Minerals

The Axi gold deposit, a low-sulfidation epithermal deposit in the Western Tianshan, China, hosts over 50 t of gold resources and is widely regarded as the result of coupled processes of rock deformation, heat transfer, pore fluid flow, and chemical reactions. However, research on the ore-forming processes of this gold deposit from a coupled perspective remains limited, resulting in its ore-forming mechanisms being incompletely understood. In this paper, we use the concept of mineralization rate based on computational modeling to indicate the 3D spatial distribution of mineralization. The simulation results reveal the following: (1) temperature gradients play a key role in influencing mineral precipitation, whereas the effect of pore fluid pressure gradients is relatively negligible; (2) gold precipitation, characterized by a negative mineralization rate, predominantly took place along fault zones that exhibit vertical transitions from steep to gentle slopes or lateral bends, which are further distinguished by the accumulation of fluids and the presence of significant temperature gradients. Notably, this particular distribution pattern of gold precipitation closely mirrors the spatial arrangement of known gold orebodies. These findings suggest that the coupling of multiple physical and chemical processes at specific fault sites plays a critical role in ore formation, providing new insights into the mechanisms governing the development of the Axi gold deposit. Furthermore, based on these observations, it can be inferred that the deeper regions of the Axi gold deposit hold considerable mineralization potential.

The mechanisms of massive gold migration and enrichment are challenging issues in mineral deposit research. The evolution of the elements and structures of gold-bearing minerals is the key to revealing the mechanisms of gold enrichment and migration. The Sawayardun gold deposit has an ore reserve of 127 t located in the southwestern Tianshan, Xinjiang, China. It is an ideal place for studying the mechanisms of massive gold migration and precipitation. However, the occurrence and distribution of gold are unclear, preventing an understanding of the massive gold enrichment and precipitation mechanism in the Sawayardun gold deposit. Therefore, in this study, the microscopic structural characteristics and chemical compositions of sulfides and gold minerals in the deposit were comprehensively analyzed using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) techniques. The mineralization evolution is divided into a metamorphosed sedimentary period and a hydrothermal mineralization period, with the latter further subdivided into four mineralization stages: the quartz–pyrite stage, the arsenopyrite–pyrite stage, the polymetallic sulfide stage, and the carbonate stage. EPMA analysis reveals no clear compositional trends among different pyrite generations. Arsenopyrite (Apy) is more enriched in Au and Sb than pyrite. Overall, arsenopyrite is S-rich and As-deficient. Compared to Apy2, Apy1 is enriched in Fe and S but depleted in As. Stibnite is closely associated with native gold and contains elevated Au (up to 3.63%). Invisible gold exists in a form that is visible at the micrometer-to-atomic scale within pyrite and is lattice-bound in arsenopyrite. Visible gold occurs as native grains in quartz fractures or within sulfides. The composition of pyrite indicates that the Sawayardun gold deposit formed in a reducing, medium-depth, meso-epithermal environment. Au extraction by Sb-rich melts, dissolution–reprecipitation, and adsorption by As-bearing pyrite were the primary mechanisms for Au migration and precipitation. This study contributes to understanding the enrichment and precipitation processes of gold in orogenic-gold deposits in southwestern Tianshan.

Shale pores and throats are key factors controlling the enrichment and development efficiency of shale oil and gas. However, the characteristics and formation mechanisms of shale pores and throats remain unclear. Taking the Permian continental shales in the Mahu Sag of the Junggar Basin as an example, this paper studies the formation mechanisms of pores and throats in shales of different lithofacies through a series of experiments, such as high-pressure mercury injection and scanning electron microscopy. The results show that the Permian continental shales in the Junggar Basin are mainly composed of five lithofacies: rich siliceous shale (RSS), calcareous–siliceous shale (CSS), argillaceous–siliceous shale (ASS), siliceous–calcareous shale (SCS), and mixed-composition shale (MCS). The pores in shale are dominated by intergranular and intragranular pores. The intergranular pores are mainly primary pores and secondary dissolution pores. The primary pores are mainly slit-like and polygonal, with diameters between 40 and 1000 nm. The secondary dissolution pores formed by dissolution are irregular with serrated edges, and their diameters range from 0.1 to 10 μm. The throats are mainly pore-constriction throats and knot-like throats, with few vessel-like throats, overall exhibiting characteristics of nanometer-scale width. The mineral composition has a significant influence on the development of pores and throats. Siliceous minerals promote the development of macropores, and carbonate minerals promote the development of mesopores. Clay minerals inhibit pore development. Diagenesis regulates the development of pores and throats through mechanical compaction, cementation, and dissolution. Compaction leads to a reduction in porosity, and cementation has varying effects on the preservation of pores and throats. Dissolution is the main factor for increased pores and throats. These findings provide a lithofacies-based geological framework for evaluating effective porosity, seepage capacity, and shale oil development potential in continental shale reservoirs.