تعیین نوع ذخایر مس با سنگ میزبان آتشفشانی در ناحیه معدنی کوه‌جارو (جنوب اشتهارد) بر اساس شواهد زمین شناسی، کانه زایی و میان‌ بارهای سیال

ناحیه معدنی کوه­جارو (ذخیره احتمالی 2 میلیون تن و عیار متوسط 3 درصد مس) با راستای شرقی غربی در دنباله آتشفشانی مردآباد بویین زهرا با سن ائوسن بالایی الیگوسن، قرار دارد. سنگ های آتشفشانی ائوسن بالایی با جنس ریوداسیت، تراکی آندزیت، آندزیت و توف تراکیتی، مهم‌ترین میزبان‌های ماده معدنی هستند. کانه‌زایی در سه مرحله شامل: 1) پیش از کانه زایی با حضور پیریت های دانه ریز افشان، 2) کانه زایی گرمابی اصلی شامل گامه های کانه زایی رگه‌ای کوارتز کالکوپیریت± بورنیت (گامه i)، کانه‌زایی برشی بورنیت کالکوسیت (گامه ii) و کانه زایی افشان گالن و اسفالریت (گامه iii) و 3) رگه های باریت و کلسیت تاخیری رخ‌داده است. طبق شواهد پتروگرافی، میان‌بارهای سیال در کانسارهای مس کوه جارو شامل دوفازی غنی از مایع (lv)، دوفازی غنی از گاز (vl)، دارای فاز جامد هالیت (lvs) و تک فازی‌های مایع (l) و گاز (v)، هستند. تغییرات دمای همگن‌شدن در این میان‌بارها بین 200 تا 350 درجه سانتی گراد و محتوای شوری بین 2 تا 37 درصد وزنی معادل نمک طعام، به‌ دست آمده است. میان‌بارهای سیال والد با چگالی بیش از 1 گرم بر سانتی‌متر مکعب و عمق تقریبی 400 متر تشکیل شدند که پس از آن، میان‌بارهایی با چگالی کمتر از 1 گرم بر سانتی‌متر مکعب و عمق کمتر از 300 متر در اثر کاهش فشار ناشی از گسلش و ورود آب های جوی کم دما، باعث وقوع فرایند اختلاط سیال و رخداد کانسنگ مس در سنگ های آتشفشانی منطقه شده اند. در مجموع کانسارهای مس کوه جارو قابل مقایسه با ذخایر مس نوع مانتو در شیلی یا مس طبقات سرخ آتشفشانی در شمال امریکا هستند.

Genesis of Eocene volcanic-hosted copper deposits in the Kuh-e-Jarou Mining District (South Eshtehard): Constraints from geology, mineralization and fluid inclusions

IntroductionThe SavehKashanQom copper belt, in the northern part of the UrumiehDokhtar Magmatic Arc (UDMA) consists of two of the oldest (gold and copper) zones in Iran (Samani, 1998; Rajabpour et al., 2017) where Upper EoceneOligocene Mard AbadBouin Zahra volcanic suite is situated. This volcanic suite hosts several copper deposits including Jarou, Gomosh Dasht, Ghezel Cheshme, Bidestan and Afshar Abad that are known as the &KuheJarou Mining District&. The KuheJarou Mining District has a total potential ore reserve of 2 Mt Cu with an average grade of 3 wt.% (ZarAzin Gostar Consultant Engineering Co., 2009). Upper Eocene volcanic and pyroclastic rocks of rhyodacite, trachyandesite, andesite, and trachytic tuff with highK calcalkaline to shoshonitic affinity consist of the main host rocks for Cu mineralization. These units are primarily intruded by post Eocene intrusive bodies.  The geochemistry and genesis of ore bodies have not been fully understood since most previous studies in this area have been focused on petrology of volcanic and intrusive rocks. Moreover, the main purpose of this study is to investigate mineralization style, geometry, and texturalstructural features of orebodies, alterations, and fluid inclusions with implication for genesis of Jarou, Gomosh Dasht, Ghezel Cheshme, Bidestan and Afshar Abad copper deposits. In addition, this research provides more insight into understanding geology and mineralization conditions in the study area with an implication for future exploration.Materials and methodsSeventeen thin polished sections from the ores and the host rocks were prepared and they were studied by a transmitted/reflected polarizing microscope in the Iran Mineral Processing Research Center (Karaj, Iran). Five rock powdered samples were also analyzed using Xray diffraction (XRD) spectrometry (X′ pert Philips) in order to identify the mineralogy of clay minerals in the mineralogy laboratory of Salamanca University (Spain). Fluid inclusion microthermometry was performed using a Linkam THMS600 heatingfreezing stage (190 to +600 °C) mounted on a ZEISS Axioplan2 microscope in the fluid inclusion laboratory of the Iranian Mineral Processing Research Center (Karaj, Iran). Salinities (wt.% NaCl eq.), density (g/cm3) and pressure (bars) were calculated using the FLINCOR v.1.4 (Brown, 1989) and FLUIDS (Bakker, 2012). Results and discussionThe orebody is controlled by a series of featherlike ruptures and faults and its dominant mineral compositions are chalcopyrite and chalcocite with minor amounts of pyrite, galena, bornite and sphalerite. The gangue minerals are quartz, barite, calcite and chlorite. Four types of hydrothermal alterations including chloritization, sulfidization, silicification and epidotization were recognized. Based on field and petrographic studies, three mineralization stages were distinguished including (1) the preore mineralization stage characterized by finegrained disseminated pyrites, (2) the main hydrothermal stage consisting of three substages: I) an early quartzchalcopyrite ± bornite vein, II) middle bornitechalcocite ± covellite breccia ore, III) late galena and sphalerite inclusions, and (3) latestage barite and calcite veins.Based on petrographic studies, five types of aqueous fluid inclusions have been distinguished in the quartzchalcopyrite ± bornite and barite veins including twophase liquidrich (LV), twophase vaporrich (VL), liquid monophase (L), vapor monophase (V) and minor halitebearing liquidrich fluid inclusions (LVS). The results show that parental fluids with a density of >1 g/cm3 and an approximate depth of 400 meters were formed and they were followed by fluid inclusions with a density of <1 g/cm3 and a depth of <300 meters due to fluid depressurization, faults. Moreover, introducing low temperature meteoric waters have caused fluid mixing and subsequently copper ore deposition (Henley et al., 2015; Cheng et al., 2019). Considering all geological mineralization styles, textures and structures of the orebody, types of alteration and fluid inclusions in copper deposits of the KuheJarou Mining District, it can be suggested that these deposits have similarities with the Mantotype copper deposits in Chile or volcanic red beds in northern America.