Sulfide trace element enrichments in the metamorphic basement-hosted Xinhua Pb-Zn-Cu vein-type deposit, eastern Guizhou province (SW China)

Cambrian carbonate formations are widespread in the western Hunan-eastern Guizhou region (southwestern China), which hosts many Mississippi Valley-Type (MVT) lead–zinc (Pb-Zn) deposits. Regional Pb-Zn mineralization is well developed in the low-grade metamorphic rocks of the basal Proterozoic Banxi Group. The mineralization is associated with quartz veins and generally distributed along NE-trending fault zones. Moreover, these deposits have an extensive distribution and high grade, and are associated with Cu-Ag endowment. However, geological and geochemical research on these Pb-Zn vein-type deposits is relatively limited, and their relationship with the regional MVT mineralization remains unclear. The representative Xinhua deposit in Danzhai district is selected as the study subject. We conducted in situ trace element analyses on the sphalerite and chalcopyrite from the various metallogenic stages, and compared them with published sphalerite trace element data from the MVT Pb-Zn deposits in the western Hunan-eastern Guizhou metallogenic belt. Seven orebodies in Xinhua Pb-Zn deposit have been discovered so far, with a metal resource of over 120,000 metric tonnes of Zn + Pb. Field geology and microscopic petrography have revealed two mineralization stages: An early-stage black sphalerite (Sp-I) followed by reddish-brown sphalerite (Sp-II) mineralization, which corresponds to the main chalcopyrite mineralization stage, and a later-stage light-yellow sphalerite (Sp-III), Cu ore-barren mineralization.

LA-ICPMS data indicate that the sphalerite from Xinhua has similar trace element compositions to those from the MVT Pb-Zn deposits in the region. They are relatively enriched in Ga, Cd, and Ge, while depleted in Fe, Co, and Mn. Critical metal Ge and Ga are particularly enriched in sphalerite, especially in Sp-I (Ge max 937 ppm, Ga max 824 ppm). The substitution mechanism of Ge and Ga in sphalerite are likely 2Cu⁺ + Ge⁴⁺ ↔ 3Zn²⁺ and Cu⁺ + Ga³⁺ ↔ 2Zn²⁺. Indium and Sn are mainly present in Sp-I and Sp-III. Chalcopyrite contains Zn and Sn both exceeding 100 ppm. Contents of Se, Ag, In, and Sn in chalcopyrite are significantly higher than those in sphalerite. Calculation of the sphalerite trace element geothermometer (GGIMFis) suggests that the average sphalerite ore-forming temperatures are 164 °C (Sp-I), 156 °C (Sp-II), and 205 °C (Sp-III), implying medium- to low-temperature mineralization. This indicates a possible influx of high-temperature, in-bearing fluid during the late-stage mineralization.

In summary, the faults-controlled vein-type Pb-Zn deposits (e.g., Xinhua) may have been products of the same Kwangsian orogeny as other strata bound MVT deposits, and the Xinhua deposit features two mineralization stages with multiple ore metal sources. During the ore-forming fluid ascent, some ore-forming materials may have precipitated in the fluid conduits. And exposed after the erosion of the shallower stratiform orebodies and strata. Consequently, the Xinhua Pb-Zn deposit represents the preserved ore-fluid conduit phase (in the basement strata) of the MVT mineralization.