Studies On Late Paleozoic-early Mesozoic Magmatism And Mineralization In Yemaquan Area,Qinghai Province

Abstract: Study of mineralization in orogenic belts is a contemporary frontier of metallogeny.Yemaquan region is located in the west of East Kunlun orogenic belt. Mineralization in latePaleozoic-early Mesozoic is strongly developed, but the relation between orogeny and mineralization hadnot yet been thoroughly studied in Yemaquan region. Thus, detailed studies on the characteristics ofigneous rocks, such as temporal and spatial distribution, petrotectonic assemblages, petrography,petrochemistry and geochemistry, and related typical deposits have been conducted in the thesis andaccordingly new progresses have been made as follows.（1） The late Paleozoic-early Mesozoic orogenic cycle is composed of four phases, i.e., the formationand spreading of oceanic basin, the subduction, the syn-collision and post-collision, subsequently. Thetiming of subduction is defined as284.3Ma（？）～271.1Ma. Syn-collision and post-collision stagesdeveloped continuously, which is recorded by intrusive rock formed in235.4Ma～204Ma.（2） Association of intrusive rocks of subduction is granodiorite-prophyritic monzonitic granite-monzonitic granite, that in syn-collision stage is quartz monzodiorite-granodiorite-prophyriticmonzonitic granite, and in post-collision stage is monzonitic granite-diorite-syenogranite-porphyry,respectively.（3） Intrusive rocks in both subduction and syn-collision stages are mainly T-type granite, and those inpost-collision stage have characteristics of I-type and A-type granites. Intrusive rocks in early period ofsubduction are moderately differentiated and those in late suduction stage highly differentiated. Intrusiverocks in early syn-collision stage are moderately differentiated, withεSr（0）100.67～141.25，εNd（0）-6.20～-5.70, implying to some extent contribution of mantle-derived substances. Intrusive rocks in lateperiod of syn-collision stage are highly differentiated. Intrusive rocks in early period of post-collision stagelikely have different magma source regions and degree of differentiation. While diorites and somemonzogranites are characterized byεSr（0） of408.97～509.35, andεNd（0） of-10.61～-9.99, with signsof EMⅡfor source region and features of fractional crystallization, other monzogranites are characyerizedbyεSr（0） of1450.31,εNd（0） of-4.06, implying contribution of HIMU, and invisible differentiation.Porphyries in late period of post-collision stage are highly differentiated.（4） From early period to late period of every stages, compositions of intrusive rocks graduallyenriched in SiO2, K2O,and Fe2O3but reduced in Na₂O, FeO, MgO, MnO and CaO. While the content ofNa₂O is more than that of K2O of intrusive rocks in subduction stages and early period of syn-collision, itis just opposite in late period of syn-collision and post-collision stages. Rocks in different stages have similar spider diagrams of trace elements, i.e., with strong enrichments of K, Rb and Th, moderateenrichments of La, Ce and HFSE Ta, Nb, Zr, Hf, and deficient in P and Ti, and V-shaped deficient of Baand Sr. From early to late period of subduction, total REEs is increasing. From early to late period ofsyn-collision and post-collision,∑REE and LREE/HREE ratio alternatively change and Eu-deficit getstronger from early to later..（5） It is determined that Hutouya deposit, whose origin had long been disputed, is a hydrothermalPb-Zn polymetallic deposit based on a thorough study. In addition, several typical deposits in Yemaquanregion and Changshan region have also been studied in detail. Three different metallogenic types havebeen distinguished, i.e., skarn iron-polymetallic deposits, hydrothermal Pb-Zn polymetallic and skarnpolymetallic composite deposits and porphyry Cu-Mo deposits in this area, among them, the first two typesare the most important.1) The skarn iron-polymetallic deposits formed in early period of syn-collisionstage. Ore-body is located in external contact zone between intrusives and wall rocks and often penetratethe strata. The mineralization zonation from igneous rocks to country rocks is from Sn, Mo, Co, to Fe, Cuthen to S, Zn and finally to Pb, which controled by temperature. No Eu-depletion but sometimeEu-enrichment can be seen in ores. Characteristics of fluid inclusions and lead isotope suggest thatmineralization substance originated from magma.2) The hydrothermal Pb-Zn polymetallic depositsformed in early period of post-collision stage, and have the following characteristics: mineralizationshowing layered distribution; ore-hosted rocks mainly diopside-skarn, skarnized carbonate, silicifiedcarbonate, and phyllic; wall rock alteration mainly silicification, skarnization, sericite quartzitization,chloritization, and carbonation; structure of ores including disseminated, massive, and stratified structures;the homogenization temperature of fluid inclusions ranging from211℃to407℃; the ore-forming ageranging from225.0±4.0Ma to230.1±4.7Ma, being greatly different from the age of wall rockNeoproterozoic Jixian stage; these characteristics mentioned above are obviously different from SEDEXdeposits.3) Porphyry Cu-Mo deposits formed in the late period of post-collision stage to late orogenicstage, ore-bodies are strictly controlled by porphyry distribution and alteration zone, closely related tosilification.（6） Mineralization in Late Paleozoic-Early Mesozoic is controlled by intrusive rocks, structure, wallrock and so on. Intrusive rocks formed in different geological settings, different magma sources, differentdegrees of differentiation have important impacts on the mineralization scale, mineralization types andore-forming minerals. The granodiorite and quartz monzodiorite in early period of syn-collision stage arethe main rock types that forming skarn-type iron polymetallic deposits in the region studied. In earlyperiod of post-collision stage, the mineralization capacity of intrusions whose origin mantle had madecontribution to is also strong. Monzogranite mainly formed hydrothermal lead-zinc polymetallic depositsand skarn-type polymetallic deposits. Granodiorite and quartz monzodiorite differentiated mainly formediron, copper, lead and zinc deposits, especially quartz monzodiorite formed iron doposits. The monzogranite to whose origin mantle had made contribution to formed metallogenetic type of copper, lead,and zinc. Monzogranite which originated only from crustal sources are mainly related to mineralization oftin, molybdenum and tungsten.（7） Geocronite has been found to in Hutouya deposit for the first time. Geocronite is a rare sulphide inthe world, and has significance for revealing regional tectonics and mineralization.（8） The research results show that Shaqiu region is the most favorable region of prospecting skarniron-polymetallic deposits. It is worth to pay more attention to skarn iron-polymetallic deposits andhydrothermal Pb-Zn polymetallic deposits in the west of Nalingguole river, Binggou, Hutouya, Yemaquan,Galingge and Sijiaoyang regions. And it is worth to pay attention to porphyry Cu-Mo deposits inChangshan, Wulanwuzhuer, Binggou and Shaqiu region.