As the most prevalent sulfide mineral within the Earth’s crust, pyrite plays a crucial role in various hydrothermal ore systems. These systems encompass porphyry-type deposits [1], sediment- and volcanic-hosted massive sulfide (VMS) deposits [2,3], Mississippi Valley-type (MVT) deposits [4], and epithermal-type deposits [5]. Pyrite, which has genetic connections with pre-ore, syn-ore, and post-ore deposition minerals, offers distinctive perspectives on fluid evolution. Additionally, it provides valuable information regarding the physicochemical conditions (such as temperature, oxygen fugacity, and sulfur fugacity) of hydrothermal systems throughout multiple mineralization stages [6,7,8,9].

Pyrite contains numerous solid inclusions, mainly consisting of sulfides, sulfosalts, and native metals, with high concentrations of elements like Pb, Hg, Bi, Sb, Au, Ag, Ni, Te, and As [10]. These mineral inclusions are predominantly found in low-temperature pyrite with high defect densities, and represent the principal origin of most trace elements within pyrite [10]. Hence, accurately analyzing the textures of sulfide minerals and the distributions of inclusions is essential for precisely evaluating trace-element contents and sulfur isotopes [7,8,11,12,13]. This analysis is especially important to prevent misestimating elemental concentrations, whether by overestimation or underestimation. In specific scenarios, the mineral inclusions hosted by pyrite can directly determine the fluid evolution processes, mechanisms of heavy-metal dissolution and transport, and pathways of ore mineral precipitation.

The objective of this research is to clarify the characteristics of solid inclusion assemblages in pyrite from the Bainaimiao porphyry copper deposit, which is located in the Central Asian Orogenic Belt, northeastern China, and is genetically related to Early Silurian granodiorite intrusions and porphyry dykes. Specifically, we aim to determine the compositions of these solid inclusions in pyrite from the southern ore belt across different mineralization stages. On this basis, we will assess the petrogenetic significance of the included mineral phases, focusing on their role in defining the ore-forming processes and the crystallization environment of the host pyrite. Furthermore, by combining microtextural observations of pyrite and associated gangue minerals, we seek to infer the physicochemical conditions (such as temperature, oxygen fugacity, and sulfur fugacity) during mineralization and establish a genetic model for the Bainaimiao copper deposit. This will help to deepen understanding of the deposit’s formation mechanism and provide a scientific basis for related mineral exploration and research.

The Bainaimiao region is located in the middle section of the accretionary orogenic belt, along the northern margin of the North China Craton (Figure 1a), with its northern boundary defined by the Ondor Sum subduction–accretionary complex belt [14] (Figure 1b). The dominant structural framework consists of NW-W-trending structures, including a series of folds, compression fracture zones, thrust faults, and schistosity zones, which control the distribution of rocks and ore deposits in this region.

The exposed strata in the area include the Middle–Late Proterozoic Bayan Obo Group, composed of slate and phyllite; the Paleozoic Baiyinduxi Group, composed of leptite and sillimanite-bearing biotite schist; the Ordovician Bainaimiao Group, composed of chlorite schist and minor sericite–feldspar–quartz schist; the Silurian Xuniwusu Formation, consisting of slate, phyllite, metasandstone, and andesitic volcanic rock; the Carboniferous Benbatu Formation, comprising graywacke, tuffaceous sandstone, and mudstone slate; the Permian Sanmianjing Formation, characterized by hard sandy and tuffaceous sandstone intercalated with limestone; and the Jurassic Daqingshan Formation, composed of acidic volcanic rock, continental facies sandy shale, and conglomerate (Figure 1c).

Magmatic activities in this area are frequent and diverse (Figure 1c), encompassing a spectrum of lithologies, from ultrabasic to acidic rocks. The intrusive rocks are predominantly Ordovician in age, including granodiorite and quartz diorite, which are distributed in the northeastern sector. In the southwestern region, Permian granite represents the dominant exposed magmatic rock unit.

The Bainaomiao deposit is a polymetallic deposit mainly featuring Cu, with significant associations of Au. It hosts reserves of over 0.6 million metric tons (Mt) of copper metal at an average grade of 0.8 wt.% Cu, and 20 tons (t) of Au with grades ranging from 1 to 16 g/t Au [16]. Geologically, this deposit stands out for its complex and diverse metallogenic processes, making it a key research subject for understanding the genesis and evolution of polymetallic deposits. The strata exposed within the mining area display remarkable geological diversity. The Ordovician Bainaomiao Formation provides the fundamental geological framework (Figure 2), while the Middle Silurian Xuniwusu Formation, Lower Permian Baiyinduxi Group and Sanmianjing Formation, and Upper Jurassic Daqingshan Formation each contribute unique sedimentary and tectonic features that have influenced the mineralization process. Additionally, Quaternary deposits cover the surface. The Bainaimiao Formation is the most widely distributed in the mining area. It extends nearly in an E-W direction and dips towards the S-SW. From top to bottom, it is divided into five lithologic sections. The first, third, and fifth sections are predominantly composed of greenschist; the second and fourth sections consist mainly of felsic schist, with thin interbeds of ferruginous quartzite in select areas. Orebodies predominantly occur within the third and fifth lithologic sections of the Bainaimiao Formation. They are hosted in rock assemblages consisting of chlorite–actinolite plagioclase schist, chlorite plagioclase schist intercalated with porphyroblastic rocks, and amphibolite, as well as amphibolite interlayered with biotite plagioclase schist (illustrated in Figure 2). Silicification, sericitization, chloritization, epidotization, and carbonatization are relatively developed in the Bainaomiao deposit. In the mining region, the exposed magmatic rocks are mainly quartz diorite, granodiorite porphyry, and muscovite granite.

Currently, twelve ore sections (labeled II–XIII) are either already in the process of exploitation or are undergoing development (refer to Figure 2). According to the spatial arrangement and geological characteristics of these sections, the deposit can be classified into two distinct belts: the southern ore belt, which encompasses ore sections II, III, IV, V, VI, VII, X, and XI; and the northern ore belt, including ore sections VIII, IX, XII, and XIII [17]. In the southern ore belt, orebodies are mainly presented in the forms of stratiform, lenticular, and vein structures, which are hosted within greenschist (as depicted in Figure 3). In the northern ore belt, the orebodies occur as fragmented bodies, predominantly hosted within the granodiorite porphyry. In the southern ore belt, the ores are predominantly characterized by zones of sulfide dissemination with distinctive structural features, and quartz veinlets (stockwork) are less developed. Locally, fine quartz veinlets may occur, but they are not the dominant ore-bearing structure. The metallic mineral composition is mainly composed of chalcopyrite, pyrite, and molybdenite, while pyrrhotite, sphalerite, galena, bornite, cobaltite, and native gold are present in relatively smaller quantities. Gangue minerals include quartz, calcite, biotite, K-feldspar, sericite, muscovite, epidote, chlorite, and gypsum. Ore textures are characterized by granular, veinlet, disseminated, and banded mineralization (Figure 4).

The sampling locations are indicated in Figure 2. Ten ore samples were collected from the 5th Formation of the Bainaimiao Group, X ore section in the southern ore belt. The samples were derived from metamorphosed volcanic rocks (greenschists) (Figure 4). About 30 mineral inclusions were analyzed, and the typical mineral and inclusion compositions are shown in Figure 5. The samples consisted predominantly of chlorite (20%), albite (20%), and quartz (15%), with ore minerals including pyrite, chalcopyrite, and bornite. Pyrite contained abundant inclusions.

Major- and minor-element analyses of pyrite and inclusions were conducted at the Geological Academy of Shandong Province using a JEOL JXA-8200 Superprobe electron microprobe (EPMA, Tokyo, Japan). The instrument operated with a focused beam, an acceleration voltage of 20 kV, and a beam current of 20 nA. Peak and background counting times were set as follows: 20 s and 10 s for Si, Na, Mg, Al, K, Cr, Ti, Fe, Cu, and Au; 10 s and 10 s for S, Zn, and As; and 10 s and 5 s for Pb. The detection limit was <120 μg/g for all elements.

At the Geological Academy of Shandong Province, in situ δ³⁴S analyses of pyrite, pyrrhotite, and chalcopyrite were carried out with a Thermo Fisher Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Fisher, Waltham, MA, USA). The analyses had a spatial resolution of approximately 10 μm (illustrated in Figure 4 and Figure 5). The analytical conditions were strictly regulated, with the temperature maintained at 18–22 °C and the relative humidity kept below 65%. For point ablation, specific parameters were set: the diameter was typically 20–30 μm for pyrite, the energy density ranged from 3 to 5 J/cm², and the frequency was 5–8 Hz. During isotopic measurements, ³²S and ³⁴S were simultaneously monitored using Faraday cups in static mode. Each measurement cycle had an integration time of 0.131 s, with 200 cycles conducted per analysis, resulting in a total acquisition time of around 27 s. Before the formal analysis, the instrumental parameters were optimized by utilizing sulfide reference materials to achieve the best performance. To reduce matrix effects, sulfide reference materials that were similar in composition to the sample matrices were selected, and a standard–sample–standard (SSS) bracketing method was adopted for mass discrimination correction.