# Machine Learning Reveals Migmatic Fertility of Skarn-Type Tungsten Deposits
## 5 Discussion
### Geochemical Implications of Feature Importance Analysis  
Feature importance analysis based on machine learning models of apatite trace element chemistry effectively discriminates apatite crystals derived from magmas of different origins, providing valuable insights into ore-genetic processes. In skarn-type tungsten deposits, apatite from fertile magmas exhibits distinct elemental concentration patterns compared to apatite from barren magmas. Specifically, fertile apatite is characterized by higher concentrations of La, Mn, and Yb, alongside lower concentrations of Sr, Gd, Tb, andY. These differences likely reflect lower degrees of magmatic differentiation and higher aluminum saturation indices in the parent magma. The concentrations of Sr and Y in apatite are critical proxies for evaluating magmatic differentiation processes [68]. Although feldspar is generally the primary host mineral for Sr, its crystallization has limited influence on the Sr concentrations in apatite. Previous studies indicate a relationship between Sr and Y concentrations in apatite and whole-rock SiO\({}_{2}\) content: apatite crystallizing during advanced differentiation stages typically shows decreased Sr concentrations relative to apatite crystallized in early magmatic stages [69]. Thus, the elevated Sr content observed in apatite from fertile magmas in skarn-type tungsten deposits likely indicates lower degrees of differentiation, consistent with the intermediate SiO\({}_{2}\) contents and moderately oxidized conditions typically associated with tungsten mineralization [16]. Experimental and natural data show that amphibole sequesters Y with partition coefficients an order of magnitude higher than those for most LREEs, especially in granitic to granodioritic melts where Y readily occupies the M2 site of hornblende [70]. As fractionation proceeds, titanite saturates and becomes an even stronger sink, that efficiently strips the residual melt of Y and coeval MREEs [71]. Consequently, late-crystallizing apatite inherits a pronounced Y trough relative to both LREEs and HREEs--an imprint of sequential amphibole-titanite fractionation followed by fluorine-enhanced melt evolution [72]. The systematic depletion of the MREEs Gd and Tb in apatite from W-bearing granites is best interpreted as the geochemical fingerprint of early MREE-scavenging by rock-forming mafic accessories during extreme, volatile-rich differentiation. Experimental partitioning data show that amphibole concentrates MREEs far more strongly than either LREEs or HREEs, progressively lowering the Gd-Tb budget of the residual melt as soon as hornblende saturation is achieved [73; 74]. Titanite--which nucleates alongside amphibole in many peraluminous W granites--exhibits even higher partition coefficients for Gd and Tb, stripping the evolving magma of MREEs with each increment of crystallization and leaving late-stage apatite conspicuously MREE-poor [75]. Because the MREE sink phases crystallize before voluminous fluoride-rich volatile exsolution, the concomitant enrichment of F in the melt does little to replenish Gd and Tb yet enhances the solubility and transport of tungsten, thereby coupling MREE depletion in apatite to W fertility at the pluton scale [76; 77]. The resulting concave-up \"U-shaped\" REE pattern in apatite is markedly low in Gd-Tb. The pronounced Mn enrichment recorded by apatite from tungsten-fertile granites is a direct consequence of extreme, F-charged fractionation. Experiments show that the apatite-melt partition coefficient for Mn rises sharply--by more than an order of magnitude--as silicic melts evolve from metaluminous to highly polymerized peraluminous compositions, an effect controlled by melt structure rather than oxygen fugacity [13; 78]. Global compilations confirm that apatite Mn scales positively with host-rock SiO\({}_{2}\) and aluminosity but is decoupled from redox state, indicating that Mn behaves as a \"late compatible\" cation whose uptake is dictated chiefly by the degree of differentiation [13; 79].