Abstract
Magmatic-hydrothermal gold–copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14‒0.24 wt% S and 0.1‒0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu–Au-related magmas, the lack of giant, early Cretaceous porphyry Cu–Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu–Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings.
Highlights
Magmatic-hydrothermal ore deposits emplaced in the shallow continental crust host many of the world’s largest Cu and Au accumulations (Richards 2009), including porphyry, skarn- and high-sulfidation epithermal-type ore Communicated by Gordon Moore.1 3 Vol.:(0123456789) 69 Page 2 of 18Contributions to Mineralogy and Petrology (2021) 176:69Au enrichment may occur during the subsequent evolution of these magmas, which depends largely on the type and extent of sulfide fractionation (e.g., Li et al 2019)
The data compilation suggests that Cu/Au orerelated stocks/dikes and all the other 126–145 Ma intermediate to felsic rocks from different parts of the central Taihangshan district (CTD) show highly comparable whole-rock elemental and Sr–Nd isotopic compositions (Figs. 3 and S2)
The 126–145 Ma magmatic rocks throughout the CTD likely formed during the transition from a compressional to an extensional regime (Chang et al 2021), presumably induced by the retreat of the low-angle subducting Paleo-Pacific slab, whereas the 110–125 Ma mafic to intermediate dikes might result from local mantle perturbations due to continuous slab retreat
Summary
Magmatic-hydrothermal ore deposits emplaced in the shallow continental crust host many of the world’s largest Cu and Au accumulations (Richards 2009), including porphyry-, skarn- and high-sulfidation epithermal-type ore Communicated by Gordon Moore.1 3 Vol.:(0123456789) 69 Page 2 of 18Contributions to Mineralogy and Petrology (2021) 176:69Au enrichment may occur during the subsequent evolution of these magmas, which depends largely on the type and extent of sulfide fractionation (e.g., Li et al 2019). There is growing evidence that arc-related basaltic magmas emplaced in the deep crust reached sulfide saturation relatively early, despite their high oxidation state (Lee et al 2012; Chiaradia 2014; Matjuschkin et al 2016; Chang and Audétat 2018; Du and Audétat 2020) These studies seemingly support the hypothesis that sulfide-rich lower crustal cumulates can be a potential source for Au-rich magmas (Richards 2009; Hou et al 2017), provided that the residual sulfides preferentially incorporated Au rather than Cu. recent experimental studies and natural observations suggest that these sulfides consist mainly of monosulfide solid solution (Li and Audétat 2012; Chang and Audétat 2018; Du and Audétat 2020), which phase preferentially scavenges Cu rather than Au
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