Deep Magmatic Process Of The Early Cretaceous Gold Metallogenic System In The Eastern North China Craton

The North China Craton（NCC）has undergone significant craton destruction since the Mesozoic,which is considered to be responsible for the large-scale magmatism,and short-term and explosive gold metallogeny in eastern NCC.Jiaodong gold deposit concentration area has the proven gold reserves over 7,000 tons,making it the most important gold producer in China.However,the source of huge amounts of gold and relationship between deep geological processes and shallow-level gold mineralization are still debated.Gold-rich sulfide melt inclusions were observed in both mantle xenoliths within basalts and intermediate to mafic dykes in eastern NCC which were emplaced contemporaneously with gold mineralization（120±10 Ma）,indicating that sulfide saturation existed in the magma source and during magmatic evolution.Based on geological investigations and petrological observations,detailed mineralogical,and in-situ mineral and sulfide melt inclusion geochemical studies were carried out using a combination of scanning electron microprobe（SEM）,electron probe microanalyzer（EPMA）and laser ablation-（multi-collector）-inductively coupled plasma-mass spectrometry[LA-（MC）-ICP-MS].On a basis of previous research,the thesis discussed the relationship between gold enrichment in the Mesozoic lithospheric mantle and deep crust-mantle magmatic systems and shallow-level gold mineralisation in eastern NCC,and established a gold-rich sulfide melt enrichment and migration model from mantle metasomatism to crust-mantle mixing.The main findings are as follows:（1）The mantle xenoliths within Fangcheng basalts consist of peridotites（15%）and pyroxenes（85%）.Olivine xenocrysts and pure peridotites have high Fo（91.3–90.5）,Cr^#（57.3–54.0）and Mg^#（49.6–46.8）values,indicating that these rocks are likely remnants of mantle fragments that have not undergone metasomatism.Metasomatic textures were observed in diallage peridotites and pyroxene xenoliths.The olivine from diallage peridotites contains low Fo values（82.3–81.9）,low Ni O（0.22–0.15 wt%）and high Mn O（0.26–0.20 wt%）contents,and the clinopyroxene shows obvious low Mg^#（85.4–84.2）.The clinopyroxene and orthopyroxene from pyroxene xenoliths also display low Mg of 86.3–81.7 and 86.0–83.1,respectively.The clinopyroxene shows different rare earth element（REE）compositions from that in cumulus pyroxenites of high-pressure origin.The results indicate that peridotites and pyroxene xenoliths were likely the products of reactions between peridotites and melts.（2）The clinopyroxene from diallage peridotites and pyroxenites is significantly enriched in LREE and Sr,and depleted in HREE,Zr,Hf,Ni and Ta.It has high Ca/Al（10.51–5.79）and（La/Yb）_N（12–5）ratios and low Ti/Eu（1942–1052）ratios.These characteristics are consistent with typical clinopyroxene formed from carbonate melt metasomatism,suggesting that LREE-rich carbonate melts may have involved in the formation of clinopyroxene,which was further supported by the presence of CO₂and carbonate inclusions.The enriched Sr isotope compositions(⁸⁷Sr/⁸⁶Sr:0.71048–0.70942vs.0.70989–0.70864)in clinopyroxene from olivine websterites and clinopyroxenites indicate that metasomatic melts may have inherited the features of mantle which was metasomatised by carbonate melts under the influence of the subduction of the Yangtze plate before the Late Triassic,with the possible involvement of oceanic subducting sediments.In comparison to the Paleozoic and Cenozoic mantle metasomatism in eastern NCC,it is believed that the type of carbonate-mantle metasomatism has changed since the Early Cretaceous.（3）Petrological observations show that the clinopyroxene formed from metasomatism host abundant sulfide melt,melt-fluid,CO₂and mineral inclusions.Sulfide melt inclusions consist of feather-linear sulfide inclusions（Type1）and isolated spherical sulfide inclusions（Type2）,which were analysed by LA-ICP-MS to obtain trace element compositions.Results show that most of the sulfide inclusions were captured by clinopyroxene in the form of monosulfde solid solution,with minor sulfide liquid.Calculated partitioning coefficients show that sulfide liquid displays stronger preference for Cu(D_C^SL/SM=5451>D_Cu^MSS/SM=595),Au（SL=479 ppb>MSS=128 ppb）,and Ag（SL=10.679 ppm>1.45 ppm）than monosulfide solid solution.Sulfide inclusions in mantle pyroxenites contain 2–5 orders of magnitude higher Au concentrations（7–980ppb;average 152 ppb;median 116 ppb;n=59）than whole rock Au contents in mantle peridotites from worldwide and eastern NCC,indicating that gold-rich sulfides were likely saturated in the metasomatic mantle sourced for pyroxenites.Ni and S concentrations in sulfide melt inclusions show positive and negative corrections with Ca/Al ratios and Si contents of the host clinopyroxene,respectively.Cu,Au and Ag contents show no correlation with Ca/Al and Ti/Eu ratios.It is shown that carbonate melt metasomatism promoted sulfur saturation and the formation of sulfide melt;however,chalcophile elements were not altered during such process.The presence of melt-fluid inclusions,combined CO₂bubbles and sulfide melts,is likely the key evidence for the direct interaction between fluids and sulfide melts during the mantle metasomatism.（4）Amphibole phenocrysts in the Early Cretaceous Guocheng lamprophyres commonly show multiple oscillatory bands.EPMA results show that,from core to rim,amphibole has high contents of Mg（core）→Fe and Al（mantle1）→Mg（rim1）→Fe and Al（mantle2）→Mg（rim2）,indicating that strong crust-mantle mixing occurred during the crystallization of amphibole,accompanied by multiple pulses of basic magma injection.Mineral thermometer results show that crust-mantle mixing may have resulted in the increase in temperature and pressure（970℃–1000℃;364 MPa–492 Mpa）of the magma and decrease in oxygen fugacity（?NNO+1.3–?NNO+0.5）.（5）Amphibole phenocrysts from the lamprophyres host two types of sulfide melt inclusions:1)Type1 occurring as isolated spherical shapes in the Mg-rich bands;and 2)Type2 occurring as clusters and raindrops in alternating parts of the bands and Fe-rich areas,with some inclusions necking down and coexisting with anhydrite inclusions.LA-ICP-MS data show that the sulfide inclusions in the amphibole from the lamprophyres have significantly higher Au contents（average 439 ppb;median 257 ppb;n=38）than those from mantle xenoliths.Their Au and Ag contents also show obvious correlation.The decrease in sulfur solubility caused by changes in physiochemical conditions during crust-mantle magmatism is likely the main reason for the exsolution of the sulfide inclusions.（6）The occurrence of sulfide saturation during Early Cretaceous mantle metasomatism and crust-mantle mixing processes in the eastern NCC indicates that gold has undergone continuous enrichment in the deep magmatic system.The combination of massive CO₂produced by the carbonate-mantle metasomatism and gold-rich sulfide melt is likely the main mechanism for gold migration.Based on previous studies,continuous Au enrichment model in the deep magmatic process in eastern NCC was established.Melt immiscibility,which was facilitated by mantle metasomatism,caused the initial enrichment of gold in the mantle.The supercritical CO₂fluid produced by carbonate-mantle metasomatism combined with sulfide melt/solid solution to produce melt-fluid droplets,which migrated upwards with melts produced by partial melting of the mantle due to their lower density.Continuous injection of mantle-derived magma resulted in the occurrence of strong crust-mantle mixing in the middle-lower crust,which elevated the temperature of the melt and reduced the oxygen fugacity.This could have further promoted the decrease of the melt sulfur solubility and caused the separation of sulfides and gold enrichment,resulting in the formation of sulfide-rich（Au-rich）region in the middle-lower crust.During the late stage of magmatic evolution,fluids exsolved from the magma destructed the sulfide melts,resulting in the release of metals to the fluids.The thesis therefore revealed the relationship between Au pre-enrichment at depths and Au mineralisation at a shallow level in eastern NCC.