Iron oxide-apatite (IOA) deposits are important sources of iron. The role of evaporite layers in the formation of IOA ore deposits remains controversial. The Luohe deposit in eastern China and the El Laco deposit in Chile are representative IOA deposits. In this study, the S isotope characteristics of these two deposits are revisited. The formation of the Luohe ore deposit is closely related to marine evaporite layers in the Middle Triassic Dongma’anshan Formation. At Luohe, most of the sulfides and sulfates have high δ³⁴S_V-CDT values (concentrated from 6‰ to 10‰ and 16‰ to 20‰, respectively). The δ³⁴S_V-CDT values of sulfates are similar to those of marine evaporite layers (28–30‰) in the Dongma’anshan Formation. Estimates show that 46–82% of sulfur in the Luohe deposit is derived from marine evaporite layers. Unlike the Luohe deposit, the El Laco ore deposit formed in close relation to terrestrial evaporite layers from the Cretaceous-Tertiary Salta Group. At El Laco, the sulfides and sulfates have lower δ³⁴S_V-CDT values of −2.3‰ to −0.9‰ and 6.8‰ to 10.5‰, respectively. The δ³⁴S_V-CDT values of sulfates from the El Laco deposit are similar to those of sulfates from terrestrial evaporite layers (9.5‰) in the Salta Group. Estimates reveal that more than 70% of sulfur comes from terrestrial evaporite layers. These results indicate that evaporite layers have been involved in IOA ore-forming systems of both hydrothermal and magmatic deposits. Evaporite layers are proposed to have played key roles in the ore-forming processes of the Luohe and the Laco deposits and in other IOA deposits elsewhere.

Porphyry Mo deposits are the most important type of Mo resource. They result from a high oxygen fugacity of the parent magma, which acts as an effective indicator for evaluating the mineralization. In the ore-forming system of porphyry Mo deposits, sulfur exists mainly as sulfate in highly oxidized magma but as sulfide in ores. What triggers the reduction in the mineralization system that leads to sulfide precipitation has not yet been determined. Most of the previous studies have focused on the origin and evolution of the ore-forming parent magma, and the effects of reductive surrounding rocks on porphyry mineralization have been ignored. In this study, a comprehensive geological–geochemical investigation and review have been performed on the typical porphyry Mo deposits, the Nannihu-Sandaozhuang, Yuchiling, and Shapingou deposits in China, and the Mt. Emmons deposits in America. Black carbonaceous sedimentary layers commonly surround porphyry Mo ores, which are widely altered and discolored during mineralization. CH₄ is commonly present in fluid inclusions in the main mineralization stage, and the δ¹³C_V-PDB values of calcite and fluid inclusions from the altered surrounding rocks and ore minerals are generally low and significantly different from those of marine sedimentary carbonate rocks, indicating that the involvement of reductive components from carbonaceous surrounding rocks might be key to the redox state transformation leading to mineral precipitation. On the other hand, the CH₄ produced by the thermal decomposition of organic matter or carbonaceous reaction with H₂O can diffuse into the ore-forming system along the structural fractures and reduce the SO₄²⁻ in the ore-forming hydrothermal fluids to form sulfide precipitation without direct contact between the intrusion and the carbonaceous surrounding rocks. Moreover, the CH₄ content controls the location of the orebody formation with the high content producing orebodies mainly in the porphyry intrusion, while the low CH₄ content results in the orebodies mainly occurring at the contact zone between the porphyry and carbonaceous surrounding rocks. Compared to the magmatic stage of mineralization, the involvement of reductive components in the carbonaceous surrounding rocks during the hydrothermal stage is more favorable for forming giant/large Mo deposits. The highly oxidized porphyry with reductive carbonaceous surrounding rocks or Fe-rich volcanic rocks offers a new indicator for efficiently evaluating porphyry Mo mineralization.