奥地利东阿尔卑斯山地区杂砂岩带中赋存于碳酸盐岩中的晶质菱镁矿床(英文)

Carbonate hosted sparry magnesite of the Greywacke zone, Austria/Eastern Alps

奥地利东阿尔卑斯山地区Austroalpine杂砂岩带中的石炭纪Veitsch逆冲推覆体是"Veitsch型"晶质菱镁矿化的典型地区。几十年来,对其成因的解释一直是人们争论的焦点。为了解决这一长久以来的问题,本文对Veitsch推覆体的地质学、矿物学和地球化学制约条件进行了论述。菱镁矿仅仅存在于Veitsch推覆体之中,而其它推覆体中的碳酸盐岩岩层中则无菱镁矿。赋存于Veitsch推覆体中的碳酸盐岩石中的菱镁矿以不规则的岩株状、透镜状和层状产出。块状和厚层状的菱镁矿总是被白云岩包裹。在Veitsch推覆体中,沉积作用开始于晚维宪期的后造山的类磨拉石海相建造,并晚于内部基底带的变形和变质作用("Bretonic期")形成,该基底带位于现代的东阿尔卑斯山地区。沉积序列研究表明,演化从浅海大陆架开始,有时还穿插有高盐度的泻湖和透镜状生物礁,发展到海退的海岸线伴随有发育强烈的三角洲沉积的分支海湾和河流。由于成矿作用有好几个期次,所以对地球化学数据的解释相当复杂。 在Hohentauern／Sunk矿床中,石膏和硬石膏层在菱镁矿质沉积主岩中互层,其δ34S值分别为+17.6±0.2‰和+17.2±0.2‰,表现出石炭纪海水的特征。白云岩和菱镁矿的REE浓度要高于石灰质主岩。在Hohentauern／Sunk矿床中。

The Carboniferous Veitsch nappe of the Austroalpine Greywacke zone (Austria/Eastern Alps) is the type region of the 'Veitsch type' sparry magnesite. Its genetic interpretation is the subject of intensive discussions for several decades. As a contribution to this longstanding discussion, geological, mineralogical and geochemical constraints of the Veitsch nappe are documented in this paper. Magnesite exclusively occurs in the Veitsch nappe and carbonate rock layers of the other nappes are never transformed to magnesite. The magnesite, hosted by carbonate rocks of the Veitsch nappe, is occurring in irregular stocks, lens and layers. The massive to thick bedded magnesite is always enveloped by dolomite. In the Veitsch nappe, sedimentation began as a post-orogenic molasse like marine formation within the late Visean after deformation and metamorphism ( 'Bretonic phase' ) of the internal basement zones presently included in the Eastern Alps. The sedimentary sequence reflects to the evolution from a shallow shelf, sometimes interfingering with hypersalinar lagoons and lensoid bioherms, to a regressive shore line with distributary bay and river dominated delta deposits. The interpretation of the geochemical data is complex in account of several stages of mineralizations.In the Hohentauern/Sunk deposit, gypsum and anhydrite layers are observed intercalated within magnesite host sedimentary rocks, which show 8 S values of + 17. 6 ± 0. 2 ‰ and + 17. 2 ?0. 2‰, respectively, indicating a Carboniferous seawater characteristic. The REE contents in dolomite and magnesite are higher than in the calcareous host rocks. In the Hohentauern/Sunk deposit, both the dolomite spheres and the dolomitic matrix in the orbicular dolomites display flat REE pattern with small negative Ce and Eu anomalies. Similar patterns are within the magnesite cores of the dolomite spheres, which are clearly different from the REE pattern of the magnesite of the deposit. In the Veitsch nappe, the δ13CPDB values of magnesite and dolomite are - 1. 0 to + 1. 3‰ and -1.6 to -2.3 ‰, respectively. The δ18OPDB values of magnesite are from -13. 9 to -17. 2 ‰. The 8DSMOW values of fluid inclusion water in magnesite and dolomite are -107 to -83‰ and -89 to-53‰, respectively. The 87Sr/86Sr values of the magnesite from the Hohentauern/Sunk deposit are between 0. 7087 and 0. 7103, similar to or slightly higher than the dolomitic and calcitic host rocks (0.7083 to 0.7085) and the Carboniferous seawater (0.7076 to 0.7081). The leached fluids from magnesite, calcite, and dolomite have a salinity of 20-30 % and high concentrations of Br-, Mg2+ , K+ , and SO42- and very high Br-/Cl- and Mg2+/Fe2+ ratios.All these observed data best fit to an epigenetic model for magnesite formation before Alpine ( Cretaceous) thrust tectonics and greenschist metamorphism. As indicated by the chemistry of the leached fluids, the mineralizing fluid evolved from evaporitic brines.