斯洛伐克西喀尔巴阡山脉Gemerská Poloma滑石-菱镁矿矿床中镁的交代作用

Gemerská Poloma矿床是个重要的滑石矿床(储量20万吨),位于西喀尔巴阡山脉Germeric地区。部分滑石化的镁质碳酸盐体赋存在早古生代火山沉积杂岩体中(黑色片岩,变质泥岩),在Variscan变质作用(M1)过程中受到了绿泥石-黑云母带区域变质相的改造。这种原岩是石灰岩的矿体由白色-灰白或者灰色-黑色的菱镁矿与白云石1组成,被次生的白云石2和滑石脉切割。本次研究考察了两次变质事件(M1和M2)的几个连续的矿物组合,最早的组合包括铁白云石,镁菱铁矿与菱铁矿,(并与黑电气石,铁绿泥石,磷灰石,与伊利石-白云母伴生),它们以微小残留物形式产出在菱镁矿和白云石1中,其形成可能早于M1变质作用高峰期。M1变质事件的高峰期以富铁金云母,镁绿泥石1,镁电气石(黑电气石的边缘)和石英的组合为代表。在M1退变质作用过程中,发生了镁交代作用,开始是白云石1结晶,接下来形成菱镁矿,最后是以铁菱镁矿沿裂隙的形成而终。根据碳酸盐地质测温原理,M1变质事件的高峰期温度为460~490℃,变质矿物组合特征也支持这一测温结果。滑石,白云石2,与镁绿泥石2沿着镁碳酸盐岩石裂隙的发育,主要受到M2变质事件的影响,这个变质事件与较年青的Alpine造山事件有关。 菱镁矿流体包裹体的研究表明,成矿流体具有复杂的组成,可能以MgCl2组分为主,主要来

Mg replacement at the Gemerska Poloma talc-magnesite deposit, Western Carpathians, Slovakia

Gemerska Poloma deposit is an important talc deposit (20 Mt of talc), located in the Gemeric unit within Western Carpathians. The partly steatizated Mg-carbonatic carbonate body was formed inside Early Paleozoic volcano-sedimentary complexes (black schist, metapelite) , regionally metamorphosed during Variscan metamorphic event (M1) in chlorite-biotite zone. The body,originally limestone, consists of white-grey and/or grey-black coarse-grained magnesite and dolomite 1 that is cut by veins of younger dolomite 2 and talc. Several successive mineral assemblages within two major metamorphic events M1 and M2 have been recognized.Earliest assemblage consists of ankerite, Mg-siderite and siderite (accompanied by schorl, Fe-chlorite, apatite and illite-muscovite) ,occurring as very small relicts in magnesite and dolomite 1 and probably preceding the peak of M1 metamorphism. The peak of the M1 is represented by the assemblage F-rich phlogopite, Mg-chlorite 1, dravite (the rim of schorl) and quartz. During retrograde part of the M1 successive Mg replacement occurred starting with the crystallisation of dolomite 1, followed by magnesite and terminating by formation of Fe-magnesite along cracks. Based on carbonate geothermometry, the peak of M1 occurred at 460 ～490℃, which is supported by metamorphic mineral assemblage. The M2 metamorphism, related to the younger Alpine orogeny, was responsible for the formation of talc, dolomite 2 and Mg-chlorite 2 along the faults in the host Mg carbonate body.Fluid inclusion study in magnesite showed the presence of fluids with very complex composition, probably MgCl2 dominated brines of evolved evaporitic origin. In primary brine inclusions the salinity reached up to ～35 wt% MgCl2 eq. and homogenisation temperature (Th) occurred in the range 216 to 235 ℃. Fluid inclusion in quartz showed also the presence of MgCl2 dominated brines of similar composition, however, with a higher range of Th values (248 to 313℃). If assuming that the host quartz is coeval with the peak of M1, the fluid inclusion data imply the pressure range for the peak of M1 from ～ 250 to 350 MPa (9 ～ 13 km) , which indicates a thermal gradient of about 35 to 40℃/km. Presuming this gradient remained constant also during the Mg-replacement, the related fluids had ～300 to 350℃ under ～ 180 to 280 MPa (7 ～ 11 km).Leachate analyses of fluid inclusions confirm the evaporitic signature of fluids with very high Cl/Br and Na/Br ratios. Large amounts of evaporitic brines in the Gemeric unit had been generated only from Upper Permian to Lower Triassic. Thus, the Permian extensional tectonics can be associated with the generation of the hydrothermal system forming magnesite.