Scheelite, apatite, calcite and tourmaline compositions from the late Archean Hutti orogenic gold deposit: Implications for analogous two stage ore fluids

- LA-ICP-MS data of scheelite, fluorapatite, calcite and tourmaline from the Hutti mine
- Compositional heterogeneity in scheelite due to multiple fluid fluxes of similar nature
- Analogous nature of fluids for two-stage gold mineralization archived in hydrothermal minerals
- Low saline trace element poor reduced metamorphogenic ore fluids for two stage mineralization
- Combined tourmaline-scheelite geochemistry provides more information of mineralizing fluid.

Gold mineralization at Hutti, southern India has occurred in two stages. Stage-1 mineral assemblage comprises biotite, chlorite, plagioclase, calcite, epidote, actinolite, tourmaline, arsenopyrite, pyrite, pyrrhotite and gold, whereas the stage-2 mineral assemblage is represented by sericite, chlorite, calcite, scheelite, tourmaline, apatite, titanite, pyrrhotite, arsenopyrite and gold. We report here the occurrence of scheelite from the stage-1 assemblage and compare major and trace element compositions of scheelite, fluorapatite (F ≈ 4 wt.%), calcite and tourmaline from the two stages with an emphasis on the second stage in which they are best represented. Scheelite displays three types of REE patterns. These are the type-I middle REE (MREE)-enriched with negative Eu-anomaly, type-II heavy REE (HREE)-enriched with minor positive/negative Eu-anomaly, and type-III that varies from slightly MREE-enriched to MREE-depleted with correspondingly increasing positive Eu-anomaly. The stage-1 scheelites display type-I pattern, while stage-2 scheelites display type-II and type-III patterns. Textural and compositional evidence suggest asynchronous precipitation of scheelite, fluorapatite and calcite in the stage-2 mineralization, indicating precipitation from cyclic fluid flow. The occurrence of rare minute scheelite grains in the stage-1 mineralization assemblage is most likely because of prevalent high temperature (350-477 °C), whereas during the stage-2 mineralization abundant scheelite precipitation, evidenced by the presence of scheelite-rich carbonate-quartz veins, was favored by the decrease in temperature (~ 300 °C). Additionally, repeated fluid pressure fluctuations due to fault-valve mechanism would have further enhanced precipitation of scheelite in stage-2. The fewer scheelites in stage-1 mineralization resulted in their MREE-enriched pattern with high ΣREE. The MREE-depleted patterns of stage-2 scheelites, on the other hand, do not necessarily indicate a compositionally different fluid, but are due to rapid removal of MREE in early formed MREE-enriched scheelites making them less and less available in the fluid for later crystallizing scheelites. This is supported by the observation that except for the slight remobilization of LREE, the REEs are conserved in the altered rocks. Trace element ratios (Y/Ho, Zr/Hf, Nb/Ta) in fluorapatite and calcite are chondritic indicating little fractionation of these element pairs and cannot be diagnostic of fluid composition. Tourmalines from both the stages record equally low contents of Na, LILEs, and HFSEs and comparably low Fe3 +/Fe2 + ratios, suggesting very similar nature of the two fluids which were low in salinity, trace element poor and reducing in nature, indicating a metamorphic parentage.