山西省代县碾子沟金红石矿床矿物学及年代学研究.doc

山西省代县碾子沟金红石矿床矿物学及年代学研究 金红石是一种重要的工业矿物，是提取金属钛的主要矿物原料之一。尽管我国钛资源非常丰富，但98％的钛资源来源于钛铁矿，源于金红石资源的含钛量仅占钛资源总量的2％。因此对于金红石矿产资源的研究，有很重要的实际意义。山西省代县金红石矿是我国一超大型金红石矿，目前探明总资源量达到369.67万吨。山西省代县金红石矿分布于恒山西段，矿体产于走向北西南东的超基性岩带中，矿带长达11km。目前对于该矿的形成时代、所经历的演化历史、原岩属性等基础研究目前仍为空白。因此，对该区金红石矿的区域地质、矿物特征、年代学及应用前景进行研究，对于了解该金红石矿的形成历史、演化等有重要的理论意义，对于指导该矿的开发与利用具有重要的现实意义。山西代县金红石矿床的含矿岩石按岩石中矿物组成可以划分以下几种类型阳起直闪岩、直闪阳起直闪岩、石榴石阳起直闪岩、十字石阳起直闪岩、阳起黑云直闪岩、阳起直闪岩、蓝晶石直闪岩、滑石阳起直闪岩、绿泥阳起直闪岩等。按金红石的颜色可以划分为两类，即红色金红石矿石、黑色金红石矿石。红色金红石矿一般位于矿体的上部，黑色金红石矿则位于矿体的下部。这两种矿石矿物组合上存在明显的不同，红色金红石矿中矿物组合为直闪石斜长石金红石镁角闪石碱性长石，黑色金红石矿的矿物组合为直闪石金云母绿帘石金红石。根据岩相学观察，认为红色金红石矿中存在如下方向的反应镁角闪石碱性长石→直闪石／-斜长石／-金云母，斜长石很可能只是过渡相，反应最终将形成直闪石金云母。黑色金红石矿中的矿物组合似乎是进行得更为彻底的上述反应。红色金红石矿中金红石成分均匀，未见有钛铁矿的出溶现象，黑色金红石矿中金红石存在明显的厚度可大于2μm钛铁矿出溶叶片。我们认为正是这种出溶叶片导致上述两种矿石出现颜色差异。目前的资料表明该矿为角闪岩相的变质岩，但要确定该金红石矿的原岩属性还需要更为深入的研究。代县金红石矿床的岩石地球化学特征为高TiO_2含量的样品中SiO_2含量主要位于基性岩区，MgO与TiO_2之间也无明显的相关关系，但当TiO_2含量大于2％时MgO的含量均大于16％。这一化学特征显示了与岩相学观察相一致的结果，即富含金红石的岩石为透闪石含量偏低的直闪岩类。从红色和黑色金红石矿中的锆石阴极发光图像来分析，尽管上述三个样品的矿物组合、金红石的颜色、产状及形成PT条件有所不同，但其中锆石的特征及年龄结果却有惊人的相似，甚至完全相同。按形成的先后顺序至少存在Ⅰ期、Ⅱ期和Ⅲ期等3期锆石。第Ⅰ、Ⅱ期锆石阴极发光强度最暗，但具规则平直的韵律环带，Th／U比值绝大多数大于1.00，具备典型的岩浆锆石所具有的特征。这两期锆石的204Pb校正的207Pb／206Pb年龄值几乎一致，加权平均值位于2.5Ga左右，可解释为金红石矿石的原岩形成年龄或者深熔年龄。第Ⅲ期锆石为边部或以脉状充填于第Ⅰ和和Ⅱ期锆石内的裂隙中，其阴极发光最亮，往往以以互补形态包裹早期锆石并多数以修复原锆石形态后以完好晶体形态出现，其U、Th含量较低，尤其是Th／U比值为最低，明显地低于0.50。加权平均值位于1.8 Ga左右，可解释为金红石矿石的变质年龄。另外，40Ar／40Ar阶段升温测年结果显示红色矿石形成于ca 1.8 Ga，之后几乎没有经历过大于白云母封闭温度350℃的热事件；对于红-黑色金红石矿，在金红石矿形成之后ca 1.4 Ga发生过大于白云母封闭温度350℃又低于锆石的封闭温度的热事件。可能正是这一热事件导致金红石矿颜色发生变化。;ReplaceFontEnDivSummary,As an important industry mineral, rutile is a major resource for metal titanium. In China, although Ti resource is very rich, Ti resources from rutile, however, take up only 2, since 98 Ti resources are from ilmenite. Hence, it is very important to study rutile mineral resource. The Daixian rutile deposit, with reservoir of 3.6967 million tons, is one of the largest rutile deposits in China. Located on western part of the Henshan Mountain, the Daixian rutile bodies occur as 11km long belts within NE-strike ultramafic rock belt. Till now, data about ing age, evolution and protolith of the rutile deposit have not acquired. In this investigation, combined studies of petrography, mineralogy and geochronology were conducted on this deposit.According to mineral components, ores of the Daixian rutile deposit can be classified into the following types actinolite- bearing anthothyllitite, actinolite-garnet- bearing anthothyllitite, staurolite- actinolite- bearing anthothyllitite, actinolite-biotite-bearing anthothyllitite, kyanite-bearing anthothyllitite, talc-actinolite- bearing anthothyllitite and chlorite-actinolite-bearing anthothyllitite, etc. According to color of rutile, ores of the Daixian rutile deposit, the second biggest rutile deposit in China, can be classified into two types red and black ore; the red ore generally occurred at upper part of the deposit, overlying the black ore without distinct border. These two ores have contrasting mineral associations anthothyllite Ath plagioclase PI rutile Rut magnesiohornblende Mhb alkline fledspar Afs for red ore and anthothyllite phlogopite Phi epsode Ep Rutile for black ore. A chemical reaction was suggested as Mhb Afs → Ath /- PI /- Phi based on petrographical observations in red rutile ore, PI is possibly the transitional phase and complete reactants are Ath Phi . The mineral associations of black ore are thus infeered to be more complete reactants of this reaction. As for rutiles, red rutiles are homogeneous in composition while black ones bear ilmenite exsolution lamellae of up to 2 μm in thickness. It is concluded that existence of the lamellae causes color difference between these two rutiles. The rutile-hosted rocks are metamorphic rocks with amphibolite facie, but more work should be done to discuss features of their protolith.Characteristics of zircons revealed by cathodeluminescence images from rutile ores with different colors are incredible similar. There are three stages upon ation sequence of the zircons. Stage-I and stage-ll zircons have oscillating zones with darker luminescence, Th/U ratios are mostly more than 1.00, indicative of igneous zircons. These zircons have nearly the same 204Pb -corrected 207Pb/206Pb ages, with weighted mean age of ca 2.5Ga, interpreted as ing or anatexis age of protolith of the rutile ores. Stage-Ill zircons occur as rims surrounding or as veins crosscutting stage-l or/and II zircons, have lower U, Th and lowest Th/U ratios. They yield a weighted mean age of ca 1.8 Ga, interpreted as metamorphic age of the rutile ores.Moreover, step-heating 40Ar/ 39Ar geochronology on these rutile ores shows that the red-color rutile ores are stable without experiences of more than 350C thermal events since ca 1.8 Ga, whereas the black-color rutile ores had undergone a thermal event of more than 350C, but less than enclosure temperature of zircon at ca 1.4 Ga after metamorphised. It is inferred that the event at ca 1.4 Ga resulted in color changes of the rutile ores.;