Origin of PGE-Depleted Ni-Cu Sulfide Mineralization in the Triassic Hongqiling No. 7 Orthopyroxenite Intrusion, Central Asian Orogenic Belt, Northeastern China

Abstract

The ~216 Ma Hongqiling No. 7 intrusion is located in the southern margin of the Central Asian orogenic belt, northeastern China. It was the second largest Ni producer after Jinchuan in China. The intrusion crops out in an area of ~0.013 km 2 on the surface. It intruded gneisses and marbles of the Paleozoic Hulan Formation. The intrusion is mainly composed of orthopyroxenite (~95 vol %), with minor harzburgite and norite. Over 90 vol % of the intrusion contains disseminated, net-textured or massive sulfides. The bulk sulfide ores (recalculated to 100% sulfide) have low average PGE concentrations (5.6 ppb Os, 3.1 ppb Ir, 4.7 ppb Ru, 2.2 ppb Rh, 31 ppb Pt, and 11 ppb Pd) and high Cu/Pd ratios (3 × 10 5 −3 × 10 7 ), consistent with the compositions of sulfide liquids segregated from PGE-depleted magma. Olivine composition (Fo <86 mol %) indicates a fractionated parental magma. Back calculation shows that the parental magma was possibly derived by olivine fractionation from a primary magma containing ~16 wt % MgO. Numerical modeling indicates that PGE depletion in the parental magma of the Hongqiling No. 7 intrusion was due to previous sulfide segregation in the staging chamber. Whole-rock data show light rare earth element (REE) enrichments, negative Nb-Ta anomalies, and positive eNd (t = 216 Ma) values from 3 to 5, which can be explained by mixing of a mantle-derived magma with a granitic melt formed by magma underplating in the crust. The gOs (t = 216 Ma) values of sulfide ores vary from 50 to 187. The highly variable gOs(t) values coupled with similar eNd(t) values indicate preferential assimilation of crustal sulfides during magma rock interaction. Based on the observations, we propose that the parental magma of the Hongqiling No. 7 intrusion formed by contamination of mantle-derived, S-undersaturated magma by crustally derived granitic melt, together with olivine crystallization, in a staging chamber. The contaminated magma became PGE depleted due to sulfide segregation in response to crustal contamination. The PGE-depleted magma became S undersaturated again during ascent due to the negative effect of pressure on the maximum solubility of sulfur in magma. Upon arrival near the present level of the Hongqiling No. 7 intrusion a second event of sulfide segregation took place due to addition of crustal sulfides from immediate country rocks. The immiscible sulfide liquids and orthopyroxene crystallizing from the magma were retained in a dynamic conduit to form the ore-bearing intrusion while much of the magma left the conduit to form some of the nearby sulfidepoor intrusions. We suggest future exploration in the region to focus on olivine-bearing and orthopyroxenedominant intrusions with ages similar to that of the Hongqiling No. 7 intrusion.

Petrology, mineral chemistry, and geochemistry of Late Triassic Ni–Cu ore-bearing mafic–ultramafic intrusions, Hongqiling, northeastern China: petrogenesis and tectonic implications

“…11a), which also indicates crustal contamination. This conclusion is further supported 292 by initial 187 Os/ 188 Os ratios (0.34 to 3.02) measured in the Hongqiling ore rocks and reported by 293 previous workers (Wei et al 2013;Sun 2013a;Hao et al 2014b). The values of 187 Os/ 188 Os are much higher than those of Cu-Ni sulfide deposits free of crustal contamination (e.g.…”

“…This trace element characteristics of the Hongqiling ore-bearing intrusions suggest that Hongqiling ore-bearing intrusions experienced the low-grade metamorphism or hydrothermal alteration. In addition, the Pd/Ir ratios of Hongqiling rocks are very low (Wei et al 2013;Hao et al 2014b), also suggesting hydrothermal alteration had a very small impact on the Hongqiling ore-bearing intrusions. Based on the mobility of elements, we choose HFSEs and REEs (e.g.…”

Geochronology and Petrology of the Beiligaimiao Magmatic Sulfide Deposit in the Northern Margin of North China Craton, Inner Mongolia

“…1a, the southern part of the Central Asian Orogenic Belt contains magmatic sulfide deposits formed in a subduction setting such as the Beiligaimiao deposit (this study) and the Erbutu deposit (Peng et al, 2013) in the central region, and post-subduction setting such as the Huangshandong deposit in the western region (Mao et al, 2014) and the Hongqiling No.7 deposit in the eastern region (Wei et al, 2013).…”

Newly identified Middle&ndash;Late Permian mafic&ndash;ultramafic intrusions in the southeastern margin of the Central Asian Orogenic Belt: Petrogenesis and its implications

“…Previous studies revealed that the Late Triassic maficultramafic intrusions (e.g., Nos. 1 and 7) possess positive e Nd (t) values varying from 0.2 to 5, and show enrichments in LILE and LREE relative to HFSE, with negative Nb-Ta anomalies, suggesting that the Late Triassic mafic-ultramafic magmas were derived from partial melting of a metasomatized lithospheric mantle triggered by upwelling asthenosphere Lü et al, 2011;Wei et al, 2013). Relative to the Middle-Late Permian (e.g., Nos.…”

“…(b) Tectonic sketch map of NE China (modified after Wilde et al, 2000;Wu et al, 2007;and Zhou and Wilde, 2013). (c) Geological map of the central area of Jilin Province (modified after Wei et al, 2013). 2006; Tang and Yang, 2007;Lü et al, 2011;Wei et al, 2013;Hao et al, 2014a, b). Due to minor occurrence of Middle-Late Permian magmatism in the CJP, the tectonic events of this stage are still poorly constrained.…”