Notice of three specimens of garnet

Abstract

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe3+) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids.Garnets from the Crown Jewel deposit range from Adr30Grs70 to almost pure andradite (Adr>99). Fe-rich garnets (Adr>90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE3+ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr>90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE3+ following a coupled, YAG-type substitution mechanism ([X2+]-1VIII[REE3+]+1VIII[Si4+]-1IV[Z3+]+1IV), whereas Eu2+ substitutes for X2+ cations. Thermodynamic data (e.g., Hmixing) in grossular–andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions.Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.