天津蓟县硼矿床锰方硼石矿物的硼同位素研究

天津蓟县硼矿床是世界上唯一的沉积环境特殊的锰方硼石型硼矿床。矿床呈层状产于震旦系碳酸岩泥灰质岩系中 ,矿石呈鲕粒状或球粒状集合体 ,是成因条件极其特殊的矿床类型。含矿岩系为中元古界高于庄组沉积岩系 ,含矿岩性由下而上依次为含锰白云岩、页岩互层→含锰白云岩、泥硅质岩 ,夹锰方硼石矿层→含锰页岩、白云岩→厚层白云岩→薄层白云岩→厚层白云岩 ,锰方硼石矿体在矿层中呈断续相连的团块状或透镜状分布。硼矿物成分简单 ,只有锰方硼石一种矿物 ,锰方硼石矿体呈透镜状断续成层分布 ,东西延长达 30km。东部矿段以锰方硼石为主 ,向西部渐变为菱锰矿。矿石有块状和斑点状。块状矿石全部由球粒状锰方硼石集合体组成 ,w (B2 O3 ) >2 0 % ,最高可达 4 2 %以上。斑点状矿石中锰方硼石集中呈球粒状集合体分布于泥质白云岩中 ,w (B2 O3 ) <7%。首次研究了该硼矿床锰方硼石的硼同位素组成 ,δ11B值为 +6 7‰～ +14 9‰ ,均为正值 ,并且东部大峪矿段的硼同位素值低于西部柏树沟矿段的硼同位素值 ,表明该硼矿床的成矿元素属于海水来源。由于锰方硼石的微量元素组成难以与蒸发岩比较 ,且富含F、Ba、Sn热液富集的元素和Co、Ni、Cr、Ti基性岩浆岩元素以及与Fe、Mn大洋结核有关的元素组分 ,因此锰方硼石应属于

BORON ISOTOPIC COMPOSITION OF DONGSHUICHANG CHAMBERSITE DEPOSIT,JI COUNTY,TIANJIN ,CHINA

The Dongshuichang boron deposit in Ji County is the only chambersite rich deposit known in the world. Boron occurs as oolitic aggregates of chambersite in stratiform beds interlayered with calcareous argillites of the Sinian System.These deposits are parts of the Mid Proterozoic Gauyuzhuang Group of sedimentary rocks. The stratigraphy of the deposit, listed from bottom to top, consists of a basal cross bedded dolomite, overlain by a manganoan dolomite unit, a calcareous argillites interbedded with oolitic chambersite, a thinly bedded dolomite unit, and an uppermost thickly bedded dolomite unit. The oolitic chambersite unit is laterally discontinuous, occurring as lenses that are generally 10-20 m wide and 2 m thick.Boron mineralization in the deposit grades from mainly chambersite rich rocks in the east to diallogite bearing assemblages in the west. Boron ore occurs in two forms: massive, and spotted. Massive ore consists entirely of oolitic chambersite grains, with an average grade of 20% B 2O 3, but locally containing up to 42% B 2O 3.Spotted ore consists of chambersite spheres disseminated within argillaceous dolomite and has B 2O 3 grades less than 7%.This study presents the first analysis of boron isotopic and trace element compositions of chambersite type boron deposits.Boron isotope compositions of the chambersite ore δ 11 B range from +6 7‰ to +14 9‰, with values in the chambersite dominant eastern part of the deposit lower than those in the diallogite rich western area. These values are consistent with a seawater source for the boron, modified by evaporation and hydrothermal activity. High concentrations of F, Ba, and Sn suggest the influence of hydrothermal fluids, as well as Co, Ni, Ti, and Cr that were most likely leached from mafic rock sources, and Fe and Mn. These hydrothermal components inhibit comparison of trace element compositions of the chambersite ore with evaporites elsewhere.Field relations, boron isotopes, and trace element geochemistry suggest the Dongshuichang chambersite deposit formed by hydrothermal activity on the sea floor centered on the chambersite-rich eastern part of the deposit.