CALC-SILICATE MAGMAS GENERATED BY LIMESTONE ASSIMILATION TO FORM "MAGMATIC SKARN"

Skarn deposits related to porphyry systems are generally considered to be products of hydrothermal metasomatism. However, at the Chating porphyry deposit (China), we find evidence for the formation of “magmatic skarn” by pervasive, infiltrative chemical exchanges between silicate melts and carbonate rocks in a shallow porphyry setting driven by assimilation, melting, and complex liquid immiscibility. In pods of endoskarn and associated veins from Chating, we find melt inclusions rich in sulfate and calc-silicate minerals (diopside + garnet + wollastonite + epidote). These inclusions were remelted in the heating experiments showing a variety of liquid immiscibility and often coexisting with chloride-rich salt melt inclusions and CO₂ vapor inclusions. We argue that skarn at Chating was formed by fractional crystallization of an immiscible, calc-silicate-rich melt, and these melts were generated by the assimilation of limestones with interbedded gypsum evaporites, followed by coupled decarbonation and desilication. The coeval assemblages of anhydrous salt-melt inclusions found in Chating, as well as the occurrence of phases such as thenardite, celestine, apatite, and chondrodite in the calc-silicate-bearing melt inclusions, suggests that the skarn-forming melts were stabilized by a variety of fluxes including elevated Na⁺, Sr²⁺, Cl^–, F^–, PO₄^3– and SO₄^2–. Meanwhile, the coeval vapor inclusions represent the products of decarbonation of the assimilated material and degassing of the CO₂ thus produced. Moreover, the calc-silicate bearing melt inclusions commonly contain sulfide daughter minerals, suggesting that these melts were rich in metals, and therefore may have played a role in ore mineralization. Based on these results, we argue that at Chating—and probably other skarn deposits where evaporites are part of the host sedimentary package—assimilation of carbonate rocks and generation of calc-silicate-rich melts was a key process of skarn formation.