The Gharah-Gol borate deposit, which is located about 80km southwest of Zanjan, northwestern Iran, currently is the only active boron mine in Iran. This deposit is hosted by continental sediments of the Upper Red Formation (URF) with Late Miocene age. The primary ore mineral is hydroboracite and secondary minerals are colemanite, priceite, szaibelyite and ortopinakiolite.
The ore body is not stratiform or even stratabound but rather shows cross-cut relations with its clay-marly host rocks. The ore body, with a NE-SW trend, is situated in the fractured core of an anticline as numerous individual lenses of various thickness.
Two major fault systems, with NE-SW and NW-SE directions are restricting the ore body. The first set of fault systems played important roles by creating conduits for circulation of the ore-bearing fluids and preparing required spaces for ore deposition.
Downward percolation of the meteoric waters via fractures and faults and heating due to high geothermal gradient established a thermal convective flow in the area. High boron-bearing tuffs have been leached by groundwater circulations. Upon arrival of these ascending boron-bearing fluids in the open spaces of the clayey-marly facies, due to domination of the reducing conditions, mineralization occurred.
During secondary alterations by descending meteoric fluids primary hydroboracite converted to colemanite and priceite.