| 甘肃阳山金矿同位素地球化学特征及矿床成因 | |
| Alternative Title | Isotope Geochemistry of the Yangshan Ore Belt, Gansu Province, China: Implications for Ore Genesis |
| 刘红杰 | |
| Subtype | 硕士 |
| 2005-05-31 | |
| Degree Grantor | 中国科学院地球化学研究所 |
| Place of Conferral | 地球化学研究所 |
| Degree Name | 博士 |
| Keyword | 花岗斑岩 Sr-nd-pb-s同位素 阳山金矿带 造山型 卡林型 秦岭造山带 Cfm模式 |
| Abstract | 甘肃文县阳山金矿的探明黄金储量已达308t，平均品位4.74g/t，是我国地质勘查储量最大的金矿床。该矿床产于西秦岭造山带，是一个同碰撞形成的类卡林型金矿床，矿体受EW向韧脆性剪切带控制，赋矿围岩为泥盆系碳质千枚岩-板岩-碳酸盐-硅质岩和侵入其中的花岗斑岩脉。流体成矿过程包括：形成石英-绢云母-黄铁矿组合的早阶段，形成石英-黄铁矿-毒砂-方铅矿等多金属组合的主成矿阶段，形成碳酸盐-辉锑矿-石英网脉的晚阶段。 与矿体关系较为密切的花岗斑岩富集LILE 和 LREE, 亏损 Ba, Sr, Nb, Ta, P 和Ti，ΣREE＝54.35～124.01 μg/g ，(La/Yb)N=9.72～27.80，δEu=0.70～0.89， ISr值为0.70806～0.71756，平均0.71107；εNd(t)平均-3.4；Nd模式年龄（T2DM）平均1.34（Ga）。表明花岗斑岩岩浆应源自成熟度较低的中元古代基底地壳物质。花岗斑岩的(206Pb/204Pb)220Ma、(207Pb/204Pb)220Ma和(208Pb/204Pb)220Ma的平均值分别为17.875、15.604和38.296，与秦岭微陆块的中元古代基底和碧口地体碧口群的Pb同位素组成一致。考虑到前人获得碧口群的年龄为1.235～1.367Ga，而秦岭微陆块沿勉略缝合带向南仰冲到碧口地体之上，我们认为由碧口群等组成的俯冲板片的变质脱水熔融作用导致了阳山金矿带花岗斑岩的形成。因此，阳山金矿带的花岗斑岩是扬子与华北大陆中生代碰撞造山过程中形成的同碰撞花岗岩类。 最新的S，Sr和Pb同位素研究表明：热液成矿早阶段的黄铁矿的34S值范围介于-15.5‰~6.59‰之间，总体离散性比较大，显示沉积地层来源的特征，硫同位素组成属离散型，不具有岩浆主导的成矿的塔式效应。花岗岩中黄铁矿硫同位素范围很集中，34S值处于-1.47‰～2.12‰之间，本区花岗斑岩不可能为成矿物质的主要来源。矿石硫化物的初始锶同位素比值范围较大（0.70877～0.71697，平均为0.71258），显示成矿物质并非单一来源，考虑到花岗斑岩先于矿床形成，只在后期构造作用的岩体部分成矿的地质事实，少量矿石中的低锶同位素比值黄铁矿有可能来自作为围岩的花岗斑岩，也可能来自基底物质。矿石硫化物Pb同位素206Pb/204Pb=17.552～18.853，平均18.260；207Pb/204Pb=15.574～15.928，平均15.685；208Pb/204Pb=37.894～39.293，平均38.680，变化范围比较大。μ=9.46~10.06，平均为9.65，ω值介于36.96～42.21，显示了铅源的物质成熟度较高，要求最佳物源是浅变质化学-碎屑沉积建造，恰好与本区泥盆构造层为浅变质细碎屑岩夹薄层灰岩系的特征一致，部分低Sr和Pb同位素比值的成矿物质可能来自于作为围岩的花岗斑岩和/或者基地物质。 总结前人阳山金矿床的H-O-C同位素体系的研究得出，初始成矿流体来源于碳酸盐地层或相似岩石建造的变质或/和改造脱水，成矿流体系统从早到晚、从深到浅，由变质热液演变为大气降水热液。与本文得出的结论一致。 总而言之， 阳山金矿矿成矿流体的来源早期具有变质水特征，应来自赋矿地层或相似岩性组合的改造或变质脱水作用，晚阶段大气水为主的流体性质。成矿物质主要来自于赋矿围岩。流体经过作为部分围岩的花岗斑岩时从中萃取少部分成矿物质，而导致了少部分的低锶、铅同位素的矿石硫化物组成。 在中生代扬子板块北缘（包括碧口地块）向南秦岭陆陆碰撞过程中，扬子北缘板片沿勉略断裂向北俯冲到南秦岭之下，下插板片增温增压，发生变质、脱水和部分熔融。碰撞中期，构造背景由挤压向伸展转变，减压增温的环境导致大量变质流体沿深大断裂向上运移，不断萃取围岩中大量成矿元素，并将成矿元素搬运至有利于流体聚集、成矿物质卸载的空间，使成矿物质富集成矿。阳山金矿床定位于泥盆系构造层中，成矿时代为190Ma左右，紧随花岗斑岩侵入作用（220Ma左右），主成矿作用发生于碰撞作用由挤压-伸展转变期的减压增温环境。成岩、成矿模式与CMF模式吻合。 阳山超大型金矿是世界罕见的碰撞造山带内类卡林型金矿床，其地质地球化学特征复杂、独特，流体性质主要与造山型金矿一致，矿床地质主要与卡林型金矿一致，部分特征兼与造山型和卡林型两类矿床之间，花岗斑岩本身成矿的特点又为阳山所特有，总体具有造山型向卡林型金矿过渡的性质。因此建议以“秦岭式”或“阳山式”类卡林型金矿床代表与阳山金矿具有类似成矿背景及地球化学性质的矿床。 |
| Other Abstract | The Yangshan gold deposit, Wenxian county, Gansu province, containing 308 t Au with average grade of 4.74g/t, is now ranked as the China’s largest gold deposit.. Locating in western Qinling Orogen, central China, it is a syn-collisionally formed Carlin-like gold deposit. Its orebodies are controlled by an E-trending shear zone and hosted in the Devonian carbonaceous carbonate-phyllite-slate sequence or the granite-porphyry dikes intruding into the Devonian strata. Its hydrothermal ore-forming process includes several stages: (1) the early stage forming quartz-sericite-pyrite assemblage; (2) the main stage forming quartz-pyrite-arsenide-galena etc. polymetallic mineral assemblages; and (3) the late stage forming carbonate-quartz-stibnite veinlets. The granite-porphyry dykes in the Yangshan gold belt, Gansu Province, are rich in LILE and LREE, and depleted in Ba, Sr, Nb, Ta, P and Ti, ΣREE＝54.35～124.01 μg/g, (La/Yb)N = 9.72 ~ 27.80, and δEu = 0.70 ~ 0.89, the granitic magmas were originated from partial-melting with little of plagioclase as residual phase. The porphyries have ISr ratios of 0.70806 ~ 0.71756 with average of 0.71107, εNd(t) values of -2.9 ~ -5.0 with average of -3.4, and Nd-model ages (T2DM) of 1.24 ~ 1.41 Ga with average of 1.34 Ga, which suggests that the magmas must be sourced from lowly-matured Mesoproterozoic basement. The averages of (206Pb/204Pb)220Ma, (207Pb/204Pb)220Ma and (208Pb/204Pb)220Ma ratios are 17.875, 15.604 and 38.296, respectively, similar to those of both the Bikou Group of the Bikou terrane and the Mesoproterozoic basement of the Qinling micro-continent. Considering that the Bikou Group has been dated to have developed during 1.235 ~ 1.367 Ga in previous studies, and that the Qinling micro-continent, which hosts the Yangshan gold belt at its southern margin, southwardly overthrusted onto the Bikou terrane along the Mian-Lue suture, we suggest that the interested granite-porphyries were generated by metamorphic devolatilization-related partial melting of the underthrust slab mainly consisting of the Bikou Group. Hence the granite-porphyries in the Yangshan gold belt are typical syn-collision granitoids formed during the Mesozoic collision between the Yangtze and North China continents. New S ,Sr and Pb isotopic data, presented in this paper show, 34S values for sulfides reveal large isotopic variations (-15.5‰~6.59‰), and presents no tower style distribution, suggest a contribution from sedimentary strata. 34S values for Pyrite from porphyry-granite shows a narrow range(-1.47‰～2.12‰) that can not supply such larger 34S isotopic variations for ore deposit of Yangshan. The extremely variable initial 87 Sr/ 86 Sr ratios 0.70877～0.71697 are reveal the characteristic of several different kinds of Sr source. Considering the porpgry-granite formed before the minaralizion, it may supply the source of pyrite and other metal minerals with low 87 Sr/ 86 Sr ratios, so basement does as well. Lead in sulfides of the Yangshan Au deposit is more radiogenic (µ = 9.46~10.06,ω=36.96～42.21) than average crustal, which suggests that the metallogenic material must be sourced from hightly-matured Devonian strata which comprise of carbonaceous phyllite, slate, chert and carbonate. The Pb isotope ratios from the sulfides are also variable , 206Pb/204Pb=17.552～18.853, with average of 18.260；207Pb/204Pb=15.574～15.928, with average of 15.685；208Pb/204Pb=37.894～39.293, with average of 38.680, the Pb-isotope ratios range within the range of those of the Devonian strata. Part of sulfides with low Sr and Pb isotope may be derived from granite or/ and basement rocks. Hydrogen, oxygen and carbon isotope systematics of the Yangshan deposit from a previous work show the ore-forming fluid-system varies from early, deep metamorphic to late, shallow meteoric, and the early stage fluids were derived from metamorphism and/or reworking from the Devonian strata or/and similar lithologies which comprise of carbonaceous phyllite, slate, chert and carbonate, which are consistent with conclusions drawn from our studies in this paper. The comparison of the hydrogen-oxygen-carbon-sulfur-lead -strontium isotope systematics between the Yangshan Au deposit and the main lithologies in the Bikou Terrian, shows that a likely source of ore fluids and metallogenic materials for the Yangshan Au deposit was country-rocks of Devanian strata. The Yangshan Au deposit was hosted by Devonian strata and formed at about 190Ma, followed the syn-collsion-type granite-porphyry formed. During the Mesozoic collision events, a north-dipping A-type subduction zone was active along the Mian-Lue suture. The slab south of the Mian-Lue suture, mainly consisting of the North Yangtze Terrane (including Bikou Groups), was subducted beneath the Qinling micro-continent and subsequently metamorphosed and dehydrated. This provided fluids to the overriding slab (i.e. Qinling micro-continent) and was ultimately responsible for the development of the Yangshan Au systerm. The main metallogenic stage happened during the collisional compression-to-extension transition, this evolution of ore-forming tallies well with the CFM model. The Yangshan gold deposit could be a typical representative of a large class of gold deposits occur in continental collision orogens in the world, especially representing those found in the western Qinling gold province. Hence, we introduce a concept of “Qinling-type” or “Yangshan-type” to cover and represent a group of gold deposits whose geological and geochemical characteristics are similar to the Yangshan gold deposit. |
| Pages | 88 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.gyig.ac.cn/handle/352002/3438 |
| Collection | 研究生_研究生_学位论文 |
| Recommended Citation GB/T 7714 | 刘红杰. 甘肃阳山金矿同位素地球化学特征及矿床成因[D]. 地球化学研究所. 中国科学院地球化学研究所,2005. |
| Files in This Item: | ||||||
| File Name/Size | DocType | Version | Access | License | ||
| 甘肃阳山金矿同位素地球化学特征及矿床成因（2079KB） | 学位论文 | 暂不开放 | CC BY-NC-SA | Application Full Text | ||
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.
Edit Comment