Hydrothermal Breccias Research Articles

Geology and genesis of the North Narbaghi Cu-Ag deposit in the Urumieh-Dokhtar magmatic arc, Iran: Fluid inclusion and stable isotope constraints

The North Narbaghi Cu-Ag deposit in the central part of the Urumieh-Dokhtar magmatic arc (UDMA) of Iran is hosted by Eocene volcanic sequences and Oligo-Miocene intrusions of arc affiliation. The mineralization can be divided into five stages that are represented by barren hydrothermal breccias, Cu-Fe sulfides cementing in the breccias, sulfosalt and sulfide hydrothermal breccias, barren post-ore quartz veinlets, and barren post-ore calcite veinlets, respectively. The metallic minerals consist of pyrite, chalcopyrite, bornite, sphalerite, and tennantite-tetrahedrite, which are variably associated with gangue minerals including quartz, calcite, and sericite. The upper parts of the ores have been oxidized to form supergene minerals including malachite, azurite, chrysocolla, chalcocite, covellite, hematite, and goethite. The ore-related hydrothermal alteration includes silicification, pyritization, carbonatization, intermediate argillitization, and propylitic alteration. Primary fluid inclusion assemblages in quartz associated with main-stage sulfides and sulfosalts have homogenization temperatures ranging from 184° to 385 °C and calculated salinities between 12.0 and 29.7 wt% NaCl equiv. (avg. 21.4 wt% NaCl equiv.). The coexistence of fluid inclusion assemblages with highly variable volume percentages of vapor phase indicates that boiling commonly occurred in the hydrothermal system, contributing to Cu saturation. Sulfide minerals range in δ34S from −1.5 to 3.7 ‰, indicating a probable magmatic sulfur source. This view is further supported by the carbon and oxygen isotopes of hydrothermal carbonates, with δ13CPDB of −2.2 to −1.5 ‰ and δ18OSMOW between 5.30 and 11.75 ‰. Stable isotope and fluid inclusion data illustrate that boiling of the ore fluid and its mixing with meteoric water facilitated mineralization at the North Narbaghi. We suggest that the North Narbaghi Cu-Ag deposit can be best interpreted as an intermediate-sulfidation epithermal deposit formed due to post-Eocene magmatic-hydrothermal activity. The intersections of post-Eocene subvolcanic rocks with regional faults were the most favorable sites for focusing fluids responsible for epithermal mineralization along the UDMA.

<sup>40</sup>Ar/<sup>39</sup>Ar dating of a hydrothermal pegmatitic buddingtonite–muscovite assemblage from Volyn, Ukraine

Abstract. We determined 40Ar/39Ar ages of buddingtonite, occurring together with muscovite, with the laser-ablation method. This is the first attempt to date the NH4-feldspar buddingtonite, which is typical for sedimentary–diagenetic environments of sediments, rich in organic matter, or in hydrothermal environments, associated with volcanic geyser systems. The sample is a hydrothermal breccia, coming from the Paleoproterozoic pegmatite field of the Korosten Plutonic Complex, Volyn, Ukraine. A detailed characterization by optical methods, electron microprobe analyses, backscattered electron imaging, and IR analyses showed that the buddingtonite consists of euhedral-appearing platy crystals of tens of micrometers wide, 100 or more micrometers in length, which consist of fine-grained fibers of ≤ 1 µm thickness. The crystals are sector and growth zoned in terms of K–NH4–H3O content. The content of K allows for an age determination with the 40Ar/39Ar method, as well as in the accompanying muscovite, intimately intergrown with the buddingtonite. The determinations on muscovite yielded an age of 1491 ± 9 Ma, interpreted as the hydrothermal event forming the breccia. However, buddingtonite apparent ages yielded a range of 563 ± 14 Ma down to 383 ± 12 Ma, which are interpreted as reset ages due to Ar loss of the fibrous buddingtonite crystals during later heating. We conclude that buddingtonite is suited for 40Ar/39Ar age determinations as a supplementary method, together with other methods and minerals; however, it requires a detailed mineralogical characterization, and the ages will likely represent minimum ages.

Comparative Study of Gajah and Ijo Volcano Mineralization in Kulon Progo Dome based on Textural and Mineralogical Characteristics

Gajah Volcano and Ijo Volcano are two tertiary volcanoes located in the Kulon Progo Dome area, Yogyakarta. Gajah Volcano is located in the middle of the Kulon Progo Dome which is the oldest in the complex and belongs to Early Oligocene volcanism period (± 29 mya). On the other hand, Ijo Volcano is a product of younger volcanism period, occurred in the Late Oligocene (± 25 mya). The tectonic deformation occurred during the Late Oligocene-Early Miocene led to the formation of geological structures like faults and joints, which also serve as pathway for acid-intermediate intrusion rocks. The intrusions are associated with hydrothermal alteration and ore mineralization in both volcanoes. There has been no research comparing the characteristics of hydrothermal deposits that formed on Gajah Volcano and Ijo Volcano. This will be the main objective of this research. The research was carried out at two mineralization prospect locations representing each volcano, namely the Kaligono area (Gajah Volcano) and the Hargorojo area (Ijo Volcano). The results were obtained from geological and alteration mapping as well as representative rocks/veins sampling...Petrology,..petrography, mineragra- phy, and XRD analyzes conducted on altered rock and vein samples from the two prospects indicated some differences. Kaligono prospect area (Gajah Volcano) consists various of alteration types ie. phylic (quartz-sericite-illite-pyrite),..propylitic..(chlorite-calcite-pyrite±epi- dote±actinolite), and argillic (illite-smectite-kaolinite±quartz). The mineralized veins found on Gajah Volcano show vein swarm, brecciated, stockwork, and massive vein structure with massive vein textures. The veins in Kaligono show NE-SW and NW-SE trends and hosted by Andesite, Dacite, and Andesite Lava. Gangue minerals that are found in the vein samples are quartz, illite, iron oxide, pyrite, and carbonate minerals. The ore minerals consist of magnetite, chalcopyrite, and sphalerite. Meanwhile in Hargorojo prospect area (Ijo Volcano), the types of alteration found including phylic (quartz-sericite-pyrite),..propylitic..(pyrite-calcite±ch- lorite), and argillic (illite-smectite-kaolinite-quartz). The mineralized veins found on Ijo Volcano have a massive vein structure, brecciation, and stockwork with comb, drussy, and massive vein textures. The veins have NNE-SSW and E-W trend and hosted by Andesite and Dacite. The gangue minerals are carbonate minerals, oxide minerals, pyrite, barite, quartz and chalcedony. The ore minerals include chalcopyrite, silver, galena, and sphalerite. Based on the vein characteristics of Kaligono prospect, such as a complex stockwork structure, hydrothermal breccia, and massive vein texture, which contain high temperature hydrothermal minerals, ie. epidote, actinolite, and magnetite, maybe indicate this deposit is controlled by deep..structure..related..to..the..porphyry miner alization. Whereas in the Hargorojo prospect shows the typical textures of shallow epithermal system (open space filling), such as comb and drussy, which contain lower temperature hydrothermal minerals, such as chalcedony, silver, and galena. Based on textural and mineralogical characteristics, Kaligono prospect suggests that the alteration and mineralization takes place deeper or closer to the magmatic source. On the other hand, Hargorojo prospect suggests the alteration and mineralization process relatively far from the source.

Mineralizations related to the Tocota pluton. Southern Colangüil batholith, Cordillera Frontal, San Juan, Argentina

The southern portion of the Colangüil Batholith is formed by units of the Tocota Pluton, of Permian age, which intrude late Carboniferous-Permian (Asselian) sandstones. Sub-volcanic bodies with andesitic composition and tuffs of Cenozoic age, modern layers of piedmont sediments and both alluvial and glacier deposits also crop out. The region exhibits a set of metalliferous deposits (Cu-Au-As) and occasionally with bismuth. These are mostly filonian type. These are spatially associated with granodiorites, granite and sandstone. Their development is interpreted as belonging to the Gondwanan cycle.Within granitic rocks, mineralizations were formed between 139 ºC and about 400 °C, probably after the consolidation of the Tocota pluton and by the upward circulation of hydrothermal fluids, probably mixed with meteoric waters. In the host rock, mineralization occurred in an interval between 200 ºC and 350 °C, through hydrothermal activity detected in areas of phyllic-argillic alteration. The gangue constitutes mainly of quartz, tourmaline and and sometimes potassium feldspar and calcite. The mineralization pattern, in both the pluton and the sandstone, has been controlled by 1) a strong domain of ore filled faults and joints, with ENE and WNW as the more common strikes and NNW subordinated, 2) fault breccias and hydrothermal breccias, and 3) dykes of andesite composition with low metal content. The host rock mineralization is commonly emplaced in joints as well as finely disseminated, related to intense silicification. Such deposits contain hypogene minerals whose isotopic 34S/32S ratios are compatible with a granite-like filiation but other provenances cannot be ruled out.

Paragenesis of gold mineralization at the Kiyuk Lake Project, Kivalliq Region, Nunavut, Canada

ABSTRACT Exploration for gold in Nunavut has been primarily focused on Archean greenstone belts in the north and coastal regions of the territory, resulting in large areas of underexplored terrain in the south. The Kiyuk Lake property is located in the underexplored southwest corner of the Kivalliq Region of Nunavut within the Hearne domain of the ∼1.9 Ga western Churchill Province. The property is hosted by Proterozoic calc-silicate and clastic sedimentary units of the Hurwitz Group (<2.4–1.9 Ga) and the unconformably overlying Kiyuk Group (1.9–1.83 Ga). Gold mineralization in Proterozoic sedimentary rocks is rare in the Canadian Shield, so the Rusty Zone at Kiyuk Lake presents a unique opportunity to study the enigmatic gold mineralization hosted in such sedimentary rocks. Mineralization at the Rusty Zone is hosted by an immature lithic wacke cut by thin intermediate dikes that are associated with hydrothermal breccias composed of Fe-carbonate, calcite, calcic-amphibole, Fe-sulfide, Fe-oxide minerals, and gold. Textural and timing relationships suggest that the gold mineralization is post-sedimentary and syn- to post-intrusion of intermediate dikes. Stable isotope thermometry suggests that mineralization took place between 450 and 600 °C, and geochronological studies indicate that the intrusion and mineralization occurred before or about 1.83 Ga. Using basement breaching thrusts faults as conduits to the surface, over-pressurization along a later normal fault is thought to be the primary cause for the localized breccia pipe that controls gold mineralization. The hydrothermal fluids are postulated to be volatile-rich aqueous solutions exsolved from a source of cooling magmas at depth. Although sub-economic at present, the occurrence of high-grade gold in a Paleoproterozoic basin such as Kiyuk Lake could signal a new opportunity for exploration for gold in the Canadian Shield.

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part I: Syndiagenetic Base Metal Sulfide Mineralization Related to Dynamic Subbasin Evolution

Abstract Divergent genetic models have been proposed for clastic dominant (CD-type) massive sulfide Zn-Pb mineralization in the Proterozoic Carpentaria Zn Province. Due to varying degrees of tectonic overprint, it has been difficult to accurately constrain structural and paragenetic timing aspects of the CD-type genetic model, and the most basic timing constraints (syngenetic vs. epigenetic, synextension vs. syninversion) remain debated. The recently discovered Teena Zn-Pb deposit is hosted by an exceptionally well preserved subbasin that permits relative timing relationships to be well defined. Using a combination of geophysical, structural, sedimentological, and petrographic datasets, a new model for subbasin development and syndiagenetic hydrothermal replacement mineralization is developed for the Teena mineral system. At Teena, sulfide mineralization was deposited from lateral fluid flow beneath an impermeable seal several hundred meters below the paleosurface and maximum flood surface, after formation of fine-grained diagenetic pyrite (py1) and dolomite nodules. Sulfide mineralization resulted from syndiagenetic carbonate replacement and pore space cementation where thermochemical sulfate reduction took place. The sulfide mineralization is therefore partly cospatial but not cogenetic with its thick pyritic hanging wall, and its lateral alteration footprint is much smaller than predicted by sedimentary exhalative (SEDEX) models. An additional zone of low-grade Zn-Pb mineralization in the footwall W-Fold Shale Member represents a different style of mineralization not previously reported for Carpentaria CD-type Zn deposits: it is associated with strata-bound lenses of hydrothermal dolomite (HTD) that formed by both replacement and carbonate dissolution and infill, which yielded diverse cavity-infill textures that include coarse-bladed dolomite fans cemented by interstitial sphalerite, dolomite, and quartz. Volumetrically minor Zn mineralization is also present in a fault conduit hydrothermal breccia and in hanging-wall synorogenic vein sets derived by hydrothermal leaching and remobilization of Zn from the underlying mineralized zones. Whereas both the Teena and nearby McArthur River Zn-Pb deposits are located along the northern margin of the 3rd-order Hot Spring-Emu subbasin, they formed in separate 4th-order subbasins in association with local extensional growth faults. Growth fault movement in the Teena subbasin was initiated during deposition of the W-Fold Shale Member and persisted episodically until a weak structural disconformity associated with sedimentary facies regression developed in the Upper HYC unit. Shifting patterns in depocenter location, sedimentary facies development, mineralization, and alteration zonation are attributed to progressive growth and linkage of segments of a regionally anomalous ENE-trending, synsedimentary fault zone. Similar patterns of extensional subbasin development were repeated in other Zn-mineralized subbasins throughout the River supersequence across the northern Carpentaria Zn Province, and formed in response to a short-lived episode of north-northwest–south-southeast regional extension around ca 1640 ± 5 Ma, triggered by far-field subduction events.

Superimposing mineralization in the Zhengguang Au-Zn deposit, NE China: Evidence from pyrite Re-Os geochronology, fluid inclusion, and H-O-S-Pb isotopes

The Duobaoshan orefield is located in the eastern part of the Central Asian Orogenic Belt (CAOB) and underwent multiphase magmatism and mineralization. The Zhengguang deposit in the orefield is characterized by porphyry copper mineralization and intermediate-sulfidation (IS) epithermal gold mineralization. The porphyry-type veinlet-disseminated sulfide mineralization (~480 Ma) is hosted in granodiorite/tonalite porphyries and hydrothermal breccias, and is associated with widespread potassic (K-feldspar and biotite) alteration. Three porphyry-type mineralization stages have been identified: (I-1) quartz + medium-fine grained pyrite, (I-2) quartz + pyrite + chalcopyrite, and (I-3) quartz + calcite + minor pyrite. The IS epithermal vein/veinlet-disseminated mineralization occurs in andesite porphyry and fractures of the Duobaoshan Formation volcanic rocks, and is associated with extensive silicic, sericitic, and pyrite-sericitic-silicic alterations and minor propylitic alteration. Three IS epithermal-type mineralization stages are identified: (II-1) quartz + coarse-grained pyrite, (II-2) quartz + polymetallic sulfides, and (II-3) quartz + calcite + barren sulfides. Five stage II-2 pyrite samples yielded a Re-Os isochron age of 246.0 ± 6.4 Ma (MSWD = 0.85), suggesting a Triassic Au mineralization event after the Ordovician Cu mineralization. Stage I-1 fluid inclusions (FIs) are predominantly of VL- and LV-type and minor SL-type, with homogenization temperatures (Th) of 283–359 ℃ and salinities of 0.3–35.1 wt%. Stage I-2 is characteristically composed of VL- and LV-type inclusions with Th of 193–296 ℃ and salinities of 0.8–14.2 wt%. Only VL-type inclusions occur in stage I-3, II-1, II-2, and II-3 with Th of 149–216 ℃, 213–285 ℃, 169–236 ℃, and 125–176 ℃, and salinities of 2.4–7.9 wt%, 6.5–12.2 wt%, 3.4–8.8 wt%, and 2.1–6.3 wt%, respectively. H-O isotopic compositions for the mineralization (Ordovician: δ18OH2O = -9.9 to −0.5‰, δD = -86 to −75.6‰; Triassic: δ18OH2O = -8.4 to −2‰, δD = −134.1 to −88.5‰) indicate two different evolving fluid systems, both with an initial magmatic source and a later meteoric water incursion. Sulfur and lead isotope (Ordovician: δ34S (−3.1 to −0.7‰), 206Pb/204Pb (17.570–17.613), 207Pb/204Pb (15.427–15.441), 208Pb/204Pb (37.258–37.294); Triassic: δ34S (-5.4 to −2.5‰), 206Pb/204Pb (17.627–17.965), 207Pb/204Pb (15.466–15.534), 208Pb/204Pb (37.468–37.703)) values suggest a dominantly magmatic and a mixed magmatic-sedimentary source for the Ordovician and Triassic mineralization, respectively.

Hydrothermal alteration and sealing at Turrialba volcano, Costa Rica, as a mechanism for phreatic eruption triggering

Turrialba is a basaltic to andesitic Holocene stratovolcano that after decades of quiescence re-activated in 1996 and has been highly active ever since. Turrialba is characterized by a highly active magmatic-hydrothermal system, and we propose that hydrothermal sealing and volatile accumulation are the mechanisms responsible for the reactivation and persistent phreatic activity at Turrialba since 2010. Evidence of sealing is found in pyroclastic breccias from phreatic eruptions as high concentrations of hydrothermal minerals coupled with low intrinsic permeability. The suite of volcanic breccias studied herein erupted from the main vent between 2014 and 2019 and has an alteration mineral assemblage of SiO2 polymorphs ± gypsum ± natroalunite ± pyrite. The mineral assemblage is indicative of acid sulphate alteration within the advanced-argillic alteration facies characterized by temperatures of approximately 200–350 °C as indicated by the presence of gypsum and natroalunite, the high temperature endmember of the alunite series. Acid sulphate alteration is the result of extreme base leaching by acidic fluids (pH <4) with a high sulphate content. Measurements of permeability and porosity yielded variable porosity and very low to non-existent permeability in all hydrothermal breccia samples. Back-scatter electron (BSE) images reveal nano-, micro- and macro-scale fracture networks discontinuously filled with hydrothermal gypsum and pyrite which are responsible for diminished permeability, supporting the conclusion that hydrothermal sealing is active at Turrialba. Diminished permeability associated with the formation of a seal inhibits the escape of gases, causing them to accumulate below the seal and pressurize the system. Eventual seal failure due to overpressure results in the frequent phreatic eruptions seen at Turrialba.

New perspective Reza gold deposit (Gedabek ore district, Lesser Caucasus, Azerbaijan)

&#x0D; &#x0D; &#x0D; The article describes Reza gold deposit of Ugur exploration area located in Geda- bek Ore District of the Lesser Caucasus in NW of Azerbaijan. It is established that main mineralization in the Reza gold deposit consists of hematite-barite-quartz-kaoline veins- veinlets and breccia, pyrite stock-stockverk and quartz-sulfide veins. On the main orebody surface center secondary quartzites with vein-veinlets barite-hematite mineralization have occured over which remain accumulations of hydrous ferric oxides cementing breccias of quartz and secondary quartzites. “Reddish mass” is also observed in erosion parts, being an oxidation product of stock and stockverk limonite-hematite ores. Representing typical gossans, these accumulations by the data of trenches for thickness about 5-10 m contain gold 0.3-3.5 ppm and silver 1.0-45.0 ppm. There are three zones of gold mineralization within the Reza gold deposit: oxide mineralization; transition zone mineralization; sulfide mineralization. The oxide gold mineraliza- tion consists of clay-gravel weathering crust of kaolinite type. The gold-bearing mineralization has been oxidized to a depth of ap- proximately 50-100 meters. Typically, the gold mineralization is coarser and a minor increase in gold grade occurs within the oxides compared to the original rocks. The nugget effect increase in the gold grade of the oxides does not exceed approximately 10%. Deposit alteration signature has characteristics which suggest the current outcrop level may be near the top of a mineralized, gold-bearing high sulfidation epithermal (HSE) system. The gold mineralization at the deposit is interpreted as forming in shallow high sulfidation epith- ermal systems. The mineralization has been noted in well-confined hydrothermal breccia and associated with pyrite stock-stockwork. The majority of the deposit material and current estimates are formed within the barite-hematite-quartz-kaoline mineralization in the secondary quartzite rocks. The main brecciation and stockwork are hosted within secondary quartzite, sometime massive silicified andesite porphyritic rocks.&#x0D; &#x0D; &#x0D;

Metallogenesis of the Rhyacian Lavra Velha gold-rich IOCG deposit in the São Francisco Craton, Brazil

• The Lavra Velha deposit represents a new gold-rich IOCG mineralization at the São Francisco Craton . • The gold mineralization is associated with a hydrothermal breccia system with co-existing iron oxides and sulfides. • IOCG mineralization is related to the Rhyacian period of the northern São Francisco Craton. • Mineralization is marked by decreasing temperatures from the bottom to the top of the deposit. Lavra Velha is a newly discovered Rhyacian IOCG deposit in the northern portion of the São Francisco Craton, in the Bahia state of NE Brazil. Gold and copper mineralization is hosted by epigenetic hydrothermal breccia with large amounts of iron oxide. Multiple phases of breccia formation and hydrothermal alteration caused zoning from the base to the top of the deposit. The alteration is mainly characterized by hydrothermal calcite-epidote-magnetite, tourmaline-chlorite-magnetite, and sericite-hematite hydrothermal alteration styles. Tourmaline and chlorite chemistry indicates that low temperature alkaline fluids had variable Fe 2+ , Mg, Al, and Na contents throughout the evolution of the breccia complex. Magnetite and hematite display low vanadium content (≤0.24 wt%) and very low titanium content (≤0.06 wt%). The sulfide minerals are pyrite (with up to ~ 3 wt% Co), chalcopyrite, bornite, arsenopyrite, and covellite, in decreasing order of abundance. Coarse muscovite that is related to the latest stages of breccia displays a high barium content with an average of ≥ 1.27 wt%. Isotopic compositions of carbon and oxygen in calcite from the calcic breccia indicate that early mineralizing fluids were derived from a mixed magmatic/non-magmatic reservoir (δ 13 C ~ −2‰ and δ 18 O from 9.95 to 13.09‰ in calcite). The sulfur isotopic compositions of chalcopyrite and pyrite demonstrate that sulfur in the mineralized breccia was derived from cooling magmas that underwent weak contamination from oxidized fluids or host rocks (δ 34 S from 2.22 to 3.89‰). New U-Pb zircon ages indicate that mineralization occurred at approximately 2100 Ma, coeval with tourmaline-bearing porphyritic granite dikes that yield ages of 2098 ± 8 and 2112 ± 12 Ma respectively. Despite being considered a small deposit in relation to known reserves, Lavra Velha may be the first IOCG deposit recognized in the São Francisco Craton, particularly in the Gavião Block. Thus, it represents an important indicator for future precious and base metal exploration in the São Francisco Craton.

Metallogenic ages and sulfur sources of the giant Dahutang W–Cu–Mo ore field, South China: Constraints from muscovite 40Ar/39Ar dating and in situ sulfur isotope analyses

• In-situ sulfur isotope compositions of sulfides imply a magmatic origin for sulfur. • The hydrothermal breccia in the Shimensi deposit formed at 142.0 ± 0.6 Ma and was triggered by the BGP . • The coarse quartz vein-type mineralization in the Shiweidong deposit formed at 136.1 ± 0.5 Ma and was caused by the early episode of CPTG. • The W–Cu–Mo mineralization and ore-related granites in the Dahutang ore field formed in a compression–extension transitional setting. The giant Dahutang W–Cu–Mo ore field is one of the largest tungsten ore fields worldwide. Three main types of mineralization are developed at Dahutang, comprising veinlet-disseminated, hydrothermal breccia, and coarse quartz vein-type mineralization. In this study, precise muscovite 40 Ar/ 39 Ar ages and systematic in situ sulfur isotope compositions of sulfides from the Shimensi and Shiweidong deposits were presented to determine the ore-forming ages and mineralizing intrusions of the hydrothermal breccia type and coarse quartz vein-type mineralization, sources of sulfur in the mineralization, and tectonic setting of the Dahutang ore field. The precise muscovite 40 Ar/ 39 Ar dating suggested that the hydrothermal breccia in the Shimensi deposit formed at 142.0 ± 0.6 Ma and was triggered by the biotite granite porphyry (BGP), while the coarse quartz vein-type mineralization in the Shiweidong deposit formed at 136.1 ± 0.5 Ma and was caused by an early episode of coarse-grained porphyritic two-mica granite (CPTG; 144.2–137.5 Ma) and destroyed by the late episode of CPTG (130–128 Ma). The in situ sulfur isotope compositions of sulfides showed that the hydrothermal breccia type and the coarse quartz vein-type mineralization had a narrow range of sulfur isotope compositions (−3.38–+0.39‰), implying a magmatic origin for sulfur. The increased sulfur isotopes in the sulfides from early to late stages were probably caused by a reduction in the oxygen fugacity of ore-forming fluids in the hydrothermal breccia mineral system. The main W–Cu–Mo mineralization event at Dahutang occurred in the 146–136 Ma interval and was only associated with the early episode of magmatism (149–138 Ma), which coincided well with the Cu–Au–Mo–Fe mineral system in the neighboring Middle–Lower Yangtze River Metallogenic Belt (148–135 Ma). The late episode of magmatism (138–128 Ma), however, was commonly emplaced after the tungsten polymetallic mineralization and even destroyed early formed orebodies as ore-barren intrusions. Combined with the regional tectonic evolution, we proposed that the W–Cu–Mo mineralization and ore-related granites in the Dahutang ore field formed in a transitional setting from a compressional regime to an extensional regime.

Tourmaline as a potential mineral for exploring porphyry deposits: a case study of the Bilihe gold deposit in Inner Mongolia, China

Tourmaline is widespread at the Bilihe porphyry gold deposit in Inner Mongolia, China. Five types of tourmaline are recognized based on their occurring locations and microscopic structures: (1) Tur-A in quartz–magnetite ± K-feldspar veins and related potassic-altered rocks, (2) Tur-B in auriferous quartz veinlets, (3) Tur-C in the intermediate argillic shell, (4) Tur-D in late quartz–carbonate–tourmaline–pyrite veins, and (5) Tur-HB in hydrothermal breccia pipes, consisting of three sub-generations (Tur-HB1, -HB2, and -HB3). Almost all these tourmalines have a schorlitic–dravitic composition and belong to the alkali group. Most of the tourmalines contain high Al contents, which may suggest that the fluids are Al-enriched, and that (□Al)(NaFe)-1 and (AlO)(R2+)-1(OH)-1 exchanges have dominated their elemental substitutions. Tur-A and -B yield similar REE patterns with no strong fractionation of LREE and HREE, whereas LREE-enrichment and HREE-depletion characterize Tur-C and Tur-D; Tur-HB1 and -HB2 have flat REE patterns. Tourmalines in hydrothermal veins always show negative Eu anomalies, whereas the ones replacing plagioclase in the altered rocks yield positive Eu anomalies. The mineral precipitation and assimilation of host rocks probably both contributed to the variable compositions of Bilihe tourmaline. Tur-A−D from the main orebody all yield positive δ11B values ranging from +0.7 to +6.3‰, whereas Tur-HB1−HB3 from the hydrothermal breccia pipe show negative δ11B values of −8.8 to −3.5‰. Magmatic hydrothermal fluids enriched in isotopically light boron could have dominated the formation of Tur-HB. In contrast, the heavy boron in Tur-A−D was probably leached from the widespread marine carbonates around the deposit. Systematic contrasts in the trace element compositions of tourmaline from different mineralization types and granitic rocks suggest that (Sn + Li) vs. (Ni + V + Zn), (∑REE + Y + Zr) vs. (Ni + V + Zn) and V vs. Zn are potential elemental groups for distinguishing tourmaline from different environments.

A genetic link between albitic magmas and IOCG mineralization in the Ossa Morena Zone (SW Iberia)

A detailed geological and isotopic study of the Colmenar deposit (Ossa Morena Zone, SW Iberia) shows that the magnetite-rich mineralization formed by complex magmatic-hydrothermal processes related to the crystallization of water-rich albite-magnetite igneous rocks derived from the crystallization of unusual melts formed during anatexis in a high temperature-low pressure metamorphic regime. The most likely protolith includes a sequence of iron-rich chemical sediments, amphibolite and possible meta-evaporites of early Cambrian age. The albite-magnetite rock occurs as up to 20 cm-thick dyke and breccia bodies and show complex immiscibility relationships with an albite-K-feldspar-quartz leucogranite. Iron-rich fluids exsolved during the crystallization of these melts are responsible of the formation of hydrothermal breccias and the widespread replacement of the hosting calc-silicate hornfels by a magnetite-ferroactinolite-albite assemblage along syn-mineralization shear zones. Geochronological data obtained for mineralization and related hydrothermal alteration points to a Variscan age (ca. 340 Ma), interpreted also as the age of the high-grade metamorphism driving anatexis at the Valuengo Metamorphic Complex. Despite the low Cu and Au contents, this mineralization shares features with the IOCG systems, which in other districts show a spatial relationship with albite-rich rocks, evaporites and pre-existing iron mineralization. The observations presented from Colmenar support an alternative genetic model with prospective implications for the Ossa Morena Zone that can be extrapolated to other IOCG belts worldwide.

Geology and geochronology of the Two-Thirty prospect, Northparkes district, NSW

The Northparkes district, central New South Wales, hosts several economic Cu–Au deposits associated with discrete, thin, porphyry intrusive complexes emplaced in the Late Ordovician during formation of the Macquarie Arc. The recently discovered Two-Thirty Cu–Au–(Mo) prospect is a mineralised magmatic–hydrothermal breccia complex that is hosted by the moderately east-dipping Goonumbla Volcanic Complex on the western limb of the Milpose Syncline ∼15 km south of the Northparkes porphyry district. Generation of the magmatic–hydrothermal breccia complex is interpreted to be related to the 448.0 ± 4.4 Ma emplacement of the Two-Thirty porphyry. However, Re–Os dating of molybdenite from the breccia complex indicates a potential for a ca 440 Ma mineralising event that has similar timing to economic porphyry mineralisation in the Northparkes district. The discovery of the Two-Thirty prospect has important implications for exploration in the Northparkes district and the broader Macquarie Arc. Two-Thirty is only the second known occurrence of magmatic-hydrothermal breccia-hosted mineralisation discovered within the Macquarie Arc, with the other being Cadia Quarry. Mineralisation at Two-Thirty is potentially older than the Northparkes and Cadia deposits, and younger than the epithermal and calc-alkaline deposits at Cowal, Marsden and Ridgeway. KEY POINTS: The Two-Thirty is a polyphase magmatic–hydrothermal breccia complex that hosts Cu–Au (Mo). The Two-Thirty is the first significant breccia-hosted mineralisation found in the Northparkes district. U–Pb zircon crystallisation ages of the causative intrusion at Two-Thirty pre-date mineralisation at Northparkes. Re–Os dates of molybdenite from the Two-Thirty breccia complex are coeval with syn-mineralisation at Northparkes, supporting the model of periodic release of melts and fluids from underlying magma chambers during the formation of porphyry mineralisation in the Northparkes district.

Mineralogy of hydrothermal breccia cement of Humpa Leu East porphyry copper-gold prospect, Sumbawa Island, Indonesia

Indonesia is a country that has several world-class copper-gold deposits, particularly in eastern Sunda arc. The Hu’u complex has several prospects in the surface as lithocap of extensive epithermal style alteration, but some were detected associated with porphyry beneath the surface. The study focuses on hydrothermal breccia cement as a factor influencing the porphyry system in the Hu’u district. The methods used is mineralogical analysis with petrography and μ-XRF elemental mapping on hydrothermal breccia samples. The Hu’u district is interpreted as a paleo-volcano; a member of the Old Volcanics Rocks Formation. The Humpa Leu East lithology consists of pre-volcanics unit (lava and pyroclastics), diorite, andesite-μ-diorite, and tonalitic intrusion at the depth. Hydrothermal alteration evolved from tonalite body to outward, consist of potassic, inner propyllitic and overprinted by phyllic and advanced argillic. Several phases of hydrothermal activities occur in this system, including the hydrothermal breccia phase associated with complex fluids. The hydrothermal cement of Humpa Leu East porphyry at least have three phases of mineralogical assemblages and possibly influencing the mineralization. The mineralogical assemblage of hydrothermal cement in HLE consists of quartz-feldspar-plagioclase-biotite as a high-temperature phase; then followed by epidote-sericite-chlorites-anhydrite-carbonates in medium temperature; there are aluminum-rich clay minerals interpreted as gibbsite. Mineralization occurs in three phases including chalcopyrite-magnetite, bornite-chalcopyrite and chalcopyrite-sphalerite phases. The occurrences of chalcopyrite at all phases indicate the stability of intermediate sulfidation proses in Humpa Leu and as a possible factor to answer the abundant copper in the Hu’u hydrothermal fluid system

New sight for gold forming system in Wulong deposit: Indicated by the distal Pb-Znmineralization in hydrothermal breccia

New sight for gold forming system in Wulong deposit: Indicated by the distal Pb-Znmineralization in hydrothermal breccia

Multistage development of a hydrothermal W deposit during the Variscan late-orogenic evolution: the Puy-les-Vignes breccia pipe (Massif Central, France)

The Puy-les-Vignes W deposit, located in the northwestern French Massif Central (FMC), is a rare occurrence of a wolframite-mineralized hydrothermal breccia pipe hosted in high-grade metamorphic gneisses. We present an integrated study of this deposit aiming to characterize the ore-forming hydrothermal system in link with the Variscan late-orogenic evolution of the FMC. Based on a set of representative samples from the host rocks and mineralization, we describe a detailed paragenetic sequence and we provide the major and trace element geochemistry of the granitic rocks and W–Nb–Ta–Sn–Ti oxide minerals, in situ U/Pb and 40Ar/39Ar geochronology, and a fluid inclusion study of quartz and wolframite. We demonstrate that the formation of this W-mineralized breccia pipe results from a multistage development related to four major episodes during the late Carboniferous. The first episode corresponds to the emplacement of an unexposed peraluminous granite at ca. 324 Ma, which generated microgranite dykes exposed at the present-day surface. The second episode is the formation of the quartz-supported breccia pipe and wolframite mineralization at ca. 318 Ma at a paleodepth of 7 km. The mineralizing fluids have a H2O–NaCl–CO2–CH4–N2 composition, a moderate-salinity (&lt; 9 wt.% NaCl eq) and were trapped at high-temperatures (&gt; 400 °C) during lithostatic to hydrostatic pressure variations caused by hydrofracturing of the host rocks. Wolframite deposition is interpreted to result from a W-rich intermediate-density magmatic fluid that exsolved from an evolved leucogranite and interacted with volatile-rich metasedimentary country rocks and/or possibly mixed with low-salinity metamorphic fluids of deep origin. The third episode corresponds to magmatic-hydrothermal Nb–Ta mineralization overprinting the W-mineralized system interpreted to be related to the intrusion at ca. 311 Ma of a rare-metal granite, which is part of a regional peraluminous rare-metal magmatism during the 315–310 Ma period. Finally, the last episode corresponds to disseminated Bi ± Au–Ag mineralization emplaced at ca. 300 Ma, which shares similar mineralogical features with late Carboniferous orogenic gold deposits in the FMC. The Puy-les-Vignes W deposit records, therefore, a multistage and long-lived development that extends over a timespan of 25 million years in a regional setting dominated by protracted peraluminous magmatism and high-temperature and low-pressure metamorphism. Although the local environment of ore deposition is atypical, our results show that the mineral assemblages, alteration styles, and fluid characteristics of the Puy-les-Vignes breccia pipe are similar to those of other peri-granitic W deposits in the FMC.

The Paleoproterozoic porphyry copper–molybdenum deposit of Kourki (Liptako Province, western Niger)

Abstract The Kourki porphyry Cu–Mo deposit is located in the southern part of the Gorouol greenstone belt in Western Niger. The potential Cu–Mo mineralization is hosted by granitoids (granodiorite and diorite) and veins, in Paleoproterozoic plutonic rocks, which are the main porphyry Cu–Mo system. The scientific objective of this study was to determine the characteristics of Kourki Cu–Mo mineralization and to compare them with other porphyry systems in the world and particularly with those of the West African Craton. The methods implemented consist of a field study and a polarizing microscope analysis in transmitted and reflected light on the rocks bearing Cu–Mo mineralization. The use of reference diagrams made it possible, on the one hand, to confirm the origin of the Cu–Mo mineralization and, on the other hand, to specify conditions of the formation of this mineralization. The geology of the study area shows a granodiorite–tonalite pluton in close contact with dioritic intrusions to the north, clastic sediments to the east, and volcano-sedimentary rocks to the SE. The pluton is crosscut by an intrusion of molybdenum porphyry dyke with irregular shape that can be observed in outcrop. The geochemical study shows that the granodiorite–tonalite pluton can be related to a calc-alkaline plutonic series which characterizes an arc type environment. Large areas of hydrothermal quartz breccia have been identified within the pluton. They were set up by hydraulic fracturing along the zones of weakness. Most of the Cu–Mo mineralization in the Kourki deposit occurs as hydrothermal breccia, filled with veins, and was disseminated in the porphyry diorite and quartz–diorite stockworks. The mineralogical assemblage is marked by a predominance of pyrite, molybdenite, chalcopyrite and quartz with some bornite and chalcocite. The alterations include potassic alteration, propylitic alteration, sericitization and epidotization. All of these data show that the Kourki porphyry represents a characteristic system of an environmental arc, quite comparable with that of the Andean Cordillera.

Fluid evolution of the Humedo porphyry-related gold deposit, southern Ecuador: Evidence from the boron isotope and chemical variations of tourmaline

Coexisting porphyry Cu-Au and low- to intermediate-sulfidation epithermal Au-Ag mineral systems are well developed in the Oligocene–Miocene volcanic and porphyry rocks in the Cangrejos–Zaruma district, Ecuador, northern Andes. The Humedo deposit, with a defined gold resource of 8 t @ 5 g/t, is a recently discovered Miocene porphyry–epithermal gold deposit in the southwest Cangrejos–Zaruma district. Here the textures, geochemistry, and B isotopes of tourmaline are investigated to understand the fluid evolution of the Humedo gold deposit. Four types of tourmaline (TurA–D) associated with three hydrothermal stages have been identified at Humedo: TurA in the matrix of stage II magmatic hydrothermal breccias, TurB in pervasive phyllic altered and silicified rocks of stage III, and TurC and TurD aggregates hosted in the extensively silicified rock and/or hydrothermal breccias of stage IV. All Humedo tourmaline belongs to the alkali group, with schorlitic–dravitic compositions. The TurA is characterized by the highest Fe, Nb, W, and Sn contents, the heaviest B isotopes (δ11B = 1.4–5.6 ‰), isovalent Al–Fe substitution, and low Eu/Eu* values, suggesting that oxidized magmatic fluids dominated during stage II. The TurB grains hosted by metamorphic and intrusive host rocks have different contents of Sc, Zn, V, and Ni, indicating a local impact on compositions from the host rocks. The low values of δ11B (−9.7 to −2.2 ‰), the correlation of δ11B with Co, and the Fe–Mg substitution of TurB suggest that fluids of stage III were reducing and possibly influenced by interaction with graphite-bearing meta ultramafic–mafic rocks of the Raspas Formation and/or Palenque Unit. The TurC–D is marked by the highest Mg contents of all groups, combined with the positive correlations of δ11B with Mg and Ba, suggesting the existence of external waters that interacted with shallow metasedimentary or volcanic sedimentary rocks during stage IV. The geochemistry and isotope characteristics of tourmaline thus record changes in the ore-forming fluid compositions (magmatic vs. external) and nature (oxidized vs. reduced) caused by fluid-rock interaction with regional country rocks in the Humedo porphyry–epithermal system.

The Pliocene Ixtacamaxtitlán low sulfidation epithermal deposit (Puebla, Mexico): A case of fossil fungi consortia in a steam-heated environment

The Ixtacamaxtitlán area in northern Puebla (central Mexico) contains middle Miocene Cu-Mo-Au porphyry/skarn and Pliocene low-sulfidation Au-Ag epithermal deposits that are geologically associated with the evolution of the Trans-Mexican Volcanic Belt (TMVB). In this paper, a new 40Ar/39Ar age (2.87 ± 0.41 Ma) is provided for rhombohedral alunite from a kaolinite + alunite ± opal ± cristobalite ± smectite advanced argillic alteration assemblage. This age contributes to the definition of a metallogenic province that is confined to the TMVB, a relevant feature for regional exploration. A ~12 My gap is established between the formation of the Cu-Mo-Au porphyry/skarn and low-sulfidation Au-Ag epithermal deposits, which rules out the possibility that their overlapping was the result of telescoping. Advanced argillic alteration is conspicuous throughout the mineralized area. This alteration assemblage consists of a widespread kaolinite-rich blanket that underlies silica sinters, polymictic hydrothermal breccias, and an alunite-rich spongy layer that consists of vertical tubular structures that are interpreted as the result of gas venting in a subaerial environment. The above indicate a shallow hypogene origin for the advanced argillic alteration assemblage—that is, formation by the partial condensation within a phreatic paleoaquifer of acidic vapors that were boiled-off along fractures that host epithermal veins at depth. The formation of the spongy alunite layer and silica sinters is interpreted to have been synchronous. Within the alunite-rich spongy layer, tubular structures hosted microbial consortia dominated by fungi and possible prokaryote (Bacteria or Archaea) biofilms. Such consortia were developed on previously formed alunite and kaolinite and were preserved due to their replacement by opal, kaolinite, or alunite. This means that the proliferation of fungi and prokaryotes occurred during a lull in acidic gas venting during which other organisms (i.e., algae) might have also prospered. Periodic acidic gas venting is compatible with a multi-stage hydrothermal system with several boiling episodes, a feature typical of active geothermal systems and of low-sulfidation epithermal deposits. The microstructures, typical for fungi, are mycelia, hyphae with septa, anastomoses between branches, and cord-like groupings of hyphae. Possible evidence for skeletal remains of prokaryote biofilms is constituted by cobweb-like microstructures composed of &lt;1 µm thick interwoven filaments in close association with hyphae (about 2.5 µm thick). Bioweathering of previously precipitated minerals is shown by penetrative biobrecciation due to extensive dissolution of kaolinite by mycelia and by dissolution grooves from hyphae on alunite surfaces. Such bioweathering was possibly predated by inorganically driven partial dissolution of alunite, which suggests a lull in acidic gas venting that allowed living organisms to thrive. This interpretation is sustained by the occurrence of geometrical dissolution pits in alunite covered by hyphae. Fungal bioweathering is particularly aggressive on kaolinite due to its relatively poor nutrient potential. Such delicate microstructures are not commonly preserved in the geological record. In addition, numerous chalcopyrite microcrystals or microaggregates are found within the alunite layer, which could be related to sulfate reduction due to bacterial activity from the sulfate previously released by fungal bioweathering of alunite. Hydrogeochemical modeling constrains pH to between ~3.2 and ~3.6 and temperature to between 53 and 75 °C during the stage in which fungi and other organisms thrived. These waters were cooler and more alkaline than in earlier and later stages, which were characterized dominantly by steam-heated waters. The most likely process to account for this interlude would be mixing with meteoric water or with upwelling mature water that did not undergo boiling.