Two episodes of mineralization in the Baolun high-grade lode gold deposit, Hainan Island, South China: Insight from LA-ICP-MS analysis of hydrothermal rutile

Abstract

The Baolun is the largest gold deposit on Hainan Island, South China, with proven reserves of over 80 t Au at an average grade of 10.3 g/t. Two economic types of gold mineralization coexist in the long-lived fault system: 1) Au-bearing sulfide quartz veins (Va), dominated by pyrite, pyrrhotite, and arsenopyrite; and 2) Au-Bi assemblage quartz veins (Vb), dominated by bismuthinite, maldonite, and native bismuth. The Baolun district has a long history of tectonic reactivation, involving crustal compression and extension, magmatic activity, multiple episodes of fluid flow, and hydrothermal mineralization. Consequently, there exists no generally accepted model for the precise timing and source of the Baolun gold deposits. In this study, two different rutile grains (Rt-1 and Rt-2) were recognized separately in the Va veins and Vb veins based on paragenetic sequence. In situ U-Pb dating indicates that Rt-1 and Rt-2 yielded two group ages of 248.3 ± 2.8 Ma (MSWD = 0.8) and 78.2 ± 8.4 Ma (MSWD = 1.4), respectively. The Rt-1 contains 632.0–6051.2 ppm W, 459.1–2169.1 ppm Zr, 1215.0–8267.5 ppm Nb, 107.3–1229.6 ppm Ta, 19.0–79.2 ppm Hf, 160.2–311.8 ppm Sn, 1371.6–3500.7 ppm V, 325.3–3813.9 ppm Cr, 0.6–22.7 ppm Sb, 0.1–38.8 ppm Y, with Ca below the detection limit. The Rt-2 contains 704.9–4390.2 ppm W, 1.8–168.5 ppm Zr, 1264.6–4062.9 ppm Nb, 108.1–453.2 ppm Ta, 0.3–6.9 ppm Hf, 6.5–32.1 ppm Sn, 14.3–311.0 ppm V, 29.4–167.1 ppm Cr, 0.6–27.3 ppm Sb, 56.3–38.8 ppm Y, and 548.7–2750.3 ppm Ca. Obviously, LA-ICP-MS trace element analysis revealed significant discrepancies in composition between Rt-1 and Rt-2, suggesting that they precipitated from distinct fluid systems. Combining mineral studies, field observations, structural analysis, and geologic evolution, the results of this study indicate that the Baolun gold deposit formed during two episodes of mineralization. The Early Triassic gold mineralization is clearly classified as an orogenic type. The ore-forming materials were liberated by the deformation and metamorphism during the Indosinian orogeny of South China, which was triggered by the closure of the Paleotethys Ocean. The Late Cretaceous Au-Bi mineralization belongs to a magmatic-hydrothermal origin, which was deposited under an extension setting associated with the post-subduction of the paleo-Pacific plate. The new mineralization model proposed provides significant implications for future exploration strategies.