白云鄂博矿床稀土富集过程:来自成因矿物学的证据

doi:10.13745/j.esf.sf.2022.10.40

摘要/Abstract

摘要：

稀土富集机制一直是白云鄂博矿床的重要研究内容之一,深刻理解稀土矿物的形成过程对于高质化开发利用稀土资源具有重要意义。白云鄂博主矿区矿石样品扫描电镜观察显示:最主要的两种稀土矿物氟碳铈矿和独居石常常呈细脉状产出,局部稀土细脉明显是由微细裂隙充填所致。需要特别关注的是,稀土矿物细脉外多见氟碳铈矿或独居石呈细小的浑圆状晶体被白云石包裹或赋存于白云石晶界处。同时,稀土细脉中也零星可见一些氟碳铈矿或独居石保留浑圆状的形态特点。如上这些现象暗示稀土物质最初是伴随白云质碳酸盐岩浆侵位而迁移到地壳浅部,并伴随岩浆固结成岩过程呈细小浑圆状晶体被白云石包裹或赋存于白云石晶界处,浑圆状似“珠滴”的晶体形态抑或是白云质碳酸盐岩浆固结成岩时发生了强烈的不混溶作用所致。此外,细脉状稀土矿物中局部可见富含Nb和Cr的独居石晶体,也可见一些独居石中包裹了早期自形富Si独居石,也说明氟碳铈矿或独居石的富集成矿具有岩浆属性。还需注意的是,在氟碳铈矿或独居石组成的稀土细脉中,除了少量晶体保留浑圆状形态外,大多呈半自形或它形晶体形态。同时,多处可见白云石晶体遭受溶蚀后被氟碳铈矿、独居石或萤石等包围的现象。特别是伴随氟碳铈矿或独居石细脉,有大量的萤石富集,局部还有较多的磷灰石结晶,其中一些磷灰石晶体中也包裹有细小浑圆状独居石。这些现象暗示早期伴随岩浆作用形成的氟碳铈矿、独居石和白云石,与后期富含F和P的流体发生了强烈相互作用。即氟碳铈矿或独居石细脉的形成,同时具有显著的热液作用属性。上述矿物学现象综合表明,白云鄂博稀土富集成矿至少经历了两次明显的成矿作用。首先是伴随碳酸盐岩浆侵位作用,稀土被运移到地壳浅部,并由于不混溶作用而得以初步富集;之后由于富含F和P的热液流体作用,使稀土物质发生了再活化和循环迁移,并得以再次富集形成稀土矿体。

关键词: 矿物学现象, 稀土富集, 岩浆作用, 热液作用, 白云鄂博

Abstract:

The enrichment mechanism of rare earth element (REE) in the Bayan Obo deposit is of global interest, and it is of great importance to define the REE mineralization process for high-quality development and utilization of REE resource. According to scanning electron microscope (SEM) observation of samples from the main orebody, two main REE minerals, bastnaesite and monazite, occur commonly as veinlets formed by filling localized micro fissures of host minerals. Of note, small and globular-shaped bastnaesite and monazite are wrapped by dolomite or occur on the grain boundaries of dolomite outside the veinlets of REE-bearing minerals; occasionally they also appear within the veinlets. The above observations indicate that REE is initially hosted by dolomitic carbonate magma, then migrates to the shallow crust, and eventually assumes its occurrence state as solidification of carbonate magma occurs. The globular droplet-like crystals also might be attributed to strong liquid immiscibility in carbonate magma during solidification. Furthermore, Nb-Cr-rich monazite can be observed in the REE-bearing mineral veinlets, also seen is Si-rich monazite from the early stage wrapped around by Si-poor monazite, suggesting that the enrichment of bastnaesite and monazite is closely associated with magma at depth. In addition, most of bastnaesite and monazite are characterized by subhedral-anhedral crystals apart from a few globular ones in the veinlets. Dolomite commonly undergoes dissolution and is wrapped by bastnaesite, monazite and fluorite. Especially, abundant fluorite and apatite are enriched together with bastnaesite and monazite veinlets, and globular-shaped monazite wrapped by apatite can be observed. These features indicate that bastnaesite, monazite and dolomite crystalized from early-stage magmatism are strongly modified by late-stage F-P-rich hydrothermal fluids—that is, the formation of bastnaesite and monazite veinlets is also closely linked to hydrothermal fluids. The above mineralogical phenomenon shows that REE enrichment in the Bayan Obo deposit underwent metallogeny at least twice: REEs are firstly carried by carbonate magma and migrate to the shallow crust where magmatic immiscibility leads to the initial enrichment; then F-P-rich hydrothermal fluids cause re-mobilization and cyclic migration of REEs, resulting in further enrichment.

Key words: Mineralogical phenomenon, Rare earth element enrichment, Magmatism, Hydrothermal process, Bayan Obo

中图分类号:

P571

申俊峰, 闫国英, 张萌萌, 王昭静, 徐渴鑫, 孟文祥. 白云鄂博矿床稀土富集过程:来自成因矿物学的证据[J]. 地学前缘, 2023, 30(2): 370-383.

SHEN Junfeng, YAN Guoying, ZHANG Mengmeng, WANG Zhaojing, XU Kexin, MENG Wenxiang. REE enrichment process in the Bayan Obo Fe-REE-Nb deposit: Genetic and mineralogical evidence[J]. Earth Science Frontiers, 2023, 30(2): 370-383.

图/表 18

图1 白云鄂博矿区地质简图(据文献[24]修改)

Fig.1 Regional geological map of the Bayan Obo deposit. Modified after[24].

图2 不同类型矿石手标本特征 a—条带状矿石;b—块状铁矿石;c—浸染状黄铁矿;d—细脉浸染状稀土矿石;e—浸染状萤石与稀土矿物共生;f—条带状萤石。矿物缩写:Mag—磁铁矿;REO—稀土矿物;Fl—萤石;Dol—白云石;Py—黄铁矿。

Fig.2 Different types of ore samples from the Bayan Obo deposit

图3 几种主要矿物的扫描电镜下特征矿物缩写:Mag—磁铁矿;Ilm—钛铁矿;Fl—萤石;Mnz—独居石;Par—氟碳钙铈矿;Bas—氟碳铈矿;Ap—磷灰石。

Fig.3 Photomicrographs of major minerals from the Bayan Obo deposit

图4 不同类型磁铁矿、萤石镜下特征 a, b—磁铁矿、钠闪石、白云石和黑云母等矿物共生;c—反光镜下磁铁矿与黄铁矿共生;d—脉状磁铁矿;e—黄铁矿穿切磁铁矿;f—萤石颗粒被磁铁矿脉穿插;g—脉状萤石;h—条带状萤石穿切白云石;i—阴极发光下萤石颗粒呈现蓝色。矿物缩写:Mag—磁铁矿;Dol—白云石;Rbk—钠闪石;Bt—黑云母;Py—黄铁矿;Fl—萤石。

Fig.4 Photomicrographs of different types of magnetite and fluorite

图5 白云鄂博矿床主要矿物相与矿物生成顺序图

Fig.5 Summary of the paragenetic sequence of major minerals from the Bayan Obo deposit

图6 与氟碳铈矿共生萤石明显比与独居石共生萤石多矿物缩写:Dol—白云石;Mnz—独居石;Bas—氟碳铈矿;Fl—萤石。

Fig.6 The content of fluorite coexisting with bastnaesite is much higher than that coexisting with monazite

表1 与氟碳铈矿共生萤石的成分特征

Table 1 The compositions of fluorite coexisting with bastnaesite

矿物名称	w_B/%
矿物名称	Ca	F	Cl	O		Si		K		Na		Fe		Mn	La		Ce		Pr		Nd		Pm
萤石1	44.65	55.37	—	—		—		—		—		—		—	0.02		0.08		—		—		0.03
萤石2	46.65	53.41	—	—		—		—		—		—		—	0.02		0.10		0.03		0.06		0.02
矿物名称	w_B/%
矿物名称	Sm	U	Gd		Tb		Dy		Ho		Er		Tm			Yb		Lu		Y		Sc
萤石1	—	0.05	—		—		—		0.13		0.11		0.10			—		—		0.09		—
萤石2	—	0.02	—		—		—		0.11		0.08		0.06			—		0.03		0.11		—

表2 白云石包裹的氟碳铈矿和氟碳铈矿脉带的成分比较

Table 2 The compositions of different types of bastnaesite wrapped by dolomite

矿物名称	w_B/%
矿物名称	Ca		F		La		Ce		Pr		Nd		Pm		Sm		U
白云石包裹的氟碳铈矿1	1.58		11.70		13.31		32.77		3.67		13.65		0.39		0.46		0.64
白云石包裹的氟碳铈矿2	2.29		13.69		12.70		31.95		3.47		12.92		0.34		0.51		0.56
白云石包裹的氟碳铈矿3	1.95		13.30		13.88		32.87		3.34		12.81		0.23		—		0.61
白云石包裹的氟碳铈矿4	0.62		12.27		16.87		34.84		3.04		8.62		0.34		0.48		0.38
氟碳铈矿(早) 1	0.15		11.52		13.14		33.42		3.94		14.58		0.16		0.92		0.39
氟碳铈矿(早)2	0.33		12.04		14.17		33.20		3.68		13.23		0.33		0.75		0.48
氟碳铈矿(晚) 1	2.03		13.49		10.22		34.64		3.54		16.90		0.05		—		0.69
氟碳铈矿(晚)2	2.88		14.23		9.57		34.39		3.11		16.48		—		—		0.82
矿物名称	w_B/%
矿物名称	Gd	Tb		Dy		Ho		Er		Tm		Yb		Lu		Y		Sc
白云石包裹的氟碳铈矿1	0.24	—		—		0.06		—		—		0.04		0.13		—		0.08
白云石包裹的氟碳铈矿2	0.34	—		0.09		—		—		—		—		—		—		0.07
白云石包裹的氟碳铈矿3	—	—		—		—		0.09		—		0.04		0.05		—		0.06
白云石包裹的氟碳铈矿4	0.41	0.23		0.31		0.04		—		—		—		—		—		0.07
氟碳铈矿(早) 1	0.59	0.55		0.51		—		—		—		0.04		0.49		—		0.08
氟碳铈矿(早)2	0.48	0.29		0.27		—		—		—		—		—		—		0.06
氟碳铈矿(晚) 1	0.23	—		—		—		0.46		—		—		—		—		0.09
氟碳铈矿(晚)2	0.16	—		—		—		0.77		—		0.32		0.71		—		0.12

图7 浑圆状稀土矿物产状 a—独居石脉之外可见白云石包裹浑圆状独居石;b—白云石包裹细小浑圆状独居石;c—白云石包裹细小的浑圆状氟碳铈矿。矿物缩写:Dol—白云石;Mnz—独居石;Bas—氟碳铈矿;Rt—金红石。

Fig.7 The occurrence of globular bastnaesite and monazite

图8 早期富硅独居石被晚期贫硅独居石包裹且浑圆状独居石周围残留白云石矿物缩写:Fl—萤石;Mnz—独居石;Dol—白云石;Ap—磷灰石。

Fig.8 Si-richmonazite from the early stage is wrapped by Si-poor monazite from later stage and dolomite occurs along the globular monazite

表3 早期和晚期独居石成分比较

Table 3 The compositions of different stages of monazite

独居石	w_B/%
独居石	P	Ca	La	Ce	Nd	Si	Mn	O	Cl
早结晶	13.15	0.06	18.78	29.61	2.90	8.38	0.08	27.15	0.01
晚结晶	13.73	0.07	19.50	30.57	6.09	0.03	0.09	30.65	0.01

图9 独居石脉中可见磷灰石包裹细小浑圆状独居石矿物缩写:Fl—萤石;Mnz—独居石;Ap—磷灰石。

Fig.9 Small and globular monazite is wrapped by apatite in the monazite vein

图10 Wk13-4 独居石脉中可见独居石包裹磷灰石矿物缩写:Fl—萤石;Mnz—独居石;Mag—磁铁矿;Dol—白云石;Ap—磷灰石。

Fig.10 Apatite is wrapped by monazite in the monazite vein

表4 独居石脉带与其包裹的磷灰石成分对比

Table 4 The compositions of monazite vein and wrapped apatite

矿物名称	w_B/%
矿物名称	P	Ca	F	O	La	Ce	Nd	Fe	Mn	Na	Si
磷灰石	17.60	34.71	6.28	37.89	0.69	1.56	0.76	0.05	0.11	0.50	—
	17.06	32.94	5.85	38.44	0.99	2.52	1.26	0.01	0.11	0.96	—
独居石	13.69	0.21	0.10	30.22	19.57	30.65	5.95	—	0.07	—	0.05
	13.84	0.15	0.00	30.44	19.51	30.42	6.14	—	0.08	—	0.03

图11 脉状独居石富集La-Ce-Pr-Nd-Nb-Cr且周围白云石含有不均匀Fe-Mg和Sc 矿物缩写:Fl—萤石;Mnz—独居石;Dol—白云石;Ap—磷灰石。

Fig.11 Monazite vein is rich in La-Ce-Pr-Nd-Nb-Cr and surrounding dolomite rich in inhomogeneous Fe, Mg and Sc

表5 白云石遭受Fe,Mn交代前(暗色)后(浅色)成分对比

Table 5 The compositions of dolomite before and after Fe, Mn metasomatism

白云石类型		w_B/%
白云石类型		Mg	Ca	Fe	Mn	Sr	O
交代前白云石 (暗色)	1	12.66	23.77	4.76	1.66	0.42	56.72
	2	11.75	24.83	6.50	2.21	0.21	54.52
	3	12.57	23.63	4.97	1.95	0.17	56.71
	4	11.81	23.76	5.76	2.23	0.16	56.28
	平均值	12.20	24.00	5.50	2.01	0.24	56.06
交代后白云石 (浅色)	1	9.56	22.95	8.63	3.62	0.25	54.99
	2	9.45	22.91	9.01	3.35	0.25	55.03
	3	8.89	22.98	10.67	1.40	0.25	55.82
	4	9.12	22.78	10.59	1.42	0.22	55.87
	平均值	9.26	22.91	9.73	2.45	0.24	55.43

图12 独居石脉中白云石遭受溶蚀呈浑圆状边缘并被独居石包围

Fig.12 Dolomite occurs in the monazite vein as globular shape and is wrapped by monazite

图13 脉状独居石和氟碳铈矿富集过程模式图

Fig.13 Schematic diagram showing the enrichment process of monazite and bastnaesite vein

参考文献 31

[1]	王凯怡, 张继恩, 方爱民, 等. 白云鄂博矿床成因: 矿体内霓长岩化成矿作用与赋矿白云岩的联系[J]. 岩石学报, 2018, 34(3): 785-798.
[2]	柯昌辉, 李以科, 李立兴, 等. 白云鄂博矿区赋矿“白云岩”地质特征与成因再认识[J]. 中南大学学报(自然科学版), 2021, 52(9): 3047-3063.
[3]	YANG K F, FAN H R, PIRAJNO F, et al. The Bayan Obo (China) giant REE accumulation conundrum elucidated by intense magmatic differentiation of carbonatite[J]. Geology, 2019, 47(12): 1198-1202. DOI URL
[4]	于俊芳, 沈茂森, 郭爱芳. 白云鄂博矿萤石浮选分析[J]. 现代矿业, 2018, 34(9): 245-246.
[5]	郝美珍, 赵永岗, 张顺, 等. 白云鄂博超大型Nb-Fe-REE矿床主矿矿体形态变化及深部找矿方向的探讨[J]. 地质调查与研究, 2018, 41(3): 167-175.
[6]	杨占峰, 李强, 王振江, 等. 白云鄂博矿床萤石型铁矿石中稀土分布规律研究[J]. 中国稀土学报, 2017, 35(4): 520-527.
[7]	王凯怡, 张继恩, 郝美珍, 等. 白云鄂博赋矿白云岩的稀土地球化学及对稀土矿化的制约[J]. 地质科学, 2020, 55(2): 439-458.
[8]	郝梓国, 王希斌, 李震, 等. 白云鄂博碳酸岩型REE-Nb-Fe矿床: 一个罕见的中元古代破火山机构成岩成矿实例[J]. 地质学报, 2002, 76(4): 525-540.
[9]	中国科学院地球化学研究所. 白云鄂博矿床地球化学[M]. 北京: 科学出版社, 1988.
[10]	白鸽, 袁忠信, 吴澄宇, 等. 白云鄂博矿床地质特征和成因论证[M]. 北京: 地质出版社, 1996.
[11]	CHAO E C T, BACK J M, MINKIN J A, et al. The sedimentary carbonate-hosted giant Bayan Obo REE-Fe-Nb ore deposit of Inner Mongolia, China; a cornerstone example for giant polymetallic ore deposits of hydrothermal origin[R]. Reston: US Geological Survey, 1997.
[12]	杨占峰, 朱智慧, 王振江, 等. 白云鄂博主矿霓石型稀土铁矿石中稀土元素在独立矿物中的富集状况研究[J]. 中国稀土学报, 2019, 37(6): 769-776.
[13]	范宏瑞, 杨奎锋, 胡芳芳, 等. 内蒙古白云鄂博地区基底岩石锆石年代学及对构造背景的指示[J]. 岩石学报, 2010, 26(5): 1342-1350.
[14]	邓淼, 韦春婉, 许成, 等. 白云鄂博超大型稀土矿床成因评述[J]. 地学前缘, 2022, 29(1): 14-28. DOI
[15]	胡乐. 再论白云鄂博稀土矿床成因: 来自白云岩研究的新证据[D]. 北京: 中国地质大学(北京), 2019.
[16]	范宏瑞, 胡芳芳, 杨奎锋, 等. 内蒙古白云鄂博地区晚古生代闪长质-花岗质岩石年代学框架及其地质意义[J]. 岩石学报, 2009, 25(11): 2933-2938.
[17]	潘启宇. 白云鄂博铌、稀土铁矿的成矿地质条件及矿床成因[J]. 内蒙古地质, 1997(1): 1-17.
[18]	杨学明, 杨晓勇, 陈天虎, 等. 白云鄂博富稀土元素碳酸岩岩墙的地球化学特征及稀土富集机制[J]. 矿床地质, 1998, 17(3):527-532.
[19]	倪培, RANKIN A H, 周进. 白云鄂博地区碳酸岩墙及岩墙旁侧石英岩中的包裹体研究[J]. 岩石学报, 2003, 19(2): 297-306.
[20]	王凯怡, 范宏瑞, 谢奕汉. 白云鄂博碳酸岩墙的稀土和微量元素地球化学及对其成因的启示[J]. 岩石学报, 2002, 18(3): 340-348.
[21]	张培善, 陶克捷, 杨主明, 等. 白云鄂博稀土、铌钽矿物及其成因探讨[J]. 中国稀土学报, 2001, 19(2): 97-102.
[22]	范宏瑞, 陈福坤, 王凯怡, 等. 白云鄂博REE-Fe-Nb矿床碳酸岩墙锆石U-Pb年龄及其地质意义[J]. 岩石学报, 2002, 18(3): 363-368.
[23]	张鹏远, 李双庆, 王长尧, 等. 白云鄂博地区地质构造特征[J]. 中国地质科学院天津地质矿产研究所所刊, 1993, 28: 1-86.
[24]	YANG K F, FAN H R, SANTOSH M, et al. Mesoproterozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, North China:constraints for the mechanism of super accumulation of rare earth elements[J]. Ore Geology Reviews, 2011, 40(1): 122-131. DOI URL
[25]	谢玉玲, 曲云伟, 杨占峰, 等. 白云鄂博铁、铌、稀土矿床:研究进展、存在问题和新认识[J]. 矿床地质, 2019, 38(5): 983-1003.
[26]	韦春婉, 许成, 付伟, 等. 稀土元素在岩浆和水热系统的实验岩石学和地球化学研究进展[J]. 岩石学报, 2022, 38(2): 455-471.
[27]	袁忠信, 白鸽, 张宗清. 白云鄂博矿床赋矿岩石的自交代现象及其意义[J]. 岩石矿物学杂志, 2004, 23(1): 19-25.
[28]	LIU S, FAN H R, YANG K F, et al. Fenitization in the giant Bayanobo REE-Nb-Fe deposit: implication for REE mineralization[J]. Ore Geology Reviews, 2018, 94: 290-309. DOI URL
[29]	佘海东, 范宏瑞, 胡芳芳, 等. 稀土元素在热液中的迁移与沉淀[J]. 岩石学报, 2018, 34(12): 3567-3581.
[30]	SHE H D, FAN H R, YANG K F, et al. Complex, multi-stage mineralization processes in the giant Bayan Obo REE-Nb-Fe deposit, China[J]. Ore Geology Reviews, 2021, 139: 104461. DOI URL
[31]	朱祥坤, 孙剑. 内蒙古白云鄂博矿床的稀土矿化时代与期次[J]. 地球学报, 2012, 33(6): 845-856.