 945±11 Ma has the ɛHf(t) and TDM(Hf) values of 8.7 and 1.14 Ga, respectively, compatible with those of the Neoproterozoic arc magmatic rocks in the eastern Jiangnan orogenic belt. Therefore we inferred that Neoproterozoic arc magma might have been involved in the formation of the Daqiling granite, and that the Neoproterozoic arc magma belt and continent-arc collision belt between the Yangtze and Cathaysia Blocks might have extended westsouthward to Limu region. It is proposed that the underplating of mantle materials triggered by crustal extension and thinning resulted in partial melting of crustal materials to form the Daqiling granite in the late Indosinian under post-collisional tectonic setting.

Phase equilibria in the granite-H2O-HF system and effect of fluorine on granitic melt structure

Experiments carried out in the system granite-H2O-HF at 0.1 GPa show that the crystal-liquid equilibrium temperature of quartz rises and that of alkali-feldspar goes down with increasing F content. The calculated results of quartz and alkali feldspar crystal-liquid equilibrium show that the activity of SiO2 in melt increases and the activities of NaAlSi3O8(Ab) and KAlSi3O8(Or) decrease, with a greater decreasing extent for aAbL than aOrL. These systematic changes are believed to be caused by F complexing with Al, Na, K, but not Si in the melt, and are consistent with F decomposing AlO2− tetrahedra and more preferentially forming complexes with Na than K. The comparison between effects of F and H2O on phase equilibrium suggests that the maximum difference affecting melt structure between F and OH is F complexing without Si and OH complexing with Si in granitic melt.Experiments carried out in the system granite-H2O-HF at 0.1 GPa show that the crystal-liquid equilibrium temperature of quartz rises and that of alkali-feldspar goes down with increasing F content. The calculated results of quartz and alkali feldspar crystal-liquid equilibrium show that the activity of SiO2 in melt increases and the activities of NaAlSi3O8(Ab) and KAlSi3O8(Or) decrease, with a greater decreasing extent for a Ab L than a Or L . These systematic changes are believed to be caused by F complexing with Al, Na, K, but not Si in the melt, and are consistent with F decomposing AlO 2 − tetrahedra and more preferentially forming complexes with Na than K. The comparison between effects of F and H2O on phase equilibrium suggests that the maximum difference affecting melt structure between F and OH is F complexing without Si and OH complexing with Si in granitic melt.

Multiple-staged granite evolution and TaNb mineralization in South China

Abstract The Mesozoic post-orogenic granites in South China are widespread. Hundreds of tungsten and tin mineral deposits are closely associated with these granites. However, the number of TaNb deposits including those of the granite-type and the pegmatite-type, are relatively less. On the basis of geology, petrology, geochemistry and mineralization data from 8 ore deposits and related granites, we suggest that the TaNb mineralized granites are the special products of well-evolved granite magmas. The most important W and Sn deposits are clustered in post-Caledonian uplift and adjacent Hercynian-Indosinian depression of the South China orogenic belt. Most of the TaNb mineralizations are found within 20 km from the boundary faults surrounding the South Jiangxi post-Caledonian uplift. The paragenetic features of rare metal elements show that TaNb are accompanied by W in the uplift region, and by Sn in the depression region. The general intrusive sequence of a rare metal-bearing granite complex is: rare metal barren (porphyrytic) biotite granite—W and/or Sn ore-forming granite—TaNb (Sn) mineralized granite. The geological and geochemical data from eight mineralization districts indicate that the TaNb mineralizations are always developed in the last stage of a multiple-stage granite evolution. Albite-rich granite is the most common rock type of the TaNb ore-bearing granite, while the maximum albite contents in different deposits vary from more than 60% to less than 30%. Quartz with “snow ball” structure, topaz, and Li-micas (lepidolite, zinnwaldite, Li-muscovite and protolithionite) exist as common typomorphic minerals. The typical TaNb host are Mn-rich columbite-Tantalite and sometimes microlite and Ta-cassiterite. The pegmatoid crust (stockscheider) can be used as one of the most distinctive indicators for the degree of rare metal-tearing granite evolution based on its thickness and zonation. Compared with the normal granites, the TaNb mineralized granites have very special geochemical compositions. The TaNb mineralized granites are characterized by high Na 2 O, Na 2 O/K 2 O ratios (>1 wt%), Al 2 O 3 , Li and F and very low TiO 2 , MnO, MgO and CaO. Most typically, nearly all the TaNb mineralized granites bear very low total REE contents (less than 50 ppm), low Sr and Zr (both usually less than 30 ppm) and very high Rb (more than 700 to 3000 ppm). Geochemical features, excluding the contribution of post-magmatic alteration, change gradually from the earlier to the later phases in a granite complex, showing some trends of magmatic evolution. For example, two good negative exponential correlation curves on the RbSr diagrams (or a correlogram for other elements) of Yichun ad Dengfuxian multiple-stage granite plutons indicate a fractional crystallization mechanism which might have been an effective way to enrich the rare metal elements. The granite-type TaNb deposits, where Ta is economically more important than Nb, will be the only TaNb mineralization type discussed in this paper.

Geology, geochemistry and genesis of Yinyan porphyry tin deposit

The Yinyan porphyry tin deposit is a blind deposit associated with a small granite porphyry stock. The petrology and geochemistry of the Yinyan granite porphyry suggest that it is genetically of the transformation type, emplaced at the late stage of fractional crystallization within a high-level magma chamber. Ore-forming fluids are derived predominantly from the granitic magma and they interact with the wall rocks intensely when finding their way upwards through the granite porphyry. From the lower part of the porphyry upwards the following alteration zones can be distinguished: (a) slightly altered granite porphyry (with weak potash feldspathization), (b) protolithionite-quartz greisenization zone, (c) topaz-quartz greisenization zone, (d) sericite-quartz sericitization zone, and (e) silicification zone (quartz core at the surface). Tin mineralization is related to greisenization, especially to topaz-quartz greisenization. Rock- and ore-forming temperatures and oxygen fugacities are estimated, respectively. There are significant differences in many aspects between the Yinyan porphyry tin deposit and volcanic-subvolcanic porphyry tin deposits.

Important geological factors controlling the formation of gold deposits in East China

The major gold deposits in East China are characterized by their occurrence in preCambrian host metamorphic rocks, restriction in some specific stratigraphic-structural terranes, tendentious localization in ductile or ductile-brittle shear zones, association with the syntexistype granitoids, and significant remobilization, superimposition and enrichment by the laterstage Mesozoic tectono-magmatic activities.