罗河铁矿的硫同位素分馏机制和矿床形成的物理化学条件的研究

罗河铁矿(又称庐枞铁矿)是我国长江中下游火山岩地区大型铁矿床之一,是一种重要的矿床类型。十多年来,各研究单位和生产部门对该矿床进行了多方面、较系统的研究,取得可喜成果(张荣华等,1974;张荣华,1981;陈锦石等,1982)。然而,要进一步阐明这类矿床的成矿机理,还需要对该矿床的蚀变和成矿条件进行深入研究。本文着重讨论罗河铁矿的硫同位素分馏机制,并在此基础上结合地质和矿物包裹体的资料,近似定量地推算该矿床形成各阶段的物理化学条件范围及变化规律。

STUDY ON FRACTIONATION MECHANISM OF SULFUR ISOTOPE AND PHYSICOCHEMICAL CONDITIONS OF ALTERATION AND ORE FORMATION IN LOUHE IRON DEPOSIT, ANHUI

The Louhe iron deposit, which lies in the area of volcanic rocks of middle and low Yangtze valley, is a volcano-pneumato-hydatogenetic metasomatic deposit of late Meso-zoic. In addition to magnetite, this ore deposit contains abundand pyrite and anhydrite. They nearly occurred in whole altered process. The whole alteration can be divided into two different zones: the strongly altered zone and the weakly altered zone.The sulfur isotopic compositions of both pyrite and anhydrite represent regular changes with depth. From lower layer of strongly altered zone to upper layer of weakly altered zone, δS_py^3t value changes from +10.4‰ to -9.3‰, and δS_anR³⁴ (purple) value from +14.1‰ to +18.5‰. The regular changes of δS_py³⁴ and δS_Anh³⁴ value is the result of systemic changes of physicochemical conditions (T, fO₂, fS₂, pH, etc.) and components of the ore-forming fluid. Even if sulfur isotopic fractionation between purple anhydrite and pyrite in the strongly altered zone is not in equilibrium, whole alteration (including the strongly altered zone) still can be research as a system of isotopic equilibrium. In fact, the strongly altered stage is an extraordinary system of isotopic equilibrium (non-equilibrium between anhydrite and pyrite, but equilibrium between SO₂ and H₂S in ore-forming fluid).According to the model of sulfur isotopic equilibrium fractionation between coexisting minerals, δS_f³⁴ value and oxidation-reduction state in ore-forming process may quantitatively be estimated. The δS_t³⁴ value is about +14.1±0.9‰ (δ) and the mole ratio of oxidation- and redution-sulfur is about 2:1. Therefore, about 1/2~1/3 of total sulfur in ore-forming fluid is derived from the underlying sediments, especially Trias-sic marine anhydrite and the rest of total sulfur is derived from upper mantle.According to the model of sulfur isotopic equilibrium fractionation by Ohmoto and Rye, main physicochemical conditions of alteration and mineralization, such as T,fO₁, fS₂, fCO₂, fH₂, pH and m∑s, can be quantitatively estimated. In the strongly altered stage, the ranges of T, fO₂ and fS₂ are 600-350℃, 10^-16.5-10^-27.0 atm and 10^-2.0-10^-7.4atm, respectively. In the weakly altered stage, the ranges of T, fO₂, fS₂ and m∑s are 350-200℃,10^-26.8-10^-41.0 atm,10^-7.2-10^-14.8 atm and 10^-3.0-10^-2.7 atm, respectively, and pH value changes from 3.2-4.0(350℃) to 6.0- 6.9(200℃). In the formation of magnetite body, the ranges of T, fO₂, fS₂, fCO₂ and fH₂ are 480-350℃, 10^-20.4-10^-27.0 atm, 10^-4.0-10^-7.4 atm, 102.2-102.0 atm and 10^-0.3-10^-0.5 atm, respectively.