Mineral Geochemistry of Sulfides and Oxides and Its Implications for Ore-Forming Mechanisms in the Northeast Saveh Epithermal System, Central Urumieh–Dokhtar Magmatic Arc, Iran

Abstract

We have investigated the major- and trace-element composition of hydrothermal pyrite, magnetite, and Ti-magnetite, and of the principal Cu-minerals chalcopyrite and chalcocite, to constrain ore-forming processes in the northeastern Saveh district (central Urumieh–Dokhtar magmatic arc, Iran). Our data provide new constraints on the magmatic–hydrothermal evolution and subsequent hydrothermal–supergene modification of the ore system. Ti-magnetites hosted in monzodioritic intrusions are enriched in Ti–V–Al, plot below the magnetite–ulvöspinel join and record high crystallization temperatures (<500 °C) under relatively low oxygen fugacity. By contrast, magnetite from silica-rich hydrothermal veins is Fe-rich with very low TiO₂; it formed at intermediate temperatures (~200–300 °C) under higher fO₂ and is markedly depleted in Ti and V compared with the intrusive oxides. Textures and oxide systematics (Al + Mn vs. Ti + V; V/Ti–Fe) document repeated hydrothermal pulses, Fe²⁺ leaching and element redistribution during cooling and fluid–rock interaction. Geochemical trends indicate progressive evolution from a magmatic fluid to later meteoric water overprint, with increasing As contents reflecting cooling and mixing with meteoric waters. Vertical elemental zoning suggests that most samples represent mid- to deep-level sections of the epithermal system. Elevated Cu contents (up to 0.95 wt.%) highlight pyrite as a significant Cu host. Co/Ni ratios between 1 and 10 further corroborate a magmatic–hydrothermal origin. Chalcopyrite is the principal economic Cu carrier at Northeast Saveh. Replacement follows a temperature- and fluid-controlled pathway (chalcopyrite → covellite → chalcocite). At lower temperatures (<~200 °C) replacement proceeds more slowly, producing chalcocite/digenite under prolonged reaction conditions. Chalcocite commonly occurs as thin replacement rims and fracture fills that concentrate remobilized copper. Collectively, the investigated oxide and sulfide proxies provide robust discriminants for separating magmatic versus hydrothermal domains and for vectoring toward higher-temperature feeders and zones of remobilized copper.

1. Introduction

Trace-element concentrations in sulfide and oxide minerals (e.g., pyrite, chalcopyrite, chalcocite, and magnetite) in porphyry and epithermal systems mainly reflect fluid composition, physicochemical conditions, and the stability of the minerals during crystallization [1]. Variations in these factors often lead to significant changes in the trace element contents of these minerals, which serve as vital indicators of fluid evolution and ore formation processes.

Pyrite is the most abundant sulfide in the Earth’s crust and a key host for trace elements in magmatic–hydrothermal systems [2,3]. Its chemical composition reflects conditions such as temperature, pH, and oxygen fugacity of the mineralizing fluids and plays a critical role in controlling the distribution of economically valuable elements like Au, Ag, As, and heavy metals [4,5]. Pyrite accommodates trace elements through mechanisms like lattice substitution (e.g., Co, Ni, Cu, As) and the incorporation of nanoparticles or micro-scale inclusions [3,6]. These features make pyrite an excellent tracer for the evolution of hydrothermal systems.

On the other hand, chalcopyrite (CuFeS₂) is the most abundant copper sulfide, contributing to about 70% of global copper production [7]. Its trace element composition varies widely depending on deposit type, reflecting the physicochemical environment during mineralization [8]. Chalcopyrite often coexists with other sulfides, such as chalcocite and bornite, and serves as a primary host for elements like Ag, Bi, Cd, Zn, and Sb. However, its trace element capacity may increase in the absence of other sulfides, making it a valuable tool for understanding ore-forming processes [5,8,9,10].

As well, magnetite (Fe₃O₄) and Ti-magnetite are ubiquitous in igneous and hydrothermal systems and serve as valuable indicators of mineralization. Magnetite, with its spinel structure, incorporates a range of trace elements that reflect the conditions of fluid or magma evolution [11,12]. Magnetite and members of the magnetite-ulvöspinel solid solution series (Ti-magnetite) can incorporate other elements into their structure. Their chemistry has been widely used to reconstruct physicochemical parameters such as temperature, oxygen fugacity, and sulfur fugacity during ore formation [13,14].

This study presents the first integrated geochemical investigation of sulfides (pyrite, chalcopyrite, chalcocite) and oxides (magnetite, Ti-magnetite) in the Northeast Saveh region to unravel the evolution of the epithermal mineralization system in the central Urumieh–Dokhtar magmatic arc (UDMA). The results offer new insights into the evolution of epithermal/mineralization and magmatic processes in this region, which has received limited research attention despite its significant potential for Cu-Au-Ag mineralization.

2. Geological Background

Magmatic activity in the UDMA began in the Late Cretaceous and persisted intermittently until the Miocene (Figure 1A). The subduction and subsequent rollback of the Neo-Tethys slab during this time led to significant magmatic flare-ups, especially during the Eocene-Oligocene. This period marked a critical tectonic transition from extensional to compressional regimes, which is reflected in the structural and magmatic patterns observed across the arc. The UDMA hosts diverse intrusive and extrusive igneous rocks ranging from basaltic and andesitic volcanic flows to plutonic rocks such as diorites, monzonites, and granites, indicating multiple phases of magmatic evolution [15,16,17,18].

Structurally, the region shows a transition from an extensional regime during the Eocene to a compressional regime in the late Eocene-Oligocene, marked by the development of dextral shear zones and strike-slip faults. This tectonic evolution facilitated the emplacement of plutonic bodies in transtensional settings, with later brittle deformation shaping the observed structural trends. Overall, the magmatic and structural characteristics of the study area reflect its dynamic evolution within the broader context of the Neo-Tethys subduction and the tectonic processes of the Alpine-Himalayan orogenic system.

The study area (Figure 1B), located northeast of Saveh in the central UDMA and approximately 100 km southwest of Tehran, highlights this complex geological history. Field and petrographic studies reveal the presence of two dominant intrusive rock units: monzonite-monzo-diorite and gabbro-gabbro-diorite. These intrusions have interacted with acidic to basic volcanic rocks and are structurally controlled by major west-northwest to east-southeast trending faults associated with the Alpine orogeny [17,18].

The monzonite-monzodiorite unit (Figure 1C,D), forming prominent topographic highs, includes quartz monzonite, monzonite, and monzodiorite compositions. These rocks exhibit granoblastic and porphyritic textures, with primary minerals such as plagioclase, potassium feldspar, hornblende, and clinopyroxene, alongside accessory minerals like apatite, zircon, and chlorite. Intense alteration features, including sericitization and carbonatization, are evident, particularly in plagioclase crystals.

The gabbro-gabbro-diorite unit (Figure 1E,F) is characterized by dark-grey outcrops with hetero-granular textures. These rocks consist of plagioclase, pyroxene, and hornblende, with secondary alteration minerals such as chlorite, epidote, and carbonate. The presence of anti-rapakivi textures and chloritized mafic minerals reflects the influence of hydrothermal fluids during emplacement.

3. Analytical Methods

Seventy polished thin sections were prepared from representative outcrops and drill cores across the mineralized zones for petrographic, mineralogical and micro-analytical investigations. Twelve of these were selected for detailed textural study and BSE imaging of oxide and sulfide ore phases; all sections were carbon-coated prior to electron-beam work to ensure electrical conductivity.

Prior to quantitative microprobe work, samples were inspected by scanning electron microscopy (SEM) in back-scattered electron (BSE) mode to document textures, identify inclusions and intergrowths, and to guide microprobe spot selection. Quantitative wavelength-dispersive electron probe microanalysis (EPMA) was carried out on a CAMECA SX Five (field-emission cathode) at the Department of Lithospheric Research, University of Vienna. Operating conditions and analytical setup were as follows: an accelerating voltage of 20 kV, a beam current of 25 nA, and a focused electron beam (nominal diameter ≈ 0 µm) rastered over an area of ~60 µm to obtain representative average compositions of oxide-rich domains and coarse sulfide aggregates while minimizing beam damage and local heterogeneity effects. The instrumental configuration (spectrometers and analyzing crystals) and counting times were selected to optimize sensitivity for the targeted major and trace elements. WDS crystals used were LLIF, LTAP, PET and TAP as appropriate for each line.

Analyses were calibrated with certified/well-characterized standards matched to each analyte (e.g., FeS for Fe and S, GaAs for As, PbS for Pb, Zn metal/oxide for Zn, InSb for Sb, Sn metal for Sn, wollastonite for Si, etc.). Intensity-to-concentration calibrations were performed from these standards, and all compositions were corrected for matrix effects using the ZAF routine implemented in the probe software (version 6.2, Cameca)

Peak and background counting times were set individually for each element to maximize sensitivity while controlling analytical time. Repeat analyses of standards and internal consistency checks were performed routinely throughout each analytical run to monitor precision and accuracy; analytical drift was negligible after standard-normalization, and repeatability of matrix-matched standards was typically better than ±2%–5% for majors and within acceptable limits for trace elements.

Because some reported trace concentrations are low, limits of detection were determined empirically for each element under the exact analytical conditions used. LODs were estimated following the commonly used 3σ criterion on background counts (i.e., LOD = 3·σbg, converted to concentration using the measured sensitivity from the corresponding standard), and take into account the peak/background counting times, spectrometer/crystal combination, and instrument sensitivity (cps per nA) logged for each line.

Where measured concentrations fell below the empirically determined LOD for a given element, they are reported as below detection (n.d.) in the data tables and were excluded from log-scale plots or any statistical computations that assume a measured value.

Microprobe spot analyses for the main mineral groups are summarized in Supplementary Tables S1–S5.

4. Mineralization and Mineralogy

4.1. Field Characteristics of Ore and Stages of Mineralization

Mineralization in the Northeast Saveh area primarily occurs within andesitic host rocks, with minor development in monzonitic and gabbroic units. The mineralized zones extend from several tens of meters up to approximately 1 km in length, with copper grades ranging from 0.1 to 5 wt.%. The predominant style of copper mineralization is of a vein and veinlet type, which is generally controlled by fault-related structures trending N–S and N40°W. Based on detailed field observations, six principal mineralized zones have been identified within the host rocks (Figure 2A).

Considering the structural and textural features of ore minerals, several distinct stages of mineralization can be recognized. During the pre-ore stage, pyrite and Ti-magnetite were formed as disseminated grains within the groundmass of the monzonitic and monzodioritic units. These minerals occur as euhedral to anhedral crystals and, to a lesser extent, as vein or veinlet fillings within open spaces. This early mineralization is interpreted to be related to the development of a magmatic–hydrothermal system associated with the emplacement of quartz monzonitic intrusions in the area [19].

Based on Goudarzi et al. 2024 [19], the main hypogene mineralization comprises an early magmatic–hydrothermal pre-ore event and a subsequent principal hydrothermal (ore) pulse. Field, textural and petrographic evidence indicate two generations of pyrite: an earlier, disseminated/hosting-phase pyrite and Fe–Ti oxide dissemination in the igneous host (pre-ore, magmatic–hydrothermal), and a later hydrothermal pyrite that occurs in veins, veinlets, stockworks and occasional xenocrysts within chalcopyrite. The principal hypogene (ore) stage is characterized by quartz ± sulfide ± oxide veinlets dominated by chalcopyrite (the main copper ore), locally accompanied by bornite and specular hematite; these veins display a range of morphologies (open-space, colloform, boxwork, breccia-cement) and crosscut earlier host textures, consistent with a distinct hydrothermal pulse responsible for emplacement of economic sulfide mineralization.

Subsequent supergene evolution is an exogenic process driven by descending meteoric, oxidizing fluids. Oxidation of the upper parts of the sulfide-bearing system produced an oxidation zone, and the resulting leaching and downward transport of copper led to a supergene enrichment horizon at the redox front. Here, secondary sulfides such as chalcocite and covellite (and locally native copper) precipitated by replacement of primary sulfides (e.g., chalcopyrite), while above the redox front continued weathering and oxidation gave rise to oxide–carbonate assemblages (malachite, azurite, cuprite, chrysocolla) and iron oxides/hydroxides (hematite, goethite, limonite) that occur as coatings, earthy masses, and veinlet infillings. Thus, supergene enrichment and the oxidation zone are conjugate near-surface processes. Oxidation creates the chemical gradients that drive Cu leaching and reprecipitation at the redox front and should be treated as linked, exogenic stages rather than as separate hydrothermal events.

Figure 2. (A) Satellite image of the study area, and the location of the mineralization zones and samples. (B) General view of the Northeast Saveh area and intrusive-volcanic outcrops. (C) Mineralized silica vein containing Cu ore and magnetite. (D) A close view of the mineralized silica vein. (E) Core drilling sample containing chalcopyrite. (F) Core drilling sample containing chalcopyrite and Fe oxide (magnetite-hematite) veins. (G) Pyrite vein and Fe oxide in a core drilling sample. Mineral abbreviations after [20]: Py—Pyrite, Ccp—Chalcopyrite, Hem—hematite, Mal—malachite, Mag—magnetite, Qtz—Quartz.

Figure 2. (A) Satellite image of the study area, and the location of the mineralization zones and samples. (B) General view of the Northeast Saveh area and intrusive-volcanic outcrops. (C) Mineralized silica vein containing Cu ore and magnetite. (D) A close view of the mineralized silica vein. (E) Core drilling sample containing chalcopyrite. (F) Core drilling sample containing chalcopyrite and Fe oxide (magnetite-hematite) veins. (G) Pyrite vein and Fe oxide in a core drilling sample. Mineral abbreviations after [20]: Py—Pyrite, Ccp—Chalcopyrite, Hem—hematite, Mal—malachite, Mag—magnetite, Qtz—Quartz.

4.2. Mineralogy and Textural Characteristics of Sulfide Minerals (Pyrite, Chalcopyrite, Chalcocite)

Pyrite is the most abundant sulfide in the Northeast Saveh area. Detailed examination of hand specimens, reflected-light petrography, and BSE imaging shows two principal generations of pyrite plus a very limited subordinate occurrence of fine inclusions. The first (early) generation occurs as euhedral to subhedral grains, typically 100 µm–1 mm in size, disseminated throughout the host rocks and commonly associated with minor chalcopyrite (Figure 3A). These grains are interpreted as products of the early magmatic–hydrothermal (pre-ore) event. In places, the original pyrite morphology is partly preserved despite partial replacement by hematite and goethite, indicating later near-surface oxidation of this generation. The second (ore-stage) generation occurs predominantly as vein- and veinlet-filling crystals and as lattice/framework or framboidal textures, consistent with precipitation from later hydrothermal fluids that exploited existing fractures and pore space (Figure 3B,C). Finally, a very limited subordinate occurrence of fine-grained pyrite (≈5–20 µm) is observed as inclusions within chalcopyrite (Figure 3D); these micro-crystals are restricted in abundance and spatial extent and are therefore interpreted as a late, minor sub-generation or exsolution/recrystallization product during the final stages of sulfide crystallization. BSE images (Figure 4) illustrate these relationships, showing disseminated first-generation grains, vein-filling second-generation pyrite, and the occasional pyrite inclusions within chalcopyrite.

Chalcopyrite is the dominant primary copper sulfide in the Northeast Saveh and constitutes the main copper host. It occurs chiefly as anhedral to subhedral grains disseminated in the groundmass and as vein/veinlet fills (Figure 3B–I). Texturally, chalcopyrite is found both in the matrix and along structural conduits, where it often forms the core of composite sulfide aggregates. Supergene processes have extensively modified the primary chalcopyrite. Oxidation of the sulfide assemblage produced iron oxides/hydroxides (e.g., goethite, specular hematite) and released copper to solution; subsequently, secondary copper minerals (e.g., malachite) form by precipitation from Cu-bearing, carbonate-rich near-surface fluids and commonly occur as coatings, fillings, or botryoidal masses in fractures and pores (e.g., Figure 5E,F). In many samples, the original chalcopyrite grains are rimmed or partially pseudomorphosed by these secondary oxides, documenting progressive oxidation and leaching of copper from the primary sulfide phase.

Chalcocite: Concomitant with oxidation, supergene enrichment produced a characteristic suite of secondary sulfides. Chalcocite is the most conspicuous product of this stage and defines the supergene sulfide zone in Northeast Saveh (Figure 3F–I). It commonly occurs as thin-rimmed replacements and fracture fillings around primary chalcopyrite and bornite, and locally as discrete aggregates in fractures and vein margins. Covellite and digenite, to a lesser extent, are also present as replacement phases, typically forming at the chalcopyrite margins or replacing thin rims of earlier-formed sulfides (Figure 3I). These textures indicate downward-percolating, oxidizing meteoric fluids mobilized copper from the upper parts of the system, followed by transport and reprecipitation as reduced Cu–S phases at or below the water table.

4.3. Mineralogy and Textural Characteristics of Oxide Ore Minerals (Magnetite-Ti-Magnetite)

Iron-oxide minerals in the Northeast Saveh area occur in two principal settings. In monzodioritic intrusive bodies, Ti-magnetite is the dominant syn-magmatic oxide, occurring mainly as disseminated grains within the groundmass (Figure 6A). In the silica veins that host or accompany Cu-sulfide mineralization, magnetite and specular hematite are predominant oxides, whereas Ti-magnetite appears less commonly (Figure 6B,C). Overall, hypogene oxide mineralization is relatively limited in volume and typically appears as scattered grains or vein infillings (Figure 6D–F).

Ti-magnetite in the intrusive units commonly occurs as euhedral to subhedral fine to coarse crystals, and locally shows intimate intergrowths with ilmenite. These intergrowths point to complex Ti–Fe oxide fractionation and exsolution histories during cooling (Figure 7B,E). In some crystals, blade-like hematite lamellae or exsolved hematite pockets are observed, indicating late-stage hematite separation that formed during the final cooling of the oxide phase (Figure 7E,F).

Magnetite associated with the mineralized veins typically forms micron-sized crystals and occurs dominantly as veinlet infillings together with chalcopyrite or as disseminated crystals within the vein selvages (Figure 6A,D and Figure 7A). Ti-magnetite is also observed as dissemination within the intrusive, commonly accompanied by subordinate pyrite and chalcopyrite.

5. Results

5.1. Mineral Chemistry of Pyrite

EPMA analyses were carried out on 43 points from 13 samples, exclusively targeting the fresh and well-preserved hydrothermal pyrite-II crystals (Figure 4). Although both pyrite-I and pyrite-II were examined petrographically, pyrite-I was not selected for quantitative analysis due to partial oxidation and replacement by hematite–goethite in many samples, which prevented obtaining reliable chemical data. In contrast, pyrite-II grains were texturally stable, unaltered, and suitable for accurate in situ EMPA measurements.

The analytical results are presented in Supplementary Table S1. The highest measured concentrations of trace elements in pyrite-II were: As (0.20 wt.%), Pb (0.26 wt.%), Cu (0.95 wt.%), Sb (0.23 wt.%), Ni (0.34 wt.%), and Co—the most abundant—at up to 1.12 wt.%. Gold was not detected in any of the analyzed points, and Ag contents were low, reaching a maximum of 0.05 wt.%. The strongest positive elemental correlations in pyrite-II were observed between Zn–Sn, Pb–As, and Pb–Mn, while the strongest negative correlations occurred between Fe–Co, S–Mn, Co–Zn, and S–Cu (

Table 1. Pearson’s correlation matrix for elements measured in Northeast Saveh Pyrites.

	Fe	Mn	Co	Ni	Cu	Zn	As	S	Pb	Ag	Sb	Sn	Si
Fe	1
Mn	−0.21	1
Co	−0.52	0.2	1					Low correlation			High correlation
Ni	−0.25	0.34	−0.14	1
Cu	−0.23	0.32	0.37	−0.19	1
Zn	−0.05	0.14	−0.36	−0.04	−0.07	1
As	0.05	0.06	−0.15	0.39	−0.20	0.48	1
S	0.04	−0.54	−0.14	−0.07	−0.36	0.23	0.05	1
Pb	−0.11	0.62	0.08	0.37	−0.01	0.49	0.76	−0.16	1
Ag	0.12	0.51	0.03	−0.19	0.28	−0.20	−0.13	−0.17	−0.19	1
Sb	−0.03	−0.31	0.08	0.24	0.08	0.28	−0.02	0.33	−0.20	−0.24	1
Sn	−0.08	−0.25	0.09	−0.11	−0.13	0.83	0.16	0.29	0.12	−0.21	0.23	1
Si	−0.20	−0.14	−0.07	−0.03	−0.12	−0.01	−0.18	−0.13	0.04	−0.10	−0.11	0.02	1

)

Violin plots displaying the distribution of trace elements in pyrites are shown in Figure 8, and elemental variation diagrams are given in Figure 9. As observed, Fe and S contents do not exhibit significant variations with increasing As concentration, suggesting that As may be incorporated into pyrite through surface adsorption in the 3+ oxidation state. Cobalt content exhibits an increasing trend. Copper content in pyrite decreases as Fe concentration increases. The variation trends of Co and Ni are nearly constant when plotted against each other. Copper concentrations show an almost increasing trend with rising Zn levels. Silver content decreases with increasing As and An concentrations but exhibits a positive correlation with Sn.

5.2. Mineral Chemistry of Chalcopyrite and Chalcocite

Chalcopyrite EPMA analysis was performed on 97 points (Supplementary Table S2). The chalcopyrite contains Fe ranging from 30.2 to 31 wt.%, sulfur from 34 to 35.5 wt.%, and copper from 33.2 to 35.1 wt.%. The concentrations of other trace elements are as follows: zinc up to 0.4 wt.%, lead up to 0.19 wt.%, silver up to 0.05 wt.%, antimony up to 0.02 wt.%, tin up to 0.05 wt.%, arsenic up to 0.02 wt.%, cobalt up to 0.01 wt.%, and nickel up to 0.02 wt.%. The strongest elemental correlations in chalcopyrite are observed between S and Fe, Fe and Cu, Sb and Ni, Cu and S, as well as Mn and As (

Table 2. Pearson’s correlation matrix for elements measured in Northeast Saveh Chalcopyrites.

	Fe	Mn	Co	Ni	Cu	Zn	As	S	Pb	Ag	Sb	Sn	Si
Fe	1
Mn	−0.21	1
Co	−0.19	−0.25	1					Low correlation			High correlation
Ni	−0.01	−0.04	0.25	1
Cu	0.44	−0.15	−0.57	−0.11	1
Zn	0.07	−0.05	−0.19	−0.25	0.01	1
As	−0.11	0.35	0.25	0.14	−0.09	0.09	1
S	0.53	0.03	−0.53	−0.07	0.43	0.11	0.02	1
Pb	−0.14	0.04	−0.03	−0.07	0	−0.09	−0.01	0.02	1
Ag	−0.10	0.07	−0.30	0.02	0.04	−0.30	−0.03	−0.19	−0.26	1
Sb	0.1	0.09	0.09	0.39	−0.05	0.11	−0.15	−0.14	−0.11	−0.06	1
Sn	0.01	−0.33	0.09	−0.32	−0.15	−0.31	−0.26	0.05	0.08	0.23	−0.16	1
Si	−0.05	0.16	−0.16	0.27	−0.15	−0.02	0.36	0.15	−0.26	0.22	0.04	0.08	1

).

Chalcocite EPMA analysis was conducted on 17 points (Supplementary Table S3). Chalcocite contains an average of 75.6 wt.% copper and 22.2 wt.% sulfur. The concentrations of other trace elements in chalcocite are as follows: zinc up to 0.58 wt.%, Pb up to 0.43 wt.%, Ag up to 0.87 wt.%, Sb up to 0.02 wt.%, Sn up to 0.04 wt.% and Ni up to 0.01 wt.%. The strongest elemental correlations in Northeast Saveh chalcocite are observed between cobalt and nickel, nickel and copper, zinc and tin, silver and tin, antimony and cobalt, as well as sulfur with lead and cobalt (

Table 3. Pearson’s correlation matrix for elements measured in Northeast Saveh Chalcocite.

	Fe	Mn	Co	Ni	Cu	Zn	As	S	Pb	Ag	Sb	Sn	Si
Fe	1
Mn	0.49	1
Co	−0.17	0.34	1						Low correlation		High correlation
Ni	−0.21	−0.86	−0.72	1
Cu	−0.09	−0.29	−0.47	0.65	1
Zn	0.41	0.09	−0.30	0.62	0.13	1
As	0.35	0.41	−0.22	−0.62	−0.02	−0.64	1
S	−0.40	0	0.46	−0.53	−0.86	−0.32	−0.08	1
Pb	−0.60	−0.31	−0.51	0.23	0.27	−0.51	−0.30	0.58	1
Ag	−0.31	−0.51	0.32	−0.03	−0.11	−0.05	−0.40	0.18	−0.08	1
Sb	0.34	0.2	0.83	−0.03	−0.40	0.68	0.25	−0.53	−0.70	−0.18	1
Sn	0.04	−0.45	−0.27	0.61	0.41	0.63	−0.37	−0.44	−0.67	0.66	0.14	1
Si	0.53	0.16	−0.67	0.32	−0.07	0.23	0.41	−0.23	0.29	−0.18	0.25	−0.21	1

).

Overall, the distribution of trace elements in copper sulfides (chalcopyrite and chalcocite) does not show significant differences, with only minor variations. Zn, Pb, and Ag concentrations are slightly higher in chalcocite, whereas Sb, As, Co, and Ni are more enriched in chalcopyrite, suggesting that these elements preferentially incorporate into the chalcopyrite structure.

5.3. Mineral Chemistry of Magnetite and Ti-Magnetite

EMPA analyses were carried out on 44 points from primary Ti-magnetite grains within the intrusive units, including monzonite, monzodiorite, and gabbro (Supplementary Table S4), and on 44 points from hydrothermal magnetite associated with mineralized veins (Supplementary Table S5). The chemical composition and inter-element correlations of magnetite and Ti-magnetite indicate distinct geochemical signatures for intrusive rock and mineralized veins, respectively. The analytical results reveal that, in the intrusive rocks, Fe₂O₃ and TiO₂ contents range between 60 and 80 wt.% and 0–16.58 wt.%, respectively. In contrast, magnetite from the mineralized veins shows markedly higher Fe₂O₃ concentrations (80.6–91.4 wt.%) and significantly lower TiO₂ contents (0–0.12 wt.%) (Figure 10, Figure 11, Figure 12 and Figure 13).

In the intrusive bodies, Fe₂O₃ correlates positively with Cr₂O₃ and V₂O₃ (

Table 4. Pearson Product correlation coefficient matrix for elements measured in magnetites and Ti-magnetite s in the Northeast Saveh intrusive rocks.

	MgO	Al2O3	TiO2	Cr2O3	FeO	V2O3	MnO	ZnO	SiO2	CaO	P2O5
MgO	1
Al2O3	0.08	1
TiO2	−0.19	−0.07	1				Low correlation		High correlation
Cr2O3	0.24	−0.16	−0.14	1
FeO	0.09	−0.76	−0.30	0.22	1
V2O3	0.07	−0.08	−0.05	0.83	0.06	1
MnO	0.31	−0.34	−0.47	0.37	0.42	0.26	1
ZnO	−0.10	0.32	−0.14	−0.07	−0.26	0.15	0	1
SiO2	0.47	0.65	−0.08	0.05	−0.50	−0.04	0.2	−0.22	1
CaO	0.52	0.04	−0.13	0.28	0.09	0.1	0.29	−0.34	0.54	1
P2O5	0.02	0.77	−0.05	−0.24	−0.83	−0.11	−0.31	−0.56	0.8	0.24	1

), consistent with co-enrichment during magmatic crystallization under relatively oxidizing conditions. In hydrothermal magnetite, Fe₂O₃ correlates most strongly with MnO (

Table 5. Pearson Product correlation coefficient matrix for elements measured in magnetites and Ti-magnetite s in mineralized veins.

	MgO	Al2O3	TiO2	Cr2O3	FeO	V2O3	MnO	ZnO	SiO2	CaO
MgO	1
Al2O3	−0.39	1
TiO2	−0.60	0.71	1			Low correlation			High correlation
Cr2O3	0.29	0.16	0.1	1
FeO	−0.11	−0.21	−0.32	0.33	1
V2O3	−0.46	0.51	0.57	0.49	0.32	1
MnO	−0.18	0.15	0.3	−0.37	−0.38	−0.21	1
ZnO	−0.58	0.27	0.61	−0.45	−0.35	0.02	0.44	1
SiO2	0.56	−0.44	−0.62	−0.34	−0.50	−0.68	0.02	−0.30	1
CaO	−0.03	−0.30	−0.33	−0.26	0.32	−0.21	−0.12	−0.16	0.03	1

). However, unlike the magmatic samples, trace elements such as Cr and V in the hydrothermal magnetite frequently approach or fall below the EPMA detection limits. Consequently, these elements do not exhibit statistically reliable correlation patterns in the mineralized veins, reflecting a significantly depleted signature compared to the intrusive oxides. This shift in chemical behavior may reflect either partial re-equilibration of the original magmatic magnetite by hydrothermal fluids or direct precipitation from compositionally distinct, evolved solutions.

TiO₂ in the magmatic Ti-bearing magnetite correlates strongly with Al₂O₃, V₂O₃, and MnO, consistent with coupled high-temperature substitution mechanisms in the spinel lattice (e.g., incorporation of Ti4+ accompanied by trivalent cations such as Al3+ and V3+ to maintain charge balance) under magmatic crystallization conditions. By contrast, magnetite precipitated from hydrothermal fluids does not display the same Ti–Al–V coupling. This difference most likely reflects contrasting physico-chemical environments: hydrothermal formation at lower temperatures, different Al and V activities in the fluid, and kinetic limitations on cation diffusion. Therefore, the strong Ti–Al–V correlations observed in the intrusive samples are interpreted as primary magmatic signatures, whereas the low and scattered Ti contents in hydrothermal samples are controlled by different substitution pathways or secondary redistribution.

Due to the inability of EPMA to distinguish iron oxidation states, the concentrations of FeO and Fe₂O₃ in magnetite and Ti-bearing magnetite were calculated using the stoichiometric method of Zhao et al. (1999) [21], based on the assumption of 3 cations and 4 oxygens per formula unit. Variation diagrams of major oxides versus TiO₂ and FeO were constructed for Ti-bearing magnetite from the intrusive bodies (Figure 10) and for magnetite from oxide–sulfide mineralized veins (Figure 11). As illustrated in Figure 10, Al₂O₃, Cr₂O₃, and V₂O₃ contents in the intrusive Ti-bearing magnetite increase systematically with rising TiO₂ content, suggesting co-enrichment of these elements during fractional crystallization. Conversely, FeO and MgO display inverse trends, decreasing with increasing TiO₂.

Regarding the magnetite grains from the mineralized veins (Figure 11), TiO₂, Al₂O₃, and Cr₂O₃ concentrations remain consistently low, often clustering near the detection limit. While MnO and MgO exhibit some variability, the extremely low abundance of Cr and V in these samples precludes the identification of robust geochemical trends. With increasing Fe content, Al₂O₃ and V₂O₃ show scattered distributions rather than systematic correlations. These features collectively highlight a clear geochemical break between the magmatic and hydrothermal domains in the Northeast Saveh system.

6. Discussion

6.1. Pyrite Trace Element Behavior and Implications for Ore Genesis

Pyrite can incorporate significant amounts of copper [22], and, in the Northeast Saveh pyrites, Cu reaches up to ~0.95 wt.% in some samples. Copper in pyrite can occur either as true structural substitution or as fine-scale inclusions of Cu-bearing minerals (e.g., chalcopyrite). Cu substitution in the pyrite lattice is principally accommodated by the similarity in oxidation state and ionic radius between Cu²⁺ and Fe²⁺ in octahedral coordination (Cu²⁺ ≈ 0.73 Å; Fe²⁺ ≈ 0.78 Å), allowing limited substitution of Cu²⁺ for Fe²⁺ on the cation site. Copper does not substitute for sulfur (S²⁻) on the anion site because of the large difference in charge and coordination chemistry. This capacity of pyrite to sequester Cu implies that pyrite may act as a significant transient sink for copper during mineralization and supergene overprint.

Lead is unlikely to enter the pyrite lattice except as Pb²⁺, due to its large ionic radius (~1.8 Å). Galena (PbS) precipitates from solution more rapidly than FeS [23,24], so galena typically forms early and, when present, is usually found as inclusions within pyrite [25]. However, no galena inclusions were observed in the Northeast Saveh pyrites. Pyrite can also trap certain chalcophile elements such as Ag, Sb and Sn during remobilization; in the Northeast Saveh samples, the maximum concentrations of these elements are low (≈500, 200 and 20 ppm, respectively).

The Co–Ni–As element suite in pyrite has long been recognized as a robust genetic indicator that reflects the geological setting and evolution of an ore system [26,27].

Plotting the Northeast Saveh pyrite data on Co–Ni–As discrimination diagrams (Figure 14; after [27,28]) reveals a predominant shift toward the Co-enriched endmember, consistent with magmatic–hydrothermal to epithermal affinities (Figure 14A). This pattern suggests a dominant magmatic water source during the early stages of mineralization, with progressive involvement of meteoric waters during later stages.

The degree of S → As substitution in pyrite is temperature-sensitive and generally increases at lower temperatures [29], so elevated As contents are commonly associated with cooler, more evolved fluids.

Nickel commonly concentrates in early, high-temperature phases formed during magmatic differentiation. As differentiation proceeds, Ni tends to decline while Co can become relatively enriched. Thus, the systematic shift in our samples toward higher Co/Ni ratios is compatible with cooling and evolution of the ore fluids. In addition, the relative abundance of As in pyrite is strongly influenced by the relative contributions of magmatic versus meteoric waters. Magmatic-dominated fluids tend to produce Co-rich pyrite compositions, whereas increasing meteoric-water input, typically associated with system cooling and fluid dilution, drives the signature toward higher As contents [27]. The Northeast Saveh pyrite data (Figure 14A) are therefore best interpreted as recording an evolution from an early magmatic-water dominated system to later stages with increased meteoric-water input.

Mixing of mineralizing fluids with high-fO₂ meteoric waters can also promote oxidation of arsenic species (As (0) → As³⁺/As⁵⁺, driving arsenic out of the pyrite lattice and into the fluid phase as oxide or oxyanion species (e.g., As₂O₃, As₂O₅). This process can reduce As concentrations in primary pyrite while increasing the mobility and redistribution of As within the evolving system, potentially causing As enrichment in other parts of the hydrothermal column or in secondary phases [30,31].

Complementary evidence from the magnetite chemistry (see Section 5.3) supports elevated fO₂ in the hydrothermal veins and a cooling trend during system evolution. Classic vertical zoning observed in epithermal systems—where the upper levels are enriched in As, Sb, Hg, Ba, and Ag, the mid-levels in Cu, Pb, Zn and Bi, and the deep levels in Co, Ni, Ti and Cr [32]—provides a useful framework for interpreting the Northeast Saveh pyrite data. As illustrated in Figure 14B, most Northeast Saveh pyrite samples plot within the middle to deeper sector of this zoning model, consistent with a predominantly mid-system hydrothermal character with some contribution from deeper, higher-temperature inputs.

The trace-element systematics of pyrite from Northeast Saveh reflect a multistage hydrothermal evolution, with an early, magmatic-fluid dominated stage (Co-rich signature) followed by progressive cooling and admixture of meteoric waters (increasing As signatures and oxidation).

Figure 14. (A) Comparison of pyrite composition in metamorphic, Carlin, and magmatic–hydrothermal or epithermal deposits on the Co-Ni-As ternary diagram (modified from [27] and the position of the measured points of Northeast Saveh pyrites on it. (B) Ternary diagram of Cu + Pb + Zn, Co + Cr, and As + Sb + Ag based on [32] and the position of the measured points of Northeast Saveh pyrites on it.

Figure 14. (A) Comparison of pyrite composition in metamorphic, Carlin, and magmatic–hydrothermal or epithermal deposits on the Co-Ni-As ternary diagram (modified from [27] and the position of the measured points of Northeast Saveh pyrites on it. (B) Ternary diagram of Cu + Pb + Zn, Co + Cr, and As + Sb + Ag based on [32] and the position of the measured points of Northeast Saveh pyrites on it.

Nickel readily substitutes into the pyrite crystal structure and, once incorporated, is relatively resistant to removal during later recrystallization [4,23,24,33]. Thus, pyrite Ni contents may record information about the source of the mineralizing fluid. Mantle- and mafic–ultramafic-related lithologies are typically Ni-rich, whereas felsic units have low Ni contents [25,34]. The Ni contents measured in the studied pyrites (up to ~0.34 wt.%) exceed typical continental-crust Ni concentrations (≈19–60 µg g⁻¹; [34]), suggesting a contribution from mafic–ultramafic or upper-mantle sources. Previous work in the region [17,18] using trace-element ratios (e.g., Ta, Nb, La) indicates a dominant mantle (spinel-lherzolitic) source modified by upper-crustal contamination, and Sr–Nd isotope ratios also point to a significant role for mantle metasomatism in the genesis of the intrusions [35,36].

Reported solubility limits suggest Ni can be accommodated in pyrite up to ~10 mol% NiS₂, whereas Co may achieve complete miscibility with pyrite at temperatures above ~700 °C [36]. This temperature dependence makes pyrite Co content a useful geothermometer [37]. Consequently, the Co/Ni ratio in pyrite is widely applied as a proxy for ore-forming conditions [25,38,39].

Empirically, low Co/Ni ratios (<1) characterize sedimentary/diagenetic pyrite [38], diagenetic pyrites commonly also show low Co/Ni (≤2), whereas hydrothermal pyrites typically display higher Co/Ni [4,40]. Co/Ni ratios between ~1 and 10 are diagnostic of magmatic–hydrothermal deposits [41,42]. The Co/Ni distribution for Northeast Saveh pyrites predominantly falls between 1 and 10 (Figure 15), consistent with a hydrothermal origin.

Given the similar geochemical behavior of Co and Ni, nickel solubility in hydrothermal fluids is primarily controlled by temperature [26]. Cooling of the hydrothermal fluid, therefore, tends to decrease the solubility of these trace metals and promotes their precipitation as discrete phases or their incorporation into sulfides (e.g., pyrite). Co and Ni are generally incorporated into the pyrite lattice by direct substitution for Fe²⁺. In contrast, As incorporation occurs through charge-balanced, coupled-substitution mechanisms rather than simple one-for-one replacement. Specifically, As³⁺ may occupy Fe²⁺ sites with charge compensation achieved by Fe vacancies or complementary anion/cation substitutions, while reduced As¹⁻ may substitute for S²⁻ with local charge balance adjustments [3,43,44]. Accordingly, Co, Ni, and As are likely present as solid-solution components in pyrites. Because of its relatively large ionic radius, although Pb commonly occurs as Pb²⁺ in aqueous solution, Pb is not readily accommodated as a point defect in the pyrite lattice; minor Pb in pyrite samples most likely reflects nanoscale galena or other Pb-bearing inclusions, or surface/secondary Pb phases, rather than true lattice substitution [45,46].

Hydrothermal pyrites with elevated Co/Ni ratios have been reported from a range of magmatic–hydrothermal deposits worldwide (for example, vein-type Ag–Pb–Zn deposits, [47]; IOCG systems, [48]; skarn Au–Cu deposits, [49]; and Pb–Zn skarns, [50], and are commonly attributed to intense fluid–rock interaction. On the basis of the evidence summarized above, pyrites from Northeast Saveh appear to be dominantly hydrothermal in origin and to have formed through interaction between magmatic–hydrothermal fluids and the plutonic host rocks.

6.2. Chalcopyrite-Chalcocite Trace Element Behavior and Implications for Ore Genesis

The evidence provided by the major and trace elements and textures confirms a fluid-driven replacement history in which primary chalcopyrite was the dominant magmatic–hydrothermal Cu reservoir that was subsequently dissolved and reprecipitated as Cu-rich sulfides along sharp reaction fronts (Figure 3 and Figure 5). The major-element trends in chalcopyrite (Figure 16A; ≈33–35 wt.% Cu, 28–30 wt.% Fe, 33–35 wt.% S) versus chalcocite (≈74–77 wt.% Cu, 3–4 wt.% Fe, 22–25 wt.% S) demonstrate a classic Fe-depletion/Cu-enrichment trajectory consistent with fluid-controlled dissolution–reprecipitation [38,51]. The near-quantitative loss of Fe from the lattice and simultaneous increase in Cu/S ratio reflect interface-coupled dissolution of chalcopyrite and local precipitation of Cu-rich sulfides at the reaction front, rather than solid-state diffusion. This replacement style is supported by thin Cct/Cv rinds, fracture-controlled infills, and partial pseudomorphs, all pointing to an open, fluid-dominated system with sharp disequilibrium boundaries.

Trace-element patterns refine this genetic scenario (Figure 16B and Figure 17). Chalcopyrite shows moderate Pb, Ag, Sb and Zn, with Pb having the broadest variability; chalcocite displays marked enrichment in Zn and Ag and retains variable Pb. This implies two complementary processes: (1) coupled lattice substitution (Pb–Bi, Sb–As, Sn–In type pairings) that reflects formation conditions and crystal-chemical accommodation [8,52], and (2) inclusion-entrainment or co-precipitation where micron/nano inclusions of sphalerite, bornite-derived phases or galena are reworked during replacement and become incorporated into secondary chalcocite [53,54]. The pronounced Zn signal in chalcocite, in particular, is best explained by remobilization of wall-rock or precursor sphalerite-derived Zn and local co-precipitation with chalcocite [8].

Redox, pH and temperature exert primary control on sequence and kinetics. Experimental and field work indicate that covellite commonly forms first, with chalcocite/digenite developing rapidly at intermediate temperatures (≈200–300 °C) or after a longer induction period at lower T (<~200 °C) [51]. However, the microtextural evidence in the Northeast Saveh samples indicates a predominantly supergene origin for the observed low-Fe copper sulfides. Specifically, rim-type replacement textures (thin chalcocite/covellite rinds around chalcopyrite and bornite), fracture-controlled infillings, partial pseudomorphism, and the close spatial association with oxidation textures and supergene oxides/carbonates point to formation during oxidative weathering and downward Cu transport at the redox front (Figure 3 and Figure 5). Microscopically, these processes produce sharp chemical zoning at grain boundaries, inclusion-bearing textures and occasional overgrowths, as documented in BSE imagery and EPMA spot analyses (Figure 5).

Sulfidic pulses in episodic, Cu-bearing near-surface fluids (or evolving ligand chemistry, e.g., chloride → sulfide complexes) would have promoted rapid local precipitation of low-Fe copper sulfides, while Fe was likely transported predominantly as Fe³⁺ (and possibly Fe²⁺) species in the H₂O-rich, moderately oxidizing fluids [37].

While hydrothermal precipitation of chalcocite/covellite from deeper, high-temperature fluids is thermodynamically and experimentally possible, such a hydrothermal origin is not supported by the paragenetic, textural and field relations in our study area. We therefore interpret the dominant occurrences of chalcocite and covellite in the studied outcrops as products of supergene enrichment and oxidation-zone processes, while noting that rare hydrothermal occurrences cannot be fully excluded without targeted fluid-inclusion or isotopic analyses.

High-T conditions (>~500–600 °C) increase the capacity of chalcopyrite to host Ga, In, Sn, Zn, Mo and noble metals [8]. Studies demonstrate strong positive correlations between temperature and trace uptake (e.g., Zn solubility rises with T; [55,56]. Reported Au contents in chalcopyrite from porphyry systems decrease markedly with decreasing formation temperature [56], and it can be said that the lack of Au observation in the chalcopyrite s in northeastern Saveh is due to the decrease in system temperature and the onset of the epithermal system e.g., [19,57].

In many porphyry/epithermal systems, observed positive correlations (e.g., Pb–Bi, Pb–Mo, Zn–Cd, Sn–In, Sb–As, Sb–Ge) imply paired or linked substitutions and/or co-entrainment in inclusions, consistent with reports of Au, Ag, As, Te, In, Zn, Cd, Ge and Ga entering chalcopyrite via coupled mechanisms [52,58].

Cobalt and nickel are typically associated with fluids derived from mafic magmas or leached from rocks with a high background content of these elements [2,22,36]. The Co/Ni ratio in chalcopyrite from porphyry, skarn, and epithermal hydrothermal deposits is typically greater than 1, whereas in Cu-Ni sulfide deposits, it is less than 1 [59]. The Co/Ni ratio in Northeast Saveh chalcopyrites is predominantly above 1, aligning with the characteristics of chalcopyrite in hydrothermal deposits. The general immobility of Co–Ni during chalcopyrite → chalcocite replacement implies the replacement front selectively partitioned Fe and Cu while leaving Co–Ni distributions largely intact.

6.3. Magnetite-Ti-Magnetite Trace Element Behavior and Implications for Ore Genesis

The composition of Ti-magnetite s in the intrusive bodies and hydrothermal veins of Northeast Saveh lies below the magnetite-ulvöspinel join, with a tendency toward the wüstite (FeO) component (Figure 18). Martitized hematite observed in thin sections indicates that martitization of magnetite occurred during the final stages of oxidation, accompanied by decreasing temperature and increasing oxygen fugacity [60].

With increasing temperature, the Ti-magnetite solid solution separates into ulvöspinel and magnetite, with magnetite acting as the host for ulvöspinel lamellae along (111) planes, forming a Widmanstätten texture [60]. This texture is characterized by a distinctive trellis-like or geometric pattern of exsolution lamellae. It typically develops as thin, needle-like or plate-shaped crystals of a secondary mineral (e.g., ilmenite) oriented along the {111} octahedral planes of the host magnetite. Widmanstätten patterns are indicative of slow cooling rates from high magmatic temperatures, allowing sufficient time for solid-state diffusion and the separation of mineral phases as the solid solution becomes unstable. Subsequently, ilmenite may form as ulvöspinel becomes unstable during cooling/oxidation. Two relevant pathways are: internal oxidation of ulvöspinel yielding magnetite plus ilmenite:

3 Fe₂TiO₄ + 1/2 O₂ → Fe₃O₄ + 3 FeTiO₃

(or equivalently 6 Fe₂TiO₄ + O₂ → 2 Fe₃O₄ + 6 FeTiO₃), and reaction of ulvöspinel with available TiO₂ to produce ilmenite:

Fe₂TiO₄ + TiO₂ → 2 FeTiO₃.

The first reaction corresponds to oxidation-driven redistribution of Ti and conversion of Ti-magnetite/ulvöspinel to magnetite + ilmenite; the second requires an external TiO₂ phase (e.g., rutile or exsolved Ti-oxide). If rutile is present in samples (or TiO₂ is liberated by exsolution/dissolution), reaction (2) will promote ilmenite formation at lower temperatures.

These reactions indicate that the initial stage involved a Ti-magnetite solid solution, which, with increasing oxygen fugacity and decreasing temperature, led to the formation of ilmenite lamellae within Ti-magnetite. Thin ilmenite lamellae represent the instability of Ti-magnetite, whereas thicker lamellae result from internal exsolution after Ti-magnetite formation [61].

The degree of martitization varies among Ti-magnetite s in Northeast Saveh. Under high oxygen fugacity, strongly martitized crystals are occasionally observed. The presence of hematite lamellae within some ilmenites likely formed during the final oxidation stage and cooling of the mineralization system, with ilmenite decomposing according to:

FeTiO₃ + O₂ = 4TiO₂ + 2Fe₂O₃

Petrographic observations indicate disruption of the cubic structure of Ti-magnetite. Thin ilmenite lamellae appear to form during early oxidation of Ti-magnetite along planes enriched in Ti, whereas thicker lamellae developed gradually along the (111) and (100) planes due to progressive oxidation of ulvöspinel lamellae coupled with Ti enrichment along these planes. As noted by [62], thick ilmenite lamellae form under advanced oxidation conditions, while thin lamellae develop during initial oxidation stages.

Figure 18. FeO–Fe₂O₃–TiO₂ ternary diagram [63], showing the locations of EPMA analysis results of magnetite and Ti-magnetite from intrusive rocks and hydrothermal veins.

Figure 18. FeO–Fe₂O₃–TiO₂ ternary diagram [63], showing the locations of EPMA analysis results of magnetite and Ti-magnetite from intrusive rocks and hydrothermal veins.

6.3.1. Origin and Compositional Evolution of Ti-Magnetite–Magnetite

The chemistry of Fe–Ti oxides is controlled not only by temperature and oxygen fugacity but also by the compositions of the parental magmas and interacting hydrothermal fluids, and by the wall-rock chemistry that is leached during fluid–rock interaction [12,13,64]. Because cation mobility (e.g., Fe²⁺ and minor substituents) is limited during moderate-temperature martitization, hematite formed by oxidation commonly preserves much of the primary magnetite signature; however, dissolution of coexisting silicates can enrich hydrothermal fluids in elements such as Mg, Al and Ti, producing hematite that is locally enriched in those added elements compared with primary magnetite [12,64].

Oxidative overprinting and partial replacement of Fe–Ti oxides are widespread. Oxidation of magnetite to hematite—commonly expressed as martitization textures along grain margins and cleavage surfaces—is attributed to Fe²⁺ leaching under acidic, oxidizing conditions. This process may be represented stoichiometrically as, for example:

4 Fe₃O₄ + O₂ → 6 Fe₂O₃,

and has been documented in comparable systems [65]. Such martitization is commonly observed along crystal boundaries and in alteration halos around oxide grains. Coexistence and textural relations between iron and titanium oxides—along with partial or complete replacement of magnetite and Ti-magnetite by hematite—suggest formation and subsequent modification under non-equilibrium thermodynamic conditions, with variable oxygen fugacity (fO₂) and temperature. The observed replacement textures are consistent with an increase in fO₂ during later stages, attributable to supergene weathering and/or late hydrothermal alteration [66,67,68]. Evidence of martitization within partly subhedral hematite crystals from Northeast Saveh further supports a hydrothermal-oxidative overprint on primary magnetite.

Comparison of intrusive Ti-magnetites and hydrothermal magnetites from the mineralized veins in Northeast Saveh demonstrates element redistribution during oxide phase transformations (Figure 19). Ti-magnetites from the intrusive bodies are enriched in Ti, Al and V, whereas magnetites from the silica-veins are depleted in those elements and show higher Fe contents—consistent with crystallization from a more mafic host for the intrusive and with later oxidation/repartitioning at elevated fO₂. Measured V₂O₅ in intrusive Ti-magnetite s ranges from ~0.016 to 1.28 wt.% (mean ≈ 0.88 wt.%), whereas V₂O₅ in vein magnetites ranges from ~0.012 to 0.39 wt.% (mean ≈ 0.12 wt.%)—confirming strong V and Cr enrichment in magmatic oxides relative to hydrothermal counterparts [12].

Vanadium partitioning is highly fO₂-sensitive: at low fO₂, V³⁺ (ionic radius ≈ 64 ppm) substitutes readily for Fe³⁺ (≈65 ppm), whereas at higher fO₂, V is oxidized to V⁵⁺ and becomes incompatible with the oxide lattice [69,70,71]. Thus, V (and Cr) concentrations in magnetite can record both the magmatic/hydrothermal redox path and fluid–rock reactions during magnetite growth [72,73].

Titanium incorporation into magnetite is likewise temperature-dependent. Higher crystallization temperatures favor greater Ti uptake into the Ti-magnetite solid solution, whereas cooling (and/or delayed increases in fO₂) promotes Ti exsolution (ilmenite/rutile) as lamellae or rims [74,75,76]. Petrographically, Northeast Saveh Ti-magnetite s display ilmenite lamellae of variable thickness (thin lamellae from early oxidation; thicker lamellae from exsolution), and martitization textures are common where repeated hydrothermal fluid pulses have leached Fe²⁺ and driven hematite replacement [61,62,77].

Chemical re-equilibration of magnetite during multi-phase hydrothermal activity can alter Ti, Al, Mg, Mn, Ca and Si contents [14,78]. A V/Ti versus Fe bivariate plot effectively discriminates magmatic Ti-magnetite s from hydrothermally re-equilibrated magnetites [76]. When our mineral data are plotted on this diagram, intrusive samples fall in the magmatic field while vein samples plot in the hydrothermal/re-equilibrated field (Figure 19), indicating the strong influence of hydrothermal fluids on oxide chemistry and the occurrence of repeated fluid release and Fe-leaching during system evolution.

Figure 19. V (wt.%)/Ti (wt.%) versus Fe (wt.%) diagram to distinguish magmatic, hydrothermal and re-equilibrated magnetites [75]. (A) Samples related to intrusive bodies. (B) Samples related to mineralized veins.

Figure 19. V (wt.%)/Ti (wt.%) versus Fe (wt.%) diagram to distinguish magmatic, hydrothermal and re-equilibrated magnetites [75]. (A) Samples related to intrusive bodies. (B) Samples related to mineralized veins.

6.3.2. Physicochemical Drivers of Magnetite Compositional Re-Equilibration During Ore Formation

These compositional differences clearly reflect the contrasting physicochemical conditions between magmatic and hydrothermal systems, particularly the relative oxidation state and temperature during mineral formation. To constrain the formation temperatures of Ti-magnetite –magnetite in Northeast Saveh, we plotted the element data on bivariate diagrams. An Al + Mn versus Ti + V (wt.%) plot (Figure 20) indicates that Ti-magnetite s from the intrusive bodies record high crystallization temperatures, whereas magnetites from silica-rich mineralized veins formed at intermediate temperatures (≈200–300 °C), consistent with fluid-inclusion constraints from quartz in the veins [19]. This trend is interpreted as cooling of the system coupled with progressive input of meteoric/oxidizing fluids during hydrothermal activity and subsequent magnetite precipitation at lower temperatures.

A Ti (wt.%) vs. Mg + Al + Si (wt.%) diagram (after [74]; Figure 21A) further supports extensive fluid–rock interaction during vein magnetite formation. Likewise, plotting Ti (wt.%) versus V (wt.%) effectively discriminates magmatic from hydrothermal magnetites [12]; Northeast Saveh samples separate clearly into magmatic (intrusive) and hydrothermal (vein) groups on this diagram (Figure 21B).

Vanadium and Ti distributions record redox and thermal evolution (Figure 22). Intrusive Ti-magnetites are relatively enriched in V and Ti compared with vein magnetites, consistent with crystallization from a lower-fO₂, higher-T magmatic environment [79,80]. Because V speciation and partitioning are highly oxygen-sensitive [69,81], the observed decrease in V and Cr from intrusive to vein magnetites indicates oxidation and element redistribution during hydrothermal re-equilibration. Plotting V (or V₂O₃) variations suggests a progressive increase in fluid fO₂ from the parental magma toward the mineralized veins, accompanied by cooling and crustal contamination, evidence for intense fluid–rock interaction during ore formation [82].

In sum, oxide geochemistry (Ti, V, Al, Mn) and mineral textures in Northeast Saveh record a high-T, low-fO₂ magmatic origin for intrusive Ti-magnetite s, followed by hydrothermal overprint at intermediate temperatures and higher fO₂ during vein magnetite formation. Multi-stage fluid release and Fe-leaching during hydrothermal events appear to have driven the observed compositional re-equilibration (Figure 11, Figure 12, Figure 13 and Figure 14); [12,73,79].

7. Conclusions

This study integrates geology, back-scattered imaging and electron-microprobe analyses to constrain the origin and evolution of Cu–Fe sulfide–oxide mineralization in the Northeast Saveh district.

Mineralization is structurally controlled by fault corridors (N–S and N25°E–N50°W trends) with mineralized zones from tens of meters to ~1 km and Cu grades of ~0.1–5 wt.%. Combined oxide and sulfide geochemistry provides practical discriminants to separate magmatic vs. hydrothermal signatures and to vector toward higher-grade Cu domains.

Ti-magnetites in the shallow monzodiorite intrusions record a high-temperature, low-fO₂ magmatic signature (Ti, V, Al enrichment), whereas magnetites from silica-rich veins formed at lower, intermediate temperatures (~200–300 °C) and under higher fO₂. EPMA data plot the two groups in separate magmatic vs. hydrothermal fields, documenting oxide re-equilibration during hydrothermal overprint. Measured Fe₂O₃ and TiO₂ contents differ markedly between intrusive and vein oxides, reflecting crystallization from different parental fluids and subsequent element redistribution during oxidation and fluid–rock interaction.

Pyrite trace-element systematics (notably Co–Ni–As and dominant Co/Ni values ∼1–10) point to a principally magmatic–hydrothermal source with progressive cooling and increasing meteoric-water input. Pyrite chemistry thus records changes in temperature, ligand chemistry and redox during mineralization.

Chalcopyrite is the primary Cu-host. Replacement textures and micro-textures document a multi-stage evolution: early magmatic–hydrothermal chalcopyrite ± bornite, later hydrothermal reworking and sulfide replacement (covellite → chalcocite/digenite depending on T and fluid composition), and a pronounced supergene stage producing chalcocite, covellite and oxide-hydroxides (malachite, azurite, goethite).

The abundance and mode of incorporation of trace elements in chalcopyrite (solid solution vs. inclusions), and partitioning in Fe–Ti oxides (V, Cr, Ti) are controlled by temperature, fO₂/fS₂, fluid composition and competition among co-precipitating sulfides. Repeated hydrothermal pulses and Fe-leaching explain observed martitization and hematite replacement textures.

The Northeast Saveh Epithermal system is dominantly magmatic–hydrothermal in origin (early Fe–Ti oxides and primary sulfides), subsequently overprinted by lower-temperature hydrothermal re-equilibration and extensive supergene enrichment/oxidation that redistributed Cu and trace metals.