كاني‌شناسي، ژئوشيمي و ژنز كانسار مس- اورانيوم رسوبي آغل مسي، بلوك طبس، ايران مركزي

The mineralogy, geochemistry and genesis of Aghol-Messi sedimentary copper - uranium deposit, Tabas block, Central Iran

حوضه رسوبي ژوراسيك-كرتاسه در ناحيه آغل مسي را لايه هاي قرمز رنگ سازند گره دو تشكيل مي دهد كه شامل توالي ماسه سنگ دانه ريز، ماسه سنگ دانه متوسط تا درشت و در نهايت كنگلومرا سرخ اكسيدان (هماتيت دار) مي باشد كه در بخش هايي احيايي شده اند. كانه زايي در افق هاي احيايي و در ارتباط با كانال هاي ديرينه رخداده است. در محدوده رخداد مس - اورانيوم آغل مسي، دو افق اصلي احيايي و چهار زير افق قابل تشخيص است كه هريك، از سه پهنه قرمز اكسيدان، پهنه شسته شده و پهنه احيايي كانه زايي تشكيل شده اند. با توجه به ويژگي-هاي سنگ شناسي، ژيومتري عدسي وابسته به لايه، گسترش ناحيه اي، ساخت و بافت جانشيني و دانه پراكنده، كاني شناسي ساده، وجود آثار و بقاياي گياهي و تمركز كانه زايي مس در ارتباط با آن، رخداد مس آغل مسي را مي توان تيپ رسوبي لايه اي قرمز با ميزبان نهشته هاي رسوبي1 در نظر گرفت. براي ارزيابي كاني شناسي و نحوه تشكيل رخداد مس- اورانيوم در منطقه، مطالعات صحرايي و آزمايشگاهي نظير تهيه مقاطع نازك و صيقلي و مطالعات ژيوشيميايي نظير XRF، XRD و ICP-MS انجام شده است. ماسه سنگ هاي ميزبان اورانيوم طبق نمودار فولك از نوع ليت آرنايت بوده و حداكثر ميزان اورانيوم در ماسه سنگ ها ppm 96 مي باشد.

IntroductionSediment-hosted copper deposits are the 2nd most important copper deposits after porphyry type deposits in the world. They supply more than 23% of world copper production (Brown, 2003). These types of deposits have been considered in Iran in recent years. In this regard, numerous copper deposits have been identified such as Markasheh and Tesuj (Mahdavi, 1387; Taghizadeh et al., 1386). The purpose of this article is to investigate the mineralogical, geochemical characteristics and genesis of Aghol-mesi sedimentary Copper-Uranium deposit in the vicinity of the Markasheh copper deposit.Materials and methodsAfter field studies and picking the right samples, 30 thin sections and 30 thin-polished sections were prepared and studied. Afterwards, geological, alteration and mineralization maps were prepared on a scale of 1: 5000 within an area of about 10 km2. 62 samples were analyzed by XRF and 17 samples by ICP-MS in Atomic Energy Organization. In order to accurately identify minerals, 10 rock samples were taken from the area for the XRD analysis. DiscussionAghol-mesi Cu deposit is located northwest of Ravar, in southwest of Tabas block. Host rocks of Aghol-mesi deposit in this part of central Iran subcontinent, include redbed sediments (Garadu redbeds) of Jurassic-Cretaceous age that extend in large areas between Ravar to Tabas, in a distance of more than 400 km. These sediments overprint salts of central Iran with upper Jurassic age and are overlaid with Cretaceous evaporite-carbonate units.Gradu redbeds include alternates of red oxidized sandstone, conglomerate and siltstone that partly change to light grey in color. In the Aghol-mesi area, two main reducing horizons are obvious within these grey parts that each of these horizons include three zones:Red oxidized zone: oxidized zones include main parts of Garadu redbeds that encompass reduced horizons from top and bottom. Neotocite is the only copper bearing mineral in this zone.Bleached zone: This zone is the altered part of Garadu Redbed sequences. Under influence of reduced fluid, color of red-orange sandstones convert to white, yellowish brown, grey or green color. Neotocite is the only copper bearing mineral in this zone that is very abundant as red zone.Reduced mineralized zone: In Aghol-mesi deposit, ore bearing zones formed non-continuous bleached reduced zones as lentiform. Abundance of plant fossil fragments as reductant and permeability of rocks in bleached zone is a principle factor in formation of this zone. Principle ore minerals in this deposit are chalchosite, bornite, covellite, chalcopyrite, pyrite, argentite and copper carbonates such as malachite. Chrysocholla, neotocite and atacamite are the other ore minerals at Aghol-mesi Cu deposit. Principle ore texture is replaced, disseminated, framboidal pyrite and solutioned seams.ConclusionExistence of upward coarse grain sedimentary cycles with woody fragments and plant fossils, sedimentary structures including ripple mark and cross bedding, rich paleochannel organic matter, biogenic sedimentary structures and caliche reveal that depositional environment of these sediments was a tidal-delta retrograde, marine environment. Considering the sedimentary sequences in the area, and paleotectonics of this part of central Iran in Jurassic-Cretaceous, Aghol-mesi copper deposit was formed in an extensional environment of the continental margin. Based on characteristics of Aghol-mesi Cu deposit such as lithology, lenses and layered shape (in geometry), regional extent, lamination, dissemination, vein and veinlets and replacement textures, mineralogy, presence of plant fossils and concentration of copper mineralization relative to these fossils, the copper mineralization in Aghol-mesi Cu deposit is a red bed type sediment-hosted copper that was formed soon after early diagenesis and before deep burial.