THE TWO METALLOGENIC STAGES FOR SARTOHAY CHROMITITE

More than 20 mineral species, such as ultra-high pressure, highly reduced and crustally-derived minerals, were firstly discovered from 847kg sartohay high-Al chromitite. These minerals can totally compare with the minerals from high-Cr chromitite from Luobusa,Tibet and Polar Ural , Russia, which would indicate that Sartohay high-Al chromitite may have the metallogenic stage in deep mantle like the high-Cr chromitites.

A large number of hydrous in situ inclusions were discovered in the chromites from Sartohay chromitite, including Na-phlogopite(aspidolite), Na-Cr pargasite. The rutiles separated from chromitites came from metamafic rocks, and the zircons from mineral separation contain crustally inclusions. All evidence imply that the metallogenic stage in shallow hydrous upper mantle. The contents of Al₂O₃ and TiO₂and value of FeO/MgO of parental magma got by calculation are similar to MORB, which means the chromitite formation was related closely to the hydrous MORB-like melt.

The model that the metallogenic stages both in deep mantle and in shallow upper mantle for the genesis of Sartohay high-Al chromitite has been proposed. The strongly reduced fluid enriched in Si, Cr, C, Fe, Ni produced by the partial melting of subducted slab in mantle transition zone including diamond, moissanite, native Si, Fe, Cr, wüstite and FeNi alloy. These minerals then moves upward beneath oceanic spreading centers and are encapsulated in chromite near the top of the transition zone (<14GPa). At this stage, the chromite preliminarily enriches and upwells into the upper oceanic lithosphere mantle with the UHP and highly reduced minerals. The upward-migrating magmas result from slab breakoff pass through the slab and assimilate the subducted materials containing crustal rutiles and zircons. The magmas then go into the magma chamber beneath the back arc spreading ridge and lead to the melt/rock interaction happen on a large scale, which facilitates Sartohay chromitite crystallization. Neither PGMs nor BMSs have been found as inclusions in chromite in that the parental MORB-like magma carried very little PGEs and was extremely S-poor. The BMSs such as millerite, heazlewoodite, and PGMs began to crystallize in the crack or interstitial matrix between chromite grains at the last stage of magmatic evolution.