Magmatic-hydrothermal System Research Articles

Hydrothermal Alteration Mineralogy Associated with Gold Mineralization in Buladu Area, Gorontalo, Northern Sulawesi, Indonesia

Geologically, the Buladu area is part of the North Sulawesi magmatic arc, located in the morphology of the mountains along the coastline of North Gorontalo district, Indonesia. Multiple subductions of North Sulawesi in the northern arm of Sulawesi and east Sangihe subduction in the east has triggered an active magmatism and volcanism. Formation and occurrences deposits of precious- and base metals have been found in the North Sulawesi Volcanic Arc. This paper describes a recent study on hydrothermal alteration mineralogy associated with one of the prospects in the region, namely Buladu gold prospect that for years has been operated by local artisanal miners. The study is focused on hand specimen and microscopic observation as well as XRD analysis. Lithologically, the study area is arranged by sandstone are members of Dolokapa volcano-sedimentary formation, above unconformity andesitic-basaltic volcanic breccia are members of the Pliocene Wobudu Formation and is intruded by granodiorite rocks. The petrographic study reveals that host rock of the mineralization is flow-volcanic breccia which shows porphyritic and flows structures. Resulting assemblages of secondary minerals are mineral associations characteristic of the magmatic-hydrothermal system the following imperative alteration zones could be recognized an outer zone of chloritic alteration and the inner zone of argillic alteration. The alteration showed characteristics of pervasive to selective alteration, where clay and quartz totally replaced groundmass, whereas chlorite and clay minerals were selectively altered plagioclase phenocrysts. Clay mineral species identified by XRD include chlorite and illite. These mineralogical features indicate that the hydrothermal alteration is an argillic type, which is spatially distal to the mineralization. Mafic mineral in andesite lava partly altered into mineral assemblage chlorite-epidote-quartz-carbonate show propylitic type, which is the outermost zone of mineralization.

Genesis of the Gaosong Sn–Cu deposit, Gejiu district, SW China: Constraints from in situ LA-ICP-MS cassiterite U–Pb dating and trace element fingerprinting

The Gaosong Sn–Cu deposit in the Gejiu ore district, SW China, is a large cassiterite–sulfide deposit, hosting total ore reserves of 67.4 Mt @1.3% Sn and 19.0 Mt @1.1% Cu. Two types of mineralization developed in the deposit, i.e. cassiterite-sulfide ore within the proximal skarn in contact with the Gaofengshan biotite granite, and fault-controlled stratiform-like cassiterite–iron oxide ± sulfide ore in the distal carbonate country rock. Cassiterite grains are disseminated in both ores. In situ U–Pb dating of one cassiterite sample from the cassiterite-sulfide ore and two from the cassiterite–iron oxide ± sulfide ore yielded weighted mean 206Pb/238U ages of 84.3 ± 1.4 Ma, 85.1 ± 1.0 Ma and 83.5 ± 2.1 Ma, respectively. The three mineralization ages are consistent with the emplacement age of the Gaofengshan granite (85.8 ± 0.6 Ma) within error, suggesting a close temporal link between tin mineralization and granitic magmatism.Cassiterite grains can be divided into two types based on cathodoluminescence (CL) illustrations, namely euhedral Cst I and veinlet-like Cst II. In the cassiterite–sulfide ore, Cst I has clear oscillatory zonation and is dull luminescing. In the cassiterite–iron oxide ± sulfide ore, Cst I core is dull luminescing (blue in OM−CL) and surrounded by a bright luminescing Cst I rim (yellow in OM−CL). Cst I is replaced by Cst II veinlets with bright luminescing (yellow in OM−CL). The SEM−CL luminescing intensity of cassiterite is mainly controlled by Ti content. Elevated W and Fe but low Nb, Ta, Zr and Hf concentrations of cassiterites from the Gaosong deposit, are distinctly different from those of cassiterites in VMS/SEDEX tin deposits, but similar to those from granite-related tin deposits. The Late Cretaceous crystallization ages and trace element characteristics demonstrate that cassiterites from both ores were formed in a magmatic-hydrothermal system associated with the Gaofengshan granite, and preclude the SEDEX origin of the stratiform-like cassiterite–iron oxide ± sulfide ore.

Archean Sanukitoid Gold Porphyry Deposits: A New Understanding and Genetic Model from the Lac Bachelor Gold Deposit, Abitibi, Canada

In the Abitibi subprovince of the Superior craton, Canada, recognition of intrusion-related gold deposits is improving, but their formation remains poorly understood. The recently reopened Lac Bachelor deposit in the northeastern part of the Abitibi subprovince is located on the edges of a syenitic complex, and this deposit is an example of a highly fractionated, Fe-Mg–poor, quartz-syenite to alkali-granite centered deposit. Gold is associated with pyrite in localized alkaline, fluorine, and hematite-rich metasomatized zones in magnetite-rich host rock. Minor mineralization is present in a quartz vein stockwork on the edges of the syenitic complex. A genetic model is proposed, wherein alkaline-oxidized magma, which favors sulfur and gold solubility and transport, migrates through the crust via major and subsidiary faults. The presence of miaroles, transolvus, and low-temperature crystallization conditions confirms the shallow emplacement of the magma. The high fluorine content of the magma, which lowers both viscosity and solidus temperature, favored the shallow emplacement of the magma and permitted the development of a long-lived magmatic-hydrothermal system. Oxygen and hydrogen isotopes, miaroles filling, and mineralized zone paragenesis confirm that oxidized hydrothermal fluids were exsolved from the magma. The planar geometry of the metasomatized ore zones indicates that hydrothermal fluid circulation and metasomatism reactions occurred preferentially in high permeability pathways, such as preexisting faults in the host rock. The redox contrast between the oxidized magmatic-hydrothermal fluid and the intermediate volcanic host rock caused sulfide precipitation and gold deposition. This genetic model is applied to the Abitibi subprovince and other Archean cratons.

Evolution of High-Level Magmatic-Hydrothermal Systems: New Insights from Ore Paragenesis of the Veladero High-Sulfidation Epithermal Au-Ag Deposit, El Indio-Pascua Belt, Argentina

AbstractThe world-class Veladero high-sulfidation epithermal Au-Ag deposit is located in the Andean cordillera of Argentina near the northern end of the El Indio-Pascua metallogenic belt. The deposit comprises two nearly coalescing subhorizontal orebodies that are centered on an extensive zone of intense hydrothermal alteration. Intensely altered volcanic rocks are composed of fine-grained groundmass quartz that formed as a result of extreme acid leaching. These quartz grains contain ubiquitous rutile inclusions as well as healed microfractures of vapor-filled inclusions that record magmatic vapor streaming through the Miocene volcanic host succession. Condensation of the magmatic vapor into ambient groundwater generated the highly acidic waters responsible for the alteration. Alunite is present in the fine-grained groundmass quartz and fills vugs in the altered rocks. Stable isotope data indicate that the alunite formed through the disproportionation of SO2 in the condensed magmatic vapor.The fine-grained groundmass quartz is crosscut by later fracture-controlled euhedral quartz that is texturally associated with ore minerals. The euhedral quartz crystals show oscillatory growth zoning and contain rare primary fluid inclusions suggesting that quartz formation occurred at ~200°C from a moderately saline (<5 wt % NaCl equiv) liquid-phase hydrothermal fluid. High-fineness native Au grains are hosted in euhedral quartzlined void spaces and along fractures. In addition to native Au, vugs and fractures in the silicified volcanic rocks host Fe oxide/hydroxide and jarosite that are interpreted to represent the oxidation products of hypogene sulfide minerals that formed during and after the late stages of quartz formation. Results of previous jarosite dating suggest that pervasive oxidation of the orebody commenced during the waning stages of the hydrothermal activity or immediately thereafter. Oxidation of the orebody continued in the supergene environment for at least 3 m.y. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) showed that jarosite, which formed as a result of the oxidation of the orebody, is the principal host for Ag in Veladero ore, explaining the low (ca. 10%) Ag recovery from the oxide ore.The Veladero high-sulfidation epithermal deposit is interpreted to have formed in the shallow part of a magmatic-hydrothermal system. Early alteration related to magmatic vapor discharge was followed by later mineralization from liquid-phase hydrothermal fluids under reduced and slightly acidic to near-neutral conditions. This change from early vapor-dominated to later liquid-dominated magmatic-hydrothermal fluid flow was key in formation of the deposit.

The Magmatic to Magmatic-Hydrothermal Evolution of the El Laco Deposit (Chile) and Its Implications for the Genesis of Magnetite-Apatite Deposits

Abstract The geology and geochemistry of the El Laco iron oxide deposit (Central Andes, Chile) support a genesis related to the ascent, degassing, and subvolcanic emplacement of an unusual oxidized silica-poor but water-and iron-rich melt that took place during the growth of the host Pliocene-Holocene andesitic volcano. The model proposed in this paper for the evolution of the deposit involves the formation of a shallow telescoped magmatichydrothermal system with complex melt-fluid unmixing in a vertical column of less than 1 km. The dominant mineralization occurs as large stratabound apatite-poor magnetite bodies interfingered with an andesite host and rooted in vertical dikes of magnetite with minor apatite. The stratabound mineralization is crosscut by abundant coeval diatreme-like structures indicative of vigorous degassing. The andesite underlying the mineralization is pervasively replaced by a high-temperature alkali-calcic alteration assemblage (K feldspar-diopside-magnetite-scapolite) that includes coarse-grained diopside-magnetite-anhydrite veins and large subvertical bodies of magmatic-hydrothermal breccias. The host andesite also shows a large strata-bound steam-heated acid alteration that is devoid of any magnetite but has produced the replacement of a significant proportion of the early magnetite by hematite. The El Laco system is rich in anhydrite but poor in sulfides, suggesting that there were persistent oxidizing conditions that inhibited the formation of a sulfide-bearing mineralization. Field evidence, oxygen isotope geothermometry, and thermodynamic constraints suggest that the magnetite mineralization formed close to the surface at temperatures above 800°C. The magnetite textures, similar to those of subaerial low-viscosity basalts, and the presence of melt inclusions in the host andesite recording the presence of immiscible Fe-Mg-Ca-(Si-Ti-P-S) and Si-K-Na-Al melts, suggest that the magnetite ore formed by direct crystallization from an iron-rich melt; its chemistry inhibited the formation of most other magmatic phases except minor apatite, anhydrite, and diopside. The crystallization of the iron-rich melt at shallow depths promoted the separation of large amounts of two immiscible aqueous fluids: a dominant low-density vapor phase and a small volume of hypersaline fluid. Diopside-magnetite-anhydrite veins are interpreted as the product of the crystallization of the residual melts, whereas the interaction of the brine with the host andesite formed the deep alkali-calcic hydrothermal assemblage. The condensation and mixing of the low-density magmatic vapor with meteoric water produced the steam-heated alteration. Isotope data from the host andesite (87Sr/86Sr: 0.7066–0.7074; εNd: −5.5 to −4.1; δ18Owhole rock: 7.2–9.6‰; δ18Omagnetite: 5.1–6.2‰) and an underlying andesite porphyry (87Sr/86Sr: 0.7075–0.7082; εNd: −5.9 to −4.6) reflect the interaction of a primitive mantle melt with Andean crustal rocks. The isotope geochemistry of the magnetite ore (87Sr/86Sr: 0.7083; εNd: −5.4 to −5.1; δ18O 3.5–5.5‰) and the alkali-calcic alteration and related diopside-magnetite-anhydrite veins (87Sr/86Sr: 0.7080–0.7083; εNd: −5.1 to −4.6; δ18Odiopside: 7.2–8.2%c; δ18Omagnetite 4.4–6.3‰) show that the mineralization has a more crustal signature than the host andesite and all the volcanic rocks of the Central Andes. Therefore, ore-forming fluids/melts were not equilibrated with the host volcanic rocks and are interpreted as related to a deep yet undiscovered batch of highly contaminated igneous rocks. Crustal contamination is interpreted as due to major interaction of a juvenile melt with the underlying Late Mesozoic-Tertiary Salta Group, located 1 to 6 km beneath the volcano and which has high 87Sr/86Sr values (0.7140–0.7141).

Geological and Chronological Constraints on the Long-Lived Eocene Yulong Porphyry Cu-Mo Deposit, Eastern Tibet: Implications for the Lifespan of Giant Porphyry Cu Deposits

Abstract The Yulong porphyry Cu-Mo deposit, the third largest porphyry Cu deposit in China, contains proven reserves of > 6.5 million metric tons (Mt) Cu and 0.4 Mt Mo. Previous radiometric dating studies have provided numerous ages for this deposit, but the timing and duration of the process governing the deposition of Cu and Mo remains not well constrained. In this paper, we first document multiple stages of mineralization and hydrothermal alteration associated with distinct magmatic pulses at Yulong by field and textural relationships, and then present high-precision molybdenite Re-Os ages of 14 quartz-molybdenite ± chalcopyrite veins representing these stages to precisely constrain the timing and duration of Cu-Mo mineralization. The ore-hosting Yulong composite stock consists of three successive porphyry intrusions: (1) monzonitic granite porphyry (MGP), (2) K-feldspar granite porphyry (KGP), and (3) quartz albite porphyry (QAP). The vein formation, Cu-Mo mineralization, and ore-related alteration are grouped into early, transitional, and late stages with respect to the intrusive history. The first two porphyry intrusions are followed by cyclical sequences of veining that are mainly associated with potassic alteration and have formed (1) ME vein/USTT, (2) EBE/T veins, (3) A1E/T veins, (4) A2E/BT veins, and (5) A3E/T veins. A2E/BT and A3E/T veins of the early and transitional stages are dominated by quartz and chalcopyrite ± pyrite, respectively, and represent the main Cu-Mo mineralization events. More than 80% of Cu and Mo at Yulong were deposited in the early stage with the remainder being formed in the transitional stage. The late-stage pyrite-quartz veins (DL), which are characterized by sericitic alteration halos, postdate the intrusion of QAP dikes and have no economic significance. Molybdenite Re-Os ages of A2E and BT veins indicate that sulfide deposition at Yulong was episodic over a prolonged history lasting over 5.13 ± 0.23 m.y. (1σ). However, the bulk Cu-Mo ores formed in a shorter time interval of 1.36 ± 0.24 m.y. (1σ) with most Cu precipitated in a more restricted timespan of 0.82 ± 0.24 m.y. (1σ) in the early stage. These results, combined with geochronologic data from porphyry copper deposits elsewhere, confirm that multiple magmatic-hydrothermal pulses with a lifespan of tens to hundreds of thousands of years are sufficient to form a giant porphyry copper deposit. Factors such as metal concentration, volume, and focusing efficiency of ore-forming fluids could have played important roles in producing a giant porphyry Cu deposit regardless of a short- or long-lived magmatic-hydrothermal system.

Mueilha rare metals granite, Eastern Desert of Egypt: An example of a magmatic-hydrothermal system in the Arabian-Nubian Shield

The Mueilha granite pluton is one of the rare-metals bearing peraluminous granitic plutons in the Arabian-Nubian Shield. It represents the apical part of a highly evolved magma chamber emplaced at a shallow level subsequent to the post Pan-African orogeny. The pluton can be seen as a highly leucocratic medium-grained albite/oligoclase framework infilled with quartz, K-feldspar and muscovite that are variably overgrown by K-feldspar, muscovite, quartz and topaz megacrysts. The increasing number and size of the K-feldspar megacrysts at the expense of the whitened albite/oligoclase framework imparts variably red color to the Mueilha granite. Contacts between the milky white and red granites are usually gradational, but may be locally sharp or may form narrow transition zones resulting from abrupt variations in texture and lithology. Textural relations indicate an initial stage of hydrothermal albitization of magmatic plagioclase and crystallization of topaz megacrysts resulting from infiltration of Na-rich fluorine bearing fluids. A subsequent stage of metasomatic enrichment is characterized by extensive growth of large K-feldspar, quartz and muscovite megacrysts at the expense of the albite/oligoclase crystals as a result of infiltration of K-Si rich hydrous fluids. Post-magmatic infiltration of hydrous fluids along the fault planes is shown by the intense replacement of alkali feldspar megacrysts by quartz, development of myrmekitic intergrowth pockets along the K-feldspar megacrysts and sealing of the micro-fractures by cryptocrystalline mixtures of clay minerals, iron oxides, sericite and chlorite. Compositionally, the red granitic rocks have higher SiO2, Fe2O3total, K2O/Na2O, Σ REE, Y, Th, U, Zr and Zn and lower Al2O3, Ga, Ta, Nb and Mo compared to the milky white granites. LILE and Sn do not show clear variation trends throughout the Mueilha granite pluton, suggesting their immobility during hydrothermal alteration. Microthermometric measurements indicate that the interactions with the hydrothermal fluids started at a minimum temperature >400°C, most likely during the late-stage crystallization of the Mueilha granite and continued after their complete solidification (i.e. subsolidus conditions) at a temperature as low as 120°C. The high fertility of Mueilha granite is most plausibly the result of partial melting within the undepleted juvenile crust of the Arabian–Nubian Shield that has formed during the Pan-African orogeny.

Diversity of Ti–Sn–W–Nb–Ta oxide minerals in the classic granite-related magmatic–hydrothermal Cínovec/Zinnwald Sn–W–Li deposit (Czech Republic)

The diversity of Ti–Sn–W–Nb–Ta oxide minerals within a complex granite-related magmatic–hydrothermal system was studied by electron microprobe on the example of the Cinovec (Zinnwald) granite cupola containing a well-known Sn–W–Li deposit. The Cinovec granite cupola is a late Variscan ( ca . 322 Ma), strongly fractionated A-type pluton composed of upper zinnwaldite granite and lower biotite granite. The zinnwaldite granite contains numerous subhorizontal quartz veins and greisen bodies, both composed of quartz, zinnwaldite and topaz and minor amounts of fluorite and Ti–Sn–W–Nb–Ta oxide minerals. Common Nb–Ta-rich rutile and rare cassiterite, columbite-(Fe) and -(Mn), and minerals of the pyrochlore-supergroup were found in biotite granite in deeper parts of the pluton, whereas common columbite occurs with rare W-rich ixiolite, qitianlingite, pyrochlores, and scheelite in some varieties of zinnwaldite granite. In the uppermost part of the cupola, Ta-rich cassiterite, microlite and pyrochlore are frequently observed. The greisens host predominantly wolframite, scheelite and Ta-poor cassiterite, which are accompanied by columbite-group minerals, rare W-rich ixiolite and qitianlingite. Throughout the pluton, the molar Mn/(Fe + Mn) ratio in columbite increases from 0.15–0.40 in the deeper part of the zinnwaldite granite (depths of 500–635 m) to 0.5–0.9 in granite and greisen in the uppermost part of the pluton (depths of 0–180 m), but no systematic changes in the Ta/(Nb + Ta) ratio were found. On the scale of individual zoned crystals, the rims are always Ta-enriched, whereas the Mn/(Fe + Mn) values in the rims are equal or lower. The Nb,Ta-rich rutile, interiors of the columbite crystals and Ta-rich cassiterite disseminated in the granites are interpreted as products of magmatic crystallization. The Ta-rich rims of columbite-(Fe) to -(Mn), Ta-poor cassiterite and all wolframite found in the greisen should be attributed to the early hydrothermal (greisenization) stage, whereas W-rich ixiolite, qitianlingite, and pyrochlore-supergroup minerals appear to belong to the later, hydrothermal event at comparatively lower temperature.

Confinement Effects on Carbon Dioxide Methanation: A Novel Mechanism for Abiotic Methane Formation

An important scientific debate focuses on the possibility of abiotic synthesis of hydrocarbons during oceanic crust-seawater interactions. While on-site measurements near hydrothermal vents support this possibility, laboratory studies have provided data that are in some cases contradictory. At conditions relevant for sub-surface environments it has been shown that classic thermodynamics favour the production of CO2 from CH4, while abiotic methane synthesis would require the opposite. However, confinement effects are known to alter reaction equilibria. This report shows that indeed thermodynamic equilibrium can be shifted towards methane production, suggesting that thermal hydrocarbon synthesis near hydrothermal vents and deeper in the magma-hydrothermal system is possible. We report reactive ensemble Monte Carlo simulations for the CO2 methanation reaction. We compare the predicted equilibrium composition in the bulk gaseous phase to that expected in the presence of confinement. In the bulk phase we obtain excellent agreement with classic thermodynamic expectations. When the reactants can exchange between bulk and a confined phase our results show strong dependency of the reaction equilibrium conversions, {{text{X}}}_{{bf{C}}{{bf{O}}}_{{bf{2}}}}, on nanopore size, nanopore chemistry, and nanopore morphology. Some physical conditions that could shift significantly the equilibrium composition of the reactive system with respect to bulk observations are discussed.

Isotope geochronology, geochemistry, and mineral chemistry of the U-bearing and barren granites from the Zhuguangshan complex, South China: Implications for petrogenesis and uranium mineralization

The Zhuguangshan complex carries some of the most important granite-hosted uranium deposits in South China. Here we investigate the Changjiang and Jiufeng granites which represent typical U-bearing and barren granites in the complex, using zircon U-Pb ages, whole-rock geochemistry, Sr-Nd isotopic and zircon Hf isotopic data, and mineral chemistry, to constrain the petrogenesis and uranium mineralization. LA-ICP-MS zircon U-Pb dating shows that both the Changjiang and Jiufeng granites were emplaced ca. 160Ma. These rocks show high silica, weakly to strongly peraluminous compositions, enrichment in Rb, Th, and U, and depletion in Ba, Nb, Sr, P, and Ti. These features coupled with the high initial 87Sr/86Sr ratios, negative εNd(t) values and εHf(t) values, and the Paleoproterozoic two stage model ages of these two granites suggest that the two granites belong to S-type granites, and the parental magmas of the two granites were derived from the Paleoproterozoic metasedimentary rocks. However, the granitoids show different mineralogical characteristics. The biotite in the Changjiang granite belongs to siderophyllite, marking higher degree of chloritization, whereas the biotite in the Jiufeng granite is ferribiotite, characterized by only slight chloritization. Compared with the Jiufeng granite, the biotite in the Changjiang granite has lower crystallization temperature and oxygen fugacity, but higher F content, and the uraninite has higher UO2 content but lower ThO2 content, and stronger corrosion. The chemical ages of uraninites from both granites are (within error) consistent with the zircon U-Pb ages and are considered to represent the emplacement ages of granites. Chemical ages of pitchblende in the Changjiang granite yield 118±8Ma, 87±4Ma, and 68±6Ma, representing multiple episodes of hydrothermal events that are responsible for the precipitation of U ores in the Changjiang uranium ore field. Our study suggests that the degree of magma differentiation and physicochemical conditions of the magmatic-hydrothermal system are the key factors that control the different U contents of these two granites. The mineralogical characteristics of uraninite and biotite can be used to distinguish between U-bearing and barren granites, and serve as a potential tool for prospecting granite-hosted uranium deposits.

Evidence of a modern deep water magmatic hydrothermal system in the Canary Basin (eastern central Atlantic Ocean)

AbstractNew seismic profiles, bathymetric data, and sediment‐rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800–5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer‐shaped, parallel, or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone‐shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer‐shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4 and CO2‐rich fluids formed by degassing and contact metamorphism of organic‐rich sediments around sill intrusions. Type III are hydrothermal‐volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB‐type, basanites with an outer ring of blue‐green hydrothermal Al‐rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78 ± 0.05 and 1.61 ± 0.09 Ma (K/Ar methods) and on tephra layers within cores at 25–237 ky. The Subvent hydrothermal‐volcanic complex constitutes the first modern system reported in deep water oceanic basins related to intraplate hotspot activity.

Noble gas data from Goldfield and Tonopah epithermal Au-Ag deposits, ancestral Cascades Arc, USA: Evidence for a primitive mantle volatile source

The He, Ne, and Ar isotopic composition of fluid inclusions in ore and gangue minerals were analyzed to determine the source of volatiles in the high-grade Goldfield and Tonopah epithermal Au-Ag deposits in southwestern Nevada, USA. Ar and Ne are mainly atmospheric, whereas He has only a minor atmospheric component. Corrected 3He/4He ratios (with atmospheric He removed) range widely from 0.05 to 35.8 times the air 3He/4He ratio (RA), with a median of 1.43 RA. Forty-one percent of measured 3He/4He ratios are ≥4 RA, corresponding to ≥50% mantle He assuming a mantle ratio of 8 RA. These results suggest that mafic magmas were part of the magmatic-hydrothermal system underlying Goldfield and Tonopah, and that associated mantle-sourced volatiles may have played a role in ore formation. The three highest corrected 3He/4He ratios of 17.0, 23.7, and 35.8 RA indicate a primitive mantle He source and are the highest yet reported for any epithermal-porphyry system and for the Cascades arc region. Compiled 3He/4He measurements from epithermal-porphyry systems in subduction-related magmatic arcs around the world (n=209) display a statistically significant correlation between 3He/4He and Au-Ag grade. The correlation suggests that conditions which promote higher fluid inclusion 3He/4He ratios (abundance of mantle volatiles and focused upward volatile transport) have some relation to conditions that promote higher Au-Ag grades (focused flow of metal-bearing fluids and efficient chemical traps). Results of this and previous investigations of He isotopes in epithermal-porphyry systems are consistent with the hypothesis posed in recent studies that mafic magmas serve an important function in the formation of these deposits.

Geology, fluid inclusion and isotope constraints on ore genesis of the post-collisional Dabu porphyry Cu–Mo deposit, southern Tibet

The Dabu Cu-Mo porphyry deposit is situated in the southern part of the Lhasa terrane within the post-collisional Gangdese porphyry copper belt (GPCB). It is one of several deposits that include the Qulong and Zhunuo porphyry deposits. The processes responsible for ore formation in the Dabu deposit can be divided into three stages of veining: stage I, quartz–K-feldspar (biotite)±chalcopyrite±pyrite, stage II, quartz–molybdenite±pyrite±chalcopyrite, and stage III, quartz–pyrite±molybdenite. Three types of fluid inclusions (FIs) are present: liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and solid bearing multi-phase (S-type) inclusions. The homogenization temperatures for the FIs from stages I to III are in the ranges of 272–475°C, 244–486°C, and 299–399°C, and their salinities vary from 2.1 to 49.1, 1.1 to 55.8, and 2.9 to 18.0wt% NaCl equiv., respectively. The coexistence of S-type, V-type and L-type FIs in quartz of stage I and II with similar homogenization temperatures but contrasting salinities, indicate that fluid boiling is the major factor controlling metal precipitation in the Dabu deposit. The ore-forming fluids of this deposit are characterized by high temperature and high salinity, and they belong to a H2O–NaCl magmatic–hydrothermal system. The H–O–S–Pb isotopic compositions indicate that the ore metals and fluids came primarily from a magmatic source linked to Miocene intrusions characterized by high Sr/Y ratios, similar to other porphyry deposits in the GPCB. The fluids forming the Dabu deposit were rich in Na and Cl, derived from metamorphic dehydration of subducted oceanic slab through which NaCl-brine or seawater had percolated. The inheritance of ancient subduction-associated arc chemistry, without shallow level crustal assimilation and/or input of the meteoric water, was responsible for the generation of fertile magma, as well as CO2-poor and halite-bearing FIs associated with post-collisional porphyry deposits. The estimated mineralization depths of Qulong, Dabu and Zhunuo deposits are 1.6–4.3km, 0.5–3.4km and 0.2–3.0km, respectively, displaying a gradual decrease from eastern to western Gangdese. Deep ore-forming processes accounted for the generation of giant-sized Qulong deposit, because the exsolution of aqueous fluids with large fraction of water and chlorine in deep or high pressure systems can extract more copper from melts than those formed in shallow systems. However, the formation of small-sized Dabu deposit can be explained by a single magmatic event without additional replenishment of S, metal, or thermal energy. In addition, the ore-forming conditions of porphyry Cu–Mo deposits in GPCB are comparable to those of porphyry Cu±Au±Mo deposits formed in oceanic subduction-related continental or island arcs, but differ from those of porphyry Mo deposit formed in the Dabie-Qinling collisional orogens. The depth of formation of the mineralization and features of primary magma source are two major controls on the metal types and ore-fluid compositions of these porphyry deposits.

The genesis of the Hashitu porphyry molybdenum deposit, Inner Mongolia, NE China: constraints from mineralogical, fluid inclusion, and multiple isotope (H, O, S, Mo, Pb) studies

The Hashitu porphyry molybdenum deposit is located in the Great Hinggan Range Cu-Mo-Pb-Zn-Ag polymetallic metallogenic province of NE China, in which the Mo-bearing quartz veins are hosted in approximately coeval granites and porphyries. The deposit contains more than 100 Mt of ore with an average grade of 0.13 wt.% Mo. This well-preserved magmatic-hydrothermal system provides an excellent opportunity to determine the source of the molybdenum, the evolution of the hydrothermal fluids and the controls on molybdenite precipitation in a potentially important but poorly understood metallogenic province. Studies of fluid inclusions hosted in quartz veins demonstrate that the Hashitu hydrothermal system evolved to progressively lower pressure and temperature. Mineralogical and fluid inclusion analyses and physicochemical calculations suggest that molybdenite deposition occurred at a temperature of 285 to 325 °C, a pressure from 80 to 230 bars, a pH from 3.5 to 5.6, and a ∆log fO2 (HM) of −3.0, respectively. Results of multiple isotope (O, H, S, Mo, and Pb) analyses are consistent in indicating a genetic relationship between the ore-forming fluids, metals, and the Mesozoic granitic magmatism (i.e., δ 18OH2O from +1.9 to +9.7‰, δDH2O from −106 to −87‰, δ 34SH2S from +0.3 to +3.9‰, δ 98/95Mo from 0 to +0.37‰, 206Pb/204Pb from 18.2579 to 18.8958, 207Pb/204Pb from 15.5384 to 15.5783, and 208Pb/204Pb from 38.0984 to 42.9744). Molybdenite deposition is interpreted to have occurred from a low-density magmatic-hydrothermal fluid in response to decreases in temperature, pressure, and fO2.

Geology, mineralization, and fluid inclusion characteristics of the Lermontovskoe reduced-type tungsten (±Cu, Au, Bi) skarn deposit, Sikhote-Alin, Russia

The Lermontovskoe deposit (∼48Kt WO3; average 2.6% WO3, 0.24% Cu, 0.23g/t Au) is situated in a W-Sn-Au metallogenic belt that formed in a collisional tectonic environment. This tungsten skarn deposit has a W-Au-As-Bi-Te-Sb signature that suggests an affinity with reduced intrusion-related Au deposits. The deposit is associated with an intrusion that is part of the ilmenite-series, high-K peraluminous granitoid (granodiorite to granite) suite. These rocks formed via mantle magma-induced melting of crustal sources.The deposit comprises reduced-type, pyroxene-dominated prograde and retrograde skarns followed by hydrosilicate (amphibole-chlorite-pyrrhotite-scheelite-quartz) and phyllic (muscovite/sericite-carbonate-albite-quartz-scheelite-sulfide, with abundant apatite) alteration assemblages. Fluid inclusions from the skarn assemblages indicate high-temperature (>500°C), high-pressure (1400–1500bars) and high-salinity (53–60wt% NaCl-equiv.) magmatic-hydrothermal fluids. They were post-dated by high-carbonic, methane-dominate, low-salinity fluid at the hydrosilicate alteration stage. These fluids boiled at 360–380°C and 1300–1400bars. The subsequent phyllic alteration started again with a high-temperature (>450°C), high-pressure (1000–1100bars) and high-salinity (42–47wt% NaCl-equiv.) fluid, with further incursion of high-carbonic, methane-dominated, low-salinity fluid that boiled at 390–420°C and 1150–1200bars. The latest phyllic alteration included the lower-temperature (340–360°C), lower pressure (370–400bars) high-carbonic, methane-dominated (but with higher CO2 fraction), low-salinity fluid, and then the low-temperature (250–300°C) H2O-CO2-CH4-NaCl fluid, with both fluids boiled at the deposit level. The high-salinity aqueous fluids are interpreted to have come from crystallizing granitoid magma, whereas the reduced high-carbonic fluids probably came from a deeper mafic magma source. Both of these fluids potentially contributed to the W-Au-As-Bi-Te-Sb metal budget. Decreasing temperatures coupled with high aCa2+ and fluid boiling promoted scheelite deposition at all post-skarn hydrothermal stages.The deposit is characterized by limited downdip extent of mineralized zones and abundant coarse-grained muscovite-quartz (+apatite, scheelite) aggregates that formed at the phyllic alteration stage. Together with presence of high-temperature, high-pressure and high-salinity fluids directly exsolving from crystallizing magma, this suggests a root level of the mineralized magmatic-hydrothermal system of reduced W skarn deposits.

Satellite observations of fumarole activity at Aluto volcano, Ethiopia: Implications for geothermal monitoring and volcanic hazard

Fumaroles are the surface manifestation of hydrothermal circulation and can be influenced by magmatic, hydrothermal, hydrological and tectonic processes. This study investigates the temporal changes in fumarole temperatures and spatial extent on Aluto, a restless volcano in the Main Ethiopian Rift (MER), in order to better understand the controls on fluid circulation and the interaction between the magmatic and hydrothermal systems. Thermal infrared (TIR) satellite images, acquired by the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) over the period of 2004 to 2016, are used to generate time series of the fumarole temperatures and areas. The thermal anomalies identified in the ASTER images coincide with known fumaroles with temperatures >80°C and are located on or close to fault structures, which provide a pathway for the rising fluids. Most of the fumaroles, including those along the major zone of hydrothermal upwelling, the Artu Jawe Fault Zone, have pixel-integrated temperature variations of only ~2±1.5°C. The exception are the Bobesa fumaroles located on a hypothesised caldera ring fault which show pixel-integrated temperature changes of up to 9°C consistent with a delayed response of the hydrothermal system to precipitation. We conclude that fumaroles along major faults are strongly coupled to the magmatic-hydrothermal system and are relatively stable with time, whereas those along shallower structures close to the rift flank are more strongly influenced by seasonal variations in groundwater flow. The use of remote sensing data to monitor the thermal activity of Aluto provides an important contribution towards understanding the behaviour of this actively deforming volcano. This method could be used at other volcanoes around the world for monitoring and geothermal exploration.

Evidences of volcanic unrest on high-temperature fumaroles by satellite thermal monitoring: The case of Santa Ana volcano, El Salvador

On October 1st, 2005, Santa Ana volcano (El Salvador) underwent a VEI 3 phreatomagmatic eruption after approximately one century of rest. Casualties and damages to some of the local infrastructures and surrounding plantations were followed by the evacuation of the nearby communities. The analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) infrared data reveals that the main explosion was preceded by a one-year-long thermal unrest, associated to the development of a fumaroles field, located at the western rim of the summit crater lake. By combining space-based thermal flux and ground-based measurements (seismicity, sulfur emissions and lake temperatures), we suggest that the activity observed at Santa Ana between 2004 and 2005 was driven by the gradual intrusion of an undegassed magma body at a very shallow depth. Magma injection induced thermal anomalies associated with sustained degassing from the fumaroles field and promoted the interaction between the magmatic-hydrothermal system and the overlying water table. This process culminated into the VEI 3 phreatomagmatic eruption of October 2005 that strongly modified the shallow structure of the crater area. The subsequent three-years-long activity resulted from self-sealing of the fracture system and by the opening of a new fracture network directly connecting the deeper hydrothermal system with the crater lake. Our results show that satellite-based thermal data allow us to detect the expansion of the high-temperature fumarolic field. This may precede an explosive eruption and/or a lava dome extrusion. In particular, we show that thermal records can be analyzed with other geochemical (i.e. SO2 emissions) and geophysical (seismicity) data to track a shallow magmatic intrusion interacting with the surrounding hydrothermal system. This provides a remarkable support for volcano monitoring and eruption forecasting, particularly in remote areas where permanent ground data acquisition is hazardous, expensive and difficult.

Zircon and molybdenite geochronology and geochemistry of the Kalmakyr porphyry Cu–Au deposit, Almalyk district, Uzbekistan: Implications for mineralization processes

Copper, gold and molybdenum mineralization of the Kalmakyr porphyry deposit in Uzbek Tien Shan occurs as stockworks, veinlets and disseminations in the phyllic and K-silicate alteration zones developed predominantly in a middle to late Carboniferous intrusive complex composed of monzonite and granodiorite porphyry. Zircon U–Pb dating yielded an age of 327.2±5.6Ma for the ore-hosting monzonite and an age of 313.6±2.8Ma for the ore-bearing granodiorite porphyry. Re–Os dating of seven molybdenite samples from stockwork and veinlet ores yielded model ages from 313.2 to 306.3Ma, with two well-constrained isochron ages of 307.6±2.5Ma (five stockwork ores) and 309.1±2.2Ma (five stockwork ores and two veinlet ores), respectively. These results indicate that Cu–Au mineralization post-dated the emplacement of the monzonite, started right after the emplacement of the granodiorite porphyry, and lasted for ca. 7Ma afterward. The geochronological and geochemical data suggest that the Kalmakyr deposit was formed in a late Carboniferous mature magmatic arc setting, probably related to the latest subduction process of the Turkestan Ocean beneath the Middle Tien Shan. The εHf(t) values of zircon grains from the monzonite vary from +11 to +1.7, with an average of +5.1, and those of zircon grains from the granodiorite porphyry range from +5.7 to −1.8, with an average of +2.4. These data indicate that the magma of both monzonite and granodiorite porphyry was derived from partial melting of a thickened lower crust with input of mantle components and variable crustal contamination, and that there was more mantle contribution to the formation of the monzonite than the granodiorite porphyry. The high rhenium concentrations of molybdenite (98–899ppm) also indicate major mantle contribution of rhenium and by inference ore metals. The relatively high EuN/EuN∗ values (average 0.68), Ce4+/Ce3 values (average 890) and Ce/Nd values (average 36.8) for zircon grains from the granodiorite porphyry than those from the monzonite (average EuN/EuN∗=0.33, average Ce4+/Ce3=624, average Ce/Nd=3.9) suggest that the magma for the syn-mineralization granodiorite porphyry has higher oxygen fugacity than that for the pre-mineralization monzonite. Based on these data, it is proposed that while the monzonite was emplaced, the oxygen fugacity and volatile contents in the magma were relatively low, and ore metals might disperse in the intrusive rock, whereas when the granodiorite porphyry was emplaced, the oxygen fugacity and volatile contents in the magma were increased, favoring copper and gold enrichment in the magmatic fluids. The Kalmakyr deposit formed from a long-lived magmatic-hydrothermal system connected with fertile magmatic sources in relation to the subduction of the Turkestan Ocean beneath the Middle Tien Shan.

Geology and Genesis of the Cerro la Mina Porphyry-High Sulfidation Au (Cu-Mo) Prospect, Mexico

The Cerro la Mina Au (Cu-Mo) porphyry-high sulfidation prospect is located in Chiapas State, southeastern Mexico, outside of the major metallogenic provinces of Mexico. The prospect is hosted by Pleistocene alkaline volcanic rocks of the Chiapanecan volcanic arc that formed in a complex triple-junction tectonic setting. Cerro la Mina's stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1.04 ± 0.04 Ma (U-Pb, zircon), and that were overlain by debris flows and synvolcanic trachyandesite domes. The volcanic stratigraphy of Cerro la Mina is dominated by pyroclastic flows and rare basalts that are cut by the Cerro la Mina breccia pipe, a matrix-rich granular, vertically oriented, downward-tapering, polymict lithic rock unit that is host to all of the significant alteration and mineralization. A NW-trending sinistral wrench fault, which was active throughout the history of Cerro la Mina, is responsible for dismembering the prospect after mineralization. The magmatic hydrothermal system was composed of early porphyry-style potassic veins (quartz + K-feldspar ± biotite) and stage 1 pyrite that are preserved in clasts within the breccia pipe, suggesting that brecciation disrupted an embryonic porphyry system. Late potassic alteration occurred after the formation of the breccia pipe, as its matrix is strongly K-feldspar altered. Hydrothermal fluids then produced phyllic alteration composed of quartz, muscovite, illite, illite-smectite, and chlorite that is associated with stage 2 pyrite ± chalcopyrite ± molybdenite ± quartz veins. An unusual zoned pattern of advanced argillic-argillic alteration overprinted potassic and phyllic alteration. This zoning included a low-temperature ( 120°C) quartz + dickite ± kaolinite ± pyrophyllite ± alunite that occurs from 250 m to the present-day surface. The advanced argillic-argillic altered rocks host the most significant Au-Cu mineralization, which is associated with stage 3 marcasite, sphalerite, galena, and barite, and stage 4 arsenian pyrite ± enargite ± covellite. The magmatic hydrothermal system at Cerro la Mina began sometime between monzodiorite emplacement (1.04 ± 0.04 Ma; zircon U-Pb) and the precipitation of porphyry stage 2 molybdenite at 780 ± 10 ka (Re-Os). 40Ar/39Ar dating of biotite (689 ± 13 ka) records the age at which the hydrothermal system cooled below the biotite closure temperature of ~300°C and provides a maximum estimate for the onset of advanced argillic-argillic alteration. Sulfur isotope results of sulfides (–2.5 to +4.9‰; mean +0.7‰; n = 20) and a sulfate (barite; +10.5‰; n = 1) suggest a magmatic source of sulfur for all four stages of mineralization. The lack of residual quartz, rare alunite, and anomalous halloysite-kaolinite alteration may be explained by the high acid-buffering capacity of alkaline volcanic host rocks, high CO2 contents of the alkaline magma, and/or potentially by a highly reduced magmatic hydrothermal fluid. At the regional metallogenic scale, the Cerro la Mina prospect along with the nearby Santa Fe mine and Campamento deposit represent parts of a porphyry copper system—specifically, a porphyry/high-sulfidation, proximal skarn and intermediate sulfidation deposit, respectively. The characteristics of Cerro la Mina (i.e., anomalous halloysite-kaolinite alteration) broaden the window for additional discoveries to be made in the porphyry-epithermal environment.

Zonation of Sulfate and Sulfide Minerals and Isotopic Composition in the Far Southeast Porphyry and Lepanto Epithermal Cu–Au Deposits, Philippines

AbstractThe world‐class Far Southeast (FSE) porphyry system, Philippines, includes the FSE Cu–Au porphyry deposit, the Lepanto Cu–Au high‐sulfidation deposit and the Victoria–Teresa Au–Ag intermediate‐sulfidation veins, centered on the intrusive complex of dioritic composition. The Lepanto and FSE deposits are genetically related and both share an evolution characterized by early stage 1 alteration (deep FSE potassic, shallow Lepanto advanced argillic‐silicic, both at ~1.4 Ma), followed by stage 2 phyllic alteration (at ~1.3 Ma); the dominant ore mineral deposition within the FSE porphyry and the Lepanto epithermal deposits occurred during stage 2. We determined the chemical and S isotopic composition of sulfate and sulfide minerals from Lepanto, including stage 1 alunite (12 to 28 permil), aluminum–phosphate–sulfate (APS) minerals (14 to 21 permil) and pyrite (−4 to 2 permil), stage 2 sulfides (mainly enargite–luzonite and some pyrite, −10 to −1 permil), and late stage 2 sulfates (barite and anhydrite, 21 to 27 permil). The minerals from FSE include stage 2 chalcopyrite (1.6 to 2.6 permil), pyrite (1.1 to 3.4 permil) and anhydrite (13 to 25 permil). The whole‐rock S isotopic composition of weakly altered syn‐mineral intrusions is 2.0 permil.Stage 1 quartz–alunite–pyrite of the Lepanto lithocap, above about 650 m elevation, formed from acidic condensates of magmatic vapor at the same time as hypersaline liquid formed potassic alteration (biotite) near sea level. The S isotopic composition of stage 1 alunite–pyrite record temperatures of approximately 300–400°C for the vapor condensate directly over the porphyry deposit; this cooled to <250°C as the acidic condensate flowed to the NW along the Lepanto fault where it cut the unconformity at the top of the basement. Stage 1 alunite at the base of the advanced argillic lithocap over FSE contains cores of APS minerals with Sr, Ba and Ca; based on back‐scattered electron images and ion microprobe data, these APS minerals show a large degree of chemical and S‐isotopic heterogeneity within and between samples. The variation in S isotopic values in these finely banded stage 1 alunite and APS minerals (16 permil range), as well as that of pyrite (6 permil range) was due largely to changes in temperature, and perhaps variation in redox conditions (average ~ 2:1 H2S:SO4). Such fluctuations could have been related to fluid pulses caused by injection of mafic melt into the diorite magma chamber, supported by mafic xenoliths hosted in diorite of an earlier intrusion.The S isotopic values of stage 2 minerals indicate temperatures as high as 400°C near sea level in the porphyry deposit, associated with a relatively reduced fluid (~10:1 H2S:SO4) responsible for deposition of chalcopyrite. Stage 2 fluids were relatively oxidized in the Lepanto lithocap, with an H2S:SO4 ratio of about 4. The oxidation resulted from cooling, which was caused by boiling during ascent and then dilution with steam‐heated meteoric water in the lithocap. This cooling also resulted in the sulfidation state of minerals increasing from chalcopyrite stability in the porphyry deposit to that of enargite in the lithocap‐hosted high‐sulfidation deposit. The temperature at the base of the lithocap during stage 2 was ≥300°C, cooling to <250°C within the main lithocap, and about 200°C towards the limit of the Lepanto orebody, approximately 2 km NW of the porphyry deposit. Approximate 300°C and 200°C isotherms, estimated from S isotopic and fluid inclusion temperatures during stage 1 and stage 2, shifted towards the core of the FSE porphyry deposit with time. This general retreat in isotherms was more than 500 m laterally within Lepanto and 500 m vertically within FSE as the magmatic–hydrothermal system evolved and collapsed over the magmatic center. During this evolution, there is also evidence recorded by large S isotopic variations in individual crystals for sharp pulses of higher temperature, relatively reduced fluid injected into the porphyry deposit.