سنگ نگاری، زمین‌ شیمی و منشأ احتمالی افقهای سنگ آهن ‌اوولیتی کم عیار در سازند لشکرک، البرز شرقی

نوع مقاله : علمی- پژوهشی

نویسندگان

گلستان

چکیده

افقهای آرژیلیتی دارنده اووئیدهای آهن، از سازند لشکرک البرز شرقی (اردویسین زیرین- میانی) و بر اساس توالی کاملاً آواری کم‌ژرفا و بدون حضور سنگهای آتشفشانی واقع شده‌اند. هماتیت و شاموزیت کانیهای آهن دار اصلی هستند و گوتیت و گلاکونیت در مقدار جزئی شناسایی شدند. افقهای مورد بررسی، با میانگین 21 درصد وزنی آهن در گروه سنگ آهنهای اوولیتی کم عیار رده بندی شده‌اند. آهن مورد نیاز برای تشکیل اووئید های آهن در مناطق مورد بررسی، از آب دریای معمولی در دسترس قرار گرفته است و این آهن می تواند به‌صورت ذرات آواری همراه با رسها، از هوازدگی و فرسایش سنگهای شیلی و از خشکیهای مجاور به دریا حمل شود. بالا بودن میزان پتاسیم و سیلیکا در افقهای اوولیت آهن، حضور زیرکن، کوارتز و کانیهای رسی در داخل اووئیدها و همچنین در زمینه این سنگها ضمن تأیید این الگو، نقش رسهای آهن‌دار را در پیدایش اووئیدهای اولیه شاموزیتی در محیطی با pH=5-9 و Eh اکسیدان متوسط- ضعیف با اهمیت نشان داده است. فراوان بودن اووئیدهای هماتیتی نسبت به گوتیتی، اووئیدهای متراکم و بیضی شکل و رخ‌داد فراوان رگه‌چه های کلسیتی قطع‌کننده اووئیدها به فرآیند دیاژنز و تبدیل شاموزیت و گوتیت به هماتیت نسبت داده شده است.

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