安徽向山铁硫矿区硅质岩特征及成因意义

Hydrothermal Origin of Silicalite in the Xiangshan Pyrite Ore District, Anhui Province

宁芜向山—皇姑山地区铁硫矿床中及附近存在大片硅质岩,属热泉沉积的硅华。其微量元素Al、Mn、Cu、Pb、Zn含量较高,且具有稀土元素总量较低,Ce明显亏损,Eu明显富集等特征。据岩石学、岩石化学特征,结合微量元素、稀土元素与同位素等综合研究表明,矿床附近硅质岩为热水成因,硅质可能来自下伏及深部地层。

SiO2 rich rocks are of a wide distribution, especially in connection with the pyrite deposits in the Xiangshan pyrite ore district. They have ever been considered to be the results of hydrother-mal alteration, resulting from the intrusion of volcanic magma. Recent investigations reveal that the formation of the majority of the SiO2 rocks is closely related to volcanic hot springs which are called silicalites in this paper.Silicalites do not appear like a "silicification cap "covering over a porphyrite body, but extend along the trending of the wall strata in planar or lenticular forms, and with lengths of hundreds to about a thousand meters and thicknesses of ten to a hundred meters. Most of silicalites are impure or dirty, containing variable contents of tuffaceous material, pyrite, sericite, anhydrite and even silts. Pure silicalites are only found in the uppermost part of the deposits. They are found in the pyroclastic rocks, predominantly in the upper part of the pyrite-bearing horizons, serving as host-rocks or as cover beds of the sulfides. Bedded structures are rather common. Silicalite bands can be interbedded with pyrite or anhydrite thin beds. Syndepositional siliceous fragments are also found in the ore-horizons. Sometimes, clasts of different features can be seen occurring in one sample or even in silicalites themselves. They are also characterized by extremely fine-grained texture (0. 01 - 0. 03mm)and very small inclusions with low vapor/liquid ratios (< 10%) and low homoginization temperatures (100℃ or so). Widely outcropping near the Xiangshan pyrite deposits, Anhui, and being derived from siliceous sinter induced by hot springs, the silicalite have trace elements similar to modern hot spring sediments. Studies of petrology, petrochemistry, trace elements, rare earth elements and isotopes give abundant eveidence that the silicailte should owe its origination to hot water deposition, with a source of siliceous materials from the underlying or deep rocks.