四川攀枝花大田地区铀矿化透镜地质体特征、成因及其对深源铀成矿的启示

王凤岗; 姚建; 吴玉; 陈友良; 孙泽轩; 张玉顺; 李秋实

doi:10.12029/gc20200409004

四川攀枝花大田地区铀矿化透镜地质体特征、成因及其对深源铀成矿的启示

Characteristics and genesis of uranium mineralized lenses and its implications for deep−source uranium metallogenesis in Datian area of Panzhihua, Sichuan Province

摘要

摘要:
研究目的 近期在四川省攀枝花大田地区发现了呈雁列式展布铀矿化透镜体群，无论是在铀矿化特征上还是在成因上均极为特殊，具有重要的研究价值。
研究方法 通过对铀矿化透镜体开展岩石学、矿物学、岩石地球化学、同位素地质学及年代学、铀赋存状态及铀矿化与透镜体之间的成因联系等研究。
研究结果 铀矿化透镜体基本由斜长石组成并发生了强烈钠黝帘石化，具有岩浆岩常见的镶嵌结构，形成年龄为821 Ma（SIMS锆石U–Pb）。化学组成上具有富Na₂O（含量3.95%~5.68%，平均为5.09%）、CaO（含量4.40%~7.35%，平均为5.46%），贫SiO₂（含量51.52%~55.09%，平均为53.34%）的特征。微量元素分析结果显示铀矿化透镜体具有极低的ΣREE含量（含量9.96×10⁻⁶~33.63×10⁻⁶，平均为22.03×10⁻⁶），特殊的铕正异常（δEu=1.59~5.51，平均为2.68）稀土配分模式。I_Sr值介于0.7060~0.7088，平均为0.7074，具有地幔来源特征。透镜体中铀主要呈独特的“铀钛矿物聚集体”形式存在，主要由“金红石+铀钛混合物+钛铀矿+晶质铀矿+锆石”等矿物组成，且上述矿物具有由“金红石（Ti）→铀钛混合物（Ti>U）→钛铀矿（Ti<U）→晶质铀矿（U）”的演化特征。
结论根据铀矿物与透镜体的关系及铀矿物稀土元素示踪等综合判断，铀矿化具有岩浆成因属性，推测在深部高温（>700℃）高压（>15 kbar）的环境中，U与Ti具有极强的亲和性，形成以NaU⁴⁺(Ti⁴⁺)TiO₄⁴⁺(F,Cl)为主要形式的络合物，并在熔体中向富钠的部位迁移、富集。铀与透镜体具有同源、同成因特征，而“铀钛矿物聚集体”是在等压降温过程中因温度降低从浆状体中分离出来所形成。攀枝花大田地区铀矿化透镜体的发现提供了深源铀成矿的地质实例，为探讨深源铀成矿提供了参考。

Abstract:
This paper is the result of mineral exploration engineering.
Objective A very special and echelon arrangement uranium mineralized lenses group were found in the Datian area of Panzhihua, Sichuan Province, with important research value.
Methods Through the comprehensive studies, including petrology, mineralogy petrochemistry, isotope geology and chronology, uranium occurrence state the genetic relationship between uranium mineralization and lenses.
Results The lenses mainly consist of plagioclase with strongly sodium zoisitization, and have typical magmatic mosaic texture, with age of 821 Ma (SIMS zircon U–Pb age). They have the chemical composition characterices of high Na₂O (3.95%–5.68%, average 5.09%), CaO (4.40%–7.35%, average 5.46%), low SiO₂ (51.52%–55.09%, average 53.34%). The analysis of trace element show that lenses have very low ΣREE content (9.96×10⁻⁶–33.63×10⁻⁶, average 22.03×10⁻⁶), positive Eu anomalies (δEu=1.59–5.51, average 2.68) and special REE distribution pattems. The results of I_Sr (0.7060–0.7088, average 0.7074) indicate that the raw material of lenses coming from the mantle. The mainly uranium occurrence state in lenses is the unique "U–Ti minerals aggregates". The "U–Ti minerals aggregates" are mainly composed of rutile, uranium–titanium mixture, brannerite and uranium, and the minerals in "U–Ti minerals aggregates" have the evolutionary characteristics of "rutile (Ti)→uranium–titanium mixture (Ti>U)→brannerite (Ti<U) → uranium(U)".
Conclusions According to the relationship between uranium minerals and lenses, and the REE tracer method of uranium minerals, it is confirmed that the uranium mineralization genesis is relation to magmatism. It is speculated that the NaU⁴⁺(Ti⁴⁺)TiO₄⁴⁺(F,Cl) is mainly complex in the deep environment with high temperature (>700 °C) and high pressure (>15 kbar), and it can move and concentrate in the sodium–rich site in the lenses. The U and raw material of lenses originate from same magma, and the "U–Ti minerals aggregates" are separated from magma body in the process of isobaric cooling. The discovery of uranium mineralized lenses in Datian area of Panzhihua provides the direct geological case that uranium can be enriched in the mantle, and this discovery provides a reference for the discussion of deep-source uranium metallogenesis.

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