四川盆地东部中二叠统茅口组热液活动特征

四川盆地中二叠统茅口组存在丰富的热液流体活动。在野外剖面和岩心观察的基础上,利用多种分析测试方法,对四川盆地东部茅口组碳酸盐岩中的热液活动特征进行了探讨。研究区茅口组热液活动包括2类,一类为硅质热液,表现为薄层硅质岩或硅质团块;另一类为碳酸盐型热液,表现为粗晶方解石和白云石充填在张性构造裂缝中或呈"雪片状"集合体,偶见少量黄铁矿、闪锌矿等金属硫化物伴生。硅质岩主量元素之间的关系表明硅质来源具有多样性,是热液硅与壳源硅的混合;黄铁矿、闪锌矿原位S同位素δ34S介于-3.91‰~-6.87‰之间,推测可能受到了微生物和基性岩浆岩的双重影响;方解石脉和具鞍状双晶的白云石脉普遍具有Ca O高于标准计量、相对富Sr和U、贫REE和Ti、Ce/Ce*明显负异常、Eu/Eu*主体正异常、Y正异常等特征,表明碳酸盐脉为热液活动的产物,流体来源也呈现出火山活动相关流体与富钙地下水混合的特点。闪锌矿富集Ge和Cd,Zn/Cd及Zn/Fe值反映了中等成矿温度。硅质岩和碳酸盐流体包裹体均一温度变化范围较大,介于54.7~294.3℃之间,与稀土元素和微量元素比值所揭示的特征吻合。研究结果表明,穿层状硅质结核和团块的首次出现往往标志着茅口组热液活动的开始,而张性构造裂缝和碳酸盐型热液活动带来的围岩白云化作用则有效地改善了白云岩储集层的物性。

Hydrothermal activities in the middle Permian Maokou Formatio n in eastern Sichuan Basin

The abundant hydrothermal activities have been found in the Maokou Formation of middle Permian in the Sichuan Basin. Based on the field investigation and core observation,many kinds of measured methods are applied to discuss the hydrothermal activity characteristics of the carbonate rocks in the Maokou Formation of middle Permian in the Sichuan Basin. Two types of hydrothermal activities in Maokou Formation are identified in eastern Sichuan Basin: one is hydrothermal silica,formed as thin layered silicalite or nodules;the other is featured by hydrothermal carbonate minerals,including coarse calcite and dolomite that are filled in the tensile fractures or formed as the "snow flake"aggregates,sometimes they occur as few metal sulfides including the pyrite and sphalerite. The Al-Fe-Mn end-member diagram of cherts and correlation diagrams between silica and other main elements indicate the hydrothermal origin,and the silica comes from both the hydrothermal and terrestrial sources. The in situ sulfur isotopes of pyrite and sphalerite range between -3. 91‰ and -6. 87‰,implying the mixed sulfur sources of microbial and basic magma. Both coarse calcite and saddle dolomite in the veins have similar geochemical features,such as higher CaO content than those in stoichiometry calcite and dolomite,rich in Sr and U,poor in REE and Ti,obvious negative anomalies of Ce/Ce*,and positive anomalies of Eu/Eu*,and very positive Y anomalies,illustrating the hydrothermal origin and the mixed sources of fluids related with volcanic activity and the Ca-bearing underground water. High concentration of Ge and Cd,and ratios of Zn/Cd and Zn/Fe in sphalerite reflect the medium mineral-forming temperatures. The homogenization temperature ranges between 54. 7℃ and 294. 3℃ measured from fluid inclusions in microcrystalline quartz,calcite and dolomite,which are consistent with the features illustrated by the REEs and trace elements. The results show that the first occurrence of chert nodules and chips penetrating into the rocks is the indicant of the beginning of hydrothermal activities in Maokou Formation. The tensile fractures and the hydrothermal dolomitization caused by carbonate-rich fluid in host rocks significantly improve the reservoir properties of dolomite.