Characteristics of fluid inclusions in the Alqal Copper-Lead-Zinc deposit, Xinjiang

The Alqal Copper-lead-zinc deposit isl ocated in the Tarim plate of the Xinjiang. It has undergone multiple stages of tectonic mineralization and is an important deposit in the Kuslafu metallogenic belt of the West Kunlun area. Based on the analysis of the metallogenic geological conditions of the deposit, The composition of fluid inclusions in mining area was studied, The characteristics of metallogenic fluid are discussed. The results show that the uniform temperature of gas-liquid two-phase fluid inclusions in the Alqal deposit is between 145°C and 267°C, the average uniform temperature is 191°C; the salinity is 0.71% to 14.87%, and the average salinity is 6.33%. The estimated metallogenic pressure is about 135MPa, and the capture temperature is concentrated around 320°C. The ore-forming fluid has undergone multi-stage superimposed transformation, which is inconsistent with the characteristics of the typical MVT deposit, and it is a sedimentary transformation type deposit.


Introduction
West Kunlun Orogen in Xinjiang Uygur Autonomous Region is a key area in China's strategic deployment of ore exploration, having several Cu-Pb-Zn-Au-Ag metallogenic belts with rich reserves due to the superior ore-forming geological conditions. The Kusilapu metallogenic belt is located at the junction of the northeastern margin of the West Kunlun-Pamir area and the southwestern margin of the Tarim Basin. As an important deposit discovered in this metallogenic belt, the Alqal copper-lead-zinc polymetallic deposit has received extensive attention. The genesis of its deposits is controversial, and different opinions reflect the complexity of mineralization and the versatility of deposits. Preliminary studies have controversy over the ore genesis of carbonate-type Pb-Zn deposit, which greatly restricts Figure1.The Geologica lsketch map of the study area(according to Ref.6) 1-Carbonate；2-Clastic；3-Fault；4-Axis of syncline；5-Copper-lead-zinc ore body；6-Iron ore body Metamorphic siltstone: The main minerals are quartz (80%), feldspar (15%) and muscovite (2~5%). Quartz particles are semi-automorphic-heteromorphic granules, with low roundness, and overall a slightly directional distribution, with a particle size of about 0.04mm. Feldspar is mostly distributed in the interstices of quartz grains and has strong alteration. The altered minerals are mainly sericite and chlorite. Containing a small amount of muscovite, it is a long strip, scattered among the quartz particles.
Limestone: The main mineral components are calcite (90%), quartz (10%), etc. The calcite is semiautomatic-heteromorphous-semi-rhombic-irregular, with obvious double crystal grains, and the grain size is mostly between 1 and 2 mm. Quartz particles are filled between calcite particles in irregular shapes with distinct particle sizes.
Crystalline limestone with biological clastics: the main mineral components are calcite(85%), quartz (10%) and biological clastics(5%). Calcite is mostly produced in the form of fine crystals, with a particle size between 0.1 and 0.2 mm, and the recrystallization effect is obvious. There are bands of flint nodules with a width of about 2mm. From the perspective of crystal shape, the biological debris is dominated by crinoid stems.
Dolomite bioclastic crystalline limestone: the main minerals are calcite(60%), dolomite(30%) and bioclastic(10%). The calcite can be seen in the form of bands, the degree of self-shape is general, the phenomenon of agglomeration is obvious, and the particle size is 0.4~0.8mm. Dolomite is a metasomatous origin, distributed on the surface of calcite, with a more euhedral quadrilateral shape, with a particle size of about 0.1mm. The types of bio detritus may be diverse. Judging from the shape, the stems of crinoids are the main ones with a small amount of fossils.
The ore mineralscompriseCcp, Gn, P, Sp, and their oxide; gangue minerals include Dol, Cc, Qrz, Ms, etc.; ore textures are crushed porphyritic crystal texture, strawberry-like texture, replacement remnant texture, and idiomorphic granular texture; ore structure are mainly block structure, disseminated structure and vein structure, most of which are generated through replacement and fissure filling; and the surrounding rock alteration of the ore deposit includes silicification, sulfidation, carbonatization and argillization, in which carbonatization is found in a wide area, and silicification, argillization and sulfidation, which are closely related to mineralization, belong to close-to-ore alternation types. (Fig.2). The interspersed metasomatism between the minerals was observed under a microscope, and it was found that sphalerite and galena replaced pyrite along the periphery and fissures of the pyrite, so that the residual structure of the transaction was distributed in the galena. Some galena and sphalerite are directly interspersed with pyrite in the form of veins, and the phenomenon of mutual substitution between galena and sphalerite is obvious. The common pyrite calcite veins are filled and developed. The above characteristics indicate that the mining area has the characteristics of multi-stage mineralization, and the generation sequence is roughly as early as dolomite--pyrite+calcite vein--galena+sphalerite.

Inclusion petrography
As the only ancient ore-forming fluid contained in the mineral crystal lattice, fluid inclusion research is the most direct means to understand the physical and chemical properties of ore-forming fluids [8−10]. By selecting samples from different sections in the mine district, this inclusion research divided the fluid inclusions of Alqaldeposit into two main types: inclusions of gas-liquid aqueous solution (Type-I), and liquid inclusions (Type-II, which were mostly formed in later stages and rarely related to mineralization). Inclusions of gas-liquid aqueous solution have a V/T ration ranging mostly between 10% and 25% at room temperature (20℃), are small in size and irregular in shape with diameters mainly between 2μm～4μm, and are distributed in isolationin quartz (Fig.3). Note: V/TI type inclusions represent the percentage of the gas phase to the total volume of the inclusions; Tm (ice)-the melting temperature of ice; Th (to L)-complete homogenization temperature.
For type I inclusions, L represents the homogeneous liquid phase; The main mineral of the sample inclusions is quartz.

Discuss
Based on the average thickness of the covered strata in the research area-about 5km [14] , the average density of overlying strata is assumed as 2.7g/cm 3 , and the metallogenic pressure is estimated to be 135 MPa based on lithostatic pressure (P=hρg). The homogenization temperatures of the tested inclusions were calibrated by the relation figure of homogenization temperature and pressure of NaCl solution of varying concentration, and the calibration value is approximately +125℃ [15]. As a result, the trapping temperature is calculated and estimated to be inthe range of 270℃～412℃, mainly around 320℃.
It can be known from Fig. 4 that there are two relatively temperature-salinity concentrated sections in the gas-liquid fluid inclusions of Alqal copper-lead-zinc deposit, one with the homogenization temperature ranging from 140℃ to 190℃, and the salinity varying between 7% and 16%; the other with the homogenization temperature ranging from 200℃ to 260℃, and the salinity between 0% and 6%. These two concentrated sections of different temperature-salinity indicate that there potentially exist two stages of ore-forming fluids in Alqal copper-lead-zinc deposit. The inclusion with trapping temperature (320℃ on average) probably comes from the deep-seated magmatic hydrothermal fluids, while the inclusion with low temperature may derive from meteoric water or basinal hot brine.
The ore-forming fluids of standard MVT-type lead-zinc deposit mostly originate from basinal hot brine, with relatively high salinity (>15%) but relatively low trapping temperature (usually no more than 150℃ ～ 200℃) [16]. The original ore-forming fluids in Alqal copper-lead-zinc deposit is probably similar to MVT-type lead-zinc deposit. However, along with the later invasion of deepseated magma, the deep fluids moved into the spots with weak geological structure, mixed with early fluids and finally formed superimposed mineralization at appropriate structural positions.
The physical and chemical characteristics of the two stages of ore-forming fluids demonstrate that Alqal copper-lead-zinc deposit is a superimposed and modified deposit, and differs to some degree from the standard MVT-type (Mississippi valley type) lead-zinc deposit.
A kind of mineralized breccia is widely distributed in the study area, which mainly appears in or near the junction of the lower clastic rock and the upper carbonate rock (paleo aquifer). Most breccias are closely related to mineralization. Through microscopic observation of the mineralized dolomitic breccia, it is found that the composition of the breccia is dominated by more euhedral rhombohedral dolomite particles, closely inlaid with each other. Sandy quartz and silicified quartz veins can also be seen in the breccia. The cement is also mainly dolomite, and some particles have obvious arc-shaped cleavage and double crystal lines. Pyrite mineralization can be seen at the edge of the breccia. Although mineralized breccia is distributed around the fault, it does not have the characteristics of structural breccia from the perspective of the composition of cement and breccia. The cement is mainly composed of calcareous cement, which is consistent with the surrounding strata. Fracture marks such as friction surfaces may be caused by hydrothermal metasomatism of carbonate rocks, and the surrounding fractures may play the role of hydrothermal transport channels.

Conclusion
After researching and analysing the characteristics of ore-forming fluids in Alqal copper-lead-zinc deposit, it can be concluded that: (1) The homogenization temperature of the gas-liquid fluid inclusions in Alqal deposit ranges between 145℃ and 267℃ (191℃ on average); the salinity varies from 0.71% to 14.87% (NaCl quality percentage, 6.33% on average). The metallogenic pressure is estimated to be 135 MPa, and the trapping temperature (after calibration) of the fluid inclusions centres around 320℃.
(2) The fluids of Alqal copper-lead-zinc deposit take on a feature of multi-phase superimposition. The ore-forming fluids of the two phases are respectively related to meteoric water-basinal hot brine and the deep-seated magma.
(3) In its early stage, the characteristics of ore-forming fluids in Alqal copper-lead-zinc deposit resemble the lead-zinc deposit of standard Mississippi valley type; in its later stage, the fluid characteristics do not conform to those of Mississippi type, but taking on the fluid characteristics of superimposed and modified deposit.