Episodic fluid pulses in the Baiyun gold deposit, Liaodong Peninsula, Eastern China: Insights from in-situ trace elements, sulfur isotopes, and texture characteristics of pyrite

Abstract

The driving mechanism behind the mineralization process remains ambiguous, whether it is propelled by a single fluid evolution or multiple fluid pulses. Minerals have the capacity to precisely document the fluid evolution. Hence, an accurate understanding of mineral formation is essential for a precise interpretation of fluid evolution. The Baiyun gold deposit is located in the Qingchengzi ore field in the Liaodong Peninsula of the North China Craton. The textural characteristics and geochemical composition of pyrite provide evidence for the formation process of the Baiyun gold deposit. Based on mineral assemblages, ore textures, and cross-cutting relationships, the Baiyun gold deposit can be divided into four stages: (I) disseminated/stockworked quartz-pyrite-K-feldspar, (II) quartz-pyrite-chalcopyrite-native gold vein, (III) quartz-pyrite-native gold ± galena ± sphalerite vein, and (IV) ore-barren calcite-quartz veinlets.

Native gold predominantly accumulates in stages II-III, each subdivided into two generations (Py2a, Py2b, Py3a, Py3b), with significant gold precipitation in the later Py2b and Py3b. Notably, Py2a displays high concentrations of As (avg. 412.66 ppm), Au (avg. 2.12 ppm), Ag (avg. 34.65 ppm), Co (avg. 471.88 ppm) and Te (avg. 39.79 ppm), contrasting with lower concentrations in Py2b for As (avg. 100.11 ppm), Au (avg. 0.99 ppm), Ag (avg. 8.33 ppm), Co (avg. 281.28 ppm) and Te (avg. 11.11 ppm). Stage III compares to stage II, with elements like Co (Py3a: avg. 2240.00 ppm; Py3b: avg. 170.15 ppm), Au (Py3a: avg. 0.74 ppm; Py3b: avg. 0.65 ppm), Ag (Py3a: avg. 5.33 ppm; Py3b: avg. 2.50 ppm), and As (Py3a: avg. 1132.91 ppm; Py3b: avg. 245.90 ppm) exhibiting similar trends of change. The δ³⁴S value trend (Py2a → Py2b: avg. 15.7 ‰ → avg. −8.0 ‰; Py3a → Py3b: avg. 11.5 ‰ → avg. −3.4 ‰) aligns with the trace element variations. Thermodynamic simulations, based on mineral compositions and sulfur isotopes, reveal difference fluid natures between Py2a (T = 300 °C; pH = 5.1–6.5; moderate fO₂ = −33.1 to −31.1) and Py3a (T = 250 °C; pH = 5.6–6.6; lower fO₂ = −39.2 to −36.1). Noteworthy differences exist not only in trace elements and sulfur isotopes between stages II-III but also in the micro-deformation of pyrite. Stage II is marked by plastic deformation (dominated by low-angle boundaries; 2–5°), resulting in a non-significant contribution to gold precipitation. In contrast, stage III exhibits brittle deformation (dominated by high-angle boundaries; >5°), where gold primarily precipitates and enriches. The significant variations in trace elements, sulfur isotopes, fluid natures, and pyrite deformation indicate the occurrence of episodic fluid pulses. Furthermore, sulfur isotopes display both enrichment and depletion characteristics. This phenomenon may be related to sulfate reduction. The Paleoproterozoic strata serve as significant sulfur reservoirs within the region and could potentially serve as the principal source of sulfur.