کاربرد روش های زمین شناسی در پی جویی نهشته های کرومیت انبانی در پهنه افیولیتی خوی، شمال باختر ایران

اکتشاف کانسارهای کرومیت انبانی به‌دلیل نبود هاله‌های دگرسانی و ژئوشیمیایی اولیه، ابعاد کوچک کانسارها، رخداد غیرقابل پیش‌بینی آنها و وجود جابه‌جایی‌های زمین‌ساختی شدید، همواره چالش‌برانگیز بوده است. در این پژوهش، از روش‌های زمین‌شناسی در پی‌جویی توده‌های کرومیتی در افیولیت خوی استفاده‌شد. این بررسی‌ها به شناسایی یک میدان کروم‌دار با تعداد 18 توده معدنی منجر‌شد. معیارهای زمین‌شناسی در پی‌جویی این کرومیت‌ها عبارتند از: تفکیک دقیق هارزبورژیت و دونیت (برداشت زمین‌شناسی)، رخنمون هاله دونیتی، ارتباط مکانی کرومیت‌ها با نفوذی‌های گابرویی، پراکندگی توده‌های کرومیت در یک راستای شرقی غربی، وجود خرده‌سنگ‌های حاوی کرومیت در آبراهه‌ها، وجود ریخت برآمدگی رخنمون توده‌های کرومیتی نسبت به سنگ میزبان و پوشش گیاهی فقیر مناطق کرومیت‌دار علی‌رغم بالا‌بودن میزان بارندگی. کانسارهای کرومیت شناخته‌شده، ابعاد کوچکی دارند و تناژ آنها در بازه 385 تا 56210 تن تغییر می‌کند.

Application of geological methods for prospecting of podiform chromite deposits in the Khoy ophiolite zone, Northwestern Iran

Introduction;Podiform chromite deposits are small magmatic chromite bodies formed in the lower section of an ophiolite complex. Podiform chromite mines have produced 57.4 percent of the world’s total chromite production. Most ore bodies are irregularly dispersed and relatively small, between 0.0004 and 1 Mt, averaging 0.011 Mt (United States Geological Survey, 2012), and reserves greater than 1Mt are most uncommon (Evans, 1998).;The Khoy ophiolite covers an extensive area in the northwest of Iran along the IranTurkey border. This ophiolite zone comprises one of the most promising areas for prospecting of chromite deposits as a result of extensive outcrops of ultramafic rocks. The Kochuk area is located in the western domain of the Khoy ophiolite (Fig. 1). Some geological and geochemical investigations have been carried out for recognition of chromite deposits in this area by the authors during the last 15 years. The geological criteria of prospecting for chromite deposits from the Khoy ophiolite are discussed in this study.;  ;Materials and methods;In this research, geological methods were used to identify chromite deposits in the Khoy ophiolite. Geological surveys at scale of 1:20000 were implemented in an area of about 70 km2. Lithogeochemical sampling (eighty five samples), petrography (five samples), ore microscopy (eighteen samples) and sampling for determination of specific gravity of ore (eleven samples) were performed in these study.;Testing of bedrock mineralization was performed in a relatively straightforward manner by sampling of outcrops in areas where chromite orebodies cropped out or were underlain below a thin soil cover. In contrast, chromite mineralization prospecting in locations with thick cover were carried out by pitting and trenching. Chemical analysis of the samples was carried out by the XRF method in the Kansaran Binaloud laboratory.;  ;Results;In this study, geological mapping allows discrimination of ophiolite lithologies (ultramafic rocks, chromitite, basaltic pillow lava, gabbrodiorite, ophiolite mélange, listwaenite and other associated rocks). Ultramafic rocks are important in prospecting for chromite mineralization.;Geological prospecting led to the identification of a chromite ore field with a remarkable potential since more than 20 chromite orebodies were recognized. Five mineralized zones called A, B, C, D, E and 13 chromite indices were recognized in the Kochuk chromite field.;The A zone is located almost in the central part of the study area. Four ore bodies have been recognized in this zone. The A1 orebody extended by 68 meters in length and 69 m in thickness. This subzone is characterized by a lenticular shape with an eastwesttrending strike (N115S) and a 60o dipping toward THE south. The B zone is located at ~2.9 km west of the A zone. The B1 orebody consists of the largest known chromite orebody in this area which comprises N120Strending and 50 NW dipping lenticular geometry extended by 118 meters in length. The thickness of the B1 orebody varies between 6 and 12 meters averaging around seven meters. There are three chromite orebodies recognized as C1, C2, and C3 subzones in the C zone. The C1 orebody is composed of 74 meterlong lens with variable thickness between 3.5 and 9 meters. It has a N40Etrending strike, which dips 4550 degrees to the west. In the D zone, three small chromite orebodies have been identified. The D1 orebody consists of a 20 meterlong lens ranging from 1 to 5 (avg. 3.5) meters in thickness. This orebody is oriented by a N160S in strike and a 50 NE in dip.;The chromitite occurrences have lenticular, tubular and veinlike shapes host by hurzburgite. Rocks of the upper mantlelower crust transition zone and probably the associated chromite deposits have not been recognized yet in this area (Imamalipour, 2011). The typical ore textures consist of disseminated, nodular, massive, banded and cataclastic.;  ;Discussion;Exploration of podiform chromite deposits has been a challenge due to their unpredictable occurrence, the small size of most orebodies and the intensive tectonic dislocations (United States Geological Survey, 2012). Moreover, the absence of primary geochemical halos and associated alteration are matters that have led to difficulties in prospecting for podiform chromites. Chromite as an accessory mineral is associated with harzburgite host rocks. This mineral is released during the weathering process and is accumulated within the stream sediment heavy minerals. Therefore, application of the stream sediment geochemistry method may not necessarily result in useful information for determining the location of chromite outcrops. In this study, geological methods were used for podiform chromites prospecting which culminated valuable results. The Kochuk chromitebearing area was recognized as a chromite ore field in the western city of Khoy.;The most important geological criteria of prospecting for chromite deposits from the Khoy ophiolite are: 1) chromite bodies are surrounded by dunite envelopes with variable thickness; 2) the recognized chromiterich zones are mainly located near gabbroic intrusions; 3) most chromite lenses are oriented along an east–west trend; 4) the existence of chromite fragments on stream beds can be considered to be a suitable sign   to define the entry of these anomalous rocks to the stream sediment; 5) morphologically, chromite outcrops often occur protruding from the host rock because of their higher resistance to erosion. This can facilitate the recognition of their outcrops; 6) chromite bearing zones usually do not have or exhibit thin vegetation cover despite the high rate of annual rainfall; 7) outcrops of disseminated ores can indicate the presence of high grade chromite ore in the sub surface parts; 8) the main oxide contents of chromite ores vary from an individual ore body to another one.;  ;References;Evans, A.M., 1998. An introduction to economic geology and its environmental impacts. Blackwell Science, United Kingdom, 376 pp.;Imamalipour, A. 2011. Geochemistry and geological setting of chromitites of Aland area from the Khoy ophiolite complex, NW Iran. Geosciences, 20(80): 47–56. (in Persian with English abstract);United States Geological Survey, 2012. Podiform chromite deposits, database and grade and tonnage models. United States Geological Survey, Virginia, Scientific Investigations Report 2012–5157, 45 pp.