Formation of giant iron oxide-copper-gold deposits by superimposed episodic hydrothermal pulses

Abstract

Iron oxide-copper-gold (IOCG) deposits are a globally important source of copper, gold and critical commodities. Despite their relevance, IOCG deposits remain an ill-defined clan, with a range of characteristics that has complicated development of the general genetic model. Here we focus on the Candelaria IOCG deposit in Chile and reveal that by using micro-textural and compositional variations in actinolite, a common alteration mineral found in many IOCG deposits, we can constrain the evolution of these systems. We demonstrate that Candelaria formed by the superposition of at least two pulses of mineralization with a late Cu-rich event overprinting and superimposed over an early, and probably higher temperature, iron oxide-apatite (IOA) mineralization event. These distinct pulses were likely caused by episodic injections of magmatic-hydrothermal fluids from crystallizing magmas at depth. Our data provide empirical evidence of grain-to-deposit scale compositional and potentially temperature changes in an IOCG system. The results support the use of actinolite chemistry as a novel approach to understand the formation of IOCG deposits and a potential tool for vectoring in exploration.

Contrasting magma chemistry in the Candelaria IOCG district caused by changing tectonic regimes

“…The proposed two-stage model for the mineralization at Candelaria is consistent with the paragenetic sequence 11 , 30 , 31 , where an early iron oxide stage is followed by a sulfide stage characterized by the precipitation of abundant Cu sulfides, mainly chalcopyrite, which according to our data would have occurred ~ 5 Ma after the IOA-type mineralization stage. This superimposed sequence of events is further supported by: (1) the variability of magnetite trace elements concentrations interpreted as precipitation under high temperature conditions at deeper levels (IOA-type mineralization) to lower temperatures at shallower levels where the Cu sulfide ore forms (IOCG mineralization) 13 , (2) the two-stage variable chemistry of actinolite 11 , and (3) the Ni/Se ratios in pyrite, which are redox and temperature dependent 10 .…”

“…Chemical and isotopic analyses of magnetite and actinolite from drill cores in the Candelaria district reveal a vertical zonation. The S-poor deep levels are primarily associated with high-temperature processes 11 , 13 , while mineralization at shallow levels shows higher concentrations of Cu and Au 11 related to lower temperatures and hydrothermal overgrowths 11 , 13 . These observations suggest a possible connection between IOA and IOCG-type mineralization.…”

“…Several lines of evidence suggest a magmatic-hydrothermal origin for the fluids along with the influence of multiple fluid sources 3 , 8 , 9 . Specifically, studies on Andean-type IOCG deposits, which form in continental magmatic arcs under extensional conditions 4 , have shown that the ore-forming fluids are predominantly of a magmatic-hydrothermal nature, with minor but variable contributions from oxidized basinal brines 3 , 6 , 10 , 11 . Despite these advances, fundamental questions about the genesis of IOCG systems remain.…”

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

“…Iron oxide-apatite (IOA; "Kiruna-type") and iron oxide-copper -gold (IOCG; "Chileantype") deposits contain major resources of a wide range of critical (iron, copper, phosphorous, rare earths, uranium, etc.) and precious (gold, silver) metals [1][2][3][4][5][6][7][8][9][10] and are commonly found in arc-related, orogenic and post-orogenic tectonic settings [1][2][3]7,[11][12][13][14][15][16][17][18][19][20]. 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].…”

“…and precious (gold, silver) metals [1][2][3][4][5][6][7][8][9][10] and are commonly found in arc-related, orogenic and post-orogenic tectonic settings [1][2][3]7,[11][12][13][14][15][16][17][18][19][20]. 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].…”