Newly discovered Early Cretaceous Au mineralization overprinting Late Jurassic Pb-Zn-Ag mineralization in the Qin-Hang metallogenic belt (South China)

Abstract

Magmatic and mineralizing events in South China were traditionally thought to be limited between 150 Ma and 130 Ma despite the numerous Mesozoic-aged magmatic-hydrothermal deposits in the region. This viewpoint is being reevaluated in light of new age data. Kangjiawan is a representative Pb-Zn-Ag-Au deposit in the Hengyang Basin in the Qin-Hang metallogenic belt of South China that contains 1.96 Mt of Pb-Zn resource with 4.70% Zn, 6.02% Pb; 2280.5 t of Ag resource with 108.23 g/t of Ag; and 58 t of Au resource with 3.25 g/t of Au. Its mineralization can be divided into four stages: (I) pre-ore quartz−pyrite, (II) quartz−pyrite−galena−sphalerite, (III) quartz−galena−sphalerite, and (IV) calcite−pyrite. Multiple generations of pyrite were identified at Kangjiawan. Generation I pyrite (Py1, formed in Stage I) is coarse-grained, texturally homogeneous, and contains abundant silicate inclusions (e.g., K-feldspar and titanite). Generation II pyrite (Py2, formed in Stage II) is generally fine-grained and exhibits a core-rim texture, with the porous, sphalerite−galena inclusion-bearing core (Py2a) having replaced Py1 and the rim (Py2b) being homogeneous. Generation III pyrite (Py3, formed in Stage IV) is subdivided into Py3a and Py3b, with the homogeneous Py3a often replaced by the porous Py3b. The low Co/Ni ratio of Py1, along with the abundant silicate inclusions, is suggestive of intense fluid-rock interaction during Stage I. Py2a is As-Cu-Pb-Ag-Au−rich, Co-depleted, and has elevated Ag/Co (average 3.24) and As/Co (average 10,217.14) ratios, which are suggestive of extensive fluid boiling during the formation of Py2a. This is also supported by the ubiquitous Py2-cemented hydrothermal breccia. In contrast, Py2b is depleted in As-Cu-Pb-Ag-Au but enriched in Co, which is indicative of non-boiling conditions. Subsequent sealing of fractures by veins may have caused the formation of coarse-grained sphalerite and galena in Stage III. The similar 207Pb/206Pb values among Py1 (average 0.849), Stage II galena (average 0.849), Stage III galena (average 0.849), and the nearby Shuikoushan granodiorite stock (corrected average 0.848) suggests that mineralization in stages I−III may represent distal skarn-type Pb-Zn-Ag mineralization genetically related to the Shuikoushan stock. This interpretation is supported by the similar ages of mineralization Stage I (apatite U-Pb: 159.4 ± 1.0 Ma) and Stage II (fuchsite 40Ar-39Ar: 158.1 ± 0.4 Ma) to the age of Shuikoushan stock (zircon U-Pb age: ca. 158 Ma). As the primary Au-hosting mineral at Kangjiawan, the As- and Au-rich Py3a occurs in sharp contact with porous Co-Ni-Se-Bi-Mo-As−poor Py3b, which commonly contains abundant fine-grained galena and sphalerite inclusions. This suggests that Py3b formed via coupled dissolution−re-precipitation of Py3a. This coupled dissolution−re-precipitation reaction favors the remobilization of Au from within the Py3a structure and its re-enrichment as Au inclusions in Py3b; this is supported by the anomalous Au peaks in time-resolved signal profiles from laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and the abundant outliers for Au concentrations in Py3b. The markedly distinct trace-element geochemistry of Py3 compared to those of Py1 and Py2, and the U-Pb age of Stage IV calcite (ca. 138 Ma), are indicative of an Early Cretaceous Au mineralization event. Published calcite C-O isotope data, coupled with the narrow range of δ34S values (0.6‰−2.2‰) of Py3, suggest that the Kangjiawan Au mineralization may be associated with a concealed Early Cretaceous pluton. We propose that the Kangjiawan deposit formed via the overprinting of Late Jurassic distal skarn Pb-Zn-Ag mineralization by Early Cretaceous magmatic-hydrothermal Au mineralization, which demonstrates that 150−135 Ma may also be an important period for the formation of the magmatic-hydrothermal mineralization belt.