The skarn and quartz-vein type of mineral deposits:With emphasis on the Shizhuyuan W-Sn-Mo-Bi

Skarn: Lime-bearing silicates derived from nearly pure limestone and dolomite with the introduction of large amounts of Si, Al, Fe and Mg.

Schematic diagram of skarn formation

含鎢礦物約 15 種，有工業價值的僅兩種：黑鎢礦（ Wolframite ） (Mn,Fe)WO4 ，含 WO3 約 76%白鎢礦（ Scheelite ） CaWO4 ，含 WO3 約 81%

已知含錫礦物的 20 種，重要的有： 錫石 (Cassiterite ， SnO2) ，含 Sn 79% ； 黝錫礦 (Stannite ， Cu2FeSnS4) ，含 Sn 27.5% ；

已知鉬礦物約 20 種，工業礦物只有輝鉬礦 (Molybdenite) (MoS2) 。

已知的鉍礦物有 50 多種，其中的工業礦物為：自然鉍 (Native bismuth) 含 Bi 95-99% ，輝鉍礦（ Bismuthinite ， Bi2S3 ） 含 Bi 82% ，泡鉍礦（ Bismutite ， Bi2(CO3)(OH)4 ） 含 Bi 87% ，

The supergiant Shizhuyuan W–Sn–Bi–Mo deposit is hosted by the Qianlishan granite, a small, highly fractionated granitic pluton (ca.10 km2) with multiple phases of intrusions within the Early Yanshanian granitoid province of SE China.

Strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and Devonian limestone is responsible for the polymetallic mineralizations.

SHRIMP U–Pb zircon analysis indicates that the two early phases of the Qianlishan granite formed contemporaneously at 152±2 Ma. Metasomatic minerals (garnet, fluorite and wolframite) separated from the skarn and greisen yield a Sm–Nd isochron age of 149±2 Ma that is interpreted as the formation age of the Shizhuyuan deposit.

Therefore, the mineralization of the super giant Shizhuyuan polymetallic deposit formed contemporaneously with, or very shortly after, the intrusion of the small, highly fractionated Qianlishan granite.

Geological maps of Qianlisha area:Shizhuyuan deposits

Sketch of skarn in Shizhuyuan

Progressive mineralizationof Shizhuyuan deposits

Previous view

The Qianlishan complex comprises five separate intrusions, including fine-grained porphyritic biotite granite (182–187 Ma, granite 1), medium-grained biotite-K feldspar granite (158–163 Ma, granite 2), fine-grained biotite and K feldspar granite (granite 3), granitic porphyry (144–146 Ma, granite 4), and diabase (142 Ma).

The upper part of the granite complex is characterized by extensive greisen alteration. Skarn zones distributed around the intrusions are mainly calcic, consisting of garnet, garnet-pyroxene, vesuvianite-garnet, and wollastonite-vesuvianite, progressively outward from the intrusion. Following the primary skarn formation, some of the skarn zones underwent retrograde alteration. The high garnet/pyroxene ratio and the diopside-rich and andradite-rich compositions of the pyroxene and garnet indicate that the skarn belongs to the oxidized type.

Mineralization consists of Sn-Be veinlet ore (type I) in marble and porphyry, massive W-Bi-Mo-Sn skarn ore (type II), stockwork W-Sn-Bi-Mo-F ore (type III), and W-Sn-Mo-Bi greisen ore (type IV), mainly associated with granite 2.

Emplacement of granite 2 was accompanied by late, intense fracturing characterized by stockwork mineralization (type III), which was superimposed on massive skarn and greisen zones. The stockwork ore consists mainly of greisen and skarn veins and veinlets with scheelite, wolframite, molybdenite, cassiterite, bismuthinite, and fluorite.

The Sm-Nd data for pyroxene and garnet in the massive skarn (type II ore) associated with granite 2 yields an isochron age of 157 Ma. This age corresponds closely to the age of granite 2.

柿竹園 黃砂坪 錫礦山

Quartz-veintype

Skarn type

Fluid inclusion studies of four types of ore samples reveal four types of inclusions: aqueous two-phase inclusions, H2O-CO2 inclusions, gas-rich inclusions, and daughter mineral-bearing inclusions containing halite or calcite. Homogenization temperatures of fluid inclusions in skarn minerals range from 350° to 535°C. The homogenization temperatures of fluid inclusions in greisen and stockwork are lower than that in skarn, and range from 200° to 360°C. The fluid inclusion data indicate that there are two types of fluids associated with the massive skarn and greisen, having salinities from 26 to 41 wt percent NaCl equiv, and from 1 to 21 wt percent NaCl equiv, respectively. The high-salinity inclusions are daughter mineral bearing, whereas the low-salinity inclusions are dominantly aqueous with a few gas-rich inclusions. The results suggest the evolution of the ore-forming fluids either by immiscibility or by mixing between a high-temperature, high-salinity magmatic water and a low-temperature, low-salinity fluid such as meteoric water, consistent with previously published isotopic studies.

Quartz-vein type mineral deposits

An example of Xihuanshan tungsten deposits at Dayu, Jiangxi (China)

Geological setting, mineralogy and genesis of tungsten mineralization in Dayu district, JiangXi (China):

 The city of Dayu is the center of the most important tungsten district of China. Valuable quantities of Sn, Mo, Bi, Nb, Ta, REE, Cu, Pb, Zn, Ag, Be and Li are also recovered.

At the Xihuashan and Dangping mines, hundreds of the typical Jiangxi wolframite-quarz veins, located at the top and marginal parts of a Jurassic biotite granite intrusion, are mined. At Piaotang the mined ore bodies consist of a stockwork-type of mineralization which cuts the roof-rocks (Cambrian metasediments: phyllite, quartz sandstone, hornfels) of the biotite granite.

Around the mineralizations, the country rocks display extensive alteration: namely K-feldspar alteration and greisenization in granite; tourmalinization, muscovitization, silicification, pyrophillitization in the metasediments.

Three stages of mineralization:1. Oxides,2. Sulfides,3. Carbonates.

The main mineralogy and parageneses of the veins and veinlets can be summarized as follows:

1st stage — oxide — (wolframite, cassiterite, molybdenite, quartz, K-feldspar, beryl, fluorite, topaz), the best represented;

2nd stage — sulphide — (chalcopyrite, galena, sphalerite, pyrite, pyrrhotite, bismuthinite, cassiterite, wolframite, quartz) well represented at Piaotang;

3rd stage — carbonate — (pyrite, scheelite, chlorite, sericite, fluorite, quartz, calcite).

The fracture-controlled mineralizations of Dayu appear to be the product of a continuous multistage process related to the late phases of the Jurassic Yanshanian magmatism.