Gold deposits in the Taihang Mountains, northern China, mainly consist of quartz sulfide veins in granitoid plutons. This paper describes the geological setting of the gold deposits, and presents the results of microthermometric, Fourier transform infrared spectra, and stable isotope analyses of ore-forming fluids for the purpose of examining the characteristics of these fluids. The ore-forming fluid was of high temperature (up to 380ºC) and high salinity (33–41 wt% NaCl equiv.), represented by type I inclusions (with daughter minerals). This fluid evolved to low salinity at low temperatures recorded in type II (liquid-rich) and III inclusions (vapor-rich). Primary type II inclusions coexist with type III inclusions in quartz. Type III inclusions have almost the same homogenization temperatures as type II inclusions. This probably reflects boiling. The secondary fluid inclusions homogenized at lower temperatures, and have lower salinities than primary inclusions. Based on microthermometric data, we propose that the high-temperature fluid that separated from residual magma corresponded to the ore-forming fluid represented by type I inclusions. This fluid mixed with meteoric water in the upper part of the granitic pluton and was diluted. The diluted fluid boiled, probably due to abrupt pressure decrease, and formed liquid-rich type II inclusions and vapor-rich type III inclusions. The deposition of sulfide minerals and gold probably occurred during boiling.

Kokchetav ‘lamproite’ occurs in the east end of Kokchetav massif and consists of phenocryst (mainly clinopyroxene) and matrix (mainly feldspar). The compositions of clinopyroxene, magnetite and biotite phenocryst were determined using wavelength dispersive spectrometry on a JEOL Super-probe 8900 electron microprobe for the purpose of revealing the process of magma evolution. Analyses revealed a core-rim variation, which is consistent with three stages of magmatic evolution: Mg-rich clinopyroxene cores (diopside) and biotite cores (phlogopite) crystallized in a deep magma chamber (stage I); Fe-rich clinopyroxene rim (salite) and biotite rim crystallized at low pressure in a shallow magma chamber (stage II); Magnetite phenocryst core also crystallized in a shallow magma chamber, and coexists with Fe-rich clinopyroxene rim and biotite rim. The magnetite rims probably formed during magma eruption at the same time when groundmass crystallized (stage III).The calculated temperatures for ilmenite-magnetite pair range from 679 to 887ºC, log fO2 values range from 311.1 to 314.9 log units. These values represent the latest conditions of magma as ilmenite exsolution in magnetite probably occurred during magma eruption from the shallow chamber to surface. © 2002 Elsevier Science B. V. All rights reserved.

The chemical composition of a barren biotite granite, a two-mica granite, and a rare metal-bearing pegmatite (the No. 3 vein in Keketuohai, Altay Mountains, northwest China), along with mineral separates of apatite and mica, were analyzed and compared in the present work. Bulk chemistries of biotite granite, two-mica granite, and pegmatite rim are consistent with fractional crystallization trend from granites toward pegmatite. Changes in pegmatite composition can be explained mainly by fractional crystallization of biotite. Rb–Sr isochrons yield 248.8±7.5 Ma for the biotite granite, 247.8±6.3 Ma for the two-mica granite, and 218.4±5.8 Ma for the pegmatite rim. Similar geochemical characteristics, including initial εNd (T) values (ranging from -0.76 to -3.04) for apatites, similar initial 3Nd (T) values for whole-rock samples of studied granites and pegmatite (ranged from-1.40 to-3.21) and for biotites from granites (ranged from-2.75 to-3.15) suggest that these rocks were derived from a common magma source. Old continental crust may have played a significant role in magma generation. Based on these data, we interpret the evolution of the Keketuohai granite–pegmatite by magma differentiation from a common source. Biotite granite was the first to crystallize, followed by two-mica granites. The residual melt was probably stored in a granitic batholith. The pegmatite veins were injected ～30 M. y. later during another episodic tectono-magmatic event in the Altay Mountains. © 2005 Elsevier Ltd. All rights reserved.