The Paleoproterozoic Raimunda Porphyry-Type Gold Deposit, Juruena Mineral Province, Amazonian Craton (Brazil): Constraints Based on Petrological, Fluid Inclusion and Stable Isotope Data

Abstract

The Juruena Mineral Province is an emerging world-class mineral province in the southern Amazonian Craton, due to numerous Au-Cu and base metal deposits, such as the Raimunda deposit, related to Novo Mundo 2.03–1.98 Ga I-type calc-alkaline granites. Its hydrothermal alteration zones comprise Na-metasomatism, microclinization, propylitic and sericitic alteration, silicification, a sulfide stage, and late carbonate alteration. The disseminated mineralization, associated with the sulfide stage, the main mineralization stage, is represented by gold inclusions and fracture-filling grains in pyrite, chalcopyrite, and Cu-Bi sulfides. Chlorite geothermometer and fluid inclusion data indicate temperature conditions of 325–380 °C for the mineralization. The coexistence of high-temperature aqueous and aqueous-carbonic fluid inclusions, based on a microthermometric study of fluid inclusions, reveals a mixing of medium-saline hot fluids with cooler and low-saline fluid. The δ¹⁸O_fluid (3.11–7.86‰) and δ³⁴S_py data (−1.4–0.1‰) are coherent to the magmatic origin of the mineralizing fluid. Gold was initially transported as chlorine complexes in a hot, high-salinity, acidic, and oxidized fluid from the magma chamber, and later as H₂S⁻ complexes. The chemical-physical instability during fluid ascent is interpreted as a triggering factor for ore precipitation. The results offer valuable insights into the genesis of porphyry–Au deposits and their implications for prospecting in the Amazonian Craton.

1. Introduction

The Amazonian Craton (Brazil) has undergone a series of calc-alkaline granitogenic events over its geological history, some of which led to gold and other metal mineralization [1]. The Tapajós and Juruena Mineral provinces are the major metallogenic provinces in Brazil, located in the southern Amazonian Craton. The Juruena Province, especially, has been an important gold producer in the country. Its main mineralization is predominantly spatially and temporally related to Paleoproterozoic calc-alkaline granites formed in volcanic-arc settings [2,3,4,5].

In the Juruena Mineral Province, mainly in the surroundings of the cities Nova Santa Helena, Peixoto de Azevedo, Matupá, Guarantã do Norte, and Novo Mundo, primary gold mineralization associated mainly with granite systems and, subordinately, with volcanic sequences occur either disseminated or hosted into veins and stockwork-veins [5,6].

The emplacement of these bodies into continental magmatic arcs has been essential for forming disseminated, vein-type, and Au ± Cu ± base metals stockwork-type magmatic-hydrothermal systems. The systematic and integrative study of these orebodies points out to a magmatic-hydrothermal model comparable to that of porphyry gold deposits [7], low-, intermediate-, and high-sulfidation epithermal systems [8], and structurally controlled [9] placed at variable crustal levels.

The Novo Mundo granite complex, which hosts the Raimunda deposit, the main object of this study, is in the homonymous municipality in the mid-northern region of the state of Mato Grosso, east of the Juruena Mineral Province. This granite body hosts several mineral deposits, including the Raimunda deposit, located in its northwestern region.

The percolation of hydrothermal fluids promotes anomalous and systematic mineralogical and geochemical changes in the host rocks, the extent and magnitude of which depend on their alteration state, porosity, permeability, and degree of chemical reactivity of the host rocks. The fluid-rock interaction generates alteration footprints that may assist in determining the exploratory guides in mature terrains [10].

Despite recent advances in knowledge about Proterozoic mineralizing events in the Amazonian Craton, prospecting complex Paleoproterozoic gold-copper deposits in Juruena Mineral province is a challenge, especially due to the few studies that have investigated the origin of mineralizing fluids using stable isotopic data to constrain the genesis of mineral deposits in the southern region of the Amazonian Craton.

This paper integrates new petrological (i.e., drill core samples, whole-rock assay results), ore mineral chemistry (e.g., chlorite, muscovite, sulfides and gold), fluid inclusion, and stable isotope data (O and S) from a preserved Paleoproterozoic deposit, located in the southern Amazonian Craton, and proposes a metallogenetic model for the Raimunda deposit and other analogous deposits in Brazil and worldwide.

2. Geological Settings

The Amazonian Craton has been a tectonically stable area since the Grenvillian Orogeny (1.2 Ga). It has been understood for decades as the result of the successive amalgamation of accretionary arcs from the Archean to the Paleo- and Mesoproterozoic [11,12]. However, newer alternative concepts and models have been proposed based on new isotopic and geophysical data (Figure 1).

Based on the progressively younger igneous bodies striking NE, at 1.88 Ga, the central region of the Amazonian Craton underwent a period of flat-slab subduction, which caused the migration of arc-related units towards the Central Amazonian block [13]. The sequence related to the Paleoproterozoic Tapajós Province arc was developed on an Archean basement and the well-defined E-W-striking lineament, which extends from the Tapajós Province to the Carajás Province and represents the ancient cratonic basement [14]. The lack of fold belts and/or metamorphic terranes, as well as the orogenic and post-orogenic tectonics, led Juliani et al. [15] to suggest weakly extensional to slightly compressive settings for the region.

Magnetotelluric profiles carried out in the eastern region of the province found a high-resistivity anomaly approximately 20 km deep, which is ascribed to the Peixoto de Azevedo Domain, surrounded by conductive anomalies related to the Statherian magmatism and mafic enclaves [16]. Furthermore, this domain presents a controversial tectonic evolution linked to long-lived shear deformation comparable to that of the Tapajós Province [17].

In the southern segment of the Amazonian Craton, there are two extensive Paleoproterozoic domains: an older one, named Peixoto de Azevedo (2.05 to 1.98 Ga), and a younger one, named Juruena (1.85–1.75 Ga). The Peixoto de Azevedo Domain comprises the Cuiú-Cuiú Magmatic Arc and post-magmatic arc rocks, mainly composed of Paleo- to Mesoproterozoic volcano-plutonic sequences formed in arc settings [6,18,19].

The eastern segment of the Juruena Province is mainly composed of oxidized, medium- to high-K calc-alkaline, volcanic-plutonic, and meta- to peraluminous rocks. These rocks mostly belong to the magnetite series (I-type granites) and are volcanic, subvolcanic, and alkaline granites [20].

Figure 1. Geological and tectonic environment of the Juruena Mineral Province in the Amazonian Craton, based on [5,21]. Geochronological provinces based on [11].

Figure 1. Geological and tectonic environment of the Juruena Mineral Province in the Amazonian Craton, based on [5,21]. Geochronological provinces based on [11].

2.1. Geology of the Deposit

The exploration area of Novo Mundo Paleoproterozoic Complex exhibits surface coverage that contains dozens of mineral deposits that host Paleoproterozoic Au mineralization [22], Au-Cu-Mo [9], and Cu [23] along a NW-SE-striking lineament.

The Novo Mundo complex intrudes Cuiú-Cuiú Archean rocks (granitic- to tonalitic gneisses and migmatites) and shows U–Pb data between 2.03 and 1.98 Ga [24]. It has an approximately 60-km diameter and is composed mainly of granites, along W-NW, which is compatible with the preferred orientation of the major regional lineaments mapped from the oldest terranes in that region [3], although there is no evidence of structural controls [19].

The Raimunda deposit is associated with monzonite, biotite granodiorite, monzogranite and syenogranite facies. The last two are the main host rocks of the deposit. The granites are cut by dikes of basaltic–andesite and andesite compositions.

The overview of the deposit is shown in Figure 2A,B, with a detailed view of the open-pit constructed by artisanal miners in the late 1990s and early 2000s for ore extraction. Figure 2C–E show rare blocks of outcrops classified as syenogranite, monzogranite and mafic bodies, respectively, all found around the deposit. Contact between the granitic facies is rarely observed. In drill holes, subangular to rounded microgranular enclaves are locally registered, especially monzonite enclaves into syenogranite.

Although the study area is in a region with local NW-SE-striking ductile-brittle shear zones (Novo Mundo Shear Zone–NMSZ) [25], which host several gold-bearing mineral deposits in the eastern region of the Juruena Mineral Province [6] and display a high density of microfractures, the rocks of the Raimunda deposit have not undergone significant deformation. The strongly fractured and crushed appearance of the minerals results from the action of the hydrothermal system over those rocks. Centimeter-to-millimeter-thick veins and veinlets are mostly non-oriented and contain alteration minerals (e.g., quartz, chlorite, muscovite, epidote, calcite, pyrite and rutile).

2.1.1. Monzonite Facies

The monzonite is dark gray, medium-grained, inequigranular, and exhibits a phaneritic texture (Figure 3A). It contains andesine crystals (An_40–30 tending to An₂₅) (20–25%), orthoclase (18–20%), quartz (18–20%), biotite (3–5%), and hornblende (2–5%). Accessory minerals are magnetite (2%), titanite (1%), apatite 1%), and zircon (<0.1%), as well as hydrothermal minerals, such as epidote (1–2%), chlorite, muscovite, and carbonate (~5%) (Figure 4A).

2.1.2. Monzogranite Facies

The monzogranite is the more representative facies of the deposit. It has a greenish-gray color and is composed of quartz (25–30%), albite (15–20%), orthoclase (20–25%), and biotite (3–5%); accessory minerals (magnetite < 1%), and hydrothermal minerals (muscovite, chlorite, carbonate, epidote, and rutile, 12–15%) (Figure 3B). Microtextures show widespread veins and veinlets through the rock in association with potassic alteration (Figure 4B–D). Locally, open-space filling textures (“comb texture”) are also observed, especially proximal to or in sulfide-bearing strongly hydrothermalized zones. Quartz megacrysts (0.5–3 mm) are restricted to strongly silica-altered zones (Figure 4B).

2.1.3. Syenogranite Facies

The syenogranite is reddish to pinkish and, in places, displays a porphyritic texture with quartz megacrystals/phenocrysts (Figure 3C). It is composed of quartz (30–35%), orthoclase (20–25%), albite (15–20%), and biotite (1–2%). Accessory magnetite (<1%), apatite (<0.1%), and zircon (<0.1%) feature as inclusions in quartz, plagioclase, and potassium feldspar. Muscovite, carbonate, and epidote commonly replace feldspar and plagioclase (12–15%) (Figure 4F,G). These samples also contain quartz megacrystals (0.5–3.5 mm) in strongly silicified zones, which are similar to those of monzogranites.

2.1.4. Granodiorite Facies

The granodiorite is greyish, medium-grained, composed of quartz (25–30%), andesine (An_40–30 tending to An₂₅) (25–30%), orthoclase (15–20%), biotite (2–5%), and hornblende (1–3%) (Figure 3D). The accessory minerals are magnetite (1–3%), titanite (1%), apatite (<0.1%), and zircon (<0.1%); and the secondary minerals are muscovite, carbonate, epidote, and rutile (10–12%) (Figure 4E).

3. Materials and Methods

3.1. Fieldwork, Sampling, and Petrography

Three fieldwork campaigns were carried out for mapping and sampling. In total, 80 samples were collected for petrographic analysis to identify the main gangue and ore mineral associations of hydrothermal alteration and ore-bearing zones, as well as their patterns and styles [26,27]. This enables a better understanding of their evolution throughout time in the system.

The polished thin sections and polished sections were made at the thin-section laboratory of the Geosciences Institute of the University of Brasília (LABLAM-UnB). The doubly polished thin sections were made at the thin-section laboratory of the Petrografia-BR company (Contagem–MG). The petrographic descriptions were carried out at the Microscopy Laboratory of the UnB Geosciences Institute using an Olympus BX-50 petrographic microscope (Olympus Corporation, Tokyo, Japan), as well as at the UnB Fluid Inclusions Laboratory (LIF-UnB) using an Olympus BX-50 petrographic microscope.

3.2. Electron Microprobe

The mineral chemistry analyses were determined using a JEOL^® JXA-8230 Electron Probe Micro-analyzer (EPMA) (JEOL Ltd., Tokyo, Japan), at the Institute of Geosciences, University of Brasília. For silicates, the chemical analyses were obtained using an acceleration voltage of 15 kV, a beam current of 10 nA, and a nominal beam diameter of 1 μm in the spot mode. The counting time for each element during the analysis was 10 s on the peak and 5 s on the background. Interference correction was implemented for Ti and V. Matrix effects were corrected using an in-house ZAF program. The natural and synthetic calibration standards used were albite (Na), forsterite (Mg), orthoclase (Al, Si, K), topaz (F), vanadinite (Cl, V), apatite (P), andradite (Ca, Fe), pyrophanite (Ti, Mn), barite (Ba), Ni oxide (Ni), and vanadinite (V).

For the sulfides and gold, an accelerating voltage of 20 kV, a beam current of 20 nA, and a counting time of 10 s, except for selenium (25 s) were used; the counting time for backgrounds was always half of the used-on peaks values. The X-ray Kα-lines were used for sulfur, selenium, copper, iron, nickel, and cobalt; Lα for arsenic, silver, antimony, palladium, cadmium, tellurium, and molybdenum; and Mα for lead, mercury, gold, and platinum; Mα and Mβ were used for bismuth. The natural minerals applied as standards in both electron probes were pyrite (Fe, S), chalcopyrite (Cu), galena (Pb), and stibnite (Sb), and the native elements gold, silver, platinum, palladium, bismuth, copper, and antimony. The synthetic standards were gallium arsenide (As), zinc selenide (Zn, Se), and cinnabar (Hg). The following detection limits were obtained (in wt.%): 0.018 for gold, 0.012 for silver, 0.008 for palladium, 0.016 for platinum, 0.032 for bismuth, 0.025 for lead, 0.016 for copper, 0.013 for iron, 0.017 for mercury, 0.025 for arsenic, 0.009 for antimony, 0.017 for selenium, 0.005 for sulfur, 0.012 for cadmium, 0.013 for nickel and cobalt, 0.017 for tellurium, and 0.030 for molybdenum.

3.3. Chlorite Geothermometry

The studies of formation temperature and pressure conditions at the Raimunda deposit were complemented using a chlorite-independent geothermometer. The structural formulas of chlorite and muscovite were calculated based on 28 and 11 oxygen atoms, respectively. The temperatures are based on the cationic proportion of tetrahedral aluminum (Al^IV) in chlorites, which allows the crystallization temperature to be calculated using the following equation: T (°C) = 321.98 × (Al^IV/2) − 61.92 [28,29]. In total, 18 analyses of chlorite were carried out on carbon-coated polished thin sections at the LME-UnB using a JEOL^® JXA-8230 EPMA (JEOL Ltd., Tokyo, Japan). The WDS operated under the conditions of an acceleration voltage of 15 Kv, a beam current of 10 nA, and a count time of 10 s.

3.4. Fluid Inclusions

The study of fluid inclusions was performed on doubly polished thin sections from six different mineralized samples. Petrography of fluid inclusions was carried out at room temperature (±25 °C) under a Zeiss Axioplan 2 petrographic microscope (Zeiss, Oberkochen, Germany) according to the criteria of [30,31,32].

Microthermometric measurements were obtained with a LINKAM THMSG600 heating-freezing system (Linkam Scientific, Salfords, Surrey, UK) mounted on an Olympus BX-50 petrographic microscope (Olympus Corporation, Tokyo, Japan), at the UnB Fluid Inclusions Laboratory (LIF-UnB). Rock fragments were observed at variable temperatures between −195 °C and 600 °C under a 10× to 100× objective lens following the procedures of [31].

The calibration was carried out using synthetic H₂O–NaCl two-phase and H₂O–CO₂ three-phase fluid inclusions at speed rates of 2 to 15 °C/min, with an estimated accuracy of ±0.5 °C for freezing (±25 at −150 °C), and ±2 °C for heating (above 350 °C).

3.5. Stable Isotopes

The study of stable isotopes was performed on minerals of representative mineralized samples from the Raimunda deposit. Sample selection and sample preparation took place at the sample preparation laboratory at the University of Brasília (LPA-UnB). The pure mineral concentrates were handpicked under a binocular microscope at LME-UnB.

Analyzes were performed at the Queen’s Facility for Isotope Research (QFIR) laboratory at Queen’s University, Ontario, Canada. Results are reported as delta (δ) values per mil (‰) relative to V-SMOW (Vienna Standard Mean Ocean Water) for oxygen, and V-CDT (Canyon Diablo Troilite) for sulfur.

Oxygen isotope data were obtained for nine samples, between 20 and 28 mg each, using the Thermo Finnigan Delta V Advantage and Finnigan Delta XP mass spectrometers (Thermo Fisher Scientific, Waltham, MA, USA). The analytical protocol applies the laser fluorination method to release oxygen from minerals through the reaction with BrF₅ by using a CO₂ laser as a heat source. It is then either converted into CO₂ through the reaction with graphite at 750 °C, or it is collected as O₂ in a molecular sieve and analyzed as O₂. The analytical uncertainties for δ¹⁸O were lower than 0.1 and 0.16‰.

For sulfur, six samples of pyrite were prepared, each weighing between 80 and 95 mg. For this analysis, the SO₂ produced through combustion of about 0.5 mg of sample is loaded into tin capsules reacting at 1080 °C. The gases were transported through current and reduced by contact with high-purity copper wires. The SO₂ was eventually measured using gas chromatography. Analyzes were performed using a Flash Elemental Analyzer (EA) coupled with a ThermoFisher MAT 253 Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific, Waltham, MA, USA). Analytical uncertainties are estimated at 0.2‰.

4. Results

4.1. Hydrothermal Alteration and Mineralization

The Au (±Cu) Raimunda deposit sits in the NW of the Novo Mundo Complex, being hosted by variably hydrothermally altered granite facies (i.e., monzonite, biotite granodiorite, monzogranite, and syenogranite).

The types of hydrothermal alteration identified at the Raimunda deposit, and their respective mineral assemblages, are summarized in

Table 1. Hydrothermal alteration stages and paragenetic evolution table of the Raimunda Au (±Cu) deposit, Juruena Mineral Province, Brazil.

Event		Pre-Mineralization			Mineralization Stages			Post-Mineralization
	Alteration Stages	Na-Metassomatism	Microclinization	Propylitic Alteration	Sericitic Alteration (Ms-Qz2-Chl2-Py2-Au1)	Silicification (Qz3)	Sulfide Stage (Py3-Au2-Ccp-Cc-Bin)	Carbonate Alteration (Cal-Chl3-Qz4)
Mineral
Quartz 1
Quartz 2
Quartz 3
Quartz 4
K- Feldspar
Albite
Magnetite
Titanite
Rutile
Chlorite
Muscovite
Calcite
Epidote
Pyrite 1
Pyrite 2
Pyrite 3
Chalcopyrite
Chalcocite
Larosite
Bismuthinite
Hodrushite/krupkaite/wittchenite
Gold
		Traces	Minor	Abundant
Temporal evolution

. The hydrothermal system and the associated mineralization are mainly observed through pervasive zones, and, subordinately, along veins and veinlets and the more affected rocks are the monzo- to syenogranite facies.

Based on overprinting relationships, mineralogy, and alteration assemblages, seven alteration zones are recognized: (1) Early sodic metasomatism (albite); (2) Microclinization (orthoclase/microcline-quartz1-hematite); (3) Early or intermediate propylitic alteration (epidote-chorite1-carbonate-rutile-titanite-pyrite1); (4) Intermediate to proximal sericitic alteration (muscovite-chlorite2-quartz2-pyrite2-gold1); (5) Silicification (quartz3); (6) Main mineralization phase: sulfide stage (gold2-pyrite3-chalcopyrite-bismuthinite and other Bi- and Cu sulfides); and (7) Late-stage carbonate alteration and chloritization (carbonate-chlorite3-quartz4).

The alterations occur throughout the ore-bearing host granite at the deposit. At the field scale, and even when analyzing the different drill cores, it is not possible to macroscopically delineate hydrothermal alteration zones.

The early Na-metasomatism alteration modified the essential mineralogy of the granites. The oligoclase is completely or partially replaced by albite and the magmatic biotite is locally preserved.

Subsequently, microclinization represents another outward alteration halo surrounding the previous one, and it is commonly observed at the periphery of the alteration profile. Microclinization commonly occurs within all granite facies; although more intensely in the syenogranite, through the alteration of K-feldspar and the formation of microcline rims. The orthoclase is microclinized and the feldspar and biotite crystals are partially to completely altered to fine muscovite. Hydrothermal quartz grains (quartz1) are generally milky.

The propylitic alteration can be noticed in all granitic facies of the Raimunda deposit but is more perceptive in non-mineralized samples, including monzonite and granodiorite facies. It is pervasive, affecting especially feldspar crystals, which are partially or completely replaced by epidote-chlorite1-carbonate. It is common to occur in veins and/or veinlets with epidote-chlorite1-carbonate crossing the samples and rutile, titanite, and fine-grained pyrite1 in agglomerates mass.

Sericitic alteration is marked by its pale green color throughout all granite facies, where they occur pervasively or selectively altered and become more intense towards the monzogranite facies. The alteration mineral assemblage is mainly composed of muscovite, chlorite2 (Chl2), pyrite2 (Py2), quartz2 (Qz2), and chalcopyrite. The main feature is the texture obliterated by the replacement of feldspars with fine muscovite, along with disseminated fine-grained pyrite and quartz, locally. In this case, pyrite (Py2) occurs disseminated either into quartz veins and veinlets or associated with quartz2 and muscovite. This phase was identified as the first stage of mineralization (i.e., muscovite, pyrite2, quartz2, and gold1). Euhedral to subhedral pyrite is classified as pyrite2, which is poorly fractured and displays a low degree of fracture filling, occurring disseminated in host rocks and, locally, into veins or veinlets that cut through the rock (Figure 5A). Gold (Au1) occurs either as inclusions or as fracture-fills in pyrite2.

Silicification occurs pervasively or as veins and veinlets (Figure 5C). The veins/veinlets consist of microcrystalline quartz (massive and mosaic texture—Qz3), associated with sulfides and other minerals, such as muscovite and carbonate.

The sulfide stage is both disseminated and forming massive sulfide zones that crosscut the rocks. At the deposit, that corresponds to the main mineralizing stage with a high content of associated sulfides (i.e., pyrite3, chalcopyrite, chalcocite, bismuthinite, hodrushite, krupkaite, wittchenite) and gold2. The pyrite3 occurs in massive sulfide levels, strongly fractured, and with a high degree of fracture filling (Figure 4E,G). Gold (Au2) occurs as fracture-fills in pyrite3 and as seldom inclusions in quartz3. The samples highlight pressure relief features, vuggy silica in quartz veins/veinlets, and levels of massive sulfide (Figure 5E,F).

Late hydrothermal alteration is represented by carbonate alteration. It is less expressive than the last two stages and restricted to the final stages of hydrothermal alteration. In general, potassium feldspar and albite are affected by carbonate alteration. Carbonate-quartz4-chlorite3 veins and veinlets crosscut the rock samples (Figure 5E).

4.2. Ore and Hydrothermal Mineral Chemistry

Tables S1–S7 display EPMA analyses of pyrite, chalcopyrite, gold, copper-rich minerals, bismuth-rich minerals, chlorite and muscovite which can be found in the Supplementary Material. The selection considered the variable styles of mineralization (veins/veinlets and disseminated) for each mineral as well as their ore mineral paragenesis.

4.2.1. Sulfides and Gold

The analysis of pyrite focused on types 2 (Py2) and 3 (Py3), which are associated with the mineralization stages at the deposit (Table S1, Figure 6). Py3 (Figure 6A) contains slightly higher contents of Au (Py3: 0–0.09%; Py2: 0.02–0.07%), Te (Py3: 0–0.24%; Py2: 0–0.12%), and Mo (Py3: 0.01–0.17%; Py2: 0–0.08%). Furthermore, there are higher contents of Pb (Py3: 0.1–0.34%; Py2: 0.19–0.23%), as well as higher contents of Cu (Py3: 0–0.82%; Py2: 0–0.04%).

The significant amount of these elements in Py3 compared to Py2 reveals the genetic differences between those types based on chemical evidence, which had been previously corroborated by textural features. This points out variable timings and fluctuating element availability throughout ore precipitation.

The chalcopyrite occurs in fracture-fills and inclusions in Py2 and, more intensely, in Py3 (Figure 6A,C,D,F). Chemically, there are no significant differences between them, with average Au contents of 0–0.06%, Ag contents of 0–0.16%, Te contents of 0–0.23%, Mo contents of 0–0.12%, Pb contents of 0.04–0.30% and Bi contents of 0–0.23%.

Chalcocite (Cu₂S) occurs together with chalcopyrite and other copper sulfides, such as larosite ((Cu, Ag)₂₁(Pb, Bi)₂ S₁₃), filling fractures in pyrite3. Bismuthinite is the most abundant bismuth mineral and, likewise, occurs as fracture-fills. Other Cu- and Bi-rich sulfides identified as part of the mineral paragenesis at the deposit are hodrushite (Cu₈Bi₁₂S₂₂), krupkaite (PbCuBi₃S₆), and wittchenite (Cu₃BiS₃) (Figure 6D).

All sulfides are in equilibrium with Py3 making up the main mineralization stage at the Raimunda deposit. Gold occurs either as inclusions (Au1) or fracture-fills (Au2) (Figure 6B–F). In both cases, gold inclusions and fracture-fills in pyrite showed no significant compositional variation between them (Table S3, Supplementary Material). Fracture-filling gold grains (Au2) display average Au values of 57.03–66.77%, and Ag values of 28.45–28.74%, while gold inclusions yield Au contents of 65.91–66.80% and Ag contents of 27.90–28.70%. Therefore, their chemistry reveals relative constant silver values and variable gold values, which are related to Cu, Te, and Mo input.

4.2.2. Chlorite Chemistry and Geothermometry

The chemical composition of chlorite contributes to the study of hydrothermal alteration processes to delineate the former physical–chemical conditions (T, P, fO₂, pH, and host rock composition) of the geological environment at the time of its formation, which makes it an important indicator of the rock formation temperature [28,33].

Table S6 (Supplementary Material) presents 18 microprobe analyses of chlorite (i) after biotite (Chl1), (ii) in equilibrium with muscovite and epidote (sericitic alteration) (chl2), and (iii) in association with the late-stage carbonatization (Chl3). The compositional variation in chlorite might be related to the partial replacement of igneous biotite by chlorite, which had been depicted from textural features and is used for a classification scheme.

Magnesium-rich chlorite yields Fe/(Fe + Mg) ratios varying between 0.33 and 0.56, and Si/Al ratio varying between 1.02 and 1.39. Si atoms range between 5.24 and 5.98 a.p.f.u., and Al^IV ranges from 2.02 to 2.76 a.p.f.u. Iron-rich chlorite exhibits Fe/(Fe + Mg) ratios of 0.67–0.84 and less varying Si/Al ratios of 1.03–1.09. Si atoms range between 5.31 and 5.51 a.p.f.u., and Al^IV values fall within the range of 2.49–2.70 a.p.f.u. Based on the classification of [34], these compositions plot at the transition between clinochlore-chamosite (Figure 7A,B).

Empirical geothermometers were proposed by several authors [28,29,34,36,37]. Each one applies correction criteria based on the characteristics of the given chlorite, as follows: (i) tetrahedral Al (Al^IV) contents [28,29] and (ii) corrected Al^IV contents based on the relationship between Al^IV and Fe/(Fe + Mg) ratios [36,37,38,39]. Empirical geothermometers used in this study are summarized in Table S6 (Supplementary Material). The peak temperature was obtained through the equation of Cathelineau [29]. The values calculated according to Cathelineau and Nieva [28] are similar to those calculated according to Cathelineau [29]. The lowest temperatures were obtained using the equation of Zang and Fyfe [37]. Based on the features and compositions of the different types of chlorites here identified, the formation temperatures estimated through Cathelineau [29] were selected.

The chlorite geothermometry data are coherent with the petrographic data: the hydrothermal alteration had its onset with propylitic alteration, marked by the replacement of biotite by chlorite (Chl1) along with the formation of other Fe-rich minerals (e.g., rutile) at temperatures between 355 and 382.4 °C. As alteration progressed, Chl2, which is associated with sericitic alteration, began to nucleate, at temperatures between 338.9 and 372.8 °C, under the conditions where the first mineralization stage took place. During late hydrothermal alteration, Chl3 was predominant in association with late quartz and carbonate veinlets, at temperatures ranging between 263.3 and 297.87 °C.

4.2.3. Muscovite

Electron microprobe data and the chemical formula of muscovite from the Raimunda deposit are available in Table S7 (Supplementary Material). The muscovite samples chosen for analysis are those associated with the first (1) mineralization stage (sericitic alteration) due to their greater occurrence. The 16 spot analyses reveal a relatively homogeneous composition with small variations in Al₂O₃ (29.8–31.4%), MgO (0.6–1.18%), FeO (1.54–5%), and Na₂O (0.13–0.4%) [35] (Figure 7C).

4.3. Fluid Inclusions Data

4.3.1. Petrography and Classification

The fluid inclusion study was carried out on quartz grains from the mineralization stage (Qz2 and Qz3) and late alteration stage (Qz4), using doubly polished thin sections from the Raimunda deposit. Primary and secondary fluid inclusions were described according to the fluid inclusion assemblage (FIA) criteria proposed by [40], which is based on the classification criteria of [31].

Based on the distribution, number of phases present at room temperature, patterns of occurrence, phase transitions throughout heating and freezing runs, and Raman spectrometry data, nine subtypes of fluid inclusions were recognized. The main characteristics are summarized in

Table 2. Petrographic and microthermometric data of fluid inclusions from the Raimunda deposit, Juruena Mineral Province, Brazil. Abbreviations: L—liquid; V—vapor; S—daughter mineral (saturation solid); Ts—trapped solid. Te: eutectic temperature; Tm_hh—hydrohalite melting temperature; Tm_ice—ice melting temperature; Tm_CO2—CO₂ melting temperature; Tm_cla—clathrate melting temperature; Th_CO2—CO₂ homogenization temperature; Th_t—total homogenization temperature.

Fluid Inclusion Type	Host Rock	Hydrothermal Alteration Stage	Mineral Association	Petrographic Characteristics	Microthermometric Data	Model System
Type 1-a (L + V); n = 28	Monzogranite; syenogranite	Sulfide stage	Py3 + Ccp + Qz3 + Au2	Primary; random; colorless; ellipsoidal, oval to irregularly shaped; 8–20 μm; F: 0.70–0.8	Te = −24 to −21 °C; Tmice = −6.4 to −0.8 °C; Tht = 233.5 to 471.2 °C; salinity = 1.396–9.72 wt.% NaCl eq.; density; 0.409–0.895 g/cm3	H2O-NaCl
Type 1-b (L + V); n = 25	Monzogranite; syenogranite	Sericitic alteration	Ms + Qz2 + Au1	Secondary; planar grouping and trails; oblate; 3–12 μm; F: 0.7–0.8	Tmice = −4.2 to −0.5 °C; Tht = 200.5 to 297 °C; salinity = 0.879–6.737 wt.% NaCl eq.; density = 0.715–0.918 g/cm3	H2O-NaCl
Type 2 (V; L + V; L + L + V); n = 40	Monzogranite	Sulfide stage	Py3 + Ccp + Qz3 + Au2	Primary; random distribution or in clusters; grayish; dark bubble; elongated and oval; 5–20 μm; F: 30–50	TmCO2 = −57.3 to −56°C; Tmice = −3.8 to −2.9 °C; Tmcla = 6.2–8.5 °C; ThCO2 = 19–30.1 °C; Tht = 210 to 364.7 °C; salinity = 3.00–5.94 wt.% NaCl eq.; density = 0.87–0.88 g/cm3	H2O-NaCl-CO2
Type 3-a (L + V; L + V + S); n = 3	Monzogranite	Carbonate alteration	Cal + Chl3 + Qz4	Primary; random distribution; square-shaped, elongated, and sub-rounded; colorless or greenish; 8–22 μm; F: 60–70; with one cubic crystal (halite daughter mineral)	Tmhh = −10.7 to −3.8 °C; Tht = 290.5 to 367.5 °C; salinity = 38.88–50.25 wt.% NaCl + CaCl2 eq.; density = 0.78–0.82 g/cm3	H2O-NaCl-CaCl2
Type 3-b (L + V; L + V + S + Ts); n = 19	Monzogranite	Sericitic alteration	Py3 + Ccp + Qz3 + Au2	Secondary; planar grouping and trails; oblate square-shaped, elongated, and sub-rounded; 3–12 μm; F: 60–80; with colorless cubic crystals (halite daughter mineral) and trapped solid locally	Te: −55 to −51.2 °C; Tmice = −43.2 to −37.8 °C; Tmhh = −10.6 to −1.1 °C; Tht = 100.6 to 251.5 °C; salinity = 33.02–38.21 wt.% NaCl + CaCl2 eq.; density = 0.87–0.97 g/cm3	H2O-NaCl-CaCl2
Type 4-a (L + V); n = 19	Monzogranite; syenogranite	Silicification; Sulfide stage	Py3 + Ccp + Qz3 + Au2	Primary; clusters and isolated; colorless; 8–20 μm; F: 0.8–0.85	Tmice = −5.4 to −1.0 °C; Tht = 199.2 to 288 °C; salinity = 1.57–8.41 wt.% NaCl eq.; density = 0.74–0.92 g/cm3	H2O-NaCl
Type 4-b (L + V); n = 41	Monzogranite; syenogranite	Sericitic alteration	Ms + Qz2 + Au1	Secondary; trails/aligned along micro-fractures in the quartz; 8–20 μm; F: 0.8–0.85	Tmice = −4.7 to −0.1 °C; Tht = 101 to 195 °C; salinity = 0.53–6.59 wt.% NaCl eq.; density = 0.87–0.97 g/cm3	H2O-NaCl
Type 5 (L + V); n = 8	Monzogranite; syenogranite	Sericitic alteration	Ms + Qz2 + Au1	Secondary; irregular to sub-rounded shaped; colorless; 15 and 50 μm; F: 0.75–0.8	Tmice = −4.2 to −0.8 °C; Tht = 98 to 190 °C; salinity = 1.39–6.74 wt.% NaCl eq.; density = 0.92–0.97 g/cm3	H2O-NaCl
Type 6 (L; V)	Monzogranite; syenogranite	All assemblages		Colorless to greenish; rounded/oval to irregular; 4 and 20 μm; it occurs in all samples studied		H2O-NaCl

.

Type 1 (t1) are two-phase (liquid + vapor) aqueous liquid-rich primary (t1-a) and secondary (t1-b) inclusions. Type t1-a inclusions show clear evidence of primary fluid, and occur in Qz3, which is associated with sulfide stage, as clusters or isolated inclusions with a liquid fill ration (F) of 0.70–0.8. They are usually colorless, with a light-colored bubble surrounded by a dark rim. They are rounded, oval, and, more rarely, irregularly shaped, varying between 8–20 μm (Figure 8A,C). Type t1-b inclusions encompass secondary inclusions in Qz2 associated with sericitic alteration. They mostly occur along trails and subordinately as clusters. They are colorless and display an F of 0.7–0.8. The light-colored bubble moves at room temperature in some samples. They range between 3 and 12 μm and are rounded, sub-rounded, elongated, and, more rarely, rectangular.

Type 2 (t2) are primary, single-phase carbonic vapor-rich, two-phase (L_H2O + V_CO2) and three-phase (L_H2O + L_CO2 + V_CO2) aqueous-carbonic fluid inclusions that occur in Qz3. Single-phase inclusions are gray, with an F of 100% CO₂, varying between 5 and 15 μm. Two-phase and three-phase inclusions display bubbles with a dark rim, varying between 5 and 20 μm. They are sub-rounded, elongated, and, more locally, irregularly-shaped. Their F is 30–50 vol% (Figure 8A–C). They occur as single inclusions or in clusters being oval to irregularly shaped. Three-phase inclusions usually display CO₂ bubbles only during reheating, before the homogenization temperature for CO₂ is reached. They are considered part of the same fluid inclusion assemblage (FIA).

Type 3 encompasses three-phase primary (t3-a) and three-phase secondary (t3-b) inclusions (L_H2O + V_H2O + S₁) containing daughter minerals and multiphase-solid phases (L_H2O + V_H2O + S₁ + S₂). The t3-a inclusions correspond to the primary in Qz4, associated with late-carbonatization. It varies between 10 and 20 μm, and is elongated, and sub-rounded; colorless to greenish. They contain cubic solid phases (daughter mineral) that are colorless, isotropic, and interpreted as halite based on petrographic and microthermometric data as well as in the lack of a Raman signal. Overall, the F for both subtypes is 15–20 vol% (vapor), 15–20 vol% (solid), and 60–70 vol% (liquid).

The t3-b fluid inclusion is secondary in Qz3 and occurs as trails and/or aligns associated with sulfide stage. They vary between 8 and 22 μm and are square-shaped, elongated, sub-rounded, and greenish. The cubic solid phases (daughter mineral) are colorless and, locally, also occur as a trapped solid (opaque mineral) in the fluid inclusion (Figure 8D). The F for both subtypes is 15–20 vol% (vapor), 15–20 vol% (solid), and 60–70 vol% (liquid) (Figure 8D,E).

Type 4 (t4) are liquid-rich two-phase aqueous inclusions. Primary fluid inclusions (t4-a) in Qz3 are associated with sulfide stage. Secondary inclusions (t4-b) predominate in all samples studied and are associated with sericitic alteration (Qz2). In both types, the liquid phase is dominant, with an F of 0.8–0.85 and with a vapor bubble content of 15–20 vol%. They are colorless, 8 to 20 μm, and display rounded, sub-rounded, elongated, and irregular shapes. They are generally found as trails/aligned along micro-fractures in the crystal (t3-b), clusters, and isolated (t3-a) (Figure 8E).

Type 5 (t5) are secondary, liquid-rich two-phase aqueous (L + V) inclusions (Figure 8F). The degree of fill (F) is 0.75–0.8. They are irregular to sub-rounded, vary between 15 and 50 μm, and occur only in Qz2 which is associated with sericitic alteration.

Type 6 (t6) are single-phase liquid-rich and/or vapor-rich inclusions. They are colorless to greenish, rounded/oval to irregular, and locally occur along trails varying between 4 and 20 μm. Vapor-rich inclusions are dark, irregular, and vary between 2 and 15 μm. They occur in all samples studied.

4.3.2. Microthermometric Data

Table 2 summarizes the fluid inclusions microthermometric data of the Raimunda deposit. Microthermometry was carried out on all types of inclusions in quartz grains previously selected according to petrographic criteria. A total amount of 182 valid measurements were obtained (Table S8, Supplementary Material). The salinity and density of types 1, 4, and 5 aqueous fluid inclusions were calculated based on the equation of Bodnar [40], using the final ice melting temperature (Tm_ice) to estimate salinity in the H₂O–NaCl system. For the fluid inclusions interpreted to be of the system H₂O–NaCl–CaCl₂, salinities were calculated using the Steele-Macinnis et al. equation [41]. The salinities in H₂O–NaCl–CO₂ inclusions (type 2) were estimated using the final CO₂-clathrate melting temperatures [42], and the densities were estimated using the equations of Parry 1986 [43].

Type t1-a shows the temperature of the first visible melting (Te) ranging between −24.5 and −21.0 °C and encompasses inclusions with the final ice melting temperatures (Tm_ice) between −6.4 and −0.8 °C (Figure 9A). The total homogenization temperature (Th_t), into the liquid phase, took place between 233.5 and 471.2 °C (Figure 9C), with average values of 325–380 °C. The salinities data are between 1.40 and 9.72 wt.% NaCl eq. and the density values between 0.41 and 0.89 g/cm³. In type t1-b, Tm_ice took place between −6.4 and −0.5 °C (Figure 9B), with an average from −4.2 to −0.5 °C and the Th_t varies from 200.5 to 297 °C (Figure 9D). The salinity varies from 0.88 to 6.74 wt.% NaCl eq. and densities of 0.71–0.92 g/cm³.

Type 2 has average CO₂ melting temperature (Tm_CO2) values ranging between −56.8 and −56.6 °C for two-phase and three-phase aqueous-carbonic inclusions (Figure 10A). For carbonic inclusions, Tm_CO2 is −57.3 to −57 °C. These values vary to either more positive or more negative values compared to the pure H₂O-CO₂ system (−56.6 °C). This points out the presence of other volatile compounds apart from CO₂ (e.g., CH₄ and N₂). The clathrate melting temperatures (Tm_Cla) are between 6.2 and 8.5 °C and the CO₂ homogenization temperatures (Th_CO2) vary between 26.8 and 30.1 °C for aqueous-carbonic inclusions, and between 19 and 26 °C for single-phase carbonic inclusions. Th_t varies from 210 to 390 °C, but the main range for three-phase (L_H2O + L_CO2 + V_CO2) inclusions take place between 280 and 340 °C; and, for two-phase (L_H2O + V_CO2) inclusions, the main temperature range is 300–364.7 °C. Salinities are between 3.00 and 5.94 wt.% NaCl eq. and the densities range from 0.87 to 0.88 g/cm³.

Type t3-a primary inclusions showed Tm_hh values ranging between −10.7 and −3.8 °C. These inclusions homogenized into the liquid and vapor phase between 290.5 and 367.5 °C (Th_t). The salinities are between 38.88 and 50.25 wt.% NaCl + CaCl₂ eq. with densities varying from 0.78 to 0.82 g/cm³.

Type t3-b inclusions show the first observed melting temperature range between −55 to −51.2 °C, interpreted as a eutectic temperature (Te) and suggesting the composition of the H₂O-NaCl-CaCl₂ system [41]. The final melting of ice is between −43.2 and −37.8 °C, and the hydrohalite dissolution (Tm_hh) occurs between −10.7 and −1.1 °C, with a main interval between −9 to −5 °C. During heating, the vapor bubble homogenizes first (132 to 202 °C), and, as temperature increases, daughter minerals homogenize (Th_t) in the range of 192–278.6 °C, with an average between 210 and 250 °C. The salinities show range values of 33.02–38.21 wt.% NaCl + CaCl₂ eq. and density values range from 0.87 to 0.97 g/cm³. The H₂O–NaCl–CaCl₂ ternary diagram plots the composition for type t3–b inclusions at the Raimunda deposit based on Steele-MacInnis et al. [41] (Figure 10B).

Type 4-a primary inclusions display Tm_ice values between −5.4 and −1.0 °C (Figure 9A) and Th_t took place in the range between 200 and 288 °C (Figure 9C). The corresponding salinity of 1.56–8.41 wt.% NaCl eq. and the density was estimated between 0.74 and 0.92 g/cm³. Type t4-b secondary inclusions yielded Tm_ice values in the range of −4.7 and −0.1 °C (Figure 9B). This type homogenized into the liquid phase between 101 and 195 °C (Figure 9D). The salinity is between 0.53 and 6.59 wt.% NaCl eq. with densities between 0.74 and 0.92 g/cm³.

In type 5, the Tm_ice values fall between −4.2 and −0.8 °C and total homogenization temperatures (Th_t) into the liquid phase range between 98 and 190 °C. The salinity is between 1.39–6.74 wt.% NaCl eq. and densities between 0.92 and 0.97 g/cm³ (Figure 9B,D). This type homogenized both into the liquid and vapor phase and the latter showed some inclusions crepitating while other inclusions did not return to their original state before heating.

4.4. Stable Isotopes Data

Stable isotope data are summarized in

Table 3. Isotopic composition of hydrothermal minerals and fluids from the Raimunda deposits, Novo Mundo granite, Juruena Mineral Province, Brazil.

Sample	Mineral	Hydrothermal Alteration Stage	δ34SPy (‰)	δ18OQz (‰)	δ18O H2O (‰) [44]	Estimated Isotopic Equilibrium Temperature (°C) from Fluid Inclusion Data
F75-05	Qz2	Sericitic alteration		9	3.11–3.99	340–370
F70-02				9.6	3.72–4.59
F73-02				10.3	4.72–5.29
F70-03	Py2		−0.5
	Qz2			11.5	5.62–6.50
F73-03	Py3	Sulfide stage	0.1			325–380
	Qz3			12.6	6.22–7.86
F48-A15	Py3		−1
	Qz3			12.2	5.82–7.46
F70-01	Py3		−0.1	11.1	4.73–6.36
F78-05	Py3		−0.7
	Qz3			11.6	5.23–6.85
F73-04	Py3		−1.4
	Qz3			10.7	4.32–5.95

. The oxygen data were obtained for Qz2, which is associated with sericitic alteration (Ms-Qz2-Py2-Au1), and for Qz3, which is related to the sulfide stage (Py3-Au2-Ccp-Cc-Bin and Cu sulfides); sulfur data were obtained for Py2 (sericitic alteration) and Py3 (sulfide stage), both associated with the mineralization stages at the Raimunda deposit (Table 1).

The δ¹⁸O values of quartz vary between 9.0 and 12.6‰. The isotopic values of the fluid (δ¹⁸O_H2O) were calculated using the total homogenization temperature range (Th_t) of primary t1 fluid inclusions (325–380 °C) as the isotopic equilibrium temperature for the main mineralized phase, sulfide stage, and the chlorite geothermometric temperature (340–370 °C) for the sericitic alteration stage.

These fluid inclusions are associated with the main mineralization stage at the Raimunda deposit. For the quartz-water system, the oxygen isotopic fractionation factor and the isotopic balance equation were used [44]; for oxygen, the following formula was applied to determine the temperature range between 250 and 500 °C: 1000 lnα_Qz-Water = 3.34 × (10⁶T⁻²) − 3.31.

δ³⁴S_py values for sulfides slightly varied between −1.4 and +0.1‰. Pyrite associated with sericitic alteration (Py2) yielded a δ³⁴S value of −0.5‰; pyrite from massive sulfide zones (Py3) provided δ³⁴S values from −1.4 to +0.1‰. The isotopic fractionation factor between pyrite–H₂S was retrieved from [45].

5. Discussion

5.1. Tectonic Settings of Host Rocks, Hydrothermal Zonation and Mineralization

The Au (±Cu) Raimunda hydrothermal deposit is hosted in the monzogranite (2029 ± 4 Ma) and syenogranite (1987 ± 7.4 Ma to 1964 ± 1 Ma) facies, which are part of the Cuiú-Cuiú granite complex (2.03 and 1.98 Ga), eastern region of the Juruena Mineral Province [24]. These rocks are related to a plutonic-volcanic event in volcanic arc settings [46].

The zones more intensely affected by hydrothermal alteration are indicative of ore precipitation at intermediate to shallower levels. This is corroborated by pressure-release structures and vuggy quartz as well as breccias and brittle-like structures typical of shallower depths (Figure 5E).

Gold-rich mineral deposits in Novo Mundo were studied by [19,22]. These authors found disseminated gold hosted in intensely hydrothermally-altered syenogranite and monzogranite crosscut by andesitic to basaltic dikes. Those authors observed that mineralization was associated with intense phyllic alteration overprinting strong pervasive potassic alteration [22]. These other deposits share strong similarities with the Raimunda deposit, which might be indicative of similar mineralization events in the region. Other neighboring mineral deposits are hosted in 1.99–1.97 Ga granites [22].

In Novo Mundo, the mineralogical composition, the magmatic and hydrothermal textural features, the mineral parageneses, the cutting and overprinting relationships among host rocks and alteration and mineralization zones, microthermometric data, as well as chlorite and muscovite chemistry, allow us to propose the following paragenetic sequences for the Raimunda deposit presented in Table 1.

The evolution of the mineralizing hydrothermal system goes through the development of sericitic alteration (muscovite + pyrite2 + quartz2 + gold1), a lower-temperature stage compared to the previous ones (Na-metasomatism, microclinization, and propylitic alteration). Sericitic alteration represents the first mineralization stage, with a temperature estimate between 340 and 370 °C based on chlorite chemistry (Table S6, Supplementary Material).

This stage is followed by the main ore precipitation stage, with average temperature estimates between 325 and 380 °C, based on the homogenization temperature of type 1-a fluid inclusions. The following post-mineralization stage is marked by cooling and de- compression of hydrothermal fluids [47] and is represented by chloritization–carbonate alteration, with average temperatures of 262–280 °C, based on chlorite geothermometer (Chl3).

5.2. Origin, Hydrothermal System, and Evolution of Fluid Composition

Isotopic and fluid inclusion data indicate an evolution from hot, medium-salinity fluid (>450 °C and ≤10 wt.% NaCl eq.) to cooler, medium- to low-salinity fluids (<300 °C and ≤5 wt.% NaCl eq.).

The petrographic characteristics and fluid inclusion microthermometric data reveal the existence of two fluids: a low to medium salinity H₂O–NaCl–CO₂ aqueous-carbonic fluid (3.0–5.94 wt.% NaCl eq.) and a low-temperature high-salinity H₂O–NaCl–CaCl₂ aqueous late fluid (33.02–50.25 wt.% NaCl + CaCl₂ eq.), and that make up the fluid system of the Au (±Cu) Raimunda deposit.

The CO₂ content in aqueous-carbonic and carbonic fluid inclusions might be related to hydrolysis during hydrothermal alteration of the host rock (propylitic alteration), which consumed water from the fluid, enriching it in CO₂. The high homogenization temperatures observed at these inclusions (up to 472 °C) corroborate this interpretation.

Aqueous fluids are divided into two chemical systems, based on the relationship Te versus salinity: (1) H₂O-NaCl fluid in Qz2 and Qz3, with Te between −24.5 °C and −21.4 °C, and salinity between 0.87 and 9.72 wt.% NaCl eq.; (2) H₂O-NaCl-CaCl₂ fluid in Qz3 and Qz4, with the first melting temperature observed, interpreted as a Te, between −55 and −51.2 °C, and salinity between 33.02–50.25 wt.% NaCl + CaCl₂ eq. (Table S8, Supplementary Material).

The saturated fluid displays homogenization temperatures Th_t (290.5–367.5 °C, for t3-a; 200–250 °C, for t3-b main interval) with complete dissolution of the daughter mineral, thereby indicating homogeneous entrapment. And, despite the occurrence of two primary fluids (H₂O–CO₂ and H₂O–NaCl) deriving from a single fluid and the variable homogenization into the vapor and liquid phases (t3 and t5), theses inclusion types do not belong to the same assembly (FIA), and the homogenization occurs in a medium to a large range of temperature, which is compatible with a homogeneous entrapment.

The occurrence of brines in aqueous and aqueous-carbonic primary fluid inclusions can yield the fluid mixing hypotheses, leading to brine dilution and an increase in salinity for aqueous-carbonic fluid, therefore, explaining the presence of the late-carbonate alteration at the deposit. Fluid mixing may have taken place at the time of the formation of Qz3, with ongoing brine activity through the formation of Qz4 at a lower temperature, when the aqueous-carbonic fluid began to reduce its activity and precipitate calcite, which is corroborated by late hydrothermal alteration (Cal + Chl3).

Thus, the evolution of the medium-salinity and very hot system (>470 °C) is linked to the stages of hydrothermal alteration prior to mineralization (sodic metasomatism, microclinization, and propylitic alteration); and the medium to low salinity (<10 wt.% NaCl eq.) cool system or progressively cooler (<380 °C) is associated with the main to late alteration stages sericitic alteration and contemporaneous sulfide stage associated with silica veins and carbonate alteration). Furthermore, the transition from microclinization into late carbonate alteration can suggest a progressive increase in pH values [48,49,50], which likely lasted throughout ore precipitation.

The calculated isotopic composition of the hydrothermal fluid responsible for both mineralization stages shows δ¹⁸O_H2O values for stage 2 in the range between +4.32 and +7.86‰ (sulfide stage), while those calculated for stage 1 vary between +3.11 and +6.49‰ (sericitic alteration) (Table 3), which is coherent to isotopic compositions of fluids associated with felsic magmas [51] and are compatible with the magmatic compositional isotopic range expected for world-class porphyry-type magmatic-hydrothermal deposits, with a homogeneous isotopic fluid (δ¹⁸O_H2O, 6–10‰) [52,53,54,55]. The results are similar to paragenetic and fluid inclusion data, and point to mixing between magmatic and meteoric fluids, that have values within the δ¹⁸O range of −10 to −20‰ according to Craig [56] but remained predominantly with characteristics of magmatic fluid [57].

Figure 11 compares δ¹⁸O_H2O values, based on temperatures provided by chlorite chemistry, which is associated with sericitic alteration and the first mineralization stage, and δ¹⁸O_H2O values based on homogenization temperatures from fluid inclusions associated with the main mineralization stage at the Raimunda deposit. The two groups show an isotopic range similar to those obtained for magmatic-hydrothermal deposits with mantle-derived fluid.

The coexistence of t1-a and t2 primary inclusions at the deposit, with similar temperatures, can indicate simultaneous fluid entrapment. Phase changes among the different types of inclusions may suggest that fluid immiscibility preceded their entrapment; homogenization temperatures decrease from type t1-a to t4-a on the salinity vs. homogenization temperatures diagram (Figure 11B), which is likely due to progressive cooling and/or mixing with relatively lower-temperature fluids, such as meteoric water [58].

δ³⁴S data directly reflects its source given that under proper redox and pH conditions for precipitation, much or all of the sulfur available in the system may have been converted into H₂S at the first mineralization stage [59]. In turn, once the second mineralization stage is composed of sulfides only, the isotopic composition of sulfur in pyrite matches that of the sulfur source [59].

Figure 12 correlates δ³⁴S values of the Raimunda deposit (−1.4 to +0.1‰) with the isotopic ranges of several deposits in the Tapajós and Juruena Provinces, as well as with world-class deposits and deposits and geological environments available in the literature and are compatible with those of other world-class magmatic-hydrothermal deposits [60,61,62].

Reference values of δ³⁴S from [63], combined with petrological and mineral textural evidence at the Raimunda deposit, are indicative of mineralization of magmatic origin, which may be related to the fluids that hydrothermally altered the Novo Mundo granite. This is corroborated by the similarity between δ³⁴S values and hydrothermal patterns at the Raimunda deposit and those reported for other world-class magmatic-hydrothermal deposits and/or granite-related mineral deposits [64,65,66,67,68].

The occurrence of primary gold in the Juruena Mineral Province demonstrates a close spatial relationship with (i) oxidized I-type calc-alkaline granites, (ii) volcanic rocks, and (iii) volcano-sedimentary sequences linked to continental magmatic arcs [4]. The common occurrence of native Au as inclusions in pyrite and chalcopyrite is indicative of early Au oversaturation in the fluid which we relate to sulfide precipitation and phase separation, destabilizing the AuHS⁰ or Au(HS)²⁻ complex, leading to the accumulation of Au particles [67,69].

At the Raimunda deposit, the ore is composed of native gold and has a mantle-like polymetallic signature (Au ± Ag ± Cu ± Bi ± Te ± Pb) associated with pyrite, chalcopyrite, chalcocite, larosite, bismuthinite and other bismuth sulfides emplaced at a zone of intense sulfidation.

This copper-rich sulfide ore assemblage suggests that the Raimunda deposit formed under fO₂ conditions above the PPM buffer, pyrrhotite + pyrite + magnetite, which is close to mSO₂/mH₂S = 1 [69,70]. It is compatible with moderate to high fO₂ conditions where fluids oscillate from low to high temperatures; the latter being represented by moderate-salinity aqueous-carbonic fluids (up to ≤370 °C and up to ≤10 wt% NaCl eq.). This scenario suggests that ore precipitation is linked and/or similar to porphyry magmatic-hydrothermal systems [71].

Figure 12. Sulfur isotope values (δ³⁴S_CDT) of different geological mineral deposits, isotopic data from the Tapajós and Juruena Province, Amazonian Craton. Numbers: 1—this study; 2—[2]; 3—[19]; 4—[8]; 5—[68]; 6—[71]; 7—Mineral deposits from Shanks [51]. The magmatic sulfides range (−3 to +3‰) corresponds to [62].

Figure 12. Sulfur isotope values (δ³⁴S_CDT) of different geological mineral deposits, isotopic data from the Tapajós and Juruena Province, Amazonian Craton. Numbers: 1—this study; 2—[2]; 3—[19]; 4—[8]; 5—[68]; 6—[71]; 7—Mineral deposits from Shanks [51]. The magmatic sulfides range (−3 to +3‰) corresponds to [62].

Hence, our hypothesis is that during the final stage of granite crystallization (~2.03–1.98 Ga), high-salinity and higher-temperature single-phase fluids, with an intermediate density, were exsolved from a magma at high depth. These fluids went through immiscibility and separated into H₂O-NaCl and CO₂-rich fluids. During their ascent, fluids decompressed and cooled down [56], while ions (e.g., Au⁺, Ag⁺, Te²⁻ and S²⁻) precipitated along favorable structures, under proper P–T conditions, giving rise to the first mineralization stage.

Other ore-forming elements (i.e., Au, Cu⁺, Ag⁺, Fe²⁺, S²⁻ and Te ²⁺) were exsolved from the magmatic fluid and precipitated along microfractures and stockwork sulfide veins.

This last stage gave rise to the main stage of mineralization at the deposit, which is coherent with the ore mineral paragenesis.

The ore precipitation was triggered by the cooling of magmatic fluid expelled from magmatic chambers at shallower depths [67]. This model outlines that the input of meteoric water would only affect peripheral and/or post-mineralization hydrothermal alteration, such as propylitic alteration [57,67], without playing a significant role in the main mineralization stage, and the homogeneous δ¹⁸O_fluid values (3.11–7.86‰) corroborates this scenario. The relative depth of gold precipitation and, consequently, deposit formation can be helpful to preserve the Raimunda deposit until these days (Figure 13).

All these features are compatible with those of porphyry magmatic-hydrothermal deposits, commonly found in Phanerozoic volcano-plutonic settings, e.g., [72]. Although they lack some classic features of Phanerozoic analogous surrounding the Raimunda deposit, which may be due to variable erosion levels, it is likely associated with mineralizing events reported earlier for the Tapajós-Parima Paleoproterozoic Province [8].

6. Conclusions

The current study allows us to draw the following conclusions about the Au (±Cu) Raimunda deposit:

The gold mineralization is hosted in arc I-type calc-alkaline, peraluminous to metaluminous granitic rocks, subdivided into four granite facies (monzonite, monzogranite, biotite granodiorite, and syenogranite), with U–Pb zircon ages of 2.03–1.98 Ga, named Novo Mundo Complex, eastern region of the Juruena Mineral Province, Amazonian Craton.

The hydrothermal system resembles that of mineral deposits previously studied in the Novo Mundo Complex and in the eastern region of the province, which reveals an important regional association.

Hydrothermal alteration data and mineral and fluid chemistry data allow us to establish the following relationships: (a) the initial stages of alteration indicate temperatures > 470 °C; (b) progressive evolution of the hydrothermal system led to a temperature decrease that culminated into the first mineralization stage associated with sericitic alteration (340 and 370 °C); (c) it was followed by the main stage of ore precipitation, sulfide stage (Py3-Ccp-Cc-Bin-Au2), with average temperatures of 325–380 °C, as demonstrated by the homogenization temperature of type 1-a primary fluid inclusions; (d) the post-mineralization stage, marked by carbonate alteration and chloritization (Chl3), yielded average temperatures from 262 to 280 °C for the system.

Au (±Cu) mineralization occurs as Ag-rich (25–28%, electrum) gold inclusions and fracture-fills in pyrite, and the main mineralization stage mineralization at the deposit is composed of the following sulfide assemblage: pyrite, chalcopyrite, chalcocite, larosite, bismuthinite, hodrushite, krupkaite, and wittchenite.

The fluid inclusion study identified nine different subtypes of inclusion, varying from H₂O–NaCl, H₂O–NaCl–CO₂, and H₂O–NaCl–CaCl₂ model systems. Based on the relationship between Te versus salinity, it was possible to identify two chemical systems in the Raimunda deposit: (1) H₂O–NaCl fluid in Qz2 and Qz3, with Te between −24.5 and −21 °C and salinity between 0.87 and 9.72 wt.% NaCl eq.; and (2) H₂O–NaCl–CaCl₂ fluid in Qz3 and Qz4, with Te between −55.3 and −51.2 °C, and salinity of 33.02–50.25 wt.% NaCl + CaCl₂ eq.

The Raimunda deposit evolved from essentially high to medium temperature (≤472 °C) and medium salinity (≤10 wt.% NaCl eq.) magmatic fluids, mixed with fluids meteoric (Th_t: 98–190 °C; lower salinity: 1.39–6.74 wt.% NaCl eq.).

Ore precipitated under considerably high fS₂ and fO₂ conditions, and relatively medium to high temperatures (e.g., homogenization temperature of t1-a primary fluid inclusion suggests ~325 to 380 °C).

The isotopic composition of the mineralizing fluid points out to magmatic fluid, with δ¹⁸O_H2O values of 3.11–7.86‰, indicating magmatic origin with a meteoric fluid contribution. The main stage of ore deposition was triggered by the instability of magmatic fluids with δ³⁴S_py values between −1.4 to +0.1‰, similar to several regional and world-class magmatic-hydrothermal deposits.

The overall features of the Raimunda deposit are genetically compatible with those of magmatic-hydrothermal systems related to Au-mineralized porphyry formed from oxidized granitic magmas. These findings have implications for mineral exploration of gold and associated metals in the Juruena Mineral Province and Tapajós Province, Amazonian Craton.