Involvement of Evaporite Layers in the Formation of Iron Oxide-Apatite Ore Deposits: Examples from the Luohe Deposit in China and the El Laco Deposit in Chile

Abstract

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 δ34SV-CDT values (concentrated from 6‰ to 10‰ and 16‰ to 20‰, respectively). The δ34SV-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 δ34SV-CDT values of −2.3‰ to −0.9‰ and 6.8‰ to 10.5‰, respectively. The δ34SV-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.

The Role of Reductive Carbonaceous Surrounding Rocks in the Formation of Porphyry Mo Deposits

“…(1) For Reductive Fe-rich volcanic rocks, SO 4 2− from highly oxidized mineralized magma-hydrothermal fluids is reduced to S 2− by Fe 2+ from surrounding Fe-rich volcanic rocks, while Fe 2+ is oxidized to form magnetite/hematite, and S 2− combines with residual Fe 2+ and Mo 4+ to form pyrite and Mo ores (Formula ( 2)) [91][92][93][94]. Reductive Fe-rich volcanic rocks are widely distributed in many porphyry Cu-Mo deposits across the world, such as ferrimafic andesite in porphyry Cu-Mo deposits in South America [57,58], the gabbrodiabase-andesite complex in the El Teniente porphyry Cu-Mo deposits in Chile [56,59], and a series of tholeiitic basalts in the Oyu Tolgoi porphyry Cu-Mo deposit in Mongolia [90].…”

Iron–Titanium Oxide–Apatite–Sulfide–Sulfate Microinclusions in Gabbro and Adakite from the Russian Far East Indicate Possible Magmatic Links to Iron Oxide–Apatite and Iron Oxide–Copper–Gold Deposits

“…The formation of most current models involves multi-stage magmatic-hydrothermal processes and hydrous halogen-rich fluids, which scavenge metals from primary mantle-sourced, metal-rich silicate melts [9,[21][22][23][24][25][26][27][28][29][30][31]. Evaporitic basin-derived sources were also invoked for ore-forming fluids in some IOCG-IOA systems; for example, the giant Olympic Dam Fe-REE-Cu-Au-U district in Australia, Fe-Cu-Au-mineralized systems in Central Chile and magnetite-apatite deposits along the Middle and Lower Yangtze River in China [6,[32][33][34][35][36][37][38]. Several IOA and IOCG deposits in orogenic and post-orogenic environments contain magnetite-rich lavas, suggesting the involvement of Fe-rich melts in their formation [10,[39][40][41][42][43][44][45][46][47][48][49].…”

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“…These deposits are found in coastal areas and shallow marine basins all around the world and have been studied extensively for their geological, economic, and environmental significance [2][3][4]. The importance of studying marine evaporite minerals lies in their role as indicators of ancient climates, sources of economically valuable minerals, and potential applications in various fields [5][6][7]. By examining the geological processes that lead to their formation, we can gain insights into the past and present oceanic and atmospheric conditions [8].…”