Investigation of Antimony mineralization and mass transfer in host rocks, Baout, west of Zahedan (southeast of Iran)

The Baout antimony deposit is located 80 km west of Zahedan. Antimony occurs as a trace element in Earth crust, introduced in many minerals, especially sulfides and sulfosalts and occurs as small high grade ore deposits in different parts of the earth. Antimony mineralization in Iran is mainly in the form of hydrothermal veins associated with volcanic and plutonic activities. The Sistan suture zone (SSZ) in east and southeast of Iran hosts high-grade Sb-veins in several areas from north to south such as Sefidabeh, Baout, Lakhshak, Sefidsang and Shurchah (Boomeri et al., 2018). The aim of this paper is petrography and mineralogy of the host rocks and ore-bearing veins, loss and gain of various elements, especially antimony and related elements in alteration zones, and investigation of fluid inclusions in quartz associated with stibnite 

Geology:

The Baout is located in the SSZ and consists of Cretaceous ophiolitic rocks, Eocene flysch sedimentary rocks (turbidite), Oligo-Miocene intermediate dikes and recent sediments. The ophiolites and flyschs are metamorphosed and altered and host several NE quartz-stibnite veins. The area is a shear zone and has been affected by strike-slip faults. The NE faults are dominant in the mineralized area.

Method and material:

20 thin sections and 10 polish and 6 thin-polish sections were examined by polarizing microscope under transmission and reflected light for petrography, mineralogy and alteration and mineralization studies. A few samples from the fresh and altered rocks were analyzed by XRF and ICP-MS for major, trace and rare earth elements, respectively. Sb ores were analyzed by ICP-OES to study and interpret grade and variation of Sb, Cu, Au, As, Ag, and Zn. After optical observations three representative samples from Baout were chosen for subsequent micro thermometric measurements. The micro thermometric measurements were carried out by Linkham THMS600 heating-freezing stage (-196 to +600˚C) at Iran processing research center.

Petrography The igneous rocks in the Baout rock are serpentinized harzburgite, gabbro, diorite, basalt, diabase and dacite. These rocks mainly contain plagioclase with or without clinopyroxene, amphibole and biotite. The turbiditic rocks are sandstone, siltstone and metamorphosed shale (phyllite). Limestone and list waenite are other rocks of the area.

Alteration and Mineralization:

The host rocks including igneous rocks in the study area are extensively altered. The propylitic alteration occurs in the mafic rocks and sericitic alteration in the turbiditic rocks. The propylitic alteration is characterized by quartz, actinolite, epidote and calcite. Quartz and calcite are dominant secondary minerals that occur as vein, veinlets, and open space infillings in the host rocks. Serpentinization and list waenitization occur in harzburgite. Mass changes of altered igneous rocks are calculated by the Isocon method (Grant, 2005).  The altered rocks are depleted relative to less-altered rocks of Baout from mobile elements of Sb and As while they are enriched by SiO2 and immobile elements such as Pb. However, the hosrocks in the Baout area have more Sb than equal rocks from non-mineralized area of the Kurin to the south.The Sb mineralization is structurally controlled and occurs as NE quartz-stibnite veins. The stibnite is the most abundant sulfide and ore mineral and it occurs as open space filling mainly later than quartz.  There are also locally variable amounts of valentinite, senarmontite and stibiconite calcite, and iron oxides in the veins. The Sb grade is mainly high and reaches up to more than 30 wt. %. Other anomalous elements are Au, Pb, Zn, As and Cu

Fluid inclusion:

Fluid inclusions in quartz from the Baout area are primary, secondary, and pseudo-secondary in type. The fluid inclusion homogenization temperature and salinity range from 130 to 215˚ C and 2.07 to 3.06 wt. % NaCl eq., respectively. They all fall within the range of those from epithermal ore deposits and metamorphic waters.

The Sb mineralization in Baout occurs as quartz-stibnite veins. The ophiolitic and flysch units are host of the veins. The oldest veins are non-mineralized quartz veins followed by quartz-stibnite and carbonate veins, respectively. These veins are structurally controlled by NE Faults. The altered rocks are more depleted of Sb and As, and enriched in Pb and SiO2 as compared with less altered rocks. According to homogenization temperatures and salinities of liquid-rich two-phase primary fluid inclusions in quartz, Sb mineralization was formed by metamorphic hydrothermal solutions.