纳米比亚罗辛地区白岗岩成因及铀成矿作用

doi:10.13745/j.esf.sf.2023.5.10

摘要/Abstract

摘要：

罗辛地区存在多期白岗岩,分为A、B、C、D、E和F 6种类型,但仅D和E类白岗岩形成了白岗岩型铀矿床。微量元素、铅同位素显示各类白岗岩具有壳源特点,两阶段Nd模式年龄与前达马拉基底大体一致表明源区为高放射性的前达马拉基底。黑云母电子探针数据显示D类白岗岩相对A、B、C和F类白岗岩黑云母具有更高的F含量,D类白岗岩Nb/Ta明显高于A、B、C和F类白岗岩,A、B、C和F类白岗岩的Ba含量与Rb/Sr值呈负相关关系,符合白云母脱水熔融的特征,D和E类白岗岩则表现为更复杂的Ba含量与Rb/Sr值的关系,为黑云母脱水熔融模式。区域上存在D1、D2、D3和D4 4期变形作用,A、B和C类白岗岩侵位与D3期变形同期或早于D3期,D和E类白岗岩则与D4期变形同期,D4期变形的应力体制转换使古老基底的深熔作用由白云母熔融模式转向了黑云母熔融模式,黑云母熔融带入了铀的矿化剂氟离子,所以古老基底熔融的不均一性是罗辛地区各期白岗岩铀含量差异的根本原因。白岗岩经历了较强的结晶分异作用,分离结晶矿物主要为钾长石、黑云母、磷灰石、钛铁矿和独居石等。黑云母的分离结晶可能带走铀元素,这对残余岩浆成矿不利;钾长石、钛铁矿和独居石的分离结晶有利于铀的富集和晶质铀矿的生成。

关键词: 深熔作用, 白云母脱水熔融, 黑云母脱水熔融, 结晶分异, 罗辛地区

Abstract:

The multiphase leucogranite of Rossing can be subdivided into six types, types A-F, but only types D and E form leucogranite uranium deposits. Trace element and Pb isotopic analyses show that the Rossing leucogranite has the characteristics of mixed crustal source, and the two-stage Nd model ages suggest the highly radioactive pre-Damara basement is the source area. The biotite electron probe data show that biotite in type D leucogranite has higher fluorine content compared to types A-C and F, and type D obviously has higher Nb/Ta contents. Types A-C and F leucogranite show a negative correlation between Ba content and Rb/Sr ratio, consistent with muscovite dehydration melting; whilst types D and E show a more complex relationship consistent with biotite dehydration melting. The Rossing area experiences four deformation stages (D1-D4), where the emplacement of types A-C leucogranites occurs no later than D3 while types D and E coincide with D4. The stress transformation during D4 changes the mode of anatexis in the ancient basement from muscovite to biotite melting, and the biotite melt provides fluoride ion, a uranium mineralizer. Therefore, the heterogeneous melting of the ancient basement is the cause of differential uranium enrichment in leucogranite of different phases. The Rossing leucogranite experiences strong crystallization differentiation, where the resulting crystalline minerals are mainly potassium feldspar, biotite, apatite, ilmenite and monazite. The fractional crystallization of biotite may cause uranium depletion, which is unfavorable to the mineralization of residual magma; whereas the fractional crystallization of K-feldspar, ilmenite and monazite is beneficial to the enrichment of uranium and formation of uraninite.

Key words: anatexis, muscovite dehydration melting, biotite dehydration melting, crystal fractionation, Rossing area

中图分类号:

陈旭, 范洪海, 陈东欢, 陈金勇, 王生云. 纳米比亚罗辛地区白岗岩成因及铀成矿作用[J]. 地学前缘, 2023, 30(5): 59-73.

CHEN Xu, FAN Honghai, CHEN Donghuan, CHEN Jinyong, WANG Shengyun. Genesis of and uranium mineralization in leucogranite, Rossing, Namibia[J]. Earth Science Frontiers, 2023, 30(5): 59-73.

图/表 11

图1 达马拉造山带大地构造背景及主要矿产与变质相带的分布(据文献[3]修改) A—刚果和卡拉哈里克拉通之间泛非达马拉造山带的位置。B—达马拉造山带主要特征及矿产与变质相带的分布[2],箭头指示变质程度增加方向,与同构造至后构造花岗岩有关的矿田以主要元素表示;缩写: CKZ=中 Kaoko带, EKZ=东Kaoko带, SKZ=南Kaoko带,WKZ=西Kaoko带,NMZ=北部边缘带, NP=北部地块, NZ=北部带, NCZ=北部中间带, SCZ=南部中间带,SMZ=南部边缘带,SZ=南部带,SF=南部前陆, OL=Okahandja线性构造, OML=Omaruru 线性构造,WL=千岁兰线性构造。

Fig.1 Tectonic setting of the Damara Orogenic Belt (A) and regional distribution of major minerals and metamorphic facies (B). Modified after [3].

图2 达马拉南部中央带穹窿构造、千岁兰断裂、铀矿床分布(据文献[4])

Fig.2 Distribution of dome structures, WL fault and uranium deposits in SCZ of the Damara Orogenic Belt. Adapted from [4].

表1 白岗岩分类简表(据文献[8,11])

Table 1 Classification of the Rossing leucogranite. Adapted from [8,11].

类型	主要特征	位置	年龄/Ma
A	呈不规则褶皱状,浅灰白色,细-中粒,糖粒状均质结构,以白色长石为主	欢乐谷	547.4±3.6 (LA-ICP-MS U-Pb锆石) ^[11]
B	白色,不等粒结构(细粒至伟晶结构),常见石榴子石、黑云母、电气石	欢乐谷	537.8±4.3 (LA-ICP-MS U-Pb锆石) ^[11]
C	淡红-乳白色,中粒-伟晶结构,含微斜长石和斜长石, 副矿物为磁铁矿、褐铁矿和电气石	欢乐谷	525.4±2.6 (LA-ICP-MS U-Pb锆石) ^[11]
D	呈不规则网状,白色,中-粗粒状结构,原生铀矿化的白岗岩主要由白色长石、烟灰色石英组成,见β硅钙铀矿和磷灰石	欢乐谷	506±8.1(SHRIMP U-Pb锆石)^[25] 497±5.5 (LA-ICP-MS U-Pb锆石) ^[11]
D	呈不规则网状,白色,中-粗粒状结构,原生铀矿化的白岗岩主要由白色长石、烟灰色石英组成,见β硅钙铀矿和磷灰石	湖山	496.1±4.1 (EPMA U-Th-Pb晶质铀矿) ^[26]
E	红-粉红色,颜色及粒度多变,见浅红色长石,有氧化晕圈, 矿物组成与D型相似,或者全部由烟灰色(黑色)石英和粉红色长石构成
F	红色,粗粒-伟晶结构,见粉红色粗粒条纹长石、乳白色石英, 副矿物为磁铁矿和褐铁矿	欢乐谷	511.4±4.3 (LA-ICP-MSU-Pb锆石)^[4]

图3 罗辛地区白岗岩主量及U、Th元素哈克图解(据文献[8,11])

Fig.3 Harker diagrams for major and U/Th elements of the Rossing leucogranite. Adapted from [8,11].

图4 罗辛地区白岗岩稀土元素球粒陨石标准化配分图(白岗岩数据据文献[11],球粒陨石标准化值据文献[28])

Fig.4 Chondrite-normalized REE patterns for the Rossing leucogranite (leucogranite data from [11]; normalization values from [28])

图5 罗辛地区微量元素原始地幔标准化蛛网图(数据据文献[11],原始地幔标准化值据文献[28])

Fig.5 Primitive mantle-normalized spider diagrams for the Rossing leucogranite (data see Fig.4)

图6 罗辛地区白岗岩的 Zr/Hf、Nb/Ta、Y/Ho和K/Rb值及其与TE1,3相关关系(大陆地壳平均值引自 [30],CHARCA区引自[31-32],样品数据据[11],底图据[33])

Fig.6 Plots of elemental ratio vs. TE1,3 for the Rossing leucogranite showing the REE tetrad effect (data from [11,30⇓-32]; base chart from [33])

图7 黑云母氧逸度图解(据文献[40])

Fig.7 Triangle diagram for biotite from the Rossing leucoganite showing variation of oxygen fugacity between types A, B, D, F. Modified after [40].

图8 罗辛地区白岗岩εNd(t)-t图解(据文献[43]修改,白岗岩数据据文献[11],前达拉基底资料据文献[43⇓-45],晶质铀矿数据据文献[43])

Fig.8 εNd(t)-t diagram for the Rossing leucogranite (modified after [43]). Leucogranite data are from [11]; basement data from [43⇓-45]; uraninite data from [43].

图9 罗辛地区白岗岩Rb/Sr-Ba关系图(数据据文献[11])

Fig.9 Rb/Sr-Ba diagram for the Rossing leucogranite (data from [11])

图10 白岗岩Sr-Ba(a)和La-(La/Yb)N(b)关系图及分离结晶趋势(a据文献[60];b据文献[61])

Fig.10 Sr-Ba (a, adapted from [60]) and La-(La/Yb)N (b, adapted from [61]) diagrams for the Rossing leucogranite

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