Metosomatic silcate chemistry from fenites and ore bodies at Bayan Obo, Inner Mongolia, China: multiple fluid flow events in the world's largest REE deposit

Smith, M.P. (2007) Metosomatic silcate chemistry from fenites and ore bodies at Bayan Obo, Inner Mongolia, China: multiple fluid flow events in the world's largest REE deposit Lithos, 93 (1-2). pp. 126-148. ISSN 0024-4937

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Official URL: http://www.elsevier.com/locate/lithos

Abstract

Fenite aureoles around carbonatite dykes, and alteration associated with Fe–REE–Nb ore bodies at Bayan Obo, Inner Mongolia, China, show alkali silicate assemblages containing aegirine–augite, (magnesio-)riebeckite, (magnesio-)arfvedsonite, and phlogopite, accompanied by varying amounts of apatite, albite and quartz. In both fenites and orebodies simple thermodynamic constraints indicate mineral parageneses are consistent with rock buffered cooling accompanied by the infiltration of a range of externally buffered hydrothermal fluids. Statistical analysis of amphibole chemistry indicates that even in apparently texturally well constrained paragenetic stages wide variations in chemistry occur in both the ore bodies and fenites.Much of this variation is attributable to theMg and F content of amphibole, and is therefore interpreted as a result of variation in externally controlled variables (P, T, initial fluid composition) rather than internally controlled variables such as protolith composition. Similarities in chemistry exist between fenite and some ore body amphiboles. Thermodynamic analysis of the composition of biotite and apatite allows constraints to be placed on the F-content of hydrothermal fluids, and indicates relatively consistent compositions in fenites and orebodies (log aHF/aH2O=−3.8 to −3.6 at 300 °C and 1 kbar). Amphibole and biotite associated with niobatemineralization are both enriched in fluorine relative to the rest of the paragenesis, and biotite compositions indicate significantly higher HF activities in the hydrothermal fluid (log aHF/aH2O=−2.6 at 300 °C and 1 kbar). The data presented here reinforce previous interpretations of the complex, multistage nature ofmineralisation at Bayan Obo, but are still consistent with a direct involvement of carbonatite derived fluids during ore genesis.

Item Type:Journal article
Uncontrolled Keywords:Amphibole; biotite; fenite; Bayan Obo; fluid chemistry
Subjects:F000 Physical Sciences > F600 Geology
DOI (a stable link to the resource):10.1016/j.lithos.2006.06.013
Faculties:Faculty of Science and Engineering > School of Environment and Technology > Applied Geosciences
Faculty of Science and Engineering > School of Environment and Technology
ID Code:2459
Deposited By:editor environment
Deposited On:08 Aug 2007
Last Modified:30 Apr 2012 14:10

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