Fluid-rock interaction near large vein bodies: Isotopic evidence

The processes of material transport near vein bodies and the relations of these processes with the deposition of ore mineralization are studied by examining the oxygen isotopic composition of wall rocks at two well-known ore deposits which belong to similar ore associations: Iultin (Sn-W deposit in the Chukotka Peninsula) and Akchatau (W deposit in Central Kazakhstan). The oxygen isotopic composition of rocks at the Iultin deposit was examined in three types of profiles: in metamorphosed sedimentary rocks, in the inner-contact part of a leucogranite dome, and in the altered wall rocks in the central part of the deposit, in a profile across the strike of a large orebody (62/50). The results are appended with data on the oxygen isotopic composition of the main vein minerals, quartz and muscovite, and with the hydrogen isotopic composition of selected whole-rock samples. The oxygen isotopic composition of granites at the Akchatau deposit was examined in profiles adjacent to orebodies of the quartz group (veins 220, 14) and the muscovite group (vein 152). The isotopic data were amplified with analyses of the same samples for trace elements. It is demonstrated that during the development of vein bodies at the Iultin deposit, the oxygen isotopic composition of the wall rocks was modified insignificantly. The significant depletion of the rocks in the O-18 isotope took place during the final stage, when the fluids in the system were dominated by meteoric water. Judging from the distribution of the oxygen isotopic composition, the isotopically light waters discharged immediately below the roof of the cooling intrusion. During that time, no significant ascending flows were focused within the veins, and the observed decrease in the delta(18)O and delta D values in the contact zone testify to a greater amount of solution that has percolated through this more permeable zone. The variations in the isotopic composition of the wall rocks at the Akchatau Mo-W deposit suggest that the isotopic zoning developed during the origin of the quartz greisen bodies, when the fluid was dominated by magmatic water, and that they were not modified later. The isotopically lighter waters of mixed composition discharged in the axial zone of the orebodies, which was the most permeable to solutions during the tectonic opening of the fractures.