Temperature trends in Andean IOA deposits
The temperature data obtained for magnetite from IOA deposits within the Chilean Iron Belt reveal a distinct cooling trend that broadly correlates with the relative depth of formation or structural level of emplacement of the deposits26. The deep, intrusive-type Los Colorados deposit has the highest calculated temperatures (~ 850–500°C; Fig. 2a). Intermediate temperatures (~ 780–340°C) characterise the transitional El Romeral deposit, followed by lower temperatures (~ 700–320°C) in the hydrothermal-type Cerro Negro Norte deposit (Fig. 2a). Interestingly, temperatures for early magnetite (Mgt-1) within each deposit are consistently higher than the other late textural types in most deposits (Fig. 2a). In Los Colorados, El Romeral and Mariela, this decreasing temperature trend correlates with trace element concentrations, most notably Ti, V and Ga, which are higher in Mgt-1 (Fig. 2b).
In Los Colorados, the high magnetite temperatures are consistent with oxygen isotope thermometry data (> 850°C to ~ 610°C), and re-homogenization temperatures of polycrystalline inclusions (> 950°C) in Mgt-111, 19. In addition, the core-to-rim temperature gradient—TMgt−1 (~ 730°C) > TMgt−2 (~ 630°C) > TMgt−3 (~ 600°C)—agrees with the qualitative magnetite cooling path determined by using the [Ti + V] vs. [Al + Mn] concentration plot3, 5, 9 (Fig. S1). The latter was interpreted by Knipping et al.10, 11 as the result of cooling from high-temperature, magmatic-hydrothermal (> 600°C) to lower temperature hydrothermal conditions (< 600°C). Notably, hydrothermal magnetite (Mgt-4) from the late-stage stockwork event at Los Colorados44 yields a similar average temperature (~ 620°C; Fig. 2a) to those determined for the late-stage rims; Mgt-2 and Mgt-3 (~ 630 and 600°C; Fig. 2a), formed over Mgt-1 cores in the massive magnetite orebodies.
Magnetite temperature data for El Romeral, reveal that the Ga concentration can be used to discriminate between magnetite from the high-temperature deep zone and the lower-temperature shallow zone, as well as the high-temperature Mgt-1 and low-temperature Mgt-2 (Figs. 2, S2). Furthermore, temperatures from the deep zone correlate well with those determined by the composition of primary actinolite grains at 1 kbar (~ 805 to ~ 735°C)22. In Cerro Negro Norte, the average temperature for early Mgt-1 is consistently higher (~ 572°C), although data for the other textural types (Mgt-2 to -4) cluster within average temperatures of ~ 520–450°C (Fig. 2a). Pegmatitic-type IOA deposits such as Mariela, Carmen and Fresia (~ 750–360°C; Fig. 2a) exhibit evidence of widespread dissolution-reprecipitation processes, which have been interpreted as caused by multiple pulses of both magmatically-derived and externally-derived meteoric fluids and/or basinal brines26, 47. Fluid inclusion studies by Velasco and Tornos42 at Carmen are consistent with this notion, suggesting the involvement of aqueous-gas rich, high-salinity (> 30 wt.% NaCl eq) Ca-Cl-Mg fluids with temperatures > 360 ºC. Average crystallisation temperatures for magnetite from Carmen and Fresia (~ 600°C; Fig. 2a) are consistent with oxygen isotope thermometry temperatures determined for coeval magnetite-actinolite pairs in Carmen42.
Temperature data for magnetite from El Laco andesitic host rocks provide further insights into the conditions of magnetite crystallisation from a silicate melt. Magnetite in the host andesite, which is undoubtedly of igneous origin20, 29, crystallised at higher temperatures (~ 1055–700°C on average) than magnetite from the orebodies (~ 900–400°C; Fig. 3b). Magnetite samples from the Laco Norte and Laco Sur orebodies at El Laco yield crystallisation temperatures that are similarly variable (Fig. 3a). Furthermore, no major temperature variations are observed with depth or with the temporality of magnetite types in these orebodies. Despite the relatively high average calculated temperature of magnetite from the shallow/surface zone (up to ~ 940°C), a lower temperature “tail” (~ 400 − 350°C) that corresponds with a depletion in Ti-V-Ga is observed (Fig. 3a, b). Previous studies reported high temperatures at El Laco as documented by fluid inclusion thermometry. Temperatures of ~ 840 to ~ 700°C and high salinities (0.2–59 wt.% NaCl eq.) were obtained from fluid inclusions in clinopyroxene and apatite intergrown with magnetite39, 40. Recently, Bain et al.33 reported liquid-vapor homogenization temperatures between ~ 951 and 800°C for polycrystalline inclusions in diopside-magnetite-anhydrite veins from the El Laco Pasos Blancos orebody. These temperatures overlap with those estimated from oxygen isotope thermometry in the diopside-magnetite-anhydrite veins (~ 1125 − 900°C)29.
Thermal evolution and genetic implications
Figure 4 presents a schematic representation of magmatic-hydrothermal stages illustrating the thermal evolution for Andean IOA deposits. We coupled the magnetite thermometry data reported here with reference temperatures calculated for magnetite crystallisation in igneous rocks (basalt, andesite, dacite) and other magmatic/magmatic-hydrothermal ore systems including Fe-Ti, V deposits, skarn and porphyry Cu-Mo-Au deposits20, 48, 49 (Figs. 4a, S3; Table S5). The new temperature data for magnetite presented here are consistent with a magmatic-hydrothermal origin for Andean IOA deposits. The magnetite thermometry data agree with δ56Fe and δ18O stable isotope data for magnetite in several IOA deposits in Chile that are consistent with magnetite crystallisating from high-temperature ore fluids sourced from silicate melts18, 19, 23, 24, 34, 37, 50. Furthermore, the data correspond well with δ56Fe and δ18O isotope information for several other IOA deposits worldwide, formed under different tectonic settings in different epochs. These include IOA deposits in the Kiruna and Gräsbergerg districts in Sweden, the Bafq District, Iran, and the Pea Ridge and Pilot Knob deposits in Missouri, USA, among others23, 51, 52, 53.
Our proposed model in Fig. 4 invokes a combination of magmatic and hydrothermal processes to explain the thermal evolution of IOA deposits in continental arc settings. Stage 1 comprises purely igneous magnetite crystallisation from a hydrous silicate melt, reflected by temperatures of ~ 1000–800°C. During this stage, upward migration of magma to 3–4 km depth26 results in decompression-induced volatile saturation of the silicate melt, facilitated by heterogenous nucleation onto magnetite microlites10, 11, 54–56. Decompression allows an efficient separation of the Fe-rich magmatic volatile phase from the parental magma reservoir and its rapid transfer to upper levels through faults, forming tabular, massive magnetite bodies such as those found at Los Colorados10, 26. During stage 2 (~ 800–600 ºC), the dissolved FeCl2 in the exsolved magmatic-hydrothermal fluid precipitates as magnetite upon cooling, which is more efficient under higher degrees of decompression22, 57. Mineralization styles include stockwork and breccias, typical of systems dominated by high water/rock ratios (e.g., El Romeral, Cerro Negro Norte, deep/intermediate zones of El Laco). Stage 3 involves magnetite precipitation from cooling hydrothermal fluids at temperatures below 600°C, with variable contributions from available external fluids, e.g., meteoric fluids and/or basinal brines23, 58. During this stage, multiple injections of fluids lead to complex magnetite growth, with increased dissolution-reprecipitation processes and formation of pegmatitic bodies and pervasive replacement horizons, e.g., Carmen, Fresia, Mariela, El Laco21, 26, 47.
The thermal trends recorded in magnetite in the studied orebodies suggest that Andean IOA deposits were formed predominantly under high-temperature magmatic-hydrothermal (~ 800–600°C) conditions that grade to lower temperature hydrothermal conditions (< 600°C; Fig. 4b, c). These results agree with most recent studies in IOA systems18, 19, 23, 24, 26, 37 and similarly to other magnetite-bearing magmatic-hydrothermal deposits such as skarns and porphyry Cu-Mo-Au systems8, 9, 16, 48 (Fig. 4a).
Considerations for magnetite thermometry in ore systems
The crystallisation temperatures reported in this study, calculated from a database of over 3000 magnetite analyses, are unequivocally consistent with geological observations and independent temperature estimations in the studied deposits, confirming the robustness of the magnetite thermometer. We highlight that the proper application of the TMg−mag thermometer depends not only on a good experimental and empirical calibration43, but also relies on detailed textural studies, including identification of chemical zonation, exsolution, Mg-bearing nano- to micro-sized inclusions and domains, as well as oxidation products (hematite, maghemite, goethite). These features are frequently observed and particularly affect magnetite in ore deposits due to the significant hydrothermal and chemical weathering overprinting in these systems. Therefore, it is likely that some of the temperatures calculated in this study may be representative of multiple episodes of hydrothermal circulation, leading to higher average temperatures. For example, magnetite from the intermediate zone in El Laco21 yields high calculated temperatures of up to ~ 940°C (Fig. 3a), possibly due the abundance of micron- to nano- scale Mg-bearing silicate particles (Table S2). Thus, it is likely that hydrothermal processes such as fluid-aided dissolution-reprecipitation lead to variations in the calculated temperatures attributable to complex magnetite textures in some deposits (e.g., El Laco, Carmen, Fresia). This may result in an overestimation of the Mg concentration in magnetite and consequently higher calculated temperatures, where the average Mg concentration in magnetite corresponds to the sum of Mg in the magnetite matrix and silicate nano-inclusions44. Additionally, variable degrees of magnetite low-temperature oxidation and replacement by maghemite, hematite and goethite—typically found at shallow levels at El Laco (Mgt-3 and-4; Table S2)—could explain the wide range of magnetite temperatures (~ 950–390°C Fig. 3a). Accordingly, relatively high-Mg and low-Ti, V, Ga contents of these magnetite grains (Fig. 3b) reflect extensive chemical modification.
FINAL REMARKS
We reconstructed the thermal evolution of Andean iron oxide-apatite (IOA) deposits by using the TMg−mag thermometer on a large magnetite geochemical dataset. Our results are the first comprehensive assessment of the thermal evolution of IOA deposits, providing a quantitative estimation of cooling trends in several IOA deposits of variable size and types. Calculated magnetite temperatures record a transition from purely igneous (~ 1000–800 ºC) to mainly hydrothermal conditions (< 600°C). Our data support a genetic model that invokes a magmatic-hydrothermal origin for Andean IOA deposits, and most importantly, reveal a predominance of fluid-dominated hydrothermal conditions. Our results demonstrate that magnetite thermometry opens new avenues to constrain formation temperatures in IOA systems, and therefore could be useful for vectoring towards magnetite-rich zones laterally and vertically, and for inferring the presence of deeper mineralized orebodies.