斯洛伐克西喀尔巴阡山Kosice-Medvedia菱镁矿床的矿物学、流体包裹体和碳-氧-锶同位素研究

Kosice矿床是斯洛伐克第二大的菱镁矿床(150Mt),位于Gemeric的东部.其镁质碳酸盐矿体赋存于石炭纪石灰石和含白云石的石灰石中,同时下盘黑色片岩中也含有被铁质碳酸盐交代的薄层碳酸盐透镜体.在华力西期造山运动(M1)中,古生代岩石受到了低级变质作用(绿泥石带).镁交代作用始于白云岩1的结晶作用,其后形成菱镁矿,最终沿裂隙形成铁菱镁矿.铁质碳酸盐包括早期铁白云石-白云石,铁白云石和后期含方解石和石英的菱铁矿.根据碳酸盐矿物对地质温度计,白云石l结晶作用发生在300～340℃.这一结果与M1的变质矿物组合(绿泥石,白云母-伊利石)吻合.铁白云石的结晶作用发生在320～370℃.少量细脉中可见白云石2,绿泥石和伊利石-多硅白云母,它们是由于阿尔卑斯期造山运动M2变质作用形成的更晚的矿物组合.菱镁矿的流体包裹体(FI)研究,显示存在不同成分的热卤水,卤水成分变化相当于NaCl含量21～42 wt%,但其它成分的盐含量高于NaCl,溶解的CO2含量也有变化.两相包裹体均一温度(Th)的范围为164～217℃,含石盐子晶包裹体均一温度的范围为217～344℃.富CO2包裹体(盐度相当于NaCl含量1～22wt%,CO2的密度为0.28～0.77 g·cm-3,均一温度为289～344℃)在菱镁矿中是次要的,但这种包裹体在与矿石伴生的石英中是主要的,并且与含石盐子晶流体包裹体共生.在后期镁交代过程中流体中的CO2逐渐增加.和铁质碳酸盐伴生的石英中只有两相包裹体,包裹体中CO2含量有所变化,盐度范围为17～24 wt%的NaCl(或者34～36 wt%的MgCl2),均一温度为152～195℃.包裹体的数据结合碳酸盐地质温度计显示镁交代作用的压力范围是180～320MPa(7～12km),铁交代作用的压力范围是280～420MPa(10～16km),说明地热梯度约为25～35℃/km.包裹体浸出液的分析表明Cl/Br和Na/Br的比值存在变化,但仍旧说明富镁的卤水来源是上二叠纪和下三叠纪的分馏蒸发岩来源.铁质碳酸盐流体的高溴和高碘含量,说明在铁交代过程中周围黑色片岩的明显影响.菱镁矿和铁交代作用,表明交代流体中的碳和二氧化碳,主要是海洋沉积的来源.菱铁矿的"Sr/86Sr比值((0.71124～0.71140),说明锶的多来源,最初应是石炭纪和二叠纪的海水,但它被当地其它陆壳中的锶混染.

Mineralogy, fluid inclusion and C, O, Sr isotope study of the Ko(s)ice-Medvedia magnesite deposit, Western Carpathians, Slovakia

Kosice deposit is the second biggest magnesite deposit in Slovakia (150 Mt), located in the eastern part of the Gemeric unit. The main Mg-carbonate body is hosted by Carboniferous limestone and dolomitic limestone, while footwall black schists contain thin carbonate lenses replaced by Fe-carbonates. Paleozoic rocks were affected by low-grade metamorphism (chlorite zone) during Variscan orogeny (M1). Mg-replacement started with the crystallization of dolomite 1, followed by magnesite and terminating by formation of Fe-magnesite along cracks. Fe-carbonates include early ankerite-dolomite, ankerite and later siderite with calcite and quartz. Based on carbonate geothermometry dolomite 1 crystallization occurred at 300 ～ 340℃, which is supported by the M1 metamorphic mineral assemblage (chlorite, muscovite-illite). Ankerite crystallization occurred at 320 ～370℃. Minor veinlets with dolomite 2, chlorite and illite-phengite represent younger mineral assemblage of the M2 metamorphism, attributed to Alpine orogeny.Fluid inclusion (FI) study in magnesite showed the presence of brines of variable composition (mostly 21 to 42 wt% NaCl eq.)with high concentration of salts other than NaCl, and variable amount of dissolved CO2. Homogenization temperatures (Th) ranged from 164 to 217℃ in two-phase aqueous FIs and 217 to 344℃ in halite(?)-bearing FIs. CO2-rich FIs (1 ～22 wt% NaCl eq. , CO2 where they co-exist with halite-bearing inclusions. CO2 shows increased participation in fluids in later stages of Mg-replacement. Quartz associated with Fe-carbonates contained only two-phase aqueous brine FIs (17 to 24 wt% NaCl eq. or 34 to 36 wt% MgCl2 eq. , Th 152 to 195℃) with variable amount of dissolved CO2. Combined with carbonate geothermometry, FI data imply pressure ranges for Mgreplacement ～ 180 to 320 MPa (7 ～ 12 kn) and for Fe-replacement ～280 to 420 MPa (10 ～ 16 kn), suggesting a thermal gradient of about 25 to 35℃/kn.Leachate analyses of fluid inclusions showed variable Cl/Br and Na/Br ratios, but the fractionated evaporitic origin of Mg-rich brines (Upper Permian and Lower Triassic) is still suggested. High Br and I content in Fe-carbonate fluids indicates appreciable influence of surrounding black schists on the Fe-replacement process.Carbon (δ13CPDB= - 1 to + 3.7‰) and oxygen (δ18OSMOW = 12.5 to 17.5 ‰) isotopes from magnesite and from carbonates in Fe-replacement suggest predominantly marine sedimentary origin of carbon and CO2 in the replacement fluids. 87 Sr/86 Sr ratio of magnesites (0.71124 to 0.71140) indicates variable sources of Sr, where the original Carboniferous and Permian seawater Sr ratio was modified by other local crustal sources of Sr.