發(fā)布時間：2018-03-20 00:01

  本文選題：秦嶺南緣　切入點：蕎麥沖金礦　出處：《中國地質(zhì)大學(xué)(北京)》2015年碩士論文　論文類型：學(xué)位論文

【摘要】：秦嶺造山帶南緣存在大片的早古生代基性火山巖,蕎麥沖金礦的含礦正長斑巖脈體呈北西向侵入基性火山巖地層中。前人在對蕎麥沖金礦進(jìn)行元素地球化學(xué)、巖石化學(xué)、同位素、包裹體、稀土元素地球化學(xué)等方面的研究中,證實了金礦成礦過程中的主導(dǎo)作用是燕山期的脆-韌性剪切變形變質(zhì)作用,得到一組模式年155Ma可能反應(yīng)礦床的成礦年齡。通過對該正長斑巖鋯石的LA-ICP-MS U-Pb定年、元素地球化學(xué)以及Hf同位素研究,探究正長斑巖脈體的侵入時代、巖石成因和大量基性巖的成巖構(gòu)造環(huán)境。結(jié)合大別山造山帶地質(zhì)背景特征,可將脈體的侵入時代可以分為兩期,鋯石加權(quán)平均年齡值分別為(415±7.3)Ma、(444±5)Ma,前者屬于早泥盆世時期,后者為寒武紀(jì)到早奧陶世時期。所測鋯石Th/U比值基本集中在1附近,結(jié)合稀土特征,典型的Ce正異常、Eu負(fù)異常,重稀土富集,可推測本次試驗鋯石為巖漿鋯石。地球化學(xué)特征上顯示含較高的Si O2含量(75%),里特曼指數(shù)δ=1.59,高Al低Ti的特點,Eu和Sr具有明顯負(fù)異常,為鈣堿性脈體。正長斑巖脈體的Hf同位素檢測結(jié)果得到εHf(t)全部為正值,其變化范圍較大(3.4~11.8),說明巖漿的主要來自虧損地幔,其中大部分集中在6~11范圍內(nèi),個別εHf(t)值偏小(4),推斷巖漿上侵過程中混入少量地殼物質(zhì),從而顯示殼�；旌咸卣�。在基性巖的地球化學(xué)特征研究中,稀土總量很高(∑REE=149.62×10-6~321.55×10-6),輕稀土強(qiáng)烈富集型,呈現(xiàn)出高度分異的特點,樣品的大離子親石元素和高場強(qiáng)元素都較MORB富集,顯示鐵鎂質(zhì)圍巖具有類似OIB的特征,巖漿源區(qū)可能來源于軟流圈地幔,或許與地幔柱有關(guān)。鋯石具有洋殼鋯石與大陸鋯石的雙重特征,樣品13QMC-011和13QMC-013的鋯石二階段模式年齡的值,變化范圍分別是913~1284Ma和749~1521Ma,為中元古代到新元古代時期。鋯石的εHf(t)值變化范圍分別為4個ε單位和8.4個ε單位,顯示了其殼�；旌系脑磪^(qū)特征。兩期正長斑巖巖脈的εHf(t)均為正值,指示了該區(qū)從大陸裂解構(gòu)造環(huán)境逐步轉(zhuǎn)變?yōu)橛邢扪笈璀h(huán)境。在早古生代時期,區(qū)域上為板內(nèi)裂解的構(gòu)造環(huán)境,軟流圈地幔上涌冷凝形成基性巖地層。在志留世末期,脈體完成侵入,將地殼深部的Au元素帶入地殼表層,為地層Au的預(yù)富集以及后期剪切成礦作用提供了Au元素的物質(zhì)來源。此后該區(qū)域處于洋盆環(huán)境,形成了一些沉積建造。在加里東末期,洋殼環(huán)境發(fā)展結(jié)束,揚子板塊再次向北發(fā)生俯沖。在三疊紀(jì)時期,與華北板塊發(fā)生碰撞。在侏羅紀(jì)時期,隨著碰撞作用的增強(qiáng),區(qū)內(nèi)再次發(fā)生巖漿熱液作用,為正長斑巖脈體中Au的活化成礦提供了熱。正長斑巖脈體形成時代的厘定,確定了該區(qū)金源層形成時代,金源層是后期金成礦的首要物質(zhì)源,這對礦床的形成具有重要意義。
[Abstract]:In the southern margin of the Qinling orogenic belt, there is a large area of early Paleozoic basic volcanic rocks, and the ore-bearing syenite veins of the buckmaichong gold deposit are in the NW direction of intrusive basic volcanic strata. The predecessors are doing elemental geochemistry and petrochemistry of the buckmaichong gold deposit. Studies on isotopes, inclusions and geochemistry of rare earth elements have proved that the dominant role in the metallogenic process of gold deposits is the brittle ductile shear deformation metamorphism in the Yanshanian period. A series of model years 155Ma may reflect the metallogenic age of the deposit. By studying the LA-ICP-MS U-Pb dating, element geochemistry and HF isotopes of the porphyry zircon, the intrusive age of the syenite porphyry vein has been explored. Based on the geological background characteristics of the Dabie orogenic belt, the intrusive age of the vein can be divided into two periods. The weighted mean zircon age is 444 鹵5Ma.The former belongs to the early Devonian period. The latter is from Cambrian to early Ordovician. The measured Th/U ratio of zircon is concentrated in the vicinity of 1. In combination with the characteristics of rare earth, the typical ce positive anomaly is negative anomaly of EU, and the heavy rare earth is enriched. It can be inferred that the zircon in this test is magmatic zircon. The geochemical characteristics show that there is a high Sio _ 2 content of 75%, the Rittmann index 未 _ (1.59), and the characteristics of high Al and low Ti are obviously negative anomalies of EU and Sr. It is a calc-alkaline vein. The HF isotopic detection results of syenite porphyry are all positive, and the range of variation is larger, indicating that the magma mainly comes from the depleted mantle, most of which is concentrated in the range of 60.11, and the magma is mainly derived from the depleted mantle, and most of the magma is concentrated in the range of 6N11, and the magma is mainly derived from the depleted mantle. Some 蔚 Hfft values are relatively small, and it is inferred that a small amount of crustal material was mixed in the process of magmatic transgression, thus showing the characteristics of crust-mantle mixing. In the study of geochemical characteristics of basic rocks, the total amount of rare earth elements is very high (鈭，

本文編號：1636587