Abstract
The Shinyemi magnesian Fe skam-type deposit develops in carbonate rocks of the Ordovician Makkol Limestone Formation in contact with Cretaceous porphyritic rocks intruded along the NNW trending faults which could have been magmatic fluid and meteoric water inflow paths. The exoskarn is mineralogically divided into three types, magnesian, intermediate and calcic skarns, which were formed simultaneously by replacement of dolostone, dolomitic limestone and limestone, respectively.
Microthermometric analysis of primary 2-phase (liquid+vapor) NaCI-H2O fluid inclusions in the skarn and ore minerals including forsterite, clinopyroxene, garnet, dolomite, calcite and sphalerite indicates that the fluid responsible for the Shinyemi magnesian Fe skam-type deposit is a medium-salinity (≤23 equiv.wt% NaC1) and hot fluid with little CO2. The formation temperatures, calculated from homogenization temperatures (360°-590°C) by pressure (500 bars) correction of prograde skarns were between 510°to 620°C (at the early first stage) and 400°C to 540°C (at the late first stage). The temperature range agrees well with that (400°- 610°C) by isotopic geothermometry. The early (440°- 540°C) and late (340°- 400°C) magnetite mineralizations in magnesian skam are considered to have occurred simultaneously with early and late sulfide mineralizations in calcic skarn, respectively. The general trend of decrease in fluid salinity and tempera-ture with time, resulted from continuous fluid-rock reaction, and/or interaction and dilution of the hydrothermal fluid with more dilute fluids in retrograde skarn evolution, agrees well with the results of oxygen and hydrogen isotope study.
