Geochemistry and controls on the mineralization of the magnetite from the Gunjiu iron deposit in the Nyixung ore field, Coqen, Xizang

The Gunjiu iron deposit in the Nyixung ore field, Coqen, Xizang is a representative iron deposit within the Mesozoic iron-copper polymetallic deposits in the Gangdise metallogenic zone. The genetic mineralogy of magnetite is dealt with on the basis of field geological investigation and mineralogical and petrological studies. The microprobe and ICP-MS analyses show that magnetite is characterized by the enrichment of the major elements SiO₂ and depletion of TiO₂ and V₂O₅, the depletion of the trace elements Ba and Ti, the enrichment of Cs and U, and highly depletion of Eu, Lu, Tb, Ho and Tm. The covariation diagrams of Ti vs. (V+Cr) and (Ti+V) vs. (Al+Mn) suggest the controlls of the oxygen fugacity (fO₂) and temperatures on the element contents in magnetite. The positively correlation of w(TiO₂) vs. w(CaO+MgO) and w(TiO₂) vs. w(Na₂O+K₂O) also indicate that the fluid-rock interaction in the skarn systems may exert a major control on geochemical signatures of magnetite. The combination of mineral sequence, Ti and V elementary variations, and previous data for fluid inclusion temperature determinations have reflected that the mineralization was initiated in the intermediate- to high-temperature environments with high oxygen fugacity and mineralization temperatures of 300℃ to 450℃. The average w(Ni) value of 8.98×10^-6, Ni/Co ratios of 0.15 to 0.59 (<1), and Ti/V ratios of 6.71 to 25.52 have disclosed that the ore-forming matter was originated from the intermediate to acidic magmatic fluid systems. The feasibility of the discriminant diagrams of TiO₂-Al₂O₃-(MgO+MnO) and (Ca+Al+Mn) vs. (Ti+V) for the genetic interpretation of magnetite have provide the evidence that magnetite from the Gunjiu iron deposit may be an important indicator mineral in the mineralization processes of the skarn deposits.