Mineralogy of Sn–W–As–Pb–Zn–Cu-bearing alteration zones in intracontinental rare metal granites (Central Mongolia)

Abstract

The metalliferous intrusive complexes in the Khentii Uplift evolved post-collisionally in an intracontinental environment. Their felsic magmatic rocks are surrounded by a small marginal facies of more granodioritic to monzonitic composition and were attributed to the A2 type granites, based on discrimination diagrams using K, Na, Si, Nb, Y and Rb. These rare metal granites have elevated contents of fluorite, topaz and tourmaline and are characterised by the light rare earth element enriched minerals allanite-(Ce) and monazite (Ce) prevailing over the xenotime, which is the only host with heavy rare earth element enrichment in these Mongolian granites. Together with zircon, these heavy minerals also show up in the clastic apron around these granites under study, providing a clue to the temperature of formation when the intrusive bodies were emplaced, and moreover provide a tool for exploration in pegmatitic and granitic terrains. Albite alteration is associated with a moderate U–Th–Ti mineralisation and greisen bodies were enriched in Sn and W of subeconomic grade. Only cassiterite re-appears as a heavy mineral in alluvial–fluvial placer-type deposits around the granite stocks, whereas only W occurs in a wide range of limonite and leucoxene like minerals. Sn-, W-, Fe, Mn- and Ti-bearing chemical residues can be used as a short-range marker to pinpoint the loci of enrichment of Sn and W, whereas the aforementioned heavy minerals are applicable as long-range tracers for metalliferous A-type granites. The Pb–Cu–Zn mineralisation is rather homogeneous and of little value as an exploration tool. Three different stages characterised by REE-carbonates and kaolinite, metalliferous duricrust abundant in Sn, W and Ti and aluminium–phosphate–sulphate minerals developed under oxidising conditions. The way how fluids moved though the granitic rocks, either as per descensum or per ascensum fluids, cannot be determined precisely. Therefore epithermal processes cannot be ruled out and have been held responsible for this near-surface alteration zone superimposed on a high-temperature Sn–W mineralisation in the Late Triassic to Early Jurassic A-type granites. These intrusive rocks are related to an extensional regime of the Triassic Indosinian orogeny in southern China.