Liandang Zhao

Alteration and Mineralization Paragenesis of the Bailingshan Fe (‐Cu) Deposit in Eastern Tianshan

Hami (Tu-Ha) basin and the central Tianshan belt, is a part of the Tianshan Orogenic Belt (TOB) and can be subdivided into several subunits by nearly EW-trending deep faults (Hou et al., 2014). It’s composed of DananhuTousuquan arc belt, the Kanggur shear zone and the Aqishan-Yamansu volcanic belt from north to south. The Aqishan-Yamansu volcanic belt is bounded by the Yamansu fault to the north and by the Aqikeduke fault to the south, respectively. Numerous Fe (-Cu) deposits in this belt have been discovered, such as Aqishan, Hongyuntan, Bailingshan, Yamansu, and Shaquanzi from west to east (Mao et al., 2005). Among these deposits, the Bailingshan Fe deposit contains a reserve of 13.065 Mt iron with average grade of 44.94%. The orebodies are hosted in the andesitic tuff breccia of the Matoutan Formation consisting of the Late Carboniferous dacite tuff, andesitic tuff breccia, andesitic to dacitic tufflava and andesitic tuff-andesitic tuff breccia from the bottom up (Wang et al., 2005). Many granitoids, such as the granodiorite, moyite and granite porphyry, intruded the above layer. Ore minerals at Bailingshan are dominated by magnetite, hematite, pyrite, chalcopyrite, and specularite, with garnet, clinopyroxene, amphibole, epidote, chlorite, quartz, and calcite as the dominated gangue minerals. The ore textures are mainly massive and disseminated. Based on the handspecimen and petrographic observation, seven stages of alteration and mineralization are identified at Bailingshan. It contains the early skarn stage, late skarn stage, main mineralization stage, late amphibole stage, quartz-sulfide stage, late veins stage and oxidation stage (Fig. 1). The earliest stage I (early skarn stage) comprises aggregates of coarse-grained garnet and fine-grained clinopyroxene. In the subsequent stage II, metasomatism occurred throughout the garnet and clinopyroxene, represented by amphibole and minor pyrite. Stage III, main mineralization stage, which formed the massive iron orebodies, comprises a variety of opaque and gangue minerals. Major mineral assemblages assigned to stage III are magnetite-epidote and magnetite-chlorite (±epidote). In stage IV and V, minor amphibole (different from those in stage II) and intense quartz-sulfide veins cut the stage III magnetite-epidote assemblages (Chen et al., 2010; Chen et al., 2011; Duan et al., 2013). Meanwhile, massive quartz-pyrite aggregates, sometimes with chalcopyrite, have void filling with magnetite. In stage VI, numerous late veins, such as quartz (without sulfide), hematite, specularite, and calcite-barite, cut the magnetite-epidote or quartz-sulfide assemblages. Oxidation (stage VII) commonly developed on magnetite and sometimes on the surface of chalcopyrite to form bornite and azurite. The characteristics based on the field and petrographic studies imply that the Bailingshan Fe deposit is a metasomatic deposit in the Eastern Tianshan belt.

Alteration and Mineralization Paragenesis of the Heijianshan Fe-Cu Deposit in the Eastern Tianshan, Xinjiang

Eastern Tianshan belt between the Junggar Basin and Tarim Basin, about 150 km south of Hami City, Xinjiang, NW China. Regional E-W-trending crust-cutting deep faults act as major boundaries of the Late Paleozoic geological unites in the Eastern Tianshan (Mao et al., 2005). The Heijianshan deposit lies between the Yamansu Fault and the Aqikuduke Fault, belonging to the AqishanYamansu volcanic belt, which hosts many high-grade iron ore deposits, such as Yamansu, Kumutag, Bailingshan, and Hongyuntan (Hou et al., 2014). The strata of the Heijianshan mine is mainly composed by the Upper Carboniferous Matoutan Formation, consisting of tuff in the lower part overlain by basalt. Orebodies are commonly hosted by tuff and brecciated tuff. Four alteration and mineralization stages at Heijianshan, including epidote alteration, magnetite stage, polymetallic sulfide stage and late veins, have been recognized largely based on the megascopic and microscopic textural relationships and mineral assemblages (Fig. 1). Stage I—Epidote alteration: Fine-grained epidote was widely distributed to replace surrounding rocks and cut by veins of magnetite and actinolite. Coarse-grained and euhedral epidote locally is intergrown with calcite. Stage II—Magnetite stage: Magnetite has different forms of texture, such as granular, brecciated, and massive; nonetheless, magnetite is closely associated with amphibole. Fine-grained magnetite and associated amphibole commonly replaced country rock and previous epidote, clinozoisite, and calcite. Locally, quartz, Kfeldspar, and titanite closely coexist with magnetite and amphibole. A hematite sub-stage, now mainly replaced by magnetite (i.e., “mushketovite”), may occur before the main magnetite mineralization. Stage III—Polymetallic sulfide stage: The main minerals in stage III are quartz, pyrite, chalcopyrite, and chlorite, with minor pyrrhotite and electrum. Magnetite was commonly replaced or cut by Stage-III pyrite and chalcopyrite. Pyrite, locally coexisting with pyrrhotite, was also fractured by chalcopyrite with electrum in veinlets. Chalcopyrite was closely associated with chlorite. Stage IV—Late veins: Late-stage hydrothermal veins are abundant in the Heijianshan deposit, but their mutual age relationship are not very clear. The veins of epidote and calcite are widely distributed to cut wall rock and other mineral assemblages; whereas the veins of quartz, hematite, clinozoisite, chlorite, and albite are only locally distributed. Specularite veins occur locally and postdate magnetite and pyrite. Supergene alteration: The supergene alteration is very common at Heijianshan, which had been recognized as an oxidizing deposit (Zhang et al.,2012). Hematite and limonite after magnetite; digenite, bornite, and malachite replacing chalcopyrite have been commonly observed. Other supergene Cu minerals, such as atacamite and chrysocolla also locally occurred. Based on the preliminary paragenesis study, we conclude that the Heijianshan Fe-Cu deposit was formed by hydrothermal metasomatism and the Cu mineralization may be separate from previous magnetite stage.