Garnet composition as an indicator of skarn formation: LA‐ICP‐MS and EPMA studies on oscillatory zoned garnets from the Haobugao skarn deposit, Inner Mongolia, China

Oscillatory zoned garnets are widespread in the Haobugao skarn‐type copper–lead–zinc–iron polymetallic deposit, and they can record garnet growing process in the early stages of metallogenesis. In order to investigate the skarn‐forming process and hydrothermal fluid evolution of the Haobugao deposit, major, trace, and rare earth element (REE) contents of oscillatory zoned garnets were analysed by electron probe microscope analysis (EPMA) and laser‐ablation inductively‐coupled plasma mass spectrometry (LA‐ICP‐MS) techniques. Three distinct generations of garnets were identified: the first generation garnets (Grt I) are Fe‐rich, euhedral, fine‐ to coarse‐grained, isotropic, show characteristic concentric oscillatory zoning, and have light rare earth element (LREE)‐enriched and heavy rare earth element (HREE)‐depleted REE patterns, with strong positive Eu anomalies and low ΣREE concentrations. The second generation garnets (Grt II) are Al‐rich, anhedral to subhedral, anisotropic, with abundant oscillatory zoning alone the growth lines, and have LREE‐depleted and HREE‐enriched REE patterns, with negligible Eu anomalies and relatively higher ΣREE concentrations. The third generation garnets (Grt III) are anhedral, anisotropic and generally occurred at the rim of pre‐existing Grt I or beside fractures that cut through the pre‐existing Grt I crystals. All these three generations garnets show oscillatory zoning under an optical microscope and have different compositions from each other, but there's limited chemical zoning (such as bell‐shaped zoning of major elements) in each individual garnet crystal. The texture and composition characteristics of the garnets indicate that the Grt I is precipitated rapidly from high temperature and oxidized magmatic fluids by advective metasomatism, in a high water/rock ratio condition; the Grt II is precipitated from low temperature residual fluids that were in equilibrium with the host rock by diffusive metasomatism, in a low water/rock ratio condition; and the Grt III is formed by retrograde hydrothermal‐metasomatic alteration of pre‐existing garnets. The incorporation of REE into garnet is controlled by its crystal chemistry and fluid composition, dominated by the YAG (yttrium aluminium garnet) ‐type substitution mechanism X2+−1VIIIREE3++1VIIISi4+−1IVZ3++1IV.