**1. Introduction**

Studies of intrusive magmatism can provide insights into the fluid activity beneath volcanoes. Fluids derived from melts are important distributors of mass and energy in the upper crust. Such fluids form a link between magmatic and hydrothermal systems and provide valuable evidence of transport processes within the crust [1,2]. Slip events, or other external events that reduce principal stresses, induce transient pressure phenomena [3] (e.g., earthquakes, sector collapse) that cause the instantaneous precipitation of ore minerals. Over-pressured fluids can release energy to an overlying hydrostatic regime and generate a series of earthquakes within a relatively short period of time, known as swarm phenomena [4].

Studies of natural [5] and experimental [6,7] systems, including geophysical observations of hypocenter migration [8,9], confirm that transient pressure shifts are common occurrences in volcanic regions. This fluid behavior is also relevant to mineral and energy exploration because transient events such as ruptures can cause ore precipitation [10], and energy can be transported from the magma by high-enthalpy supercritical fluids to be utilized as enhanced geothermal systems [11–13].

However, there is limited access to active magmatic–hydrothermal systems, so we used a porphyry copper system as a natural analogue to constrain the evolution of magmatic–hydrothermal fluids. The Erdenet Cu–Mo porphyry copper deposit, Mongolia, become our case study of magmatic–hydrothermal processes within a supercritical geothermal system. Our interpretations of petrological and fluid inclusion data provide insights into the formation of the different generations of veins and their relationship to fluid activity within the Erdenet system.

## **2. Geological Setting**

The Erdenet Cu–Mo deposit is located in northern Mongolia within the Permian–Triassic Selenge–Orkhon Trough, a volcanic–plutonic belt formed by the collision of the Siberian Craton in the north with the Central Mongolian Block in the south [14]. The volcanic activity was associated with voluminous intrusive magmatism, including the production of the Selenge Complex (SC) and the Erdenet porphyry complex.

The Erdenet porphyry complex consists of the Erdenet intrusive suites, referred to here as the porphyry association (PA), which is contained within the SC (Figure 1a). The SC consists of three phases—gabbroids, granitoids, and subalkaline granites and syenites—that were emplaced at 253–221 Ma [15]. The PA consists mainly of diorite and granodiorite porphyry and comprises five ore-related stages (Figure 1b). The first stage is syn-mineralization activity, followed by explosive events and brecciation. Field observations have documented two large explosion pipes of up to 250 m in diameter that are connected to the surface. The second stage porphyries are represented by granodiorite and granite porphyries. The granite porphyries are pink-grey and the granodiorite porphyries of the second stage are grey massive rocks with around 40% phenocrysts of plagioclase, hornblende, chloritized biotite, quartz, and very rare K feldspar, set in a fine-grained micropoikilitic and micrographic groundmass of K feldspar, plagioclase, quartz, and ore minerals. These porphyries crosscut both granodiorite porphyries and dacites of the first stage and quartz-sericite alteration. The third stage porphyries are represented by biotite plagioclase, plagiogranite, and granodiorite porphyries, which cut both first and second stages porphyries. The fourth stage consists of leucocratic porphyries and rhyodacite, truncating the last three stage porphyries as very rare dykes as well as the fifth stage. The fifth stage is described as diorite porphyries, andesite (amphibole-plagioclase), and granodiorite porphyry, which is associated with propylitic alteration [15].

K-Ar dating of the PA has yielded ages of 259–243 Ma [16], U-Pb zircon yielded ages of 245.9 ± 3.3–235.6 ± 4.4 Ma [17], and the latest obtained 40Ar/39Ar dates of 239.7 ± 1.6 and 240 ± 2 Ma for the emplacement of the intrusive sequences [18]. The ore formation at the Erdenet deposit occurred at ~240 ± 0.8–235.9 ± 1.9 Ma, based on molybdenum Re–Os [19] and sericite 40Ar/39Ar dating [16]. These ages indicate that mineralization is related to the intrusion of PA stocks and dikes, and that intrusions modified the wall rocks prior to mineralization. Three stages of alteration and mineralization are recognized at the Erdenet deposit. These stages, from the deep central parts of the deposit towards the shallower and outer parts, are (1) quartz–sericite, (2) chlorite–sericite alteration at the periphery of the ore-body, and (3) silica-rich and propylitic alteration, which is characterized by chloritized biotite [20,21].

**Figure 1.** Geology and structure of the Erdenet deposit, modified after [20]. Insert shows the location of Paleozoic magmatic belts and the study area within the outline of Mongolia. (**a**) Geological map. (**b**) Schematic cross-section through the deposit, showing multiple stages of intrusion.

#### **3. Sampling and Analytical Methods**

A total of 57 samples of granodiorite and associated porphyries were taken from a hole drilled by the Erdenet Mining Corporation and from the Erdenet open pit. Twenty-five representative samples were selected for study on the basis of hand specimen observations. Polished thin sections of these samples were examined in detail using an optical microscope. The veins in two samples from 567 m and 646 m depths show crosscutting relationships (Figure 2b); these veins represent different hydrothermal events [5]. Double-polished thick sections, ~100 μm thick, were prepared for fluid inclusion microthermometry. The homogenization temperatures (Th) were measured using a Linkam THMS600 heating stage, but the inclusions were too small (1–3 μm) to determine salinity from the ice-melting temperature (Tm-ice). The microthermometry data were combined with the Raman spectroscopy data to constrain the fluid composition of each vein type.

The textures of the quartz veins were characterized using a Hitachi-S3400N scanning electron microscope (SEM) equipped with an Oxford cathodoluminescence (CL) detector and photomultiplier at the Graduate School of Science, Tohoku University, Japan. Standard polished thin sections were analyzed at an accelerating voltage of 25 kV and a beam current of 90 μA. The textural characteristics of the different generations of veins are distinctive in SEM–CL, and qualitative differences in luminescence were used to classify the veins as CL-gray, CL-dark, and CL-bright [22].

The chemical compositions of minerals in the granodiorite and associated veins were analyzed by electron probe microanalysis (EPMA) on a JEOL JXA 8200 instrument at the Graduate School of Environmental Studies, Tohoku University, Japan. For most of the elements, the accelerating voltage was set to 15 kV, the beam current was set to 12 nA, and the counting time for each element was 20 s. The data were corrected using a ZAF correction method. Trace elements, including the Ti in quartz veins, were measured with an accelerating voltage of 20 kV, a beam current of 120 nA, and a

beam diameter of 5 μm, to minimize specimen damage. The count times were 300 s on the relevant peak and 150 s for the high and low background measurements. These conditions corresponded to detection limits of 7 ppm and 22 ppm for Ti and Al, respectively. Element mapping for Ti, Al, and Fe was performed by EPMA on selected areas where SEM–CL data were acquired, using an accelerating voltage of 20 kV, a beam current of 120 nA, a focused beam, and dwell times of 1 s per pixel [23].

#### **4. Petrography and Microstructure**

## *4.1. Host Rocks*

The drill core was brecciated locally and cut by a distinctive stockwork of veinlets that is typical of porphyry deposits (Figure 2). The rocks were divided into a gray-colored quartz porphyry (Figure 2a) and green-gray tonalite/granodiorite porphyries (Figure 2b); these rocks are referred to as the host rocks. The quartz porphyry occurred from the surface to the middle of the core (0–500 m), and the tonalite/granodiorite porphyries occurred at a >500 m depth.

**Figure 2.** Simplified core log and representative hand specimens, with sampling depths shown. (**a**) Quartz porphyry. (**b**) Tonalite/granodiorite porphyry.

The quartz porphyry was pervasively sericitized with a sericite content of 40–60%; the quartz occurred as phenocrysts within the sericite groundmass (Figure 3a). Ore minerals, such as pyrite, chalcopyrite, and molybdenite, were disseminated within the host rock (Figure 3b), and ore minerals and other sulfides also occurred within the veins. The sericite-bearing samples contained pseudomorphs of plagioclase and chlorite, but the original texture of these minerals was commonly overprinted by white mica and sericite in the more intensely altered samples. The accessory minerals included rutile, anhydrite, and apatite.

The tonalite/granodiorite porphyry consisted mainly of plagioclase (~40 vol%), quartz (~30 vol%), minor chlorite, and rare biotite. These minerals occurred as phenocrysts within a groundmass of microcrystalline quartz, K-feldspar, and plagioclase (Figure 3c). The groundmass formed ~20% of the rock by volume and filled between the phenocrysts. The plagioclase showed cryptic zoning, had a dusty/cloudy appearance, and was partially altered to white mica (Figure 3d). The quartz grains were embayed, some were rounded, and CL-dark fractures were common. Biotite is considered indicative of the PA [15,18]. It occurred as relatively large crystals of up to ~2 mm diameter; included rutile, calcite, and quartz; and was partly altered to chlorite. Chlorite occurs as an alteration after mafic minerals

(e.g., amphibole); chlorite rims were commonly altered to white mica/sericite and contained inclusions of apatite, quartz, calcite, and rare anhydrite.

**Figure 3.** (**a**) Photomicrograph of sericitized quartz porphyry (221 m depth), cross-polarized light. (**b**) Backscattered electron (BSE) image of quartz porphyry and vein. (**c**) Tonalite/granodiorite porphyry (567 m depth), plane-polarized light. (**d**) Representative BSE image of the sample shown in (**c**). Mineral abbreviations are after [24].
