当前位置: X-MOL 学术J. Geochem. Explor. › 论文详情
Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)
Formation and evolution of the calcic-magnesian Saheb Fe (Cu) skarn deposit from the Sanandaj-Sirjan Belt, NW Iran: Evidence for multistage boiling in episodes of magnetite saturation
Journal of Geochemical Exploration ( IF 3.9 ) Pub Date : 2021-03-26 , DOI: 10.1016/j.gexplo.2021.106781
Saeid Baghban , Zahra Zandi , David R. Lentz

The Saheb Fe (Cu) skarn deposit is located along the northwestern end of the Sanandaj-Sirjan Zone. This deposit occurs over an approximately 2 km distance along the contact zone between the Upper Cretaceous causative granite and the Upper Permian calcareous host rock, with several iron orebodies occurring as high-grade lenticular massive magnetite bodies (with a length of up to 90 m and a width of 25 m).

Following the formation of pyroxene hornfels facies, four main stages of skarn formation have been recognized: (I) early prograde stage (type I garnet + clinopyroxene + olivine): fluid inclusions in the type I garnet yielded a high temperature and salinity (average of 506 °C and 57 wt% NaCl equiv.), which may represent the composition of initial magmatic fluids. Higher salinities at this early stage can be associated with orthomagmatic differential fluid exsolution. (II) late prograde stage (type II and III garnets ± clinopyroxene ± disseminated magnetite): the coexistence of vapour-rich and halite-bearing fluid inclusions in type II garnet and their similar homogenization temperatures (420°–480 °C) presumably signify that fluid immiscibility from a supercritical fluid occurred at lithostatic pressures of 300 to 500 bar, corresponding to a depth of 1.5–2.0 km. (III) early retrograde stage (massive magnetite + pyrite + chalcopyrite + epidote + quartz + type I calcite + amphibole + serpentine + talc): hypersaline liquid and low-salinity vapour phases trapped within the epidote, quartz, and type I calcite are coexisting, indicating phase separation of hydrothermal fluids at temperatures of 300°–400 °C and pressures ranging from 310 to >100 bar under hydrostatic conditions, equivalent to a depth of 1.5 to 2.0 km. Hypersaline inclusions in the quartz and type I calcite show Td(NaCl) > Th(L-V), deduced to have been trapped by the over-saturated fluids or under over-pressured conditions before the hydrofracturing (up to 2200 bars). (IV) Late retrograde stage (type II calcite ± quartz ± chlorite), which is characterized by moderate temperature and salinity and non-boiling fluid inclusion assemblage (with average of 172 °C and 17 wt% NaCl equiv.).

The first boiling event is documented in the type II garnet of the late prograde stage and provoked the ore-bearing fluids to precipitate the disseminated iron ore. The main deposition of magnetite has plausibly been triggered by the second episode of fluid boiling, recorded in epidote, quartz, and type I calcite during the early retrograde stage. Two boiling events mean producing more fractured and brecciated zones, which can act as conduits for the iron-bearing hydrothermal fluids. This seems to be why there are dozens of isolated magnetite orebodies in the Saheb skarn deposit, which extend over an area of greater than two kilometers (in an E-W direction). Comparably higher salinities even in the type II calcite of the late retrograde stage (25 wt% NaCl equiv.) may reveal that significant mixing of saline magmatic fluids with external, dilute, and cold fluids is almost implausible, despite the presence of brecciated zones. Boiling, therefore, can be pondered as the most salient factor in controlling the ore precipitation at the Saheb Fe (Cu) skarn deposit. However, mixing with meteoric fluids and shifting in pH values could also facilitate the alteration and mineralization.



中文翻译:

伊朗西北部Sanandaj-Sirjan带的钙镁质Saheb Fe(Cu)矽卡岩型矽卡岩矿床的形成与演化:磁铁矿饱和事件中多阶段沸腾的证据

萨希卜铁(Cu)矽卡岩矿床位于Sanandaj-Sirjan区的西北端。该沉积物沿上白垩统致密花岗岩与上二叠系钙质基质岩之间的接触带约2 km的距离发生,有几个铁矿体以高品位的柱状块状磁铁矿体(长达90 m,宽度25 m)。

在形成辉石角岩相之后,矽卡岩形成的四个主要阶段已经被认识到:(I)早期发展阶段(I型石榴石+ Clinopyroxene +橄榄石):I型石榴石中的流体包裹体产生了高温和盐度(平均506°C和57 wt%NaCl当量),这可能代表了最初的岩浆流体的组成。在这个早期阶段较高的盐度可能与正岩浆差分流体释放有关。(II)晚期阶段(II和III型石榴石±斜ino石±弥散的磁铁矿):II型石榴石中富含蒸气和含盐的流体包裹体并存且其均质温度相近(420°–480°C),这表明在300到500 bar的岩石静压力下发生了与超临界流体的不溶混,对应于1.5-2.0 km的深度。(III)逆行早期(块状磁铁矿+黄铁矿+黄铜矿+附子+石英+ I型方解石+角闪石+蛇纹石+滑石):截留在石英,I型方解石中的高盐度液体和低盐度汽相共存表示在静水压条件下,温度在300°-400°C,压力在310至> 100 bar范围内的热液的相分离,等效深度为1.5至2.0 km。石英和I型方解石中的高盐包裹体显示T 表明在静水条件下,温度在300°–400°C和压力范围从310至> 100 bar的热液的相分离,等效深度为1.5至2.0 km。石英和I型方解石中的高盐包裹体显示T 表明在静水条件下,温度在300°–400°C和压力范围从310至> 100 bar的热液的相分离,等效深度为1.5至2.0 km。石英和I型方解石中的高盐包裹体显示Td(NaCl)  > T h(LV),据推测是在水力压裂之前被过饱和流体或在超压条件下捕集(高达2200 bar)。(IV)逆行后期(II型方解石±石英±绿泥石),其特征是温度和盐度适中,并且没有沸腾的流体包裹体组合(平均温度为172°C,当量NaCl当量为17%)。

第一次沸腾事件记录在晚期阶段的II型石榴石中,并激起了含矿流体以沉淀散布的铁矿石。磁铁矿的主要沉积可能是由第二阶段的流体沸腾触发的,在逆行早期就记录在附子,石英和I型方解石中。两次沸腾事件意味着产生更多的裂缝和角砾化区域,这些区域可以充当含铁热液的管道。这似乎就是为什么Saheb矽卡岩矿床中有数十个孤立的磁铁矿矿体,这些矿体延伸超过两公里(沿EW方向)的区域。甚至在逆行后期的II型方解石(NaCl当量为25 wt%)中,盐度也相对较高,这可能表明盐水岩浆流体与外部稀释稀溶液的显着混合。尽管存在弧形区域,但冷流体几乎是难以置信的。因此,可以考虑将沸腾视为控制Saheb Fe(Cu)矽卡岩矿床中矿石沉淀的最重要因素。但是,与大气流体混合并改变pH值也可以促进蚀变和矿化。

更新日期:2021-04-06
down
wechat
bug