Abstract:
Many talc deposits occur in the Hwanggangri Mineralized Zone (HMZ) in dolomitic marbles of the Cambro-Ordovician Samtaesan Formation within 1 km of the contact with the Cretaceous Muamsa Granite. Talc commonly forms fine-grained, fibrous aggregates, or pseudomorphs after tremolite; abundant tremolite is included as impurities in the talc ore. Talc generally was derived from tremolite in calc-silicate rock within the dolomitic marble. Calc-silicate rock, consisting mainly of tremolite and diopside, was generated from silicic metasomatism during the prograde stage, which promoted decarbonation reactions until dolomite was exhausted locally. Hydrothermal alteration of calc-silicate rock to talc is marked by the addition of Mg and Si, and the leaching of Ca; Cr, Co, and Ni were relatively immobile during the retrograde stage. Contact metamorphism related to the granite intrusion generated the successive appearance of tremolite, diopside, and forsterite, or wollastonite-bearing assemblages in the marble, depending on the bulk rock composition. The XCO2 content of the metamorphic fluids rose initially above XCO2=0.6, and decreased steadily toward a water-rich composition with increasing temperature above 600 °C in the calcitic marble, while buffered reaction of the dolomitic marble occurred at higher XCO2 conditions above 600 °C. Talc mineralization developed under metastable conditions with infiltration of large amounts of igneous fluids along a fault-shattered zone during the retrograde stage and is characterized by the loss of Ca2+ with the addition of Mg2+. Oxygen and carbon isotopic variations of carbonate and calc-silicate minerals are in agreement with theoretical relationships determined for decarbonation products of contact metamorphism. Talc formation temperatures obtained from oxygen isotope fractionation, T–XCO2 relationships, and activity diagrams range from 380 to 400 °C.