فهرست مطالب

  • سال سی و پنجم شماره 3 (پیاپی 76، پاییز 1398)
  • تاریخ انتشار: 1398/07/01
  • تعداد عناوین: 5
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  • رضا اسکندری نژاد، حامد زند *، مهدی خراسانی پور صفحات 1-22

    کانسار سرب و روی طرز یکی از معادن فعال زیرزمینی در ناحیه راور-کوهبنان است که در شمال استان کرمان قرار دارد. کانه زایی در این معدن به طور مجزا در دو بخش کربناته و سولفیدی در واحدهای کربناته تریاس میانی (سازند شتری) مشاهده می شود. بررسی های صحرایی نشان می دهند کانسارسازی در راستای گسل ها به شکل رگه ای و رگچه ای و به مقدار کمتر به شکل توده ای شکل گرفته است. مهم ترین کانی های سولفیدی اولیه در معدن سرب و روی طرز شامل گالن، اسفالریت، پیریت و کالکوپیریت هستند. کانی های ثانویه مرتبط با کانسارهای سرب و روی ازجمله اسمیت زونیت، همی مورفیت، سروزیت و آنگلزیت نیز در برخی نمونه ها شناسایی شدند. در نمونه های مطالعه شده، متوسط غلظت سرب و روی به ترتیب 19 و 24 درصد بود. عناصر دیگر مانند Cd، Ag، S، Sb، Te، Se، As و Cu نیز در نمونه های بررسی شده غنی شدگی نشان دادند. ارزیابی نتایج با روش های آماری چندمتغیره، ارتباط احتمالی بین فازهای کانی شناسی و نتایج ژئوشیمیایی را مشخصکرد؛ برای نمونه، همبستگی عناصر Pb، Ag، Tl و Sb ممکن است با کانه زایی گالن مرتبط باشد و یا ارتباط ژئوشیمیایی عناصر As، Bi، Cu، P، Fe، Co و S ممکن است از کانه زایی پیریت و کالکوپیریت متاثر باشد. روابط آماری نشان دادند عنصر Zn تنها همبستگی ژئوشیمیایی ضعیفی با عناصر Se، U و Mo دارد. همبستگی زیاد Ca و Mg نیز از کانی شناسی سنگ میزبان متاثر است که عمدتا از کانی درشت بلور (در حاشیه رگه های کانه زا) و دولومیت ریزبلور (دور از رگه های کانه زا) تشکیل شده است.

    کلیدواژگان: کانی شناسی، ژئوشیمی، کانسار سرب و روی طرز، کوهبنان، شمال کرمان
  • سعیده سنماری* صفحات 23-36

    در مطالعه حاضر، رسوبات پالئوسن در مرز سازندهای گورپی و پابده در برش آرکوازواقع در جنوب شرق ایلام بررسی شدند. بخش بالایی سازند گورپی از شیل خاکستری و بخش زیرین سازند پابده از شیل ارغوانی تشکیل شده است. در مطالعه حاضر، 23 گونه متعلق به 17 جنس از نانوفسیل های آهکی شناسایی شدند. در رسوبات بررسی شده، نه حادثه زیستی ثبت شد که بر اساس آنها، بایوزون های Ellipsolithus macellusZone (NP4/CNP6-CNP7)و Fasciculithus tympaniformisZone (NP5/CNP7-CNP8)از بخش بالایی سازند گورپی و متعاقبا زون های  Heliolithus kleinpelliiZone (NP6/CNP8) وDiscoaster mohleri Zone (NP7) / Heliolithus riedelii Zone (NP8) (NP7/8 combined Zone-CNP9/10)  از بخش زیرین سازند پابده تشخیص داده شدند. در نتیجه این مطالعه و بر اساس زون های مشخص شده، سن بخش بالایی سازند گورپی دانین پسین- سلاندین و سن بخش زیرین سازند پابده سلاندین- تانتین تعیین شد. زون CNP8 در محدوده مرز بین دو سازند قرار دارد. بایوزون های تعیین شده در مطالعه حاضر با برش هایی از دیگر نواحی زاگرس مقایسه شدند.

    کلیدواژگان: بیواستراتیگرافی، پالئوسن، زاگرس، سازند گورپی، سازند پابده، نانوفسیل های آهکی
  • معصومه کردی* صفحات 37-56
    بین فرایندهای دیاژنزی، تشکیل سیمان های کوارتزی مهم ترین عامل کاهش دهنده تخلخل و تراوایی در ماسه سنگ های با عمق تدفین زیاد (بیشتر از 2 کیلومتر) است. کانی های رسی اغلب به عنوان کاهش دهنده کیفیت مخازن هیدروکربوری ماسه سنگی شناخته می شوند؛ هرچند بررسی مقاله های گوناگون و مطالعه های دقیق فرایندهای دیاژنزی نشان می دهد تشکیل کانی های رسی اتوژنیک در برخی از شرایط دیاژنزی نه تنها موجب کاهش کیفیت مخزنی نمی شود، حفظ تخلخل و تراوایی اولیه را در پی دارد. در مقاله حاضر، بررسی پژوهش های پیشین و مقایسه نتایج با دو نمونه شاهد نشان داد اگر در شرایط ائودیاژنز، کانی های رسی با شکل پوشاننده ذرات به طور پیوسته سطح ذرات را بپوشانند، مانع از تشکیل سیمان کوارتزی در شرایط مزودیاژنز می شوند؛ این شرایط موجب حفظ تخلخل و تراوایی اولیه و درنتیجه، کیفیت مخزنی خوب درماسه سنگ های با عمق تدفین زیاد می شود. مطالعه های گوناگون نشان می دهند بین کانی های رسی اتوژنیک، کلریت در درجه اول و ایلیت در درجه دوم فراوان ترین و مهم ترین کانی های رسی اتوژنیک در ماسه سنگ ها هستند؛ بر این اساس، شناخت شرایط تشکیل و شیوه گسترش کانی های رسی پوشاننده ذرات به پیش بینی و معرفی توالی های با کیفیت مخزنی خوب در اکتشاف مخازن ماسه سنگی هیدروکربوری کمک بسیاری می کند.
    کلیدواژگان: دیاژنز، کیفیت مخازن ماسه سنگی، کانی های رسی با شکل پوشاننده ذرات، کلریت، ایلیت
  • فرزانه نوری، افشین زهدی*، حسین کوهستانی، میرعلی اصغر مختاری صفحات 57-78

    سازند روته در منطقه قارخوتلو (جنوب باختر زنجان) ازنظر سنگ شناسی عمدتا از سنگ آهک ماسه‏ای و سنگ آهک‏های کرم تا خاکستری رنگ نازک تا متوسط لایه به ضخامت 52 متر تشکیل شده است. در این منطقه، سازند روته روی ماسه سنگ های کوارتزی سازند دورود قرار گرفته و با افق لاتریتی- بوکسیتی پوشیده شده است. بررسی‏های صحرایی و میکروسکوپی، وجود شش ریزرخساره کربناته را در سازند روته در منطقه قارخوتلو نشان می دهند. تجزیه وتحلیل ریزرخساره ها و نبود آثار ریف‏های سدی بزرگ مرجانی، نبود رسوبات توربیدایتی در ریزرخساره‏های شناسایی شده و همچنین حضور آلوکم های اسکلتی مانند جلبک سبز، دوکفه‏ای، براکیوپود، فرامینیفر بنتیک و قطعه های خارپوست نشان می دهد این ریزرخساره‏های کربناته احتمالا در بخش‏های کم عمق پلت فرم کربناته از نوع رمپ رسوب گذاری شده اند. به منظور بررسی ویژگی های زمین شیمیایی سازند روته در منطقه قارخوتلو و تعیین ترکیب کانی شناسی اولیه کربنات های آن، تجزیه وتحلیل های عناصر اصلی (کلسیم و منیزیم) و فرعی (آهن، منگنز، سدیم و استرانسیم) روی 15 نمونه از سنگ آهک های این سازند (بخش های میکرایتی) انجام شدند. بر اساس نمودارهای دومتغیره Sr در برابر Na، Mn در برابر Sr/Na، Na در برابر Mn و Sr/Mn در برابر Mn می‏توان گفت ترکیب کانی‏شناسی اولیه نمونه‏های سنگ آهکی سازند روته، مخلوط کلسیت- آراگونیت است که با محیط تشکیل کربنات های سازند روته در حاشیه جنوبی اقیانوس پالئوتتیس در زمان پرمین همخوانی دارد.

    کلیدواژگان: چینه‏شناسی، ریزرخساره، محیط رسوبی، ژئوشیمی، سازند روته، پرمین، قارخوتلو، زنجان
  • محمدنبی گرگیج*، سمیه شهرکی صفحات 79-98

    میسلانیدآ، گروهی از میکروفسیل های شاخص بزرگ پالئوژن (پالئوسن میانی- اوایل ائوسن پیشین) هستند که برای بایوزون بندی نهشته های دریایی کم عمق این زمان ارزش زیادی دارند. این گروه از فرامینیفرهای بنتیک بزرگ طرح های مجزای ساختمانی و تزئیناتی دارند که سبب تفکیک آنها از نومولیتیدها و روتالیدها در سطح خانواده می شوند. میسلانیدآ تاکسون های مختلفی دارند که اساسا برای بررسی های زیست چینه نگاری وجغرافیای زیستی استفاده می شوند. به منظور مطالعه زیست چینه نگاری، تعیین سن و شناسایی سیستماتیک خانواده میسلانیدآ، نهشته های پالئوسن میانی- ائوسن زیرین رخنمون یافته در دو برش چینه نگاری در روستای پدگی در شمال زاهدان بررسی شدند. برش های یادشده ازنظر سنگ شناسی شامل تناوبی از مارن، آهک مارنی و سنگ آهک به ضخامت 5/237متر (برشPE) و 5/222 متر (برش PEP) هستند. مطالعه جنس های مختلف متعلق به خانواده میسلانیدآ در مقاطع نازک به شناسایی گونه های Miscellanea miscella ، Miscellanea juliettae، Miscellanites primitivus، Miscellanits minutus منجر شد که گویای بایوزون های استاندارد SBZ3-SBZ6 و سن پالئوسن پسین- ائوسن پیشین برای برش های مدنظر است؛ همچنین الگوی پخش و انتشار جهانی گونه های قدیمی تر از جنس Miscellanea نشان می دهد گونه های یادشده بومی حوضه تتیس شرقی بوده اند.

    کلیدواژگان: پالئوسن پسین-ائوسن پیشین، فرامینیفرهای بنتیک، زمین درز سیستان
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  • Reza EskandariNejad, Hamed Zand, *, Mehdi Khorasanipour Pages 1-22

    The Tarz Pb-Zn deposit is one of the underground active mines in the Ravar-Kuhbanan area, located in the North of Kerman province. In this mine, mineralization was observed in two distinct sulfide and carbonate divisions within the dolomitic-limestone host rock units of the Middle Triassic succession (Shotori Formation). Field observations show that mineralization occurred mainly along the faults as veins or veinlets and in lesser extent as massive textures. Galena, sphalerite, pyrite and chalcopyrite are the most important primary sulfide minerals in the Tarz Pb-Zn deposit. Secondary minerals associated with the Pb-Zn deposits such as smithsonite, hemimorphite, cerussite and anglesite were also identified in some of the investigated samples. The average concentrations of Pb and Zn were 19 and 24% in the studied samples, respectively. Other elements such as Cd, Ag, S, Sb, Te, Se, As and Cu also showed significant enrichment in the investigated samples. Evaluation of the obtained results using multivariate statistical methods can reveal the possible relationships between mineralogical phases and geochemical analysis. For example, the correlation between Pb, Ag, Tl and Sb can be related with the galena mineralization or the geochemical relationship between As, Bi, Cu, P, Fe, Co, and S is related with the pyrite and chalcopyrite mineralization. Statistical relationships also showed that Zn has only a weak geochemical association with Se, U and Mo. The strong correlation of Ca and Mg is also due to the host-rock mineralogy, which is mainly composed of course and fine crystal dolomites at the margin and far distances of ore veins, respectively. Keywords: Mineralogy, Geochemistry, Tarz Pb-Zn deposit, Kuhbanan, North of Kerman  

    Introduction

    Tarz Pb-Zn deposit is located at 30 km east of Kuhbanan and 5 km southwest of the Tarz village in the North of Kerman Province. From geological point of view, this mine is located on the southeastern margin of the Bahabad Pb-Zn belt, which is a part of Tabas-Poshte-e-Badam metallic belt of the Central Iran (Alavi 1991; Rajabi et al. 2013). In this area, there are a large number of Pb-Zn deposits that mostly occurs in the Triassic dolomite-limestone beds of the Shotori Formation. Major faults such as Kuhbanan and Behabad with NW-SE direction have been very effective role in the development of geology and mineralogy history of this region. However, The dominant trend of the faults in Tarz Pb-Zn deposit is about N40E, but field observations showed various faults and fractures in different directions, which demonstrate the active tectonic of this area. For example, one of the major mineralization parts of the mine was in a shearing zone produced by four major faults with NW-SE direction. Some authors such as Amiri et al. (2009) believe that based on the position of mineralized sections in the carbonate host rocks the Tarz Pb-Zn mine is classified as the Mississippi Valley Type (MVT) deposits. In the Tarz mine, mineralization is observed in two distinct sulfide and carbonate sections, which are located in the north and south of the mine, respectively. This study emphases on the texture, mineralogy and geochemistry of sulfide section of the Tarz Pb-Zn deposits in order to determine the elemental dispersion, and reconstruction of depositional history of Pb and Zn minerals.  

    Materials & Methods

    Fifty rock samples were collected from mineral veins and hosted-rock for mineralogical studies of sulfide section of the Tarz Pb-Zn mine. Ore mineralogy studies were done on the 30 polished-thin section and five polished-blocks by reflecting and polarizing microscope (OLYMPUS BH-2 model) at the Geology Department of Shahid Bahonar University of Kerman. Also, eight samples were selected for further mineralogical studies by X-ray diffraction (XRD) method in the Zarazma Laboratory, Tehran, Iran. After mineralogical studies, 10 samples were selected for major and trace elements analysis by the ICP-MS method in the Zarazma Laboratory. Geochemical data were used to (1) calculate enrichment factor; (2) identification of enriched elements; and (3) geochemical correlations of target elements. The spider diagrams of major and trace elements were plotted in order to determine their changes from ore veins to the surrounding hosted rocks. For drawing of spider diagrams, the data were normalized against the standard limestone.  

    Discussion of Results & Conclusions

    Mineralogy Mineralogical studies using ore microscopy and XRD showed that both primary and secondary minerals are present in the sulfide section of the Tarz deposit. The most important primary minerals were galena (about 60%), sphalerite (less than 40%), pyrite and chalcopyrite. These minerals were often found in the mineralized sections in the northern part of the mine, where mineralization occurs mainly as veins and veinlets, breccia and in lesser extent as pore-filling structures. The secondary minerals including smithsonite, hemimorphite, cerussite, anglesite and iron oxides also were identified in different parts of the mine. In addition to the mentioned minerals, dolomite and calcite were also observed as the main minerals of the hosted-rocks. The identified dolomites minerals can be classified into two groups including fine-crystalline and sudhedral, coarse crystalline or saddle dolomite. It seems that fine-crystalline dolomites have formed as primary minerals during the early time of diagenesis, while the coarse-crystalline dolomites have formed by hydrothermal fluids. Calcite in the hosted rocks was mainly in the form of microcrystalline or micrite type and it seems that this mineral is not affected by the Pb-Zn mineralization process.   Structure and texture The structures and textures of mineralization in the Tarz mine, were as veins and veinlets, massive, breccia, pore-filling and replacement types. These structures are mainly related to the active fault system of the region. Galena, sphalerite, pyrite and dolomite were the most important primary minerals in most of mineralized textures, however, secondary minerals such as smithsonite, hemimorphite and cerussite were also observe in pore-filling and replacement textures.  

    Geochemistry

    According to the results obtained from normalized enrichment factor Pb, Cd, Zn, Ag, S, Sb, Te, Se, As and Cu have moderate to high enrichments in the investigated samples, respectively. Also, U, Tl, Bi, Mo, Co and Cr showed low enrichments in some samples. The geochemical relationships between enriched elements was well compatible with the mineralogical results. For example, identified sulfide minerals (galena, sphalerite, pyrite and chalcopyrite) have a great potential for replacing of several of enriched elements in their crystalline structures as impurities. The geochemical associations of most of the target elements in sulfide minerals can be considered as possible mechanisms for enrichment of these elements in the investigated samples. These geochemical association can be revealed using multivariate statistical methods. For examples, the following groups of geochemical associations were observed in the investigated samples:

    1-     The correlation between Pb, Ag, Tl and Sb, which can be related with the galena mineralization; 2-     The correlation between P, Cu, Bi, As elements with Fe, Co and S can be related with the pyrite and chalcopyrite mineralization; 3-     The strong correlation between Ca and Mg is due to the mineralogy of the host-rock minerals; 4-     Zinc only showed a weak correlation with Mo, U and Se, which may be due to the alteration and weathering of its primary sulfide minerals mainly into the secondary carbonate minerals; The spider diagrams of major elements showed that except iron, other elements have the same values with standard limestone. Ore forming processes, especially the presence of sulfide minerals such as pyrite, chalcopyrite and even sphalerite are responsible for iron enrichment in the investigated samples. Contrasting to the most of major elements, trace elements, especially Pb, Cd, Zn, Ag, S, Sb, Te and Se, As and Cu, showed strong deviation from their values in the standard limestone. Concentration of these elements were decreased by increasing distance from ore veins to the host rock.   Possible mineralization model The Pb-Zn mineralization in the Tarz mine as well as other ore deposits in the Ravar-Bahbad region is often as veins and veinlets. A variety of lithological and structural factors control Pb-Zn mineralization in this region. According to Leach et al. (2005) diagenesis and tectonic processes have a fundamental role in the formation of MVT type Pb-Zn deposits. Field observation demonstrate that active tectonic and formation of a complex fault system have more important role than other factors in the formation of Tarz Pb-Zn deposit. Comparison of the obtained mineralogical and geochemical data in this study with the previous researches revealed that the Pb-Zn mineralization in Tarz area is more similar to the MVT type. Nevertheless, to determine the source of ore-bearing fluids and to draw up a comprehensive mineralization model, isotopic and structural data are required.

    Keywords: Mineralogy, Geochemistry, Tarz Pb-Zn deposit, Kuhbanan, North of Kerman
  • Saeedeh Senemari * Pages 23-36

    In this study the Paleocene sediments have been investigated at the upper part of Gurpi Formation and the lower part of Pabdeh Formation at Arkavaz section in southeast of Ilam. The upper part of the Gurpi Formation is mainly consists of grey shale and the lower part of the Pabdeh Formation consists of purple shale. As a result of this study, 22 species belong to 17 genera of calcareous nannofossils were detected. Nine bioevents were recorded and based on these bioevents, the Ellipsolithus macellus Zone (NP4/CNP6–CNP7) and Fasciculithus tympaniformis Zone (NP5/CNP7–CNP8) are recognized at the uppermost part of the Gurpi Formation. Subsequently, Heliolithus kleinpellii Zone (NP6/CNP8) and Discoaster mohleri Zone (NP7) / Heliolithus riedelii Zone (NP8) (NP7/8 combined Zone–CNP9/10), are identified at the base of the Pabdeh Formation, respectively. As a result of this study and based on the identified calcareous nannofossil biozones, the age of the uppermost part of the Gurpi Formation is Late Danian–Selandian and the lower part of the Pabdeh Formation is Selandian–Thanetian, and based on CNP8 the boundary between these two formations is continuous. The identified biozones in this study were compared with other parts of the Zagros Basin.     

    Introduction

    One of the most extensive Cretaceous and Palaeogene deposits in Zagros is the marine sediments of the Gurpi and Pabdeh formations, which were first identified in the Zagros Basin on the basis of stratigraphy and paleontology. Type section of the Gurpi Formation is studied in Lali oilfield located in the NE of Masjed soleiman. Also, the term Pabdeh Formation has been introduced for the argillaceous limestone, marly limestone and shale succession exposed at the Tang-e Pabdeh, from the southeastern part of Pabdeh Mountain, located in north of the Lali oilfield. The most important thing to do in these formations is to determine the exact boundary by calcareous nannofossils. At the Arkavaz section, the upper part of Gurpi Formation (with 41.5 m thick) and the lower part of Pabdeh Formation (with 64.9 m thick) consists of 106.4 m thick was selected which mainly consists of gray shale and purple shale, respectively.  

    Material and Methods I

    n this study 47 samples from the Gurpi and Pabdeh formations have been studied. Samples were prepared following a standard smear slide method (Bown and Young 1998). All slides were studied under polarized light microscope at ×1000 magnification. Calcareous nannofossil nomenclature follows the taxonomic schemes of Perch-Nielsen (1985). 

    Discussion of Results and Conclusions

    Calcareous nannofossils recorded in the Mesozoic and Cenozoic strata are believed to be an appropriate means for biostratigraphic studies. As a result of this study, 17 genera and 23 species of calcareous nannofossils have been identified. Abbreviations used in this study are the NP (Nannofossil Paleogene) and CNP (Calcareous Nannofossil Paleocene). The nannofossil zonation used in the present study is based on the Nannoplankton zonation of Martini (1971) and Agnini et al. (2014), respectively. As a result of this study, 23 species belong to 17 genera of calcareous nannofossils were detected. Nine bio events were recorded and based on these bio events, the Ellipsolithus macellus Zone (NP4/CNP6–CNP7) and Fasciculithus tympaniformis Zone (NP5/CNP7–CNP8) are recognized at the uppermost part of the Gurpi Formation. Subsequently, Heliolithus kleinpellii Zone (NP6/CNP8) and Discoaster mohleri Zone (NP7)/Heliolithus riedelii Zone (NP8) (NP7/8 combined Zone–CNP9/10), are identified at the base of the Pabdeh Formation, respectively. Two biozones of the zonation of Martini (1971) are recognized at the upper part of Gurpi Formation as follows: (1) Ellipsolithus macellus Zone (NP4/CNP6–CNP7): The first nannofossil unit recorded in this study is the NP4 zone. This biozone is recorded from the FO Ellipsolithus macellus to the FO of Fasciculithus tympaniformis. The age of this zone is late Late Danian–Early Selandian. (2) Fasciculithus tympaniformis Zone (equivalent to NP5/ CNP7–CNP8): The second nannofossil unit recorded at the upper part of Gurpi Formation is the NP5 zone. This bio zone is recorded from the FO Fasciculithus tympaniformis to the FO of Heliolithus kleinpellii. The age of this zone is Selandian. Also, two biozones of the zonation of Martini (1971) are recognized at the lower part of Pabdeh Formation as follows: (1) Heliolithus kleinpellii Zone (equivalent to NP6/CNP8): The next nannofossil unit recorded in this study is the CC19 zone. This bio zone is recorded from the FO of Heliolithus kleinpellii to the FO of Discoaster mohleri. The age of this zone is Late Selandian–Early Thanetian. (2) Discoaster mohleri Zone (NP7) / Heliolithus riedelii Zone (NP8) or (NP7/8 combined Zone–CNP9/10): The last nannofossil unit recorded at the lower part of Pabdeh Formation is the NP7/8 combined zone. This biozone is recorded from the FO of Discoaster mohleri to the FO of Heliolithus riedelii. The age of this zone is Thanetian. In this study the Paleocene sediments have been investigated at the upper part of Gurpi Formation and the lower part of Pabdeh Formation at Arkavaz section in southeast of Ilam. The upper part of the Gurpi Formation is mainly consists of grey shale and the lower part of the Pabdeh Formation consists of purple shale. The detail study based on calcareous nannofossils, enables the subdivision of the studied deposits into four bio zones. As a result of this study and based on the identified calcareous nannofossil bio zones, the age of the uppermost part of the Gurpi Formation is Late Danian–Selandian and the lower part of the Pabdeh Formation is Selandian–Thanetian, and CNP8 Zone is located in the boundary between two formations. The biozones identified in this study were compared with sections from other regions of Zagros Basin.

    Keywords: Biostratigraphy, Paleocene, Gurpi Formation, Pabdeh Formation, Calcareous Nannofossils, Zagros
  • Masoumeh Kordi * Pages 37-56
    Abstract Among different diagenetic alterations, quartz cements are the foremost porosity and permeability destroying cement in deeply buried (>2 km) sandstones. Clay minerals are also known to commonly reduce reservoir quality of sandstones, however, detailed diagenetic studies, has suggested that in some diagenetic situations, the authigenic clay minerals not only do not reduce the reservoir quality, but they can help in preserving primary porosity and permeability of sandstones. Based on this research, if during the eodiagenesis, the clay minerals, occur as well-formed, thick, and continuous clay coatings on grains, they inhibit formation of quartz cements, especially overgrowths, during mesodiagenesis. This results in preserving primary porosity and permeability, leading to high reservoir quality in deeply buried sandstones. Different studies show that among different authigenic clay minerals, chlorite is the first most and illite is the second most abundant and important clay minerals in sandstones. Investigation about the conditions of formation and extension of clay coating minerals in sandstones help us in prediction and recognition of strata with high reservoir quality for hydrocarbon exploration. Keywords: Diagenesis; Sandstones reservoir quality; Grain coating clay minerals; Chlorite; Illite   Introduction Prediction of the reservoir quality based on sedimentary and diagenetic processes within sedimentary basins is a curtail component for hydrocarbon exploration and risk assessment. Diagenetic alterations can reduce, increase or preserve primary porosity of the sediment and sedimentary rocks. Formation of authigenic clay minerals during diagenesis are commonly known to reduce reservoir quality. However more detailed petrographic studies of deeply buried sandstones have shown that under some diagenetic conditions, the clay minerals could play a different role, which is the subject of his study. This condition is mainly related to the clay minerals occurring as the form of grain coatings. In this paper, the different types of grain coating clays, source and mechanism of their development and their effects on reservoir quality of sandstones has been studied.   Material and Methods In the last two decade, the role of authigenic clay minerals was subject of numerous studies and many papers have been published based on the case studies accordingly. In this paper, based on reviewing more than 100 papers and comparing two case studies, the circumstances in which clay minerals could help increase the reservoir quality of deeply buried sandstones is demonstrated. The case studies includes the Late Cretaceous Lower Tuscaloosa sandstone reservoir in the USA and the Cambrian-Ordovician Lower Sandstone Unit in Egypt. The sandstone intervals of the Lower Tuscaloosa Formation are high quality reservoir zone in subsurface of the Mississippi Interior Salt Basin, with average porosity of 25%, and average permeability of 50 md while the Lower Sandstone Unit in Egypt have very low porosity and permeability. The role of authigenic clay minerals, as the most important factor in construction of the reservoir quality, has been compared in these two case studies.   Discussion of Results & Conclusions During diagenesis, the primary porosity may be reduced, mostly by compaction and cementation. Among all the diagenetic cements, quartz cement and specially quartz overgrowths are the most important and common alterations that could reduce or totally destroy the porosity and permeability of the deeply buried sandstones. If the authigenic clays form as well-formed, thick, and continuous clay coatings on grains in shallow to medium depth burial, they inhibit formation of quartz overgrowths in deep burial. Quartz cementation can be inhibited by the presence of grain coating clays, because quartz needs clean substrates to form and quartz crystal cannot nucleate on or through the coatings. This results in development of high reservoir quality in deep burial. By these processes, the well-formed, thick and continuous chlorite coatings in the Lower Tuscaloosa Formation inhibited formation of quartz overgrowth, resulted in high porosity and permeability after deep burial, whereas the Lower Sandstone Unit, without any clay coatings on the detrital grains, have been cemented by quartz overgrowths. Type and amount of the clay mineral coatings depend on the sandstone composition and pore-water chemistry. Based on the literature, among all different authigenic clay coatings, chlorite and illite are the most common grain coating clay minerals in the sandstone reservoirs. Availability of depositional and/or early diagenetic precursor clay minerals, primarily berthierine and odinite are important factor in development and type of the chlorite coats (Odin1988; Ehrenberg 1993; Aagaard et al. 2000; Dowey et al. 2017). To a lesser extent, magnesium and iron rich smectite tend to be chloritized during diagenesis (Chang et al. 1986; Anjos et al. 2003). Authigenic chlorite coats are usually iron-rich, but magnesium-rich chlorite has also been documented in sandstones (Pittman et al. 1992; Ehrenberg 1993; Ajdukiewicz et al. 2010). Chlorite coats usually are composed of relatively small chlorite crystals that form perpendicular to the detrital grain surface. Chlorite coatings around framework grains can prevent quartz cement growth by blocking silica from nucleating on the grain surface. This results in preserving primary porosity and permeability of sandstones during late diagenesis. Authigenic illite can form by various processes including transformation of a clay precursor (smectite, montmorillonite and/or mixed-layer illite-montmorillonite), direct precipitation from pore-waters and replacement of other minerals (e.g. feldspar). However availability of precursor infiltrated clays, especially smectite, is the main factor in development of illite coats in shallower depth burial (Worden and Morad 2003). The authigenic illite usually occurs as irregular flake with lath-like projections. These projections may be relatively short or they may develop into curled fibrous projections up to 10 µm long, as fibrous and filamentous crystal habit, which extend into intergranular pore spaces. Unlike the chlorite and flake crystal habit of illite coatings, which lead to development of both high porosity and high permeability in deep burial, the fibrous and filamentous crystal habit of illite would reduce the pore and throat space, thus significantly decreasing reservoir quality, especially permeability (Wilson and Pittman 1977; Pittman et al. 1992; Wilson et al. 2014). In conclusion, chlorite coatings, which are the most common and effective clay coats in sandstones, can prevent quartz cement growth by preventing silica from nucleating on the grain surface, resulting in preservation of primary porosity and permeability. Illite coatings could also prevent formation of quartz overgrowths. However, due to the fibrous and filamentous crystal habit of illite, illite could cause permeability deterioration in sandstone reservoirs.
    Keywords: Diagenesis, Sandstones reservoir quality, Grain coating clay minerals, Chlorite, Illite
  • Afshin Zohdi *, Hossein Kouhestani, Mir Ali Asghar Mokhtari, Farzaneh Nouri Pages 57-78

    The Ruteh Formation in the Qharkhotlou region (southwest of Zanjan) mainly consists of 52 m sandy limestones and cream to grey thin to medium-bedded limestones. In this area, the Routh Formation overlies the quartz-bearing sandstones of the Doroud Formation and is overlain by laterite-bauxite horizon. Based on fieldwork and microscopic studies, six carbonate microfacies is identified in the Routh Formation in the Qharkhotlou area. The interpretation of microfacies and the lack of coral great barrier reefs, absence of turbidite deposits as well as the presence of skeletal allochmes such as green algae, bivalve, brachiopoda, benthic foraminifera and echinoderm debris indicate that these microfacies possibly were deposited inside the shallow parts of a carbonate ramp . In order to study the geochemical characteristics of the Ruteh Formation and also to determinate the original carbonate mineralogy, the main (Mg and Ca) and trace (Sr, Na, Mn and Fe) elements were analyzed on 15 samples of these limestones (mainly micrite). Based on the two-variable diagrams such as Sr vs. Na, Mn vs. Sr/Na, Na vs. Mn and also Sr/Mn vs. Mn, it can be stated that the original carbonate mineralogy of these studied limestones in the Ruteh Formation are calcite-aragonite mixture which is consistent with the formation enviroment of these studied carbonates at the Palaeo-Tethys southern margin during the Permian.  

    Introduction

    The Ruteh Formation is one of the most fossiliferous carbonate units in the Alborz Mountains. Lithostratigraphically, the Ruteh Formation in Alborz Mountains was correlated with the Jamal Formation in Central Iran Basin. This formation, defined in Alborz Mountains by Assereto (1963), displays a carbonate sequence relatively homogeneous of grey to dark limestones with intercalated marls. The type section of the Ruteh Formation is located in central Alborz near the village of Ruteh (North of the Tehran), where it has a thickness of 230 meters and consists of dark grey, medium-bedded to massive fossiliferous limestones (Assereto 1963). Lasemi (2001) characterized the sedimentary palaeoenvironments of the Ruteh Formation as equivalents of modern carbonate environments of the southern Persian Gulf with open sea, shoal, lagoon and tidal flat, respectively. The erosional lower boundary of the Ruteh Formation rests everywhere unconformably on the older lithological units (mostly Doroud Formation) and the upper boundary of the Ruteh Formation is regionally marked by a bauxite-laterite deposits (Aghanabati 2010), in the most areas of Central Iran. In this research for the first time depositional conditions and elemental geochemistry of the Ruteh Formation in the Zanjan province (Qharkhotlou section) have been evaluated.  

    Material & Methods

    In this research to recognize the sedimentary environment and original carbonate mineralogy of the Ruteh Formation, we used one unique outcrop at the Qharkhotlou region located in the southwest of Zanjan. The section measured a total thickness of 52 m and consists of sandy limestone and cream to grey thin to medium-bedded limestones. In this area, the Routh Formation overlies the quartz-bearing sandstones of the Doroud Formation and is overlain by laterite-bauxite horizon. During the fieldwork studies, 35 rock samples from carbonate deposits have been taken for petrographic studies and geochemical analysis. In order to differentiate ferroan and non-ferroan calcite from ferroan and non-ferroan dolomite in thin sections, the staining method of Dickson (1965) was applied. Carbonate rocks were classified according to the schemes of Dunhum (1962). Flügel (2010) facies belts and sedimentary models were also used in this research. The composition of associated fauna and non-skeletal grains was considered. Sedimentologic texture and structure have been described in a semi-quantitative manner. Elemental geochemistry analyses (major and trace elements) were performed on 15 samples of these carbonates through the succession. The concentration of Ca, Mg, Fe, Mn and Sr of samples was measured at the Zarmazma Mineral Studies Company, Tehran.  

    Discussion of Results & Conclusions

    Based on the field and petrographic studies, the microfacies and depositional environment of the Ruteh Formation were recognized in the studied section. This formation has been made of six microfacies which deposited in a shallow open marine environment. These facies mainly consists of different kinds of benthic foraminifers with microgranular and porcelaneous shells (such as: miliolid), algae, echinoids, brachiopods and bivalve debris, along with some non-skeletal components (e.g., aggregates and intraclasts). These recognized microfacies from shallowest to deepest environments included as follow: (1) aggregate bioclast sandy wackestone, (2) peloid small benthic foraminifera wackestone, (3) bivalve green algae wackestone to packstone, (4) intraclast bioclast packstone to grainstone, (5) green algae brachiopoda packstone and finally (6) echinoderm brachiopoda wackestone. Gradual microfacies change, abundant micrites, the absence of calciturbidites and lack of extensive barrier reefs with considerable thickness, confirms a carbonate ramp for the studied carbonates succession. The microfacies mostly deposited in a distal inner ramp. The five microfacies (MF1–MF5), belong to distal inner ramp and just one is located in the proximal middle ramp (MF6: echinoderm brachiopoda wackestone). Whether carbonate ramps were distally steepened or homoclinal cannot be confirmed by the current study, since we are focusing on the shallowest environments. In the studied area, the boundary between Routeh and Shemshak formations is identified by thick laterite-bauxite layers with a thickness of about 20 m which clearly show an erosional surface forming during a warm and humid climatic condition. The geochemical results show that the samples are completely composed of limestones. Geochemical analysis of the limestones such as Ca, Mg, Sr (147–-582 ppm), Na (262–-974 ppm), Mn (101–-577 ppm) and Fe (400–-14100 ppm), and their bivariate plots (such as Sr, Sr/Na and Sr/Ca) indicate that the original carbonate mineralogy is calcite-aragonite mixture which is consistent with the formation of these studied carbonate deposits at the Palaeo-Tethys southern margin during the Permian. Geochemical studies also confirm that Ruteh carbonates were deposited in a shallow warm-water environment in the study area.

    Keywords: Stratigraphy, Microfacies, Sedimentary environment, Geochemistry, Ruteh Formation, Permian, Qarkhotlou, Zanjan
  • Mohammadnabi Gorgij *, Somayyeh Shahraki Mirzaei Pages 79-98

    Miscellaneidae are Paleogene larger index microfossils (Middle Paleocene–Early Eocene) that are important for biozonation of shallow marine deposits. These large benthic foraminifera have distinct architecture and ornamentation schemes which differentiated them form Nummulitidae and Rotalidae families. Miscellaneidae have different taxa that essentially are usefulness for biostratigraphic and palaeobiogeographic investigations. For biostratigraphic studies, age and systematic determinations of miscellaneidae, Middle Paleocene–lower Eocene deposits of two outcrop sections are investigated in the Padagi village in the north of Zahedan. These stratigraphic sections mainly consist of alternation of marls, marly limestones and limestones with 237.5 m thickness for PE section and 222.5 m thickness for PEP section. Study of different Misellaneidae genera led to identifications of Miscellanea miscella, Miscellanea juliettae, Miscellanites primitives and Miscellanites minutus that shows standard biozone SBZ3–-SBZ6 and therefore suggested Late Paleocene–-Early Eocene age. Moreover, global distribution patterns of older species of Miscellaneidae represents that these species belong to the eastern Tethys basin.

    Keywords: Late Paaleocene–Early Eocene, Large Benthic Foraminifera, Sistan suture zone