کانی‌شناسی، زمین‌شیمی و گوهرشناسی فلوریت در سنگ‌‏‌های آهکی کانسار پیناوند (پهنه‌ ایران مرکزی)

کانه‌زایی در کانسار فلوریت-باریت پیناوند (الیگومیوسن) تحت‌تاثیر دگرسانی گرمابی در توالی‌‏‌های آهکی و دولومیتی تریاس و کرتاسه‌ زیرین به‌صورت رگه، رگچه و عدسی‌شکل رخ داده است. کانی‌‏‌های اصلی شامل کوارتز، کلسیت، دولومیت، فلوریت و باریت و کانه‌‏‌های پیریت، کالکوپیریت، گالن، کوولیت، کالکوسیت، مالاکیت و آزوریت هستند که در زمینه‌ سنگ آهک پراکنده‌اند. بر پایه‌ روابط پاراژنتیک، کوارتز و باریت به‌ترتیب در مرحله‌ نخست و دوم پدید آمده‌اند و با کاهش فوگاسیته اکسیژن سیالات، فلوریت پدید آمده است. بررسی‌های گوهرشناسی نشان می‌دهد بلور‏‌های فلوریت پیناوند، بلور‏‌های شفاف با رنگ آبی مایل به سبز، با جلای شیشه‌ای، بدون شکست مضاعف، سختی 4، وزن مخصوص 18/3 و ضریب شکست 43/1 هستند. بر پایه‌ بررسی‌‏‌های زمین‌شیمیایی میانگین غلظت عنصر‏‌های اصلیِ ca و f به‌ترتیب نزدیک به 61 درصدوزنی و 27 درصدوزنی است. با توجه به غنی‌بودن فلوریت‌‏‌های پیناوند از عنصر‏‌های خاکی کمیاب سبک، فلوریت‌‏‌ها در مراحل نخستینِ تبلور پدید آمده‌اند. ناهنجاری مثبت کوچک در یوروپیم در فلوریت‌‏‌های یادشده جانشینی eu2+ به‌جای ca2+ در فلوریت و دمای پیدایش کانسار کمتر از 250 درجه‌ سانتی‌گراد را نشان می‌دهد که گویای پیدایش کانسار در ژرفای کم است. بر پایه‌ نمودار‏‌های تفکیک محیط ته‌نشست فلوریت‌ها، کانسار پیناوند از کانسار‏‌های گرمابی دمای کم تا متوسط به‌شمار می‌رود و نشان از شباهت آن به کانسار‏‌های mvt دارد. از سوی دیگر، دگرسانی گرمابی در این کانسار شامل سیلیسی‌شدن به‌صورت گسترده و دولومیتی‌شدن به‌طور محدود است و به‌دلیل گسترش دگرسانی سیلیسی، شباهت به دگرسانی کانسار‏‌های اپی‌ترمالی نشان می‌دهد.

mineralogy, geochemistry and gemologyof fluorite in carbonate rocks of the pinavand deposit (central iran zone)

fluorite deposits in iran are widely located in dolomites and dolomitic limestones (triassic-cretaceous age) (darvishzadeh, 1991) in alborz and the central iran zone (rajabi et al., 2013). qishlaqi (2002) investigated the geochemistry and genesis of the pinavand fluorite mines. shafahizadeh (2011) investigated the mineralogy and fluids involved in fluorite and barite mineralization in the pinavand region. heidari et al. (2021) investigated the paragenetic relationships of minerals in the alteration zone of the pinavand deposit. in the present paper, the semi-precious gem fluorine in the limestones of the pinavand deposit is investigated from the point of view of geochemistry and gemology. also, the connection of the host rock with the minerals of this area and the reaction of the ore-forming fluid with them will be determined.materials and methodsfollowing field studies, several thin sections (about 25) and polished sections were prepared for petrographic studies. the fluorine mineral of the studied deposit was subjected to sem analysis (at the isfahan university of technology) and gemological investigations (at isfahan university). also, the results of geochemical analyses of fluorites (icp-ms) (shafahizadeh, 2011) were applied.results and discussion mineralization of fluorite and barite occurred in scattered, massive, veined, breccia and lens-shaped in the lower cretaceous limestone and dolomitic host rocks. major alterations observed in the region including silicification and to a lesser extent dolomitization. the main minerals are quartz, calcite, dolomite, fluorite, and barite scattered in the limestone. the fluorite decomposed into carbonate along fractures, which indicates the continuous influx of hydrothermal solutions. a final silicification process has occurred following fluorine formation. it seems that the mississippi valley deposit type is one of the proposed models for the formation of pinavand mineral region. dolomitization and silicification processes are the characteristics of the mississippi valley-type deposits (pirajno, 2009). however, in the mississippi valley-type deposits, unlike epithermal mineralization, dolomitization process occurs with a weak silicification. the mineralogy in pinavand deposit includes pyrite, chalcopyrite, chalcocite, galena, goethite, fluorite, barite, quartz, calcite and dolomite. no proximity with evaporite rocks is observed. a low extent of dolomitization with a larger scale of silicification can be an indication of epithermal type alterations. as optical and gemological characteristics display the fluorine crystals with glassy and transparent feature and specific weight of 3.18 vary from colorless to green to pale blue in color. they are without any birefringence and their refractive index is 1.43. the presence of rare elements such as sc, sr and zr can affect the color of this mineral. yellow and transparent fluorites have a higher yb/la ratio than blue and purple fluorites (palmer and williams-jones, 1996). it seems that various of color in fluorite crystals are related to the amount of y and ∑ree (e.g., dill et al., 2011). the concentration of ree in the pinavand fluorites is generally low, and lrees are more enriched than heavy earth elements (hree). the low ree in fluorites can be attributed to the high fluid-to-rock ratio (sánchez et al., 2010). the low level of rare earth elements can be attributed to the high ph of the fluids and the reaction of the fluids with the carbonate host rock as well as the high ratio of fluid/rock. on the other hand, the low values of rare earth elements can indicate the mixing of magmatic fluids and atmospheric waters (valenza et al., 2000). according to moller et al. (1986) the fluorites formed in the early or middle stages of crystallization are enriched with lree, the amount of la element is high and the amount of tb is low, but the fluorites related to the final stage of crystallization are enriched with hree. so, the pinavand fluorites created in the early stages of crystallization. the examined fluorites have a small positive anomaly in europium, indicating the substitution of eu2+ for ca2+ and the deposit formation temperature is less than 250°c (schwin and markl, 2005). the negative anomaly of cerium points to its removal from the environment as a consequence of fluid reaction with calcareous wall rock giving rise to increment to oxygen fugacity. fluorites are divided into three sedimentary, hydrothermal and pegmatitic environments (constantopoulos, 1988). the studied fluorites are classified as sedimentary type with some features of primary crystallization and fluid’s reaction with calcareous host rocks.conclusionspinavand deposit is a fluorite-barite deposit consisting both of the non-metallic and metallic minerals. the latter minerals were formed under the influence of hydrothermal alteration. the negative anomaly of cerium in the fluorites suggests that it has been removed from the environment as a result of the fluid’s reaction with the limestone wall rock, which in turn increased the oxygen fugacity. this process highlights the significant role of fluid-rock interactions in the removal and redistribution of certain elements within the depositional environment. mineralization of the fluorite and barite formed as veins, veinlets, lenticular and breccia shapes have occurred mainly in the lower cretaceous limestone units. the main minerals of this deposit include quartz, calcite, saddle dolomite, fluorite and barite, scattered in the limestone background. the major changes in the region include silicification and, to a lesser extent, dolomitization processes. fluorite can observe as coarse-grained or fine-grained in size, and its gemological characteristics are as follows: transparent to semi-transparent crystals, pale blue colors, green and violet, hardness 4, specific gravity 3.18, vitreous luster, without birefringence and having a refractive index of 1.43. based on the geochemical studies, the concentration of lree in the pinavand deposit indicates the formation of fluorites in the early to middle stages of crystallization. the mineralogy in the pinavand deposit is simple like in the mississippi valley deposit. based on the type of minerals as well as alteration type, the pinavand mineralization shows some characteristics of mvt deposits, and due to the huge of silicification process, it also shows similarity to the alteration of epithermal deposits.