Genesis of the graphite orbicules in the Huangyangshan graphite deposit, Xinjiang, China: Evidence from geochemical, isotopic and fluid inclusion data

Abstract

The Huangyangshan graphite deposit in the East Junggar Orogen (China) contains intriguing graphite orbicules. This contribution is designed to present reasonably comprehensive assessment of their origin. In this deposit, the intrusive orebodies are mechanically juxtaposed against contacts with granitic wall rocks. The orbicules are typically concentrically zoned in texture with a notable binary configuration involving a central core and an outer shell. The orbicule cores possess mineralogical and compositional variations that are more or less similar to those of the granitic wall rocks. The orbicule shells are composed of various ferromagnesian minerals (clinopyroxene + hornblende) in stark contrast to the matrix, where hornblende, sanidine and albite have higher Mg#, Ab and An values, respectively. The replacement of silicates by graphite and sulfides (pyrrhotite + chalcopyrite + pyrite) is common. The graphite throughout the matrix and orbicules features a patchy distribution of major elements (C) and dispersed elements (Cr, Ni, Co, S, Cu, Zn, As, Pb, etc.). The carbon isotopic compositions of the orbicular ore are −20.9 to −20.5‰, analogous to those of the surrounding strata, i.e., −25.6 to −19.3‰. The sulfur isotopic compositions of sulfides in the orbicular ore are −4.8 to −2.6‰, which differ from those of the surrounding strata, i.e., 16.0–23.0‰. Fluid inclusions principally comprise the volatile component CH₄ and solid daughter minerals (graphite + chalcopyrite) with T_m > 450 °C. Vapor-liquid two-phase fluid inclusions formed under conditions of T_h = 143.6–367.5 °C, P = 3–194 bar, and fO₂ = −52.76 to −31.15. The hydrogen and oxygen isotopic values of the orbicular ore are δD = −110.3 to −107.0‰ and δ¹⁸O_water = 10.1–11.7‰. Zircons from the orbicular ore have ²⁰⁶Pb/²³⁸U ages of 304–359 Ma, with a weighted average of 319.0 ± 6.4 Ma, overlapping the geochronological age of the granitic host rocks (305.1–321.4 Ma). The orbicular ore has SiO₂ concentrations ranging from 53.30 to 72.61 wt%, falling within the compositional fields of granite, quartz monzonite, granodiorite, diorite, and monzodiorite in the TAS classification diagram. The Sr-Nd and Hf isotopic data from the orbicular ore indicate mixing between materials derived from the depleted mantle and materials derived from the crust, with a dominant mantle contribution. The petrographic, geochemical, isotopic and fluid inclusion characteristics indicate that the processes of incorporation of H₂O and wall rock fragments, liquid immiscibility and exsolution of a late magmatic fluid were responsible for orbicule formation in the Huangyangshan deposit. The following genetic model is proposed: (1) In the late Carboniferous, a new orbicular magma containing organic matter-derived CH₄ and deep-mantle SO₂ emplaced within the Huangyangshan pluton. Contemporaneously, crystal fragments and H₂O were incorporated into the orbicular magma from the granitic host rocks. (2) H₂O saturation was reached in the orbicular magma and fostered segregation of a hydrous ferromagnesian melt (orbicules) with a silicic residual melt (matrix). (3) At approximately 630 °C, 2000 bar and −18 lgfO₂, graphite and sulfides formed en masse through deuteric alteration: 3CH₄ + 2SO₂ + 2(Fe²⁺)₇[Si₄O₁₁]₂(OH)₂ (hornblende) = 3C (graphite) + 2FeS (pyrrhotite) + 8SiO₂ (quartz) + 2(Fe₃Si₄O₁₀(OH)₂ + Fe₃(OH)₆) (chlorite), following the expulsion of a late magmatic fluid from the ferromagnesian melt.