Unidirectional Solidification Textures Research Articles

Migration and focusing of porphyry deposit-forming fluids through aplitic mush of the Saginaw Hill cupola, Arizona, United States

Porphyry-type Cu ± Au ± Mo deposits form in the upper (ca. 2–5 km deep) parts of large, long-lived magmatic-hydrothermal systems in which mineralising fluids are thought to be derived from mid-to shallow-crustal magma chambers. Increasingly, however, magmatic systems are viewed as consisting of mush with minor and transient lenses of magma, with mush being a variably packed framework of crystals with interstitial melt and magmatic volatile phase (MVP). In this context, questions remain as to the source (mainly depth) and mechanisms of transport and focussing of the vast volumes of fluids required for shallow level porphyry-type mineralisation. Even more problematic is a paucity of first-order textural evidence for the presence of mush in magmatic-hydrothermal systems, including those which host porphyry-type deposits. To address this, we have studied the aplitic porphyry cupola of the Saginaw Hill magmatic system, Tuscon, Arizona, United States, where magmatic-hydrothermal features are exceptionally well exposed, including a massive silica cap, quartz unidirectional solidification textures (USTs), stockworks of multiple generations of variably mineralised quartz veins and mineralised miarolitic cavities. From field-to micro-scale textural and geochemical studies, particularly observations of vermiform quartz between earlier generations of magmatic quartz and feldspar, we evidence the development of fluid pathways through mush at the magmatic-hydrothermal transition. These are shown to connect and provide fluids and ore constituents to the mineralised miarolitic cavities and early quartz vein stockworks. We suggest that this process should be considered in all new genetic, exploration and numerical models for porphyry and similar types of magmatic-hydrothermal ore-deposits.

Quartz fluid inclusions and trace elements in the Lailisigaoer porphyry Mo deposit, Western Tianshan, Xinjiang: Fluid evolution and metallogenic mechanism in magmatic-hydrothermal system

The Lailisigaoer deposit is a typical porphyry Mo deposit located in the late Paleozoic Boluokenu metallogenic belt of Western Tianshan, NW China. This study aims to understand molybdenum mineralization and the dynamics of magmatic-hydrothermal fluid within the porphyry system, focusing on fluid inclusion, cathodoluminescence textures, and trace elemental composition in various generations of quartz. Six vein types in the Lailisigaoer deposit are classified as comb quartz veins with unidirectional solidification texture (UST, QI), actinolite/biotite ± quartz veinlets (M-type, QII), quartz-chalcopyrite vein (A-type, QIII), quartz-molybdenite vein (B-type, QIV), quartz-pyrite vein (D-type, QV) and quartz-carbonate vein (E-type, QVI). Micro thermometry data indicate decreasing homogenization temperatures in primary fluid inclusions from UST to E-type veins. The magmatic-hydrothermal fluids associated with UST and M-, A-, B-, D-, E-type veins underwent a transition from high temperature (342–495℃), high-pressure (∼1.25 kb), and variable salinity (6.7–41.1 wt%NaCl) in the H2O-CO2-NaCl system to low-temperature (142–238℃), low-pressure (∼0.25 kb), and low-salinity (1.6–5.1 wt%NaCl) in the H2O-NaCl system. This evolution is reflected in the proportion of quartz with oscillatory zones decreasing and irregular quartz increasing, indicating a shift from turbulent to stable fluid conditions. Furthermore, a gradual reduction in Ti concentration (from 79 ppm in UST to 1.4 ppm in E-type quartz) suggests a decrease in fluid temperature during the magmatic-hydrothermal evolution. Mineralization primarily occurs in A- and B-type veins, with molybdenum precipitation attributed to fluid immiscibility in a medium–high temperature (240–368 ℃) and high pressure (∼0.95 kb) environment. The A-type veins (mean 31 ppm in Ti concentrations) consist of equigranular (dark-core) (QIII-1) and patchy inhomogeneous CL-gray quartz (QIII-2). The B-type veins (mean 24 ppm in Ti concentrations) are characterized by equigranular (bright-core) inhomogeneous quartz (QIV-1) with growth zones that are cemented by later mosaic homogeneous CL-gray quartz (QIV-2). Based on the quartz-Ti thermometer, the mineralized veins (A- and B-type) were confined to be formed at depths of 2.8–3.6 km, with molybdenum mineralization occurring at 3.4–3.6 km. Molybdenum mineralization initiates during the QIV-1 crystallization period in magmatic-hydrothermal fluids and terminates in the QV-2 generation.

Tourmaline growth in the border and wall zones of the Emmons pegmatite (Maine, U.S.A.): Evidence for disequilibrium crystallization and boundary layer formation

Abstract The anisotropic textures, including unidirectional solidification textures and graphic intergrowths, characteristic for pegmatites, are interpreted to result from disequilibrium crystallization at high degrees of undercooling. Experimental studies have revealed the existence of thin boundary layers surrounding the rapidly growing crystals. Here, tourmaline-bearing samples from the outer zones of the Emmons pegmatite (Maine, U.S.A.) are used to examine if a boundary layer can also occur in natural samples. Crystal morphology is linked with geochemistry to understand the evolution of pegmatite melts and to constrain disequilibrium conditions at large degrees of undercooling. Petrographic studies and semiquantitative micro-X-ray fluorescence element mapping were conducted to identify crystal morphology and zonation, complemented with electron microprobe analyses to determine major and minor element compositions and LA-ICP-MS analyses of selected trace elements. Three textural groups were identified: comb-like tourmaline, quartz-tourmaline intergrowths, and radiating tourmaline. The intergrowths are optically coherent and are split into three different morphologies: central, second tier, and skeletal tourmaline. Most tourmaline is schorl, but chemical variation occurs on three different scales: between textural groups, between different morphologies, and intracrystalline. The largest scale geochemical variation is caused by the progressive evolution of the melt as it crystallized from the borders inwards, while the intracrystalline variations are attributed to sector zoning. A model is suggested where the systematic variation of Mg, Mn, and Fe within individual intergrowths is proposed to be the result of crystallization from a boundary layer, rich in water and other fluxing elements (e.g., Li, P, B), formed around the rapidly growing central tourmaline. Here, we show the first examples of boundary layers in natural pegmatites. Furthermore, the results bring into question whether boundary layer tourmaline can be used as a bulk melt indicator in pegmatitic melts.

Silver Behavior During Magmatic and Magmatic-Hydrothermal Evolution of a Highly Evolved Reduced Granitic System Related to the Giant Shuangjianzishan Ag-Pb-Zn-(Sn) Epithermal Deposit, Northeast China

Abstract Magmatic-related epithermal silver-rich polymetallic deposits are among the most important sources of Ag in the world, and they are found associated with magmatic systems with striking differences. Most of the time, they are associated either with I-type oxidized (magnetite-series granite) intermediate to evolved intrusions or with S-type/A-type reduced (ilmenite-series granite) highly evolved intrusions. To better understand these associations, the Ag evolution has been tracked during the magmatic differentiation and the magmatic-hydrothermal transition stage of A-type highly evolved porphyritic granites associated with the giant Shuangjianzishan Ag-Pb-Zn-(Sn) epithermal deposit, the largest known Ag deposit of Asia (145 million tonnes at 128.5 g/t Ag and 2.2 wt % Pb + Zn) located in the largest known metallogenic province for Ag in China (the southern Great Xing’an Range). At the Shuangjianzishan deposit, the porphyritic granite complex consists of three temporally distinct intrusions—a coarse-grained monzogranite porphyry, a fine-grained syenogranite porphyry, and a fine-grained syenogranite—having crystallized at ~2 kbar and ~750°C and recording a continuous magmatic differentiation trend. The silicate melt that generated the last highly differentiated intrusion (fine-grained syenogranite) is interpreted as the source of the mineralizing fluids forming the Shuangjianzishan Ag-Pb-Zn-(Sn) epithermal deposit, as it is the only intrusive unit that reached fluid saturation, as indicated by cotrapped fluid and melt inclusions in quartz phenocrysts and by the occurrences of unidirectional solidification textures (USTs). Silver evolution in the different porphyritic granite facies was reconstructed with laser ablation-inductively coupled plasma-mass spectrometry analyses of quartz-hosted silicate melt inclusions, amphibole-hosted magmatic sulfide inclusions, and chemical modeling. The silicate melt forming the porphyritic granite complex was sulfide saturated during the first crystallization stage, as shown by the occurrence of Ag-rich monosulfide solid solution (MSS) inclusions hosted in amphibole phenocrysts from the coarse-grained monzogranite porphyry and from mafic microgranular enclaves hosted in the coarse-grained monzogranite porphyry. However, these Ag-rich MSSs had only a minimal impact on the Ag budget of the magmatic system, as shown by the increase of the Ag concentration (~100–1,000 ppb) in quartz-hosted silicate melt inclusions during the further evolution of the system until fluid exsolution was reached. These results combined with mass balance modeling suggest that Ag and Sn are efficiently transferred to the evolving residual melt during crystallization and crystal-melt segregation. The results of this study indicate that highly Ag endowed epithermal polymetallic deposits can be formed from the exsolution of Ag-rich mineralizing fluids from relatively low volume, highly evolved, reduced melts, similar to those responsible for the formation of Sn-rich greisen deposits.

Textural characterization of unidirectional solidification textures related to Cu–Au deposits and their implication for metallogenesis and exploration

Ten rock samples with unidirectional solidification textures (USTs) from nine Asian and Australian porphyry and intrusion-related Au deposits and prospects were investigated in terms of mineralization sequences, textural and paragenetic relationships, melt and fluid inclusion inventory, cathodoluminescence, and trace elements of UST quartz. Despite general similar structural appearance of the investigated UST sequences, each UST locality has its own specific textural features. We provide a structural classification of observed USTs that distinguishes equigranular and heterogranular USTs, which are subdivided according to the UST layer structure into straight, wavy, or contorted/brain-like type. In addition, there is the group of disturbed USTs comprising incompletely developed UST layers, and ductile- or brittle-fragmented UST sequences. Ti-in-quartz-based calculations revealed crystallization temperatures of UST quartz between 590 and 770 °C. Primary silicate melt inclusions found in four samples indicate that quartz crystallized initially from a silicate melt. In five samples, fluid inclusion “boiling” assemblages were observed, documenting brine-vapor phase separation due to pressure drop. In one case, the “boiling” assemblage is in primary inclusions and in the other cases the assemblages have been trapped in pseudosecondary inclusions in a later stage. Primary brine inclusions without coeval vapor-rich inclusions were identified in one sample only. Our results permit UST formation in either an open-system setting where fluids exsolved from UST crystallization are periodically released by fracturing (pressure drops) after the formation of each individual UST layer, or closed-system setting where the exsolved fluids are trapped because of lack of fracturing as long the UST sequence forms. Here, the closed-system model is introduced for the first time to describe the formation of layered quartz UST sequences in porphyry systems where the open-system setting seems to be not applicable. Nonetheless, quartz UST sequences are very important exploration vectors for porphyry systems. Six out of the ten UST samples are high-grade with up to 12 g/t Au. However, in the investigated cases, the Au mineralization postdates UST formation. At Oyut Ulaan and Tampang, the UST quartz formation is co-genetic with major Cu–Fe sulfide mineralization, whereas in the other localities the UST formation predates the sulfide mineralization. In any case, UST layer sequences clearly functioned as an outer carapace and, thus, as a chemo-physical trap of passing mineralizing fluids entering the UST structure and may offer a vector to the main stockwork mineralization, which might occur 100 to 200 m above the UST zone.

Cassiterite geochemistry and U–Pb geochronology of the Shihuiyao Rb–(Nb–Ta–Be–Sn) deposit, Northeast China: Implication for ore‑forming processes and mineral exploration

The Shihuiyao giant Rb deposit in association with Nb–Ta–Be–Sn occurs within the Southern Great Xing’an Range tin belt known for its Sn, W, Cu, Ag, and base-metal deposits. The trace element compositions of four types of cassiterites hosted in the granite, greisen, unidirectional solidification texture (UST) quartz layer, and the hydrothermal quartz vein, respectively (abbreviated as Cst-I to Cst-IV) were measured by laser ablation inductively coupled plasma mass spectrometry and electron probe microanalysis to establish chemical criteria for the application of cassiterite as a useful geological tracer mineral. In situ U–Pb dating of Cst-I, Cst-III, and Cst-IV yielded Tera–Wasserburg intercept ages of 150.3±5.2, 145.8±4.6, and 146.9±3.5 Ma, respectively. These ages are consistent with the previously published monazite and columbite U–Pb ages of 150.2–141.0 Ma for the Shihuiyao ore-bearing granites, indicating a genetic relationship between granite magmatism and Ta–Nb–Sn mineralization. The composition of the studied cassiterites within the whole magmatic–hydrothermal evolution process yields some petrographic dissimilarities and geochemical trends from xenothermal to epithermal environments, which can be used to fingerprint the fluid evolution of the Shihuiyao deposit. For example, the magmatic cassiterites (Cst-I and Cst-II) are strongly enriched in Ta and yield low Nb/Ta ratios compared to the Cst-IV cassiterites from shallow hydrothermal veins. The occurrence of wodginite-group minerals and the highest W concentrations in cassiterite from the UST quartz layers suggests that the main enrichment in W is primarily controlled by magmatic fluid exsolution during the magmatic–hydrothermal transition stage. Additionally, hydrothermal cassiterite is generally lighter in color and characterized by complex internal oscillatory zoning with alternate dark (Cst-IVa) and light (Cst-IVb) bands. Cst-IVa cassiterites have similar W, Nb, and Ta contents but much higher Ti contents compared to Cst-IIIb cassiterites related to primary magmatic fluid, whereas Cst-IVb cassiterites exhibit sharply decreasing W but elevated Ti contents, suggesting pulsed exsolution of magmatic ore‑forming fluids and the involvement of external fluid in a distal open-fluid systems during the late mineralization stage. Therefore, the chemical variations of cassiterites are not only robust tracers pointing to magmatic–hydrothermal centers but also effective in discriminating the primary source of alluvial cassiterite and evaluating its metallogenic potential during a natural heavy mineral survey, supporting the use of cassiterite as an indicator mineral for exploration targeting.

Magmatic to hydrothermal evolution of the Gaogangshan Mo deposit, NE China: Mineralogical insights from quartz

Integrated study on cathodoluminescence (CL) textures and compositions of quartz is proven to be an effective way to reconstruct the ore-forming conditions and processes of porphyry system under subduction tectonic setting. However, little relevant research has been conducted on collision-type porphyry deposits. The Gaogangshan deposit in the northern Lesser Xing'an Range, NE China, consists of typical collision-type porphyry Mo mineralization related to Permo-Triassic granitic intrusions. Ore-forming process is recorded by multiple generations of quartz, including pre-ore stage quartz with unidirectional solidification texture (UST), early ore-stage quartz-molybdenite veins with K-feldspar alteration halos (Q1), late ore-stage quartz-sulfides veins with sericite alteration (Q2), and post-ore stage quartz veins (Q3) associated with calcite and fluorite. Cathodoluminescent textures, trace elements and fluid inclusions in quartz reveal physicochemical conditions, evolution of ore fluids and the Mo mineralization process. The UST quartz and Q1 veins are dominated by CL-bright homogenous and/or granular mosaic textures, containing more Ti concentrations (average = 35–42 ppm) than Q2 and Q3 veins. The molybdenite-bearing Q2 veins are dominated by CL-gray granular and zonal textures, displaying less CL intensity and lower Ti concentrations (average = 16 ppm) than early-stage quartz. The Q3 veins have the lowest CL-intensity and the lowest Ti concentrations (average = 2.5 ppm) among all quartz types. Three types of inclusions are identified in above quartz samples, including liquid–vapor aqueous inclusions, CO2-bearing liquid–vapor aqueous inclusions and halite-bearing multiphase aqueous inclusions. The ore-forming fluids in ore-stage Q1 and Q2 veins are dominated by large salinity variation (2.3–38.9 wt% NaCl equiv.), CO2-bearing (4.2–8.9 mol%) two-phase aqueous inclusions with vapor volumetric proportions of 30–65%. Intersections of fluid inclusion isochores with Ti-in-quartz isopleths yield quartz formation conditions of ∼2.2 kbar at ∼640℃ for UST quartz, ∼1.25 kbar at ∼510℃ for Q1, ∼1.0 kbar at ∼440℃ for Q2, and ∼0.37 kbar at ∼220℃ for Q3. The Gaogangshan porphyry Mo deposit formed at depths of 3.8 to 4.7 km.

Fluids and Melts at the Magmatic-Hydrothermal Transition, Recorded by Unidirectional Solidification Textures at Saginaw Hill, Arizona, USA

Abstract Fluid exsolution and melt evolution at the magmatic-hydrothermal transition are critical processes driving the metal enrichment of porphyry systems. Coeval fluid and melt inclusion assemblages in unidirectional solidification textures (USTs) at Saginaw Hill—a small, porphyry Cu system in southwestern Arizona—record a dynamic and repetitious process of fluid accumulation and release. The cores of quartz crystals throughout the UST bands host coeval silicate melt and brine inclusions but lack vapor-rich inclusions. This could indicate preferential expulsion of vapor and trapping of high-density brine during episodes of fracturing or the direct exsolution of single-phase high-salinity brine from the silicate melt. In contrast, the rims of UST quartz host abundant coeval brine and vapor inclusions, consistent with liquid-vapor immiscibility at lower pressures compared to the corresponding quartz cores. This transition from dominantly coeval silicate melt inclusions and brine in phenocryst cores to coeval brine and vapor in the rims suggests that the Saginaw Hill system underwent cyclic processes of fluid exsolution, accumulation, overpressure, and decompression at relatively stable temperatures (consistently ~650°C) during UST formation. Melt inclusion data indicate that the melt at this stage was highly fractionated and tended toward muscovite saturation. Metal concentrations in the brine were comparable to or higher than those in fluids reported in world-class porphyry Cu systems and were likely the result of both igneous fractionation and the high chloride content of the exsolved fluids. While limited in scale, Saginaw Hill provides evidence for processes that are predicted to occur at the magmatic-hydrothermal transition during the formation of large, well-mineralized porphyry systems.

Textural indicators of mineralisation potential in porphyry magmatic systems – A framework from the archetypal Yerington district, Nevada

Porphyry-type deposits are spatially and temporally associated with the relatively shallow and texturally complex parts of magmatic systems. Whilst certain textures offer snapshots into the physical processes which result in fluid exsolution and hydrothermal mineralisation, their documentation and interpretation remains disjointed. To address this, we describe a suite of magmatic and magmatic-hydrothermal textures from the classic Yerington Cu(-Mo-Au) porphyry district, Nevada, where Cenozoic extension and tilting has exposed a unique, ∼8 km palaeodepth, cross-section through the magmatic system. Within the granite cupolas that underlie the Ann Mason and Yerington porphyry deposits, these textures include pegmatitic pods and massive silica bodies. Emplaced through the cupolas, and genetically associated with ore formation, are aplite dykes that host mineralised unidirectional solidification textures (USTs), pegmatitic segregations, miarolitic cavities and early A-type quartz veins. Based on field relations, including associations with hypogene mineralisation, petrography and Ti-in-quartz crystallisation temperatures, we highlight how these textures may record the timing and location of the magmatic-hydrothermal transition and ore-formation. By doing so we provide a textural framework for exploration geologists to assess the likely 3D spatial and temporal architecture of porphyry mineralisation at the district-prospect scale before employing more invasive and expensive techniques.

Rock textures and mineral zoning – A clue to understanding rare-metal granite evolution: Argemela stock, Central-Eastern Portugal

The small leucocratic strongly peraluminous P, F, Li-rich granitic system at Argemela, central-eastern Portugal, is an ideal object for the study of relations between chemical composition of granitic rocks, rock-forming minerals and rock and mineral textures, and the associated Sn, Ta and W mineralization. To define the rock and mineral composition and to formulate an evolutionary model, traditional methods of bulk-rock chemical analyses, and EMPA and LA ICP-MS analyses of rock-forming minerals were combined with the study of macrotextures, TIMA-automated mineralogical mapping of typical rock samples, and CL study of internal texture of quartz and mica crystals. The Argemela stock forms a steep cylindrical body 1 km deep, with an elliptical outcrop 250 × 180 m in size, crosscutting Cambrian schists. The stock comprises two intrusions: a geochemically moderately evolved equigranular facies (0.3 wt% F, 1.2 wt% P2O5, 1850 ppm Li, 310 ppm Sn, 24 ppm Ta, 5 ppm W) composed of albite, quartz, muscovite and minor montebrasite followed by a rather inhomogeneous, more evolved porphyritic facies (0.25–1.2 wt% F, 1.3–1.8 wt% P2O5, 1600–4900 ppm Li, 600–1000 ppm Sn, 40–85 ppm Ta, 3–6 ppm W) composed of quartz, mica and subordinate K-feldspar phenocrysts embedded in an albite-quartz-mica-amblygonite matrix. The later facies is rimmed by stockscheider at the contact with slates, and with an up to 1 m thick layered zone with unidirectional solidification textures along contact with the equigranullar facies. The NE-part of the stock is crosscut by numerous thin quartz veinlets ±K-feldspar, phosphates and wolframite, while several aplitic dikes and thicker quartz+phosphate veins were found in two boreholes NW of the granite. Porphyritic facies, the most voluminous part of the system, contains strongly zoned phenocrysts of mica (phengite→lepidolite) and quartz (Ti-enriched to Al, Rb-enriched) indicating crystallization in two magmatic stages, while the equigranular facies and extragranitic aplitic dikes crystallized in one magmatic stage only. Hydrothermal muscovite enriched in Sn and Ta forms late overgrowths on some mica flakes in the porphyritic facies and in aplitic dikes. Based on a combination of chemical and textural observations, a four-stage evolutionary model of the Argemela stock was formulated: (i) an early magmatic stage comprises intrusion and in situ crystallization of aplitic dikes and the equigranular facies, and crystallization of quartz and mica cores in the deeper reservoir prior to intrusion of the porphyritic facies; (ii) in the late magmatic stage, crystal mush + evolved residual melt from the reservoir intruded upwards crystallizing the porphyritic facies including the stockscheider and layered zone. A small part of evolved melt formed rounded enclaves crystallizing as a phenocrysts-free ball facies. (iii) In the early (high-temperature) hydrothermal stage, magmatic fluid caused weak pervasive muscovitization of the porphyritic facies and aplitic dikes accompanied by the crystallization of disseminated cassiterite and columbite. After the transition from ductile to brittle deformation, quartz veinlets also developed, mainly in the NE part of the body. (iv) In the late, low-temperature stage, schist-derived fluid enriched in Ca, Sr and Ba caused local alteration of granite, forming crandallite–goyazite–gorceixite solid solution.

Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.

The Coupled THMC finite-element modeling of hydrothermal systems: Insights into the Jiama porphyry metallogenic system

The formation and localization of hydrothermal ore deposits largely depend on coupled thermal–hydraulic–mechanical–chemical processes (called coupled THMC processes) in a porphyry metallogenic system. However, these processes and the hydrodynamics of porphyry metallogenic systems remain unclear. In this study, we developed two-dimensional coupled THMC finite-element models to simulate the spatiotemporal evolutions and interactions among the temperature, pore pressure, deformation, and migration of Cu in the Jiama Cu-polymetallic porphyry metallogenic system and to investigate the hydrodynamic mechanisms based on the geology of this deposit. The results show that the low-permeability upper layer of the wall rock, inhomogeneous thermal expansion, and continuous and multiple intrusions of magma induce an abnormally high pore fluid overpressure zone above the Jiama porphyry intrusion. The high overpressure zone and its evolution above the porphyry intrusion play key roles in the formation of the hydrothermal metallogenic system. First, the evolution of the overpressure changes the fluid flow patterns during different periods of the hydrothermal metallogenic system, that is, the outward flow from the porphyry in the early stage and the backflow towards the porphyry in the late stage. These two different flow patterns are consistent with the observations of mineral growth bounds and unidirectional solidification textures. Second, the high overpressure leads to the appearance and development of fractures above the intrusion. These fractures conform to the ore-bearing spaces of the hornfels and porphyry orebodies above the intrusion. Finally, the evolution of the overpressure can be used to predict the Cu concentration in interlayer detachment zones, which is consistent with geological observations based on which the interlayer detachment zones contain skarn orebodies. In this study, an overpressure-driven hydrodynamic process of a hydrothermal metallogenic system is illustrated, physical and mechanical explanations for geological observations in the Jiama porphyry metallogenic system are provided, and the advantages of numerical simulations in studies of complex multi-field coupled geological systems are highlighted.

Temporal-spatial variations in Li-Fe mica compositions from the Weilasituo Sn-polymetallic deposit (NE China): Implications for deposit-scale fluid evolution

Abstract Spatial-temporal compositional variations in hydrothermal mica from different mineralization stages were determined across a single cross-section at the Weilasituo tin-polymetallic deposit, Northeast China. Hydrothermal mica from the magmatic-hydrothermal transitional stage (unidirectional solidification textures), hydrothermal tin-tungsten stage, molybdenum stage and copper-zinc stage was analysed in situ for major and trace element composition. Weilasituo mica is enriched in Li, Rb, Cs and Zn, has a moderate W and Sn content and a low Cu content. The temporal variation in mica composition shows that the Li content decreases slightly from the early to the late stage, while Zn concentration shows the opposite trend. Other elements in mica are relatively constant between stages, with only Sn, Rb and Cs being slightly enriched in the hydrothermal tin-tungsten stage mica. According to the spatial variations in mica composition, mica with a high W, Sn, Nb and Ta content occurs in granite, while mica enriched in volatile elements like Li and F is distributed within the main tin ore vein, or adjacent to the breccia pipes. The results suggest that mica W-Sn-Nb-Ta enrichments are genetically related to an early high-temperature magmatic fluid linked to the granitic intrusions at Weilasituo, while volatile elements were enriched by successive evolution of lateral fluid migration. The trace-element content of the Weilasituo mineralizing fluid (estimated from mica composition and rough estimates of mica-fluid partition coefficients) suggests that the fluid related to different mineralization originated from the same source, and does not support obvious magmatic-meteoric fluid mixing. Finally, the results suggest that spatial-temporal variations in mica compositions can be taken as a potential indicator of ore fluid compositional evolution in a hydrothermal system.

Efficient enrichment of Rb during the magmatic-hydrothermal transition in a highly evolved granitic system: Implications from mica chemistry of the Tiantangshan Rb-Sn-W deposit

Rare metal mineralization is commonly associated with highly differentiated granitic systems, which evolve into pegmatite or hydrothermal veins. The newly-discovered large Tiantangshan Rb-Sn-W deposit provides an excellent opportunity to study Rb enrichment, as the Rb resources are mainly hosted in the hydrothermal micas (dominantly veins), rather than the K-feldspar or magmatic micas in the parent alkali feldspar granite and volcanic rocks, as is typical of other Rb-rich systems. Micas at Tiantangshan are found in the alkali feldspar granites, trachyandesites, layers with unidirectional solidification textures (USTs) and hydrothermal veins. The mica Rb2O concentrations in the alkali feldspar granites range from 0.19 to 0.81 wt% (mean 0.44 wt%), in the trachyandesites from 0.47 to 1.13 wt% (mean 0.65 wt%), in the USTs from 0.41 to 1.26 wt% (mean 0.81 wt%), and in the hydrothermal veins from 0.39 to 1.28 wt% (mean 1.02 wt%), and show a positive correlation with F and Li contents. The mica and K-feldspar in the alkali feldspar granites have low K/Rb values (13–29 and 25–103, respectively), indicating the highly evolved nature of the alkali feldspar granite. K/Rb and K/Cs values decrease gradually in the mica from the alkali feldspar granites to the USTs and then to the hydrothermal veins, whereas Li, F and Rb increase. Based on the textures and geochemistry of the micas, they are cogenetic with the alkali feldspar granites and represent the magmatic stage, whereas the USTs formed during the magmatic-hydrothermal transition stage and the hydrothermal veins correspond to the hydrothermal stage. The Tiantangshan micas record mineralogical and geochemical evidence of efficient Rb mineralization during magmatic-hydrothermal evolution in the alkali feldspar granitic system.

Geochronology, geochemistry and fluid inclusions of the Yechangping giant porphyry-skarn Mo-W deposit, East Qinling, China

The Yechangping giant porphyry-skarn Mo-W deposit is located in the East Qinling molybdenum belt, Henan Province. Molybdenum mineralization occurs mainly as veins and veinlets or disseminated within dolomite and granite porphyry. The mineralized granite porphyry, emplaced at 148.1 ± 1.0 Ma, has petrological, mineralogical, and geochemical characteristics of I-type granite. The rock belongs to the high-K calc-alkaline to shoshonitic series and is characterized by enrichments in large ion lithophile elements and significant negative Eu, Ba, Sr, P, and Ti anomalies, attributed to strong fractional crystallization. The granite has negative whole-rock εNd(t) values (−13.2 to −12.4) and zircon εHf(t) values (−14.2 to −6.8), corresponding to old whole-rock Nd and zircon Hf model ages, which indicate that the primary magma was derived from partial melting of the ancient lower crust of the North China Craton.Four mineralization stages are identified in the Yechangping deposit. Stage I is a magmatic-hydrothermal transitional stage, which is respresented by the 1–5 mm-wide unidirectional solidification texture (UST), formed from fluids with high temperatures (375–504 °C), high/low salinities (46.3–51.0 vs. 7.81–13.75 wt% NaCl equiv.), and a high oxygen fugacity in an H2O-NaCl-CO2 fluid system. Stage II quartz-K-feldspar stockworks or quartz-K-feldspar-pyrite veins, with minor molybdenite, which are characterized by the K-feldspar and quartz with potassic-quartz alteration; they were formed from fluids with medium-high temperatures (320–464 °C), high/low salinities (38.8–43.4 vs. 5.41–12.85 wt% NaCl equiv.), and a relatively high oxygen fugacity. Stage III quartz-molybdenite stockworks and quartz-molybdenite-polymetallic veins, associated with quartz-sericite alteration, were formed from fluids with medium-low temperatures (273–396 °C), and variable salinities (3.38–40.0 wt% NaCl equiv.). Stage IV quartz-carbonate and fluorite veins, associated with fluid-related argillic alteration were characterized by low temperatures of 171–245 °C and low salinities of 3.23–8.00 wt% NaCl equiv.. The prolonged fractional crystallization, the high oxidization state of the magmatic-hydrothermal system and the fluid boiling process were the key factors that have contributed to the formation of the Yechangping giant Mo-W deposit.

Geological and mineralogical constraints on the genesis of the Bilihe gold deposit in Inner Mongolia, China

The Bilihe gold deposit is a rare example that economical gold mineralization occurred in unidirectional solidification textures (UST) quartz at the cupola of middle Permian granodioritic intrusion. However, there is a key dispute about whether gold precipitated from low-diffusion silicate melt or high-activity fluid phase. In this paper, based on in-situ trace elements analyzing, SEM-CL mapping, and infra-red and Raman spectroscopy, the magmatic fluids are supposed to play a critical role in the gold mineralization by metal extraction from below magma at magmatic-hydrothermal transition stage. At Bilihe, the UST quartz represents the most fertile unit. Statistic results show the quartz–K-feldspar (KV) and gray banded quartz veins (GBV) also host about 22–27% of total reserves. A decreasing trend in Au/Ag, and Au/S, but elevated Au/Cu ratios of gold compositions indicate a relatively continuous fluid evolution in the Bilihe system. Petrography and spectroscopy studies show that the Bilihe UST quartz can be divided into two different domains, gold-rich brownish and barren colorless zones. The former can contain more water, occurring as tiny fluid inclusion or as structural OH– in muscovite, corresponding to sector CL zones (UQ1) or euhedral oscillatory bands (UQ2). The latter usually contain melt inclusions and correspond to obscure CL-textures (UQ3). Besides, the LA-ICP-MS trace elements show that there is a significant decreasing trends on fluid-mobile elements (e.g., K, Fe, B, and Li) from UQ1 over UQ2 to UQ3. In particular, the trace elements in UQ3 show greater affinity with magmatic quartz phenocrysts but with higher Ge, B, and Li contents, and higher Ge/Ti and Ge/Al, but lower Li/Ti ratios, indicating a highly evolved and fluid-rich environment. Based on previous model, we consider that the Bilihe UST quartz grew in the variable growth mediums, namely fluctuant alternation between fluid-dominant pocket (for UQ1 + UQ2) and silicate melt phase (for UQ3) near the melt-fluid interface at the apex of magma body. The relatively H2O-poor nature of parent magma is an important factor to cause the specific crystal growth environment. Mineral thermobarometers demonstrate that the parent magma from deep reservoir has experienced a significant ascent or decompression (from ~ 3–5 kbar to ~ 0.5–1.8 kbar). In addition, high fluxing volatiles, such as B, F and P, effectively protract magma crystallization to form highly evolved melt, and finally resulting in water saturation at shallow level. Thus, gold saturation might reach in earlier exsolving low-salinity fluids at low pressure (as low as 0.5 kbar) and high temperature (~660–700 °C) condition. Then voluminous fluids get release accompanying with hydrofracturing, and some gold sequentially precipitate in the KV with fluids cooling (<550 °C) and later in the GBV with further cooling and flashing during transition from lithostatic to hydrostatic loads. With increasing sulfur fugacity by fluid condensation and further cooling in fluid temperature, secondary gold can precipitate with possible Cu-(Fe) or As-Sb sulfides associated with pervasive intermediate argillic alteration, which intensively overprinted on earlier potassic alteration zone.

Cathodoluminescence features, trace elements, and oxygen isotopes of quartz in unidirectional solidification textures from the Sn-mineralized Heemskirk Granite, western Tasmania

Distinctive quartz-rich unidirectional solidification textures (USTs) occur in apical carapaces of the Sn-mineralized Heemskirk Granite in western Tasmania (SE Australia). They are spatially associated with abundant tourmaline-filled orbicules and cavities that have been overprinted by widespread tourmaline-quartz veins. Multiple UST-quartz layers that are intercalated with aplitic layers, and can locally extend for hundreds of meters. Individual UST layers consist dominantly of hexagonal quartz (>95%) with minor K-feldspar, plagioclase, biotite, muscovite, and magnetite. Scanning electron microscope-cathodoluminescence (SEM-CL) reveals that the aplitic quartz is homogeneous and CL-bright with minor CL-dark patches. The bases of the UST quartz crystals are homogeneous and CL-bright with minor thin CL-dark fractures, whereas the trigonal apexes of the UST-quartz display CL-oscillatory growth zones. LA-ICP-MS analyses show that UST-quartz has lower Ti, Li, and Sn than aplitic quartz, but higher Al, Li, Na, K, Mn, Fe, Ge, Rb, and Cs concentrations. At a pressure of ca. 1.3 kbar, the Ti-in-quartz geothermometer yields temperatures of 545 ± 40 and 580 ± 20 °C for the UST and aplitic quartz, respectively. The UST-quartz has higher Al/Ti values of 5.8 to 32, and Ge/Ti values of 0.02 to 0.16, than quartz phenocrysts in aplite layers, which is consistent with crystallization from a highly evolved fluid. The O-isotopic compositions (+5.1 to +10.2‰) of UST and aplitic quartz are consistent with magmatic source circulated by minor meteoric and/or formation waters. Magnetite crystals in USTs have low Cr, V, Ni, Co, Cr, Sc, and high Ti, Al, Mn, Sn, Ga contents, and are overgrown by chlorite. These minerals are interpreted to have formed at UST-melt interface where hydrothermal fluids reacted with igneous minerals. The results show that the UST layers in the Heemskirk Granite precipitated from magmatic-hydrothermal aqueous fluid exsolved from granitic melt during emplacement into the shallow crust (6–10 km). Such UST layers are characteristics of mineralized intrusions, and therefore provide significant indications for mineral exploration.

The Passa Três lode gold deposit (Paraná State, Brazil): An example of structurally-controlled mineralisation formed during magmatic-hydrothermal transition and hosted within granite

The Passa Três granite is a NNE-SSW elongated shape intrusion located in the Paraná State (southern Brazil). This intrusion was emplaced within metapelites of the Mesoproterozoic Votuverava Group, which are part of the N040°E trending Lancinha Shear Zone. Gold mineralisation within the Passa Três granite is held by meter-scale quartz veins forming orebodies with variable massive, banded, sheared and/or brecciated internal textures. Structural data indicate the existence of two orthogonal directions of structures, N-S and E-W, dipping 60–75°W and 45–70°S respectively. Mineralised veins contain, in addition to the quartz of the gangue, and in chronological order: fluorite, pyrite, chalcopyrite, gold, aikinite, molybdenite, muscovite, carbonate and baryte. Gold occurs as native grains within fractures that affect pyrite, commonly associated with chalcopyrite and aikinite. Quartz veins are sometimes bordered by aplitic dykes. Additionally, some of the veins exhibit a thin margin of K-feldspar minerals that could represent the early stage of vein formation. These observations, the presence of Unidirectional Solidification Textures (UST), along with abundant expression of pegmatite-rich pockets within granite that crops out close to the surface allow the reconstruction of the granite architecture in terms of late magmatic evolution, magmatic-hydrothermal transition and position of the cupola. N-S and E-W systems are interpreted to be contemporaneous and conjugate. Normal displacements are predominant and main mineralised veins are essentially located within extensional pull-apart structures. The structural model suggests that the normal faults were initiated along former low-angle planes associated with aplite and/or early quartz veins that subsequently controlled the opening of the pull-apart structures that represent the economic orebodies. Zircon from the granite and muscovite grains from mineralised veins were dated by U-Pb and Ar-Ar methods, respectively. Zircons from the main facies (medium grained: GEM and microgranite: GEF) provided undistinguishable ages which were pooled to give an age of 611.9 ± 3.6 Ma. A less abundant, leucocratic facies (“white granite”: GEB) yielded a significantly younger age of 592.8 ± 7.1 Ma. Muscovites from quartz veins gave Ar-Ar ages of 612.9 ± 2 to 608.8 ± 2 Ma (transitional vein with a K-feldspar border), 611.7 ± 2 to 608.8 ± 2 Ma (mineralised veins) and 608.4 ± 2 Ma (barren quartz vein). Thus, the Passa Três granite and its gold mineralisation appear to represent a unique example of an intrusion-related gold system in which the mineralisation concentrates in the core of the magmatic intrusion in the form of meter-scale veins strongly controlled by tectonics.

Cathodoluminescence features, trace elements and oxygen isotopes of quartz in unidirectional solidification textures from the Sn-mineralized Heemski rk Granite, western Tasmania

Cathodoluminescence features, trace elements and oxygen isotopes of quartz in unidirectional solidification textures from the Sn-mineralized Heemski rk Granite, western Tasmania

Geology of the Boyongan and Bayugo Porphyry Cu-Au Deposits: An Emerging Porphyry District in Northeast Mindanao, Philippines

The Boyongan and Bayugo porphyry Cu-Au mineral deposits, discovered under postmineralization cover during the previous decade, are part of an emerging belt of intrusion-centered Au-rich Cu mineral deposits and prospects in the Surigao district of northeast Mindanao, Philippines. Since their formation in the early Pleistocene, exhumation and weathering of these deposits have led to the development of a 600-m-thick oxidation profile at Boyongan and a modest (30–70 m) oxidation profile at Bayugo. Debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben subsequently covered the weathered deposits, preserving their supergene profiles. The mineral deposits formed in association with a composite diorite complex containing at least 12 discrete intrusive stages. Three premineralization diorite porphyry stocks and a silt-sand matrix breccia complex represent early stages of magmatism and brecciation. Significant Cu and Au introduction followed these events and occurred in association with small early-mineralization diorite porphyry stocks at Boyongan and Bayugo. Within the diorite complex, the two mineral deposits are spatially distinct, separated by approximately 1 km of premineralization diorites. Inter- and late-mineralization intrusions were emplaced as the magmatic-hydrothermal system waned. A characteristic progression of vein and K silicate alteration styles affected each of the synmineralization intrusions. Rare comb quartz unidirectional solidification textures (stage 0) mark the transition from magmatic to hydrothermal conditions. Quartz-poor wispy magnetite-biotite-K-feldspar veinlets characterize stage 1. Stage 2 veins consist of quartz with selvage and/or disseminated magnetite or biotite and K-feldspar halos. Stage 3 quartz veins have K-feldspar halos but generally lack magnetite and biotite. Stage 4 veins consist of massive bornite-chalcopyrite and chalcopyrite-pyrite with K-feldspar halos. Stage 3 quartz veins and stage 4 sulfide veins host Cu-Au mineralization of the greatest volumetric significance, reflecting the general paucity of sulfide in the earlier vein stages. Despite the simplicity of this sequence, detailed paragenetic reconstructions reveal that this characteristic progression of veining and K silicate alteration was repeated with the emplacement of each synmineralization intrusive event, revealing multiple magmatic-hydrothermal cycles of alteration and mineralization. At Boyongan and Bayugo, intense and pervasive illite alteration, in association with pyrite, chalcopyrite, and tetrahedrite-tennantite, developed in narrow structures crosscutting quartz-K-feldspar veins. Debris flows in the burial sequence above and adjacent to the Boyongan/Bayugo complex also contain abundant clasts with intense, pervasive illite and alunite-pyrophyllite-dickite-kaolinite alteration assemblages that have overprinted K silicate-style quartz veins. In both mineral deposits, Cu and Au are associated with intense quartz-vein stockworks composed primarily of K silicate stage 3 veins. Despite this association, not all of these quartz-vein stockworks contain Cu and Au to the same tenor. Quartz-vein stockworks in the eastern high grade of Boyongan have been intersected over a vertical interval of 800 m, having affected much of the early-mineralization stock. However, hypogene Cu grades exceed 0.5% by weight only in the upper 300 m of the stockwork (in the cupola of the early-mineralization stock). Superior grade development in high-grade zones at Boyongan, locally exceeding 2% Cu and 3 g/t Au, developed where fertile vein stages from two or more magmatic-hydrothermal cycles affected the same wall rock. The documented paragenetic relationships demonstrate multiple discrete cycles of K silicate-stage veining and alteration associated with each synmineralization intrusive event. All such events predate formation of feldspar-destructive illite-smectite-chlorite, illite-pyrite, and quartz-alunite-clay assemblages. Existing geochronological constraints on the timing of magmatism and hydrothermal activity demonstrate that these repeated cycles supporting superior grade development transpired extremely rapidly, in a period of less than 200,000 years (2.3–2.1 Ma). Geologic and geochronological constraints on the life cycle of Boyongan and Bayugo describe an extremely dynamic history of emplacement, exhumation, weathering, and burial over a period of 2.3 m.y. The study illustrates the spectrum of metallogenic processes operative over a geologically brief period and highlights some of the key elements responsible for formation of superior grades and for deposit preservation in an extensional setting.