Process and Mechanism of Gold Mineralization at the Zhengchong Gold Deposit, Jiangnan Orogenic Belt: Evidence from the Arsenopyrite and Chlorite Mineral Thermometers
Abstract:The Zhengchong gold deposit, with a proven gold reserve of 19 t, is located in the central part of Jiangnan Orogenic Belt (JOB), South China. The orebodies are dominated by NNE- and NW- trending auriferous pyrite-arsenopyrite-quartz veins and disseminated pyrite-arsenopyrite-sericite-quartz alteration zone, structurally hosted in the Neoproterozoic epimetamorphic terranes. Three stages of hydrothermal alteration and mineralization have been defined at the Zhengchong deposit: (i) Quartz–auriferous arsenopyrite … Show more
“…Fig. 7. Classification diagram for (a) epidote group minerals (Armbruster et al 2006); (b) feldspar (Ab–An–Or plot; Deer et al 2013) from gabbroic and basaltic rocks from Mounts Kalnik and Ivanščica ophiolite mélange; and chlorite classification diagrams (c) Mg–Fe– VI (Al+Cr+Fe 3+ ) (Zane & Weiss, 1998) and (d) Fe/(Fe+Mg)–Si (after Sun et al 2019). …”
This contribution provides insights into ocean-floor hydrothermal metamorphism of the fast-evolving Dinaridic Neotethys. Mineralogical, geochemical and Sr isotope data collected from altered ophiolites and non-ophiolite basalts/andesites and tuffs of the active continental margin are consistent with hydrothermal alteration trajectories that reflect the host-rock composition. This suggests that hydrothermal fluxes were restricted within a simple closed seawater-fed system. Based on the initial isotopic ratios of Sr, two fluid–rock interaction trends are established: (a) low-to-medium degrees of metasomatism in pre-Middle Jurassic anorogenic ophiolites that progressively abated, and (b) increased intensities of metasomatism in post-Middle Jurassic orogenic ophiolites. This agrees with chlorite thermometry and Ca-Al-(Fe)-silicate phase chemistry. The metamorphic assemblages belong to the zeolite, prehnite-pumpellyite, prehnite-actinolite and greenschist facies. The facies is reliant on the temperature of hydrothermal systems and their fluid chemistry. Rare earth element (REE) phase geochemistry shows (a) variable fluid–rock ratios in chlorite and pumpellyite dependent on fluid temperatures, (b) prominent Eu and Ce anomalies that reflect the fluid oxidation state, (c) light REE/heavy REE mobilization attributed to prevalent ligand complexation, and (d) multi-phase fluid percolation across reaction zones of heterogeneous permeability. This study proposes initiation of simple hydrothermal system(s) at or near a spreading centre(s) in the infancy of the Dinaridic Neotethys. Such a system became more complex during Middle Jurassic and Early Cretaceous time with reactive hydrothermal fluids passing the recharge area and reaching the hot reaction zone. An abrupt obliteration of the established high-temperature regime ensued, following the final closure of the Neotethys.
“…Fig. 7. Classification diagram for (a) epidote group minerals (Armbruster et al 2006); (b) feldspar (Ab–An–Or plot; Deer et al 2013) from gabbroic and basaltic rocks from Mounts Kalnik and Ivanščica ophiolite mélange; and chlorite classification diagrams (c) Mg–Fe– VI (Al+Cr+Fe 3+ ) (Zane & Weiss, 1998) and (d) Fe/(Fe+Mg)–Si (after Sun et al 2019). …”
This contribution provides insights into ocean-floor hydrothermal metamorphism of the fast-evolving Dinaridic Neotethys. Mineralogical, geochemical and Sr isotope data collected from altered ophiolites and non-ophiolite basalts/andesites and tuffs of the active continental margin are consistent with hydrothermal alteration trajectories that reflect the host-rock composition. This suggests that hydrothermal fluxes were restricted within a simple closed seawater-fed system. Based on the initial isotopic ratios of Sr, two fluid–rock interaction trends are established: (a) low-to-medium degrees of metasomatism in pre-Middle Jurassic anorogenic ophiolites that progressively abated, and (b) increased intensities of metasomatism in post-Middle Jurassic orogenic ophiolites. This agrees with chlorite thermometry and Ca-Al-(Fe)-silicate phase chemistry. The metamorphic assemblages belong to the zeolite, prehnite-pumpellyite, prehnite-actinolite and greenschist facies. The facies is reliant on the temperature of hydrothermal systems and their fluid chemistry. Rare earth element (REE) phase geochemistry shows (a) variable fluid–rock ratios in chlorite and pumpellyite dependent on fluid temperatures, (b) prominent Eu and Ce anomalies that reflect the fluid oxidation state, (c) light REE/heavy REE mobilization attributed to prevalent ligand complexation, and (d) multi-phase fluid percolation across reaction zones of heterogeneous permeability. This study proposes initiation of simple hydrothermal system(s) at or near a spreading centre(s) in the infancy of the Dinaridic Neotethys. Such a system became more complex during Middle Jurassic and Early Cretaceous time with reactive hydrothermal fluids passing the recharge area and reaching the hot reaction zone. An abrupt obliteration of the established high-temperature regime ensued, following the final closure of the Neotethys.
“…The Jiangnan Orogen Belt (JOB) is one of the two lode Au-Sb-W metallogenic belts in the world, and over 250 gold-polymetallic occurrences have been reported in it, with a total Au resource of over 970 tonnes (t) [1][2][3][4][5][6]. As a Precambrian terrane extending northeastward for 1500 km, the JOB is a collision zone between the Yangtze and Cathaysia blocks, which formed the South China Block [7][8][9].…”
The Chaxi gold deposit is located in the southwestern Hunan Province (South China). Extremely high-grade (up to 3 × 105 g/t, avg. 5.3 g/t) Au mineralization is developed in quartz-sulfide veins controlled by WNW- and NNE-trending faults. The sulfide/sulfosalt assemblage is dominated by pyrite, chalcopyrite, and galena, with minor tetrahedrite and chalcocite. The alteration includes beresitization and carbonation. Based on the vein crosscutting relationship and mineral assemblages, the hydrothermal period comprises three stages: (1) pre-ore quartz-pyrite, (2) syn-ore quartz-ankerite-native gold-sulfide-sulfosalts, and (3) post-ore quartz-calcite-pyrite alteration. The Au occurrence is dominated by native gold, with minor native Au nanoparticles (inside sulfides) as indicated by EPMA. Fluid inclusions (FIs) in the ore-related quartz yielded homogenization temperatures and salinities of 139.6–267.1 °C and 2.7–17.6 wt.% NaClequiv (Stage I), 137.5–387.2 °C and 2.7–19.9 wt.% NaClequiv (Stage II), and 139.7–330.5 °C and 3.1–21.4 wt.% NaClequiv (Stage III). Such varying and high FI salinities can be attributed to fluid boiling. The calculated δ18OH2O values are of 2.40–5.63‰, and the fluid inclusion δD values for quartz are of −71.73 to −49.8‰. The auriferous sulfide δ34S values (6.26–19.33‰) overlap with those of the Chang’an formation (16.31–21.66‰) and Banxi Group metamorphic rocks. The auriferous sulfides have 206Pb/204Pb = 16.7215–17.2281, 207Pb/204Pb = 15.4413–15.6177, and 208Pb/204Pb = 36.9731–38.7232, distinct from those of the wallrocks. The analyzed pyrites yield Co/Ni ratio > 1 (0.539–77.000, avg. 10.559). The isotope (H, O, S, Pb) signatures coupled with EPMA results indicate that the ore-forming fluids were derived from the magmatic fluid and mixed with meteoric water, and the Pb was originated from the mantle. The ore sulfur was likely leached from the Neoproterozoic meta-clastic rocks. The Chaxi gold mineralization shares many geological and geochemical similarities with (albeit also with minor differences) typical orogenic gold deposits, and is best classified as broad sense orogenic deposit, as proposed for many other gold deposits in the Jiangnan Orogen.
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