Study On Gold Deposits Mineralization In Chifeng-Chaoyang Region, Northern Margin Of North China Craton

The study area is located at the binding site of the North China Craton andXingmeng orogenic belt. This area is an important gold-producing area in China withwide development prospects which has more than10medium-large gold deposits andmore than one hundred gold occurrences. This essay is based on the North ChinaCraton geodynamic evolution, and it makes the modern earth sciences andmetallogenic theory as a guide，and mineralization as the core. The essay willsummarize the mineralization in Chifeng-Chaoyang district which located at thenorthern margin of north china platform，and then establish metallogenic mode in thisarea and it will make the foundation for the next step of exploration strategy ofregional resources.According to the study of geological setting and geodynamic evolution，it’ssuggested that the North China Craton had gone through three stages as followed as：1. the formation of the Precambrian crystalline basement；2. the formation ofsediments in the stable block from the Proterozoic to Paleozoic and3. activation ofthe block from the Mesozoic to Cenozoic. When paleo-asian ocean closed，the“Schere”collision took place in North China plate and Siberia plate. The paleo-asiatictectonic domain turned into the round Pacific Ocean tectonic field. Both of themshowed characteristics as extrusion in the early time and stretching in the late timesince Mesozoic.It was no selectivity for gold mineralization to ore wall rocks，the ore wall rocksinclude the Archean metamorphic rocks，the Proterozoic and Mesozoic granite as wellas Mesozoic volcanic. Although their ore-bearing rocks were different， the ore-forming material sources are similar. Considered from the perspective ofore-hosting rock，the Archean metamorphic made the biggest contribution to themineralization，followed by the Mesozoic granites，and the Mesozoic volcanic rockshad no directly relationship. In the aspect of structure，the regional translithosphericfractures control the distribution of the deposits and secondary fault zones or ductileshear zones control the distribution of the ore bodies.According to the study of The pyrites typomorphic characteristics，it showed thatthe pyrites in the main mineralization of gold deposit in Chifeng-Chaoyang districtwith S/Fe value＜2belong to endogenous losses sulfur type and with Co/Ni valuevary in a large range （1to10），low content of Mn，As，high content of Cu，Pb，Zn，Au and Ag belong to the hydrothermal origin；the results of fluid inclusionsmicrothermometry showed that fluid inclusions were gas-liquid two-phase, pure CO₂inclusions were not well developed，or absent in most deposits. Gas-liquid two-phaseinclusions were generally small concentrated between4and8microns and gas-liquidratio range from15%to30%. the ore-forming fluids had the characteristics of lowtemperature （150℃³50℃），low salinity （1¹0wt%NaCl） and low-density. Ramananalysis showed that the gas component is mainly H₂O，CO₂secondly and it maycontain a small amount of CH4and they belonged to H₂O-CO₂-NaCl fluid systemgenerally. According to standardized calculation of the part of deposits mineralizationpressure，we know that metallogenic pressure range of all gold deposits exceptDongfeng and Anjiayingzi gold mine was from50MPa to70MPa，most of them werebetween10to35MPa；Metallogenic depth range was from1km to3.6km，showing alow-pressure environment and a shallow mineralization. On the whole，the orogenicgold deposits in this area are forming in a shallow systematic.Hydrogen and oxygen isotope characteristics showed that there were somedifferences among gold deposits. on the whole，the ore-forming fluid were mainlyfrom magma in the early stage， joined by meteoric-water later； C isotopiccharacteristics show that ore-forming materials is closely related to deep magmaticactivity，especially granitic magma activity；S isotope characteristics indicates thatsulfur of the orebodys come from the magma which inherited that the Archeanmetamorphic sulfur isotopic compositions came from the place below the crust ormantle-derived，Pb isotopic characteristics showed that gold deposits formed inorogenic environment，mantle-derived magma mixed hybridism with the lower crust during ascent，brought in some lower crust Pb，but mainly is mantle-derived lead.Meanwhile，by analysis of Pb isotopic of partial rock-mass，we found the sources ofores and intrusive rock were similar.According to the studies of geological background，geological characteristics，characteristics of fluid inclusions and isotopic，diagenesis，metallogenic epoch of thestudy area，the gold deposits in this area were orogenic gold deposits. Compared withthe typical orogenic gold deposits at home and abroad，their characteristics are asfollowing：①Tectonic positions were in the orogenic belts；②O re-controllingstructures were mostly secondary ductile brittle shear zones produced by regionaldeep faults；③Intrusive rocks and dikes in mining area developed well，and relatedwith the mineralization closely；④There existed some similarities in the sulfidemineral assemblages and wall rocks alteration；⑤Metallogenic epoches wereconsistency.And the significant uniquene are as following：①Mineralization backgroundwas multi-stage and multi-stage orogenic belt at the edge of the North China Cratonwhich underwent damage；②Mineralization occured in the system of shallow（<6km），on the whole that they had low-temperature，low-voltage characteristics ofthe mineralization；③O re-forming fluids were mainly the system of H₂O-CO₂-NaCl，showing the characteristics of shallow sub-class orogenic gold deposits fluid；④M ineralization types were mainly of quartz vein type followed by alteration type.In the area of Chifeng-chaoyang on the northern margin of north china craton Werecognized four magmatism（s1.7Ga，228Ma²45Ma，161MaMa and123Ma¹38Ma）and two gold mineralizations（about243Ma and125Ma¹32Ma）. And the time ofMineralization and magma activity were the same. Their tectonic environments areextensional environment after collisional orogenesis between Eastern and westernlandmass in the North China Craton，continental collision between the North Chinaplate and the Siberian plate，the tectonic regime of compression turn to stretch insetting of Paleo-Asian Ocean tectonic domain changing to round Pacific Oceantectonic field and the extensional environment of Pacific block subduction leding torapid thinning of the lithosphere and delamination.By the study of intermediate-acid intrusives related to orogenic gold deposits inthe study area，we know that granites had close relationship with various gold depositswhich was the near-surface response for the multiple tectonic-magmatic activity in the region and it is also a sign of formation of orogenic gold deposits in the margin of theNorth China Platform. Meanwhile it is one of the main features of orogenic golddeposits in this area. And the intermediate-acid intrusives can provide macro clues forprospecting in the region.