钱家店铀矿田草莓状黄铁矿特征及其地质意义

Characteristics and Geological Significance of Framboidal Pyrite in Qianjiadian Uranium Ore Field

钱家店铀矿田主要赋矿层位为青山口组、姚家组原生红杂色岩系,目前关于其内部灰色砂体沉积期的原生氧化还原性仍存在较大分歧,直接影响了区内找矿部署,而草莓状黄铁矿对沉积岩形成的古环境具有重要指示作用。文章通过系统研究青山口组和姚家组灰色砂体内草莓状黄铁矿的赋存、硫同位素、粒径等特征,首次利用其粒径特征反演目标层位沉积-成岩期水体的古氧化还原条件,进而探讨其对铀成矿的意义。结果表明,钱家店铀矿田灰色砂体中草莓状黄铁矿主要形成于沉积-成岩期,全岩δ34S值为-37.5‰~-23.4‰,为低温微生物还原作用成因。姚家组、青山口组的草莓状黄铁矿粒径分布范围分别为1.6~28.0μm、0.8~52.0μm,平均粒径分别为8.0μm、9.3μm,最大粒径分别为28.0μm、52.0μm,粒径大于10μm的颗粒分别占21.8%、28.3%,表现出姚家组草莓状黄铁矿粒径整体较青山口组小,青山口组具较广的分布范围,但两者具有相似的粒径统计学分布规律。综合草莓状黄铁矿平均粒径对标准偏差和偏态系数二元图解表明,含矿目标层沉积时古水体为次氧化环境,这是目前含矿层不发育翼部矿体和具有“两红夹一灰”纵向氧化还原分带的主要原因之一;而底部灰色砂体的原生还原容量有限,本身不足以形成规模性工业铀矿,这侧面说明下白垩统内的油气渗出还原是区内砂岩大规模铀成矿的关键因素和必要条件。该研究结果为原生红杂色含矿层砂岩型铀矿找矿方向提供了重要参考依据。

Framboidal pytite play an important role in indicating paleoenvironment of sedimentary rocks.The main ore-bearing strata in Qianjiadian ore field are Qingshankou Formation and Yaojia Formation,which are primary red complex rock series.At present,there are great differences in the primary redox property of internal grey sand body deposition period,which directly affects the uranium exploration deployment in the region.In this paper,the occurrence characteristics,sulfur isotope and particle size characteristics of strawberry-like pyrite in grey sand were systematically studied,and the paleo-oxidation-reduction conditions of water body in target horizon during deposition-diagenesis period were inverted for the first time by analyzing its particle size characteristics,and then the significance of uranium mineralization was discussed.The results showed that framboidal pytite in grey sand body of Qianjiadian ore field was mainly formed in sedimentary-diagenetic period.The bulk of sulfur isotope-37.5‰~-23.4‰,is the produets of microbial reduction at low temperature.The particle size distribution of framboidal pytite in Yaojia Formation and Qingshankou Formation is respectively 1.6~28μm,0.8~52μm,MD are 8.0μm and 9.3μm,MFD are 28μm and 52μm,respectively.The particles with particle size greater than 10μm accounted for 21.8%and 28.3%,respectively.The results showed that the particle size of framboidal pytite in Yaojia Formation was generally smaller than that in Qingshankou Formation,and Qingshankou Formation had a wide distribution range,but the two formations had similar statistical distribution laws of particle size.The binary diagrams of mean particle size versus standard deviation and skewness coefficient of framboidal pytite indicated that the paleowater body was hypo oxidized during the deposition of ore bearing target layer.This is one of the main reasons that no flank ore bodies was developed in the ore-bearing strata and only one ash stratum clamp redox zone occurred longitudinally in two red strata;the primary reduction capacity of the bottom grey sand body is limited,and it is not enough to form large-scale industrial uranium deposits,which suggest that the oil and gas seepage reduction in the Lower Cretaceous is the key factor and necessary condition for large-scale uranium mineralization of sandstone in the area.