خاستگاه كانسار آهن چغارت، ايران مركزي: كاربردي از زمين‌شيمي ميانبارهاي سيال

Genesis of the Choghart iron ore deposit, Central Iran: Implication on geochemistry of fluid inclusions

كانسار آهن چغارت در زون ايران مركزي و 12 كيلومتري شمال شرق بافق، 130 كيلومتري جنوب شرق شهرستان يزد واقع شده است. سنگ ميزبان غالب در اين منطقه شامل مجموعه‌اي از نفوذي‌هاي آذرين حد واسط تا اسيدي و دايك‌هاي مافيك است كه در برخي از بخش‌ها به شدت دگرسان شده‌اند و به متاسوماتيت معروف‌اند. اين سنگ‌هاي آذرين در برخي نقاط تا حد رخساره شيست سبز دگرگون شده‌اند. مگنتيت فراوانترين كانسنگ اكسيدي منطقه است كه به شكل‌هاي لايه‌اي، توده‌اي و افشان در سنگ ميزبان كانسار چغارت حضور دارد. كانه‌زايي اصلي كانسار در سازندهاي پركامبرين پسين و به شكل پر عيار (مگنتيت + آپاتيت+ آمفيبول) رُخ داده است. داده هاي حاصل از مطالعه ميانبارهاي سيال در آپاتيت همزاد با مگنتيت نشان مي دهد كه كان توده آهن چغارت در محدوده‌ي دمايي 370 تا 385 درجه‌ي سانتي گراد تشكيل شده است. بيشترين نرخ شوري بين 20 تا 39 درصد وزني معادل نمك طعام تشخيص داده شد. شواهد صحرايي و ريزدماسنجي ميانبارهاي سيال در كاني آپاتيت‌‌ كانسار چغارت نشان مي‌دهد كه سيال كانسنگ ساز بسيار شبيه به ذخاير ماگمايي- گرمابي طلا- مس- اكسيد آهن (IOCG) است.

The last decade has seen major progress in our understanding of the origin of iron ore deposits worldwide. The majority of interpretations focused on the igneous iron oxide deposit either having formed by magmatic liquid immiscibility or by hydrothermal alteration and replacement. The hydrothermal iron deposits are generally found at many locations around the Pacific basin, Central America, Australia, and Japan (Asadi and Rajabzadeh, 2014). Fluid inclusions (FIs) are small part of ore bearing fluid in crystals that are trapped in crystal growth zones or inside any deficiencies which has been created during crystal growth. Nowadays, one of the most important methods for acquiring physiochemical characteristics of thermal ore fluids is employing different analysis and purposive measurements of FIs that are trapped in ores with the basis of ore bearing fluids (Barnes, 1997). Investigating fluid geochemistry and accurate composition of different ores such as apatite in the Choghart iron ore deposit is very important for gaining appropriate understanding about primary conditions of ore formation and regional geological events. On the basis of specific characteristics of apatite in determination of ore bearing processes and ore formation conditions in the Choghart is required. The goal of this study is investigating nature of ore forming fluid and chemical composition of apatite in order to study temperature, pressure, and magma subtraction degree in the Choghart deposit. Bafgh - Posht Badam district, in the center of Iran which is the home of Choghart iron ore deposit is located in a northward-southward zone in the eastern margin of central Iran zone and west of Lut block. This district extended with an area of 2500 km and more than 41 iron deposits and including about 2 billion tons of iron ore. From the tectonic point of view this area is a part of Lut block (small continental East Central Iran) and is consisted of Precambrian continental basement that is covered by the sectors related to Infra Cambrian to Triassic (Forster and Jafarzadeh, 1994). The evaluation of the area is a consequence of relocation of large growth Gondwanaland and development of Lut block in the active phase of Pan-African have been highly 600 million years ago (Samani, 1988). The Choghart iron ore host rocks are consist of a collection of intermediate to acidic igneous rocks ones and mafic dykes which in some areas are highly heterologous and are known as metasomatisms (Forster and Jafarzadeh, 1994).