Apatite Geochemistry As a Fertility Tool for Porphyry Systems (Using the Example of the Shakhtama Mo-Porphyry and Bystrinsky Cu–Au–Fe-Porphyry–Skarn Deposits, Eastern Transbaikalia, Russia)

Abstract

The Bystrinsky Cu–Au–Fe porphyry–skarn and Shakhtama Mo-porphyry deposits are located in Eastern Transbaikalia, Russia. The host rocks in the deposits are multiphase granitoid plutons of the Middle–Late Jurassic Shakhtama complex. Economical mineralization is genetically related to small bodies of granite porphyries and granodiorite porphyries of late phases. To identify the specifics of fertile magmatic rocks, the composition of volatile components and rare-earth elements in apatite from fertile and barren intrusions was studied. Special attention was paid to the proof of the primary magmatic origin of apatite and the absence of influence of metasomatic alteration processes on their composition. It was shown that fertile intrusions at the Bystrinsky and Shakhtama deposits are characterized by an increased SO₃ content in apatite, which indicates their formation from oxidized melts. In addition, it is shown that the presence of sulfate sulfur in the melt is a necessary for the sulfide ore formation. It has been established that a high Cl content (>0.8 wt %) in fertile magmatic rocks, which ensures the transfer of chalcophilic elements, is a characteristic difference between apatites from Cu-porphyry and Mo-porphyry systems. The content of volatile apatite can be used as a sign of fertility for porphyry systems. An analysis of the trace element composition of apatite made it possible to establish that apatites from fertile granitoids of the Shakhtama and Bystrinsky deposits are characterized by Eu/Eu* > 0.4, which indicates the oxidation and high water saturation of the parental melts. The revealed characteristics of apatite from fertile intrusions and their discreteness from barren granitoids can serve as a sign of fertility of igneous rocks for porphyry mineralization.