نقش ph، ماده آلی و شدت هوازدگی بر روی ویژگی‌ های ژئوشیمیایی و کانی‌ شناختی لاتریت های نیکل دار در منطقه بوانات، استان فارس

لاتریت‌های نیکل ‌دار منطقه چشمه رستمی شهرستان بوانات در امتداد حدفاصل رشته کوه ‌های رسوبی زاگرس و کمربند دگرگونی‌ماگمایی سنندجسیرجان بر روی زمین رخنمون‌دارند. نهشته‌های لاتریتی نیکل‌دار برجا در منطقه مورد بررسی به‌طور مستقیم بر روی سنگ مادر هارزبورژیتی خود قرار می‌گیرند و از پایین به بالا شامل افق‌های پروتولیت، ساپرولیت، انتقالی و اکسیدی می ‌شوند. کانی‌شناسی این نهشته‌ها از کوارتز، الیوین، لیزاردیت در افق پروتولیت به‌طور پیوسته به‌سمت بخش‌های بالایی به مجموعه تالک، کلینوکلر، هماتیت، گوتیت در زون اکسیدی تغییر می‌کند. هم‌زمان مقادیر عناصر fe، cr، co و mn از افق ‌های پایین به سمت افق‌ های بالای لاتریت افزایش می‌ یابند. داده ‌های ph، ماده آلی و غلظت عناصر در افق ‌های مختلف نهشته ‌های لاتریتی نشان‌دهنده افزایش تدریجی مقدار نیکل در امتداد عمودی زون لاتریت به‌سمت افق اکسیدی است. این افزایش در غلظت نیکل از طریق جذب سطحی و جانشینی در اکسی‌هیدروکسید های آهن صورت می‌گیرد. با توجه به شدت هوازدگی ضعیف تا متوسط که موجب عدم خروج میزان بالایی از ni از ساختار کانی الیوین سنگ مادر شده است، نهشته ‌های لاتریتی منطقه نتوانسته‌اند به عیار بالایی از این فلز برسند.

The role of pH, organic matter and weathering intensity on geochemical and mineralogical characteristics of Ni-bearing laterites in the Bavanat region, Fars province

Introduction The Nilaterites were mostly derived from ultramafic rocks in ophiolite complexes during weathering in tropical climate. Lateritization processes result in leaching of some major elements (Si and Mg) from the source rocks and concentration of some others (Ni, Fe, Cr and Co) in the residual soils. The Nibearing laterites are divided into three major subgroups including oxides, silicates and clay types (Berger et al., 2011). Hematite and goethite are the main constituents of the oxides type, whereas garnierite is the main carrier for Ni in silicates type. In the clay laterite, saponite and smectite are the main Nicarriers. The Bavanat region contains Nibearing laterites as discontinuous outcrops which have formed on ophiolite ultramafic rocks in the northeast Fars province. These ultramafics are remnants of NeoTethys oceanic lithosphere which have been emplaced on continental margin along the Zagros Suture Zone in the Late Cretaceous era (Rajabzadeh, 1998). These laterites have recently attracted some geologists to work on them (e.g. Khademi and Hasheminassab, 2010; Rasti and Rajabzadeh, 2017). The aim of this study is to determine the effects of pH, organic matter (OM) and weathering intensity on the geochemical and mineralogical characteristics of Nilaterites in the Chesmeh Rostami area, Bavanat region.   Materials and methods Sampling was carried out along three geological cross sections on undisturbed laterite profiles. The samples were studied using refracted and reflected light microscopic methods. Nine of the representative samples were analyzed using XRD and ICPMS methods at the Iran Minerals Processing Research Center. PH and OM values of the samples from different soil horizons were determined using routine analytical methods.   Results An undisturbed laterite profile consists of four major horizons from base to top including protolith, saprolite, transitional and oxide zones. pH values vary in a narrow range through the soil profiles. The minimum (6.82) and maximum (7.99) values were determined just in the weathering front at the top of the protolith and at the top of oxide horizon, respectively. In the same way, OM of the soils increases from protolith (0.140 wt.%) to the oxide zone (1.475 wt.%). Protolith is generally a decomposed harzburgite that appears in dark green color. It mainly consists of lizardite with relicts of olivine and orthopyroxene and minor amounts of quartz, clinochlore and hematite. Protolith traditionally transforms to saprolite. The latter is easily discriminated by its softness in field and its light green to gray color. Lizardite and quartz are the major minerals which are accompanied with amorphous iron oxyhydroxides and silica. Transitional zone is located as a narrow zone between saprolite below and oxide horizon above. No relicts of the source rock are preserved here. It appears as a soil of yellow to orange in color. XRD data from this horizon indicated that calcite and hematite are present as major phases along with minor lizardite and quartz. Oxide horizon is a very soft and porous dark to light red soil that has 12m thickness. This horizon mainly consists of hematite, goethite and clinochlore with variable amounts of amorphous silica and iron oxides. Geochemically, Fe2O3 (8.3125.75 wt.%), MnO (0.110.27 wt.%), Ni (18986793 ppm), Co (40152 ppm) and Cr (1313985 ppm) concentrations increase continuously from base to top of the laterites. There is good correlation between Ni and Fe2O3. On the contrary, silica (41.17 to 37.17 wt.%) and MgO (18.45 to 10.97 wt.%) contents decrease from base towards the top of laterites. Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW) and Rate Weathering (RW) were used in determination of the weathering intensity during lateralization processes. The calculated data indicated that weathering intensity is medium to weak in the Bavanat region.   Discussion Nibearing laterites in the Bavanat region were formed during weathering of the ophiolite ultramafic rocks at semitropical conditions. Four major horizons were formed through vertical profiles of the laterites. However, low concentration of Ni in the source rock by one side and medium to weak intensity of weathering by other side result in production of lowgrade Nilaterites. This is confirmed by pH values and remnants of chromite grains in the protolith horizon. However, weathering causes decomposition of the source rocks resulting in weak liberation of elements. Some elements such as Si and Mg have leached away, but high values of OM and pH at the top of the soils helped Fe fixation (Kabata Pendias and Pendias, 1999; Thorne et al., 2009). Good correlation between Fe and Ni indicates that iron oxides and hydroxides play the role of scavenger for Ni. Mobility of Ni decreases in the presence of OM and high pH. It thus adsorbs on the Fe compounds. The Nilaterites in the Bavanat are classified in oxide type clan.   Acknowledgments The authors would like to thank the Research Council of Shiraz University for financial support of this work.          References Berger, V.I., Singer, D.A., Bliss, J.D. and Moring, B.C., 2011. Ni Co Laterite Deposits of the Worlddatabaseand Grade and Tonnage Models. U.S. Geological Survey, Reston, Virginia, 30 pp. KabataPendias, A. and Pendias, H., 1999. Biogeochemistry of trace elements. Polish Scientific Publishing Company (PWN), Warsaw, Poland, 400 pp.  Khademi, A. and Hasheminassab, M., 2010. Study on mining potential of Nilaterites from Ghader Abad, Fars province. 29th symposium of Earth Sciences, Tarbiat Moallem University Tehran, Tehran, Iran. (in Persian with English abstract) Rajabzadeh, M.A., 1998. Mineralisation en chromite et elements du groupe du platine dans les ophiolites d’Assemion et de Neyriz centrure du Zagros, Iran. Ph.D. Thesis, Institue National Polytechnique de Lorraine, Nancy, France, 358 pp. Rasti, S. and Rajabzadeh, M.A., 2017. Mineralogical and Geochemical Characteristics of SerpentiniteDerived NiBearing Laterites from Fars Province, Iran: Implications for the Lateritization Process and Classification of NiLaterites. International Journal of Environmental, Chemical, Ecological, Geological and Geophysical Engineering, 11(7): 541–546. Thorne, R., Herrington, R. and Roberts, S., 2009. Composition and origin of the Çaldağ oxide nickel laterite, W. Turkey. Mineralium Deposita, 44(5): 565–581.