Alteration, geothermometry, Raman spectroscopy and O-H stable isotopes studies on Lakehsiah 1 deposit, Yazd province, Iran

The Lakehsiah mining district is hosted in Early Cambrian volcano-sedimentary units (CVSU) of the Kashmar–Kerman zone, Central Iran. The Kashmar-Kerman belt is located between the Yazd block in the west and the Tabas block in the east and it is parallel to the Poshed badam, Tabas and Kalmard faults in the north and Koh Banan and Zarand faults in the south of the area (Ramezani and Tucker, 2003). Compositions of the volcanogenic rocks in this area vary from felsic to mafic and include rhyolitic, rhyodacitic tuff and spilitic lava and diabase. The sedimentary rocks include dolomites, dolomitic limestones and evaporites. Lakehsiah 1 deposit is one of three IOA outcrops in the Lakehsiah district which have been studied in this research.

The mineralogical study of alteration zones was carried out by light microscope with transmission light and X-ray diffraction (XRD) analysis, at the Mineralogical Laboratory of Bu-Ali Sina University and Iran Minerals Processing Research Center, respectively. Fluid inclusion and Raman spectroscopy studies were also performed to determine temperature, composition and evolution of the ore-forming fluid at the Institute of Earth Sciences SAS, Slovakia. Stable isotope geochemistry of quartz (O-H) was performed at the Cornell University, USA.

Iron deposits hosted in the Tashk Group show hydrothermal alteration. The major minerals of the Sodic-Calcic alteration are the crystals of calcic amphiboles (tremolite-actinolite), pyroxene, calcite, magnetite and apatite. Propylitic alteration (chloritization and epidotization) is very widespread and affects volcanic and intrusive rocks. It consists of chlorite, epidote, calcite, and magnetite with minor amounts of sericite. Silicification alteration, occurs as distal alteration in both hanging wall and footwall host rocks, forming fine-grained to coarse grained quartz aggregates, veins and veinlets. Sericitic-argillic alteration occurs mainly in intrusions. Feldspar (plagioclase and K-feldspar) was altered to sericite and clay minerals. Minor quartz occurs as veinlets in this alteration zone. Na- Ca alteration in volcanic and intrusion rocks is exposed in the center of the area. Amphiboles mainly occur as replacements of plagioclase. Plagioclases were altered to chlorite, epidote, and calcite. Additionally, veinlets of quartz-epidote-chlorite, chlorite-epidote, epidote-quartz, quartz-calcite, calcite, chlorite-calcite, and epidote-calcite are observed. Quartz and carbonates (calcite) are widespread and veins of these minerals crosscut all the rocks described above. Lakehsiah 1 deposit, hosted within high-silica rhyolitic tuffs and domes, forms a steeply dipping tabular lens and it includes massive magnetite ± apatite ± quartz ± specular hematite ± Fe-Mg silicates.

Fluid inclusion Petrography:

Four major types of fluid inclusions are observed based on proportions of vapor, liquid, and solid phases present at room temperature in quartz mineral. They are described as follows:1-liquid-rich inclusions (L) 2-vapor-rich inclusions (V) 3-Two-phase liquid rich fluid inclusions (L+V) 4-three phase inclusions with halite solid phase as daughter mineral (L+V+H). Study of inclusions petrography shows that most of the inclusions present within this mineral are primary in origin, although secondary or pseudosecondary types have been identified. They have different sizes (typically 5–15 μm). Fluid inclusion shapes are rounded, elliptical, irregular, negative crystal shapes and square.

Microthermometry and Raman spectroscopy Freezing and heating experiments were performed on Types 3 and 4 fluid inclusions. Stretching of inclusions was noted during heating of large fluid inclusions in quartz from mineralized quartz veins. In such samples, homogenization temperatures range from 217–428 °C for type 3 and 384-467°C   for type 4. Micro-thermometric data were obtained from both Types 3 and 4 inclusions. The data obtained revealed variation in salinity of the trapped fluids. The final ice melting temperature in Types 3 and 4 inclusions varies from −4° to -18 and -9 to -19 °C with a mode at around -12 °C. Final ice-melting temperatures are lowest in the mineralized quartz veins. The first melting temperatures in multiphase Types 3–4 inclusions are also in a similar range which varies from -21 to -34°C. Based on their final ice melting temperatures, it varies between 10 to 27 and 40 to 44 wt. % NaCl equivalent for type 3 and 4 inclusions. T°C vs. salinity plots of inclusions show mixing of magmatic hot fluids with cold meteoric waters. Raman spectroscopy revealed presence of 69 mol % N2 and 31 mol % CO2 and 33 mol % N2 and 67 mol % CO2 in types 3 and 4 inclusions. These gases can be derived from mantle degassing (Wang et al., 2018) and chemical reactions during ascent of fluids.

H-O isotopes:

Isotopic studies are among the most common methods for identifying the primary composition of ore-forming fluids in deposits (Barati and Gholipoor, 2014). In the study area, five quartz samples in quartz grains and veins were used for H-O isotope analyses, with the aim of determining the source(s) of ore-forming fluids. The δDH2O and δ18OH2O values of the ore-forming fluids in quartz samples vary from -60‰ to -80‰, and -4.71‰ to -1.42‰, respectively. The above observations reveal that the early ore-forming fluids are magmatic in origin and is characterized by high temperature and moderate to high salinity, and gradually evolve to low temperature, low salinity meteoric water. The Lakehsiah 1 Fe deposit is associated with the magmatism induced by the protracted subduction. The decrease in temperature, salinity and f(O2), as well as fluid-rock interactions, are the main factors controlling Fe deposition.