Discriminating porphyry and endoskarn-forming magmatic-hydrothermal systems: a case study from the Tonglushan Fe-Cu-(Au) deposit, Daye district, China

Porphyry magmatic systems emplaced within carbonate host rocks constitute a major source of the world’s Cu, Mo, Pb, Zn and Au [1]. Mineralisation is generally either porphyry-style or endoskarn-style within, or porphyry-, exoskarn- or manto-style outside the porphyry intrusion(s) [1,2]. Genetic models for porphyry and skarn mineralisation are well established, however questions remain as to why endoskarn- rather than porphyry-style mineralisation predominates within certain systems and regions. This is the case in Japan, for example, where there are very few signs of porphyry mineralisation despite generally favourable geological conditions, but there are large endoskarn and exoskarn deposits [3]. Recent studies show that magmas can assimilate large volumes of crustal carbonates, potentially providing a significant amount of CO₂ to late and post-magmatic hydrothermal fluids [4]. High levels of CO₂ in magmatic-hydrothermal systems may favour endoskarn formation and affect metal fractionation and solubility of ore minerals [5]. In this contribution, we test the hypothesis that endoskarn alteration may eliminate porphyry-style Cu mineralisation and mobilise Cu into other parts of the pluton and surrounding carbonate wall-rocks (exoskarns).  

To address this hypothesis, the Daye ore district in the Middle-Lower Yangtze River metallogenic belt was selected for study as it hosts porphyry-, exoskarn- and endoskarn-styles of mineralisation [6]. The porphyry and skarn deposits lie within Late Mesozoic intrusions or along their contacts with Late Triassic carbonates. From among the many porphyry-related systems, the Tonglushan Fe-Cu-(Au) endoskarn-bearing system was selected for detailed field-, light microscopy-, cathodoluminescence-, SEM- and QEMSCAN®-based genetic studies. The current study is mainly based on a comparison of samples from a single core through altered granite, endoskarn and exoskarn. From preliminary data for the Tonglushan system, the granites distal to the endoskarn were affected by Na-Ca alteration (replacement of intermediate composition plagioclase with albite, calcite and chlorite, and hornblende with calcite and chlorite), potassic alteration (replacement of plagioclase with K-feldspar), and later quartz-calcite veining. The endoskarn, which shows relict minerals and textures from the granite, underwent: 1) sericitic alteration, 2) prograde endoskarn formation, 3) retrograde endoskarn formation, 4) potassic alteration and 5) late carbonate veining stage. The textural relationships of oxide minerals in exoskarn and endoskarn indicate that magnetite and hematite likely formed during Stage 3, whereas Cu-(Au) mineralisation in the exoskarn is considered to be genetically associated with the potassic alteration phase, with precipitation of sulphides caused by acid neutralisation within the carbonates.

References:

[1] Sillitoe R (2010) Econ Geol 105:3-41

[2] Meinert L D et al. (2005) Econ Geol 100:299-336

[3] Ishihara S (1980) Mining Geol 30:59-62

[4] Carter L B and Dasgupta R (2016) Geochem Geophys Geosyst 17:3893-3916

[5] Lowenstern J B (2001) Mineral Deposita 36:490-502

[6] Zhai Y S et al. (1996) Ore Geol Rev 11:229-248