北秦岭成矿带西段天水东部地区产铀岩体特征及其与铀矿化的关系

Relationships Between the Rock Mass of Uranium and Uranium Mineralization in Eastern Tianshui in the West Section of Nothern Qinling Mountains Metallogenic Belt

北秦岭成矿带西段为花岗岩型铀成矿区,简述了北秦岭成矿带西段天水东部地区构造单元、岩浆区带的特征,天水东部为宝鸡岩浆岩区,发育多期次成因复杂的岩浆活动。通过岩体特征、产铀岩体特征的对比研究,认为印支期中酸性及部分碱性岩与铀矿化关系密切。论述了宝鸡二长花岗岩与东岔碱性正长岩特征。前者属富钾贫钠准铝—过铝质钙碱性系列(A/CNK=0.98~1.00,K_2O>Na_2O);CIPW标准矿物显示S型花岗岩特征;微量元素表现为以V,Cr,Co,Ni相对亏损、Nb,Th,Zr相对富集的壳源特征;稀土为铕亏损,轻重稀土分馏程度逐渐增高,为壳源同源岩浆的产物;A-C-F和δEu-(La/Yb)_N图解为S型壳源熔融花岗岩。后者属A型花岗岩,为壳幔源混合特征。分析了4种产铀岩体S异地重融型、弱过铝—不过铝富钠Ⅰ型、弱过铝二长花岗岩序列Ⅰ型、Ⅰ-S型过渡类型的特征,并对产铀花岗岩大地构造环境,岩石类型,围岩,U,Th和Th/U比值特点,岩石地化,副矿物和脉岩特征作了详细分析。认为天水东部地区铀成矿条件较好,多数岩体具有产铀岩体的基本特征,铀成矿潜力较大。

The west section of nothern Qinling mountains metallogenic belt belongs to granite type uranium mineralization area. The charateristics of the tectonic unit and magmatite zone in eastern Tianshui in the west section of nothern Qinling mountains metallogenic belt where belongs to Baoji magmatite zone developing muti-stages complicated magmatic activities are briefly discussed in the study. With comparing the characteristics between the wall rock and uranium-producing rock mass, the uranium mineralizations are closely related to indosinian intermediated- acid and some alkaline magmatite. The indosinian intermediated-acid magmatite fails into kalium-rich, sodium-deficient and standard-oversaturated aluminum calc-alkali rock series （A/CNK = 0.98 - 1.00, K2O 〉 Na2O） , in which CIPW index minerals indicate S-type granite charateristics, trace element analysises show crust-resource charateristics with relative-depleted V, Cr, Co, Ni and relative-enriched Nb, Th, Zr and rare earth data display depleted Eu and gradually increased fractionation degrees. The indosinian intermediated-acid magmatites are the productions of cognate magmatic activities and A-C-F and Eu-（La/Yb） N diagrams point to crust-resource melting granite. In addition, the alkaline magmatites are A-type granites having the charateristics of crust-mantle mixing resousres. Four uranium-producing rock masses are analyzed, namely allopatric remehing S-type, weak-oversatu- rated to no oversaturated A1, Na-rich I type, weak-oversaturated Al monzonite granite I type and I-S transitional type. And furthermore, the geotectonic environments, rock types, wall rocks, U, Th and Th/U ratio, petrological and chemical characteristics, accessory minerals and gangue rocks are discussed in detail too. As result of it, there are favorable uranium minerogenetic conditions in eastern Tianshui, which most of rock masses possess the essential characteristics of uranium-producing rock mass and the potentials of uranium mineralization are tremendous.