Auriferous Quartz Research Articles

Implications for metallogenic evolution of the Balong gold deposit, East Kunlun metallogenic belt: Insights from in-situ trace elements and S isotopes of sulfides

The Balong gold deposit is one of numerous lode gold deposits in the East Kunlun metallogenic belt. Gold mineralization is hosted in Triassic granitoids, typified by auriferous quartz veins. Hydrothermal alteration includes sericite, quartz, sulfide, chlorite, and calcite. Pyrite, as the most abundant sulfide in the ore, is sometimes seen in the company of arsenopyrite. Three types of pyrite have been identified. The porous Py1 exhibits low Co and Ni contents, with an absence of gold. Subhedral Py2-1 shows higher Co (median 80 ppm) and Ni (median 10.5 ppm) contents and contains various Cu-Pb-Zn-Ag mineral inclusions. Py2-2 shows an increase in As (median 17, 073 ppm) and Au (median 3.79 ppm), exhibiting obvious distinctions between Py2-1 and Py2-2.Gold in the Balong deposit consists of both visible and invisible gold. Gold occurs within micro-fractures of pyrite and arsenopyrite, appearing as irregular inclusions or as infillings. Apart from visible gold grains, the majority of invisible gold hosted in Py2-2 occurs as solid solutions (Au+). Coupled dissolution-reprecipitation reactions of early pyrite are a key factor for visible gold precipitation and later invisible gold enrichment. Pyrite records a narrow range of δ34S values from −1.6 to 5.4 ‰, reflecting sulfur from a deep magmatic source. These findings indicate a connection between the ore-forming materials and the evolved magmatic-hydrothermal fluids.

Gold Mineralization at the Syenite-Hosted Anwangshan Gold Deposit, Western Qinling Orogen, Central China

The Anwangshan gold deposit is located in the northwestern part of the Fengtai Basin, Western Qinling Orogen (WQO). The gold ore is hosted within quartz syenite and its contact zone. The U–Pb weighted mean age of the quartz syenite is 231 ± 1.8 Ma. It is characterized by high potassium (K2O = 10.13%, K2O/Na2O > 1) and high magnesium (Mg# = 55.31 to 72.78) content, enriched in large ion lithophile elements (Th, U, and Ba) and light rare earth elements (LREE), with a typical “TNT” (Ti, Nb, and Ta) deficiency. The geochemical features and Hf isotope compositions (εHf(t) = −6.68 to +2.25) suggest that the quartz syenite would form from partial melting of an enriched lithospheric mantle under an extensional setting. Three generations of gold mineralization have been identified, including the quartz–sericite–pyrite (Py1) stage I, the quartz–pyrite (Py2)–polymetallic sulfide–early calcite stage II, and the epidote–late calcite stage III. In situ sulfur isotope analysis of pyrite shows that Py1 (δ34S = −1.1 to +3.8‰) possesses mantle sulfur characteristics. However, Py2 has totally different δ34S (+5.1 to +6.7‰), which lies between the typical orogenic gold deposits in the WQO (δ34S = +8 to +12‰) and mantle sulfur. This suggests a mixed source of metamorphosed sediments and magmatic sulfur during stage II gold mineralization. The fluid inclusions in auriferous quartz have three different types, including the liquid-rich phase type, pure (gas or liquid)-phase type, and daughter-minerals-bearing phase type. Multiple-stage fluid inclusions indicate that the ore fluids are medium-temperature (concentrated at 220 to 270 °C), medium-salinity (7.85 to 13.80% NaCleq) CO2–H2O–NaCl systems. The salinity is quite different from typical orogenic gold deposits in WQO and worldwide, and this is more likely to be a mixture of magmatic and metamorphic fluids as well. In summary, the quartz syenite should have not only a spatio-temporal but also a genetical relationship with the Anwangshan gold deposit. It could provide most of the gold and ore fluids at the first stage, with metamorphic fluids and/or gold joining in during the later stages.

Utilising the 3D mineral system approach to identify the source of alluvial gold deposits in Ibala Town, Southwest Nigeria: an In-depth geological and geophysical exploration

ABSTRACT The Ibala gold field, discovered by artisanal miners in 2018, is located east of Ibala town in Osun State, southwest Nigeria. As older mines are depleted and abandoned, miners are now investigating the source of the deposits, believed to be auriferous quartz veins. This study uses the mineral system approach to trace the origin of the alluvial gold in the Ibala area. The research is conducted in three phases: first, a detailed analysis of the region’s structural patterns and rock distribution is undertaken to understand the area’s tectonic history. Next, a 3D conceptual model is created using an integrated geological and geophysical modeeling techinque to identify exploration targets and map the mineral system components. Finally, on-site verification is performed to validate the findings. The study reveals that the gold originates from auriferous shear zones characterized by high conductivity and chargeability geophysical signatures. These zones were formed by auriferous fluids released during the metamorphic devolatilization of deep volcanic rocks. As the fluids migrated through shear zones, they interacted with host rocks, precipitating gold alongside pyrite. When weathered, the gold-bearing material is transported via drainage channels, which miners now exploit. By using a mineral system approach, this research pinpoints the primary gold sources, supporting more efficient resource management and sustainable mining practices in the region.

Metallogenic model of the Lykling ophiolite-hosted lode Au deposit, Scandinavian Caledonides: Insight from fluid inclusions, mineral chemistry and stable isotope geochemistry

The Lykling lode Au deposit represents a unique example of gold mineralization in the Upper Allochthone of the Scandinavian Caledonides. The mineralization is hosted by the Early Ordovician Lykling Ophiolite Complex and the intruding trondhjemite unit and spatially associated with two generations of mafic dykes that crosscut both the ophiolitic complex and trondhjemite. Field observations indicate that the mafic dykes did not play an active role in the emplacement of Au at Lykling, however their contacts with the immediate host rocks (i.e., gabbro and trondhjemite) may have focused ore-forming fluids.Three main stages in the evolution of the hydrothermal system in the Lykling area are documented by two generations of auriferous quartz veins: 1) older quartz-carbonate veins hosted by moderately angled brittle-ductile shear zones and 2) younger quartz-sulfide veins that fill steeply dipping brittle faults. Stage 1 resulted in deposition of barren quartz and gold-free pyrite from moderately saline NaCl-CaCl2-H2O ± CO2 fluids at temperatures between ∼310–330 °C and pressures in the range from 2.7 to 3.5 kbars. Under the given physicochemical conditions Au was mobile in the form of its chloride complexes. Stage 2 is associated with multiple episodes of ductile deformation punctuated by concomitant brecciation reflecting brittle-ductile transition processes. Fluid inclusion data reveals a decrease in temperature and pressure that may result in the formation of stable Au-bisulfide complexes and making the hydrothermal mobility of Au sensitive to changes in pressure. Consequently, the fluctuation in pressure controlled by the development of brittle-ductile structures likely resulted in the precipitation of native gold in quartz veins. Stage 3 represents a pure brittle event which is accompanied by a significant decrease in the fCO2/fS2 ratio. Mixing of higher-temperature and moderate-salinity fluids with cooler fluids has been recognized as the principal trigger for deposition of sulfides and a paragenetically late phase of native gold in the Lykling hydrothermal system.The Lykling ophiolite-hosted lode Au deposit shows numerous similarities with orogenic gold deposits elsewhere, including structural controls during mineralizing events, deposition from moderately to low salinity CO2-bearing aqueous solutions, deposition from a focused fluid flow along trans-crustal fault zones with a mixed brittle-ductile character and the spatial association with regionally metamorphosed terranes. In contrast to the great majority of the known orogenic gold deposits, the Lykling ophiolite-hosted lode Au deposit records a magmatic origin of volatiles. Therefore, taking into consideration field relationships and the regional setting of the Lykling deposit, we argue that its formation is concomitant with emplacement of the Sunnhordland Batholith during post-collisional thinning of the crust and associated localized uplift within the Lykling Ophiolite Complex.

The historical Corrente gold deposit, Minas Gerais, Brazil: Geological characteristics and a reconnaissance study of nugget effect

Corrente is a historical underground working in Minas Gerais, Brazil, mined for gold in the nineteenth century, for which little geological information is available. Retrieval of unpublished exploration reports from the 1980s has enabled the recognition of a 7-km-long linear cluster of historical underground and open-cast workings, here referred to as the Cata Branca–Corrente goldfield. Gold production ceased at Cata Branca, the most economically relevant deposit of the goldfield, in 1844. Both Corrente and Cata Branca are auriferous quartz-lode deposits. Geological characteristics of the Corrente deposit are presented, as well as reconnaissance whole-rock analyses for Au using a variety of analytical methods, including conventional fire assay, atomic absorption spectrometry, instrumental neutron activation analysis and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) on pressed-powder pellets (PPPs). Corrente is characterised as a gold-only deposit. The Corrente auriferous quartz lode, hosted in the Archaean greenstone belt sequence of the Rio das Velhas Supergroup, is extensively oxidised and has conspicuous wall-rock alteration to fine-grained muscovite. The resulting sericite phyllite shows overprint by low-temperature kaolinitisation, likely the product of wall-rock hydration mediated by acidic water derived from sulfide oxidation at depth, below the modern weathering zone. The Corrente gold deposit has a high nugget effect. Time-resolved results of LA–ICP–MS on PPPs indicate the presence of particulate gold even in barren kaolinitised wall-rock phyllite.

Presence of Ecandrewsite and Zincian-ilmenite in Auriferous Quartz Veins of North Singhbhum Mobile Belt, Jharkhand

Presence of Ecandrewsite and Zincian-ilmenite in Auriferous Quartz Veins of North Singhbhum Mobile Belt, Jharkhand

Age and fluid source constraints of the Haoyaoerhudong orogenic gold deposit, North China: Evidence from geochronology and noble gas isotopes

The Haoyaoerhudong gold deposit, located in the northwestern part of the North China craton (NCC), has produced over 120 metric tonnes (t) of gold since 2007. It has a total reserve of > 240 t at average gold grade of 0.62 g/t, making it one of the largest open pit gold mines in China. The steeply dipping, large-tonnage, low-grade, vein- or veinlet-type gold orebodies are hosted in strongly-deformed Mesoproterozoic carbonaceous schist of the Bayan Obo Group. The laminated/boudinaged veins/veinlets in the sinistral ductile–brittle shear zones are dominated by quartz, biotite, gold-bearing löllingite, pyrrhotite, (arseno)pyrite, with minor native gold, titanite and xenotime. In this paper, we present new in situ U–Pb geochronological data on magmatic zircon from the pre-ore dikes, on metamorphic and hydrothermal xenotime, and on hydrothermal titanite from the hydrothermally altered carbonaceous schist and auriferous quartz–sulfides veins/veinlets, as well as He-Ar isotopic analysis on gold-bearing (arseno)pyrite in the syn-ore stage. The metamorphic xenotime U–Pb age of 426 ± 6.0 Ma (2σ) records a regional metamorphic event, possibly related to the accretion of the Bainaimiao arc onto the NCC. Two pre-ore andesitic dikes yielded similar emplacement ages at ∼ 278 Ma constrained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb zircon data. Hydrothermal xenotime grains from the altered carbonaceous schist and auriferous quartz–sulfides veins yielded U–Pb ages of 256.0 ± 4.1 Ma (2σ) and 254.4 ± 2.1 Ma (2σ), respectively, overlapping with that of the hydrothermal titanite at 255.4 ± 0.8 Ma (2σ) from the laminated quartz–sulfides veinlets. This indicates that the gold mineralization occurred at ca. 255 Ma. The ∼ 255 Ma gold mineralization age is much younger than the previously reported Early–Middle Permian regional magmatic activity (ca. 291 Ma to 268 Ma), and may be associated with the regional sinistral strike-slip event in the late orogenic cycle related to the collision between the Siberian craton and the NCC. The 3He/4He (R/Ra) and 40Ar/36Ar values of the gold-bearing (arseno)pyrite are 0.04 to 0.09 (average = 0.07) and 375.8 to 2023 (average = 1045), which reveal the ore-forming fluids dominantly originated from the crustal rocks, with limited involvement from the mantle. Collectively, our new geochronological data, noble gas isotopic analyses, and geological evidence support a typical orogenic gold deposit model at Haoyaoerhudong.

The genetic link between lode and disseminated gold ores and relationship between hypozonal and mesozonal deposits: A case study of the Yanzigou orogenic gold deposit, northwestern Yangtze Craton, China

The process controlling the formation of orogenic lode and disseminated gold ores is the basis for understanding the complexity of the orogenic gold system. The mesozonal Yanzigou gold deposit, located in the Danba ore district of the northwestern Yangtze Craton, is hosted by Devonian carbonaceous slate with coexisting fault-hosted lode and strata-bound disseminated gold ores. Three types of pyrite (PyI-q, PyII-q and Pyd) were formed during two stages of hydrothermal evolution. PyII-q in quartz-polymetallic veins and the rims of Pyd in disseminated ores have the highest Au and As contents, contrasting to distal sedimentary pyrite that have low Au and As contents. This implies that the Devonian host sequences are incapable of providing ore elements to the ore fluid. Pyd and associated quartz have intermediate isotopic values (δ34S = -1.7∼+0.8 ‰; δ18OH2O = 10.6 ∼ 12.7 ‰) between the quartz-vein ores (δ34S = 8.5 ∼ 10.7 ‰; δ18OH2O = 9.0 ∼ 10.1 ‰) and wall-rocks (δ34S ≈ −5‰; δ18OH2O ≈ 12 ‰), implicating a critical role of fluid-rock reaction in the formation of the disseminated gold ores and preservation of original ore fluid characteristics in the lode gold ores. The 40Ar/39Ar dating of hydrothermal muscovite from auriferous quartz veins at Yanzigou yields a total fusion age of 118.4 ± 0.4 Ma, later than ∼ 185 Ma for the nearby hypozonal Danba gold deposit. However, both deposits have common geological, geochemical, and isotopic signatures, suggesting a multi-stage orogenic gold mineralization on the Mesozoic dome margins associated with episodic reactivation of metasomatized mantle lithosphere during the continued crustal uplift.

Geological, fluid inclusion, H–O isotopic, and U–Pb geochronological constraints on the genesis of the Xiahuolong gold deposit, southern Jilin Province, NE China

The Xiahuolong gold deposit (3.05 t Au, average grade: 6.7 g/t), located in the Ji'an district of southern Jilin Province in the northeastern part of the North China Craton, contains orebodies in a Proterozoic metamorphosed sedimentary sequence. The distribution of the orebodies is controlled by NW- and NE-trending shear zones and faults, and the orebodies consist of auriferous quartz veins and auriferous altered rocks. The mineralization can be divided into four stages, namely (I) quartz-sericite, (II) quartz-pyrite-copper-gold, (III) quartz-pyrite-sphalerite-galena, and (IV) quartz-carbonate, with gold being introduced mainly during the second stage. Systematic analysis of the fluid inclusions suggests that two-phase vapor-rich (WV-type) and liquid-rich (WL-type) are the dominant types in the quartz veins. The early (stage I), main (stages II and III), and late (stage IV) fluid inclusions homogenize at temperatures of 273–367 °C, 201–346 °C, and 120–196 °C, corresponding to salinities of 0.68–0.79, 0.69–0.88, and 0.90–0.96 wt% NaCl equiv., respectively. The ore-forming fluid system evolved from CO2-H2O-NaCl ± CH4 to H2O-NaCl during ore formation, and fluid immiscibility and mixing caused the precipitation of sulfides and gold in the main stage. The H–O isotopes indicate that the ore-forming fluids were of magmatic origin, with a continuous input of meteoric water during ore formation. Two types of diorite porphyry have been identified, one cutting orebodies and the other filled with auriferous quartz veins. The diorite porphyry associated with the auriferous quartz vein yielded a zircon U–Pb isotopic age of 124.1 ± 1.9 Ma, indicating that Xiahuolong mineralization occurred in the Early Cretaceous, and that the high water content and low oxygen fugacity (ΔFMQ = -2.53 to +0.23; logƒO2 = -17.56 to -20.61) of the dioritic magma provided an important basis for mineralization. The results, combined with the deposit geology, fluid geology, H-O isotope geochemistry, timing of mineralization, and tectonic setting, suggest that Xiahuolong is a mesothermal gold deposit formed in an extensional setting characterized by lithospheric thinning related to the rollback of the subducting Paleo-Pacific Plate.

Genesis of the Shirengou gold deposit, northern North China Craton, based on zircon U[sbnd]Pb, fluid inclusion, sulfide compositional and S isotope data

The newly discovered vein-type Shirengou gold deposit on the northern edge of the North China Craton contains at least 8.69 tons of Au at an average grade of 9.80 g/t. Six auriferous quartz veins have been identified within Upper Jurassic rhyolitic breccia-bearing crystal-lithic tuff, and they follow NW-trending secondary faults. Gold mineralization was closely related to silicification, with pyrite as the predominant ore mineral. Four mineralization stages can be discerned, involving Stage I milky quartz, Stage II quartz-pyrite, Stage III gray quartz-polymetallic sulfide, and Stage IV quartz-calcite ± pyrite. Zircon UPb dating of a pre-ore porphyryritic rhyolite dike sets an upper age limit on the gold mineralization of ~131.1 Ma. Two-phase and single-phase aqueous inclusions were observed in quartz related to all four mineralization stages. From Stage I to Stage IV, the fluid inclusion homogenization temperatures decrease from respectively 257–340 °C to 230–277 °C, 201–250 °C and 161–200 °C, with corresponding salinities of 3.39–9.21, 2.24–8.00, 1.22–7.59, and 0.18–3.55 wt% NaCl equivalent, respectively. Thus, the ore-forming fluid can be described as moderate-high temperature, moderate-low salinity H2O-NaCl fluids. The δ34S values of sulfides vary from 0.0 to 5.3 ‰, indicating that the S was derived primarily from magmatic source(s), with some contribution from the metamorphic rocks of the surrounding Jianping Group. In-situ LA-ICP-MS trace element analysis of sulfides, together with our new age data, suggest that the ore-forming fluid was initially of Early Cretaceous magmatic-hydrothermal origin, and subsequently modified by interaction with the wall rocks of the Jianping Group, extracting the ore-forming metals from these rocks into the fluids.

Deformation and paleopiezometry of auriferous quartz veins in Archean orogenic gold deposits of the Abitibi greenstone belt

Most Archean orogenic gold deposits are associated with major faults such as the Larder Lake-Cadillac fault zone (LLCFZ) in the Superior craton and are hosted in or adjacent to second-order structures as quartz vein systems. Microstructures induced by quartz deformation in these faults or shear zones, such as typical textures of recovery and dynamic recrystallization, give indication of the P-T conditions during deformation. Electron backscatter diffraction (EBSD) analysis of grain lattice orientation and grain size determination, for flow stress calculation, are commonly performed on mylonite or quartzite samples from shear zones. This study tests the validity of such methods on hydrothermal auriferous quartz veins to better constrain the conditions of quartz vein deformation in the vicinity of the LLCFZ and evaluate the effect of depth and local variations on the microstructures and flow stress. Samples were collected in 24 gold deposits and showings along the LLCFZ. The microstructural observations and EBSD analysis allowed the distinction of six microstructural types within the studied samples. Samples analyzed with EBSD display a log-normal distribution of recrystallized grain sizes that is consistent with data for natural samples. The calculated flow stress is highly variable between samples and ranges from 38.2 + 5.1/-6.1 to 74.5 + 2.2/-3.0 MPa in surface samples only. Quartz recrystallization overprints primary textures in all but one samples, which indicates that recrystallization and its associated deformation occurred after the complete crystallization of the veins. Samples with bimodal distribution of recrystallized quartz grain sizes highlight the potential of quartz veins to record several deformation events. Samples are recrystallized at higher temperature and lower stress in the east than in the west of the LLCFZ, which could indicate the exposure of a deeper crust in the east. These data underline the potential of quartz vein microstructures to contribute to the evaluation and evolution of deformation conditions of auriferous quartz vein in an orogenic context.

In situ U-Pb dating of rutile and titanite from the Chaihulanzi lode gold deposit, northern North China Craton and its geological significance

The Chifeng-Chaoyang region, located along the northern margin of the North China Craton, is an important gold producer in China, with a proven gold reserve of about 500 tons. Most gold deposits are fault-controlled and formed in the Mesozoic, but the absolute ages of gold mineralization in this region have not been well constrained due to the lack of suitable dating minerals. This study presents in situ U-Pb ages of hydrothermal rutile and titanite for the Chaihulanzi gold deposit and obtains a distinctive age. The Chaihulanzi deposit (∼27 t Au) is located in the western portion of the Chifeng-Chaoyang region and mainly hosted in the carbonaceous mica schist of the Neoarchean-Paleoproterozoic Jianping Group along NW to NWW striking faults. It contains auriferous quartz veins and disseminated ores in hydrothermal alteration zones, with pyrite and pyrrhotite as the predominant ore minerals. Based on field and petrographic observations, four stages of mineralization process can be identified, involving stage I quartz-pyrite, stage II quartz-pyrite-pyrrhotite-chalcopyrite, stage III quartz-polymetallic sulfides, and stage IV quartz-calcite. Stage II represents the main gold stage and contains abundant hydrothermal rutile and titanite, which are intergrown with pyrite, pyrrhotite and native gold. Rutile grains have high contents of V, Nb, Fe, Cr, W and low contents of Zr and yield a Tera-Wasserburg lower intercept age of 285.4 ± 8.5 Ma (n = 37, MSWD = 3.9). Titanite grains show low Th/U and La/Yb ratios (0.03 to 0.66 and 0.26 to 3.98, respectively) and flat chondrite-normalized REE patterns (LREE/HREE = 0.88–5.72, Eu/Eu* = 0.71–1.71), with a Tera-Wasserburg lower intercept age of 282.3 ± 3.4 Ma (n = 20, MSWD = 2.1). Combined with the emplacement age of a post-ore mafic dyke (ca. 275.5 Ma), the timing of the Chaihulanzi deposit can be constrained to Early Permian, representing a new gold mineralization epoch in the Chifeng-Chaoyang region. On the basis of available geological and geochronological evidence, the Chaihulanzi deposit can be classified as an orogenic gold deposit formed during the southward subduction of the paleo-Asian Ocean. These new findings likely have important implications for future regional gold exploration.

Ore Mineralogy, Fluid Inclusion Geochemistry, and Zircon U-Pb Geochronology of the Nanmingshui Gold Deposit in East Junggar, Xinjiang, Northwest China: Implications for Ore Genesis

The Nanmingshui gold deposit, located in the eastern segment of the Kalamaili gold belt (KGB), is hosted by the sub-greenschist facies rocks of the Lower Carboniferous Jiangbasitao Formation. The genesis of this deposit, however, has been debated for decades because of controversial constraints on the P-T-X conditions and origins of hydrothermal fluid and mineralization age. In this study, we present gold-bearing sulfide compositions, fluid inclusions, H-O isotopes, and the results of hydrothermal zircon U-Pb dating to provide new insights into the genesis of the gold deposit. Three gold mineralization stages are recognized: quartz–pyrite–minor native gold veins (early), quartz–tourmaline–arsenopyrite–pyrite–gold–polymetallic sulfide veins (middle), and quartz–calcite veinlets (late). Gold predominantly occurs as native gold with high fineness ranging from 941 to 944 in sulfides and quartz, and some as solid solutions (Au+) within the lattice of pyrite and arsenopyrite. Three types of primary fluid inclusions are identified in hydrothermal quartz: CO2-H2O (C-type), aqueous (W-type), and pure CO2 (PC-type) inclusions. The early-stage quartz mainly contains C-type and minor W-type inclusions, with total homogenization temperatures (Th) of 220–339 °C, salinities of 0.4–3.7 wt.% NaCl eqv., and bulk densities of 0.66–1.01 g/cm3. All three types of inclusions are observed in the middle-stage quartz, of which the C- and W-type inclusions yield Th values of 190–361 °C, with salinities of 0.4–6.0 wt.% NaCl eqv. and bulk densities of 0.69–0.99 g/cm3. The late-stage quartz contains only W-type inclusions that have lower Th values of 172–287 °C, higher salinities of 1.4–6.9 wt.% NaCl eqv., and bulk densities of 0.79–0.95 g/cm3. Trapping pressures estimated from C-type inclusions in the early and middle stages cluster at 280–340 MPa and 220–310 MPa, respectively, corresponding to metallogenic depths of 10–13 km and 8–11 km. The H-O isotopic compositions (δ18Owater = 1.8–10.9‰, δD = −99 to −62.9‰) and microthermometric data indicate that the ore-forming fluids belong to medium–high-temperature, low-salinity, medium-density, and CO2-rich-H2O-NaCl ± CH4 ± N2 systems, probably originating from metamorphic water. Fluid immiscibility is a crucial mechanism for gold precipitation. Additionally, the U-Pb dating of hydrothermal zircons, from the auriferous quartz–tourmaline vein, yield a weighted mean 206Pb/238U age of 314.6 ± 9.6 Ma. Taking all of the above, the Nanmingshui deposit can be reasonably classed as a typical mesozonal orogenic gold deposit in the KGB, which was formed in a Late Carboniferous tectonic transition from syn-collision between the Jiangjunmiao accretionary complex and Yemaquan arc to post-collision in the East Junggar Orogen. Our results serve to better understand the gold mineralization and genesis of the Late Paleozoic orogenic system in the Kalamaili area, Xinjiang.

Au-Cu Resources in Some Mines from Antiquity in the South Gabal Um Monqul and Gabal Al Kharaza Prospects, North Eastern Desert, Egypt

Since Antiquity, sustainable resources of gold and copper have been mined at two prominent prospects in the north Eastern Desert of Egypt, namely the south Gabel Um Monqul (SGUM) and Gabal Al Kharaza (GKZ). Mineralization is hosted by Neoproterozoic shield rocks represented by dacite and monzo- to syenogranite at the SGUM prospect whereas they are diorite, granodiorite, and quartz feldspar porphyry at the GKZ prospect. These hosts have been emplaced in an island arc environment from calc-alkaline magmas with a peraluminous to metaluminous signature. They are hydrothermally altered including albitization, sericitization, silicification, epidotization, and chloritization. The Au and Cu mineralizations are confined to shear zones that lately filled with auriferous quartz veins adjacent to mineralized alteration zones. In the GKZ prospect, the old trenches trend mainly in a NW–SE direction whereas it is NE–SW and NW–SE in the SGUM prospect. Evidence of shearing ranges from megascopic conjugate fractures and shear planes in the outcrops to microscopic sheared and crumbled Au-Cu ore assemblages dominated by Fe-Cu sulfides, specularite, and barite. Microscopic investigation suggests that the formation of specularite is due to the shearing of early existing magnetite. The ore textures and paragenetic sequence indicate that pyrite in the alteration zones is oxidized, leading to the liberation of gold up to 3.3 g/t. The formulae of the analyzed electrum lie in the range Au74.5-76.8 Ag22.2-24.5. Integration of the field, geochemistry, and mineral chemistry data, combined with the gold fire assay data prove the presence of sustainable amounts of disseminated Au and Cu, not only in the mineralized quartz veins, but also in the alteration zones. Data materialized in our paper show similarities in the style of mineralization at the SGUM and the GKZ prospects with iron oxide-copper-gold (IOCG) deposits elsewhere in the Arabian-Nubian Shield (ANS) and other world examples.

Two-Stage Superimposed Gold Mineralization in the Xiejiagou Gold Deposit, Shandong Province: Insights from Fluid Inclusions, H-O-S Isotopes, and Trace Elements

The Xiejiagou gold deposit located in the Zhaoyuan-Laizhou gold belt is composed of altered-rock-type gold mineralization and superposed auriferous quartz veins, showing unique two-stage gold mineralization. Oxygen and hydrogen isotopic analyses yielded the following results: δ18OH2O = 0.8‰ to 4.4‰ and δD = −106‰ to −85‰ for altered-rock-type mineralization, and δ18OH2O = 3.6‰ to 5.6‰ and δD = −98‰ to −89‰ for auriferous quartz-veins. Combined studies on Co/Ni, Sb/Bi and As/Ag ratios of pyrites, it can be inferred that the ore-forming fluids were dominated by magmatic water mixed with very little meteoric water. The fractured altered rocks in the ore-hosting fault zones are characterized by mylonitization, cataclastic lithification, and structural lenses, reflecting a compressional (closed) ore-forming system. In contrast, the occurrence of auriferous quartz veins in fissures of altered-rock-type orebodies and the fact that altered-rock-type ores commonly occur as breccias cemented by auriferous quartz veins that reflect an extensional (open) ore-forming system for the vein mineralization. The increase in δ34S values from stage I (5.8‰–7.2‰) to stage II (6.6‰–9.0‰) indicate that the altered-rock-type mineralization was the result of intense water–rock interaction, while the occurrence of immiscible inclusions in auriferous quartz veins demonstrates that fluid immiscibility contributes significantly to gold deposition in the vein-type mineralization. Fluid-inclusion microthermometric data indicate that the fluids for the altered-rock-type mineralization are characterized by moderate-to-high temperature (262–368 °C), and low-to-moderate salinity (4.3–10.8 wt.% NaCl equivalent). In contrast, halite-bearing inclusions are found in auriferous quartz veins, and its fluids are characterized by moderate-to-high temperature (290–376 °C) and moderate-to-high salinity (5.1–41.9 wt.% NaCl equivalent). From early stages (I and II) to the late stage (III), homogenization temperature and high temperature element (W, Sn, and Mo) concentrations in pyrite first decrease and then increase, δ34S values and metallization-related element (Au, Ag, and Bi) concentrations in pyrite first increase and then decrease. Therefore, it can be inferred that the two distinct types of gold mineralization in the Xiejiagou gold deposit may be two separate mineralization events. The presence of magnetite in the auriferous veins suggests an increase in oxidation state during the vein mineralization. Importantly, the Xiejiagou gold deposit preserves two types of mineralization in a single deposit and uniquely records a metallogenic transition from a compressional, reduced environment to an extensional, oxidized environment, as a result of a regional stress field transition that occurred in the Zhaoyuan-Laizhou gold belt at ca. 120 Ma.

LA–ICP–MS U–Pb geochronology and geological significance of two types of monazite in the Xinjiazui gold deposit, northwestern margin of Yangtze Block, China

The newly discovered Xinjiazui gold deposit situates on the northwestern margin of the Yangtze Block, China. Previously, the absolute age of gold metallogenesis in this area has yet to be well defined due to the lack of suitable dating minerals, significantly hindering the study of ore genesis and metallogenesis of gold deposits. This study presents high precision in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) U–Pb ages of monazite to address this issue. Two types of monazite have been found in this deposit, with Type 1 developed in the sedimentary diagenetic stage and Type 2 formed in the ore-forming stage. Type 1 monazite collected from carbonaceous slate is symbiotic with framboidal pyrite in a skeletal texture. It exhibits a high total amount of rare earth elements (REE) and low Y, Th, and U contents, indicating a low-grade metamorphic origin. In contrast, Type 2 monazite from the auriferous quartz vein is intergrown with hydrothermal mineral assemblages including free gold, Au-bearing pyrite and arsenopyrite, ankerite and sericite, suggesting a hydrothermal origin. The monazite (Type 1) LA–ICP–MS 206Pb/238U age is 460.8 ± 7.0 Ma, representing the sedimentary age of the host rock. A combination of lithologic associations and geochemical characteristics indicates that the Ordovician strata (O2ch) are important surrounding rocks in the Xinjiazui gold deposit. Additionally, the forming ages of monazite symbiotic with free gold in the auriferous quartz–sulfide veins range from 206.4 to 207.4 Ma, representing the gold mineralization age of the Xinjiazui deposit. Field geological observation shows that the vertical vein wall of columnar quartz crystals grows inward in the auriferous quartz–sulfide vein geodes and the euhedral granular sulfides develop in geodes. Therefore, this study proposes that the tectonic system changed from compression to extension in the late orogenic period with the closure of Paleo-Tethys and gold mineralization occurred in the Back-Longmenshan tectonic belt.

Orogenic lode-gold deposits and listvenization processes in the El-Barramiya area, Eastern Desert, Egypt

Gold mineralization in the El-Barramiya region of the Eastern Desert, Egypt, is connected to the post-accretionary stage throughout the Central Eastern Desert. It is represented by quartz, quartz-carbonate veins and disseminations in listvenite rocks. The thrust contact between rock units in El-Barramiya area played an imperative part in gold mineralization where the obduction of ophiolitic rocks over the metasediments and metavolcanics caused shear zones. Mineralization in the study area formed along shear zones and the gold mineralization prefers to precipitate along the transition zone between low-grade regional metamorphic area which is represented by metasediments and high grade which is represented by actinolite schist. The gold mineralization lode of El-Barramiya gold mine area is situated in E–W trending quartz and quartz-carbonate veins along a shear zone located in the intersections between faults trend in NE–SW (Najd fault), NW–SE and thrust faults trend in NEE–SWW in metavolcanic and metasedimentary host rocks. Porphyry granite in the mine area played an important role in hydrothermal alteration process where it represents the source of K, listvenite formed when fluids rich in CO2 and bearing-K permeate and alter the previously altered ultramafic rocks, usually serpentinites of the ophiolitic mélange rocks. The listvenitization process includes silicification and carbonatization metasomatic processes, tectonized serpentinites are altered to listvenite as the carbonatization becomes more intense close to dipping transpressive faults. Geochemical studies of listvenite and mineralized veins helped to determine the ultramafic genesis of listvenite and gold transformed as gold bisulfide. The whole rock geochemical data from El-Barramiya and elsewhere indicate that the transformation of serpentinite into listvenite involves profound metasomatic modification of the bulk-rock geochemistry. The chemical changes during alteration of serpentinite to listvenite are dominated by the addition of CO2, the removal of H2O, and the redistribution of SiO2, MgO and CaO as carbonate minerals and silica replace serpentine. All listvenites at El-Barramiya lode gold deposit are enriched in CaO, Fe2O3 and K2O, but depleted in MgO compared with associated serpentinite that is presumed to represent their protoliths. The chemical changes during alteration of serpentinite to listvenite are dominated by the addition of CO2, the removal of H2O, and the redistribution of SiO2, MgO and CaO as carbonate minerals and silica replace serpentine. Alteration also caused redistribution of trace elements, with some being locally remobilized within the rock, some being added from a fluid phase, and others being leached out of the rock. Petrographic investigation and geochemical studies show different types of alterations (carbonatization and silicification) and mineralization. Mineralizations are represented by gold and sulfides (pyrite, arsenopyrite and smaller quantities of chalcopyrite, sphalerite, galena, tetrahedrite and gersdorffite) found in auriferous quartz veins and disseminated in listvenite. The area exposed to brittle–ductile deformation in addition to different types of structures such as faults and fractures controlling on the formation of mineralization and act as hydrothermal channels ways for fluid flow. Fluid inclusions studies revealed that gold mineralization was formed from heterogeneous trapping of H2O–CO2 fluids at a temperature of 280–340 °C and pressure within the range of 1.5–1.9 kbar, which is consistent with the mesothermal conditions.

Geochronology and ore-forming fluid source of the Buzhu orogenic gold deposit in southern Tibet

The Buzhu gold deposit is a newly discovered and representative gold deposit in the Indus-Yarlung Zangbo suture zone. At Buzhu, auriferous quartz veins with paragenetic sequences of quartz-pyrite-sericite hosted by Early Jurassic greenschist facies rocks, and gold occurs mainly as invisible lattice gold in pyrite as well as minor as visible micrograins inside (arseno)pyrite fractures. We have recognized two auriferous pyrite generations formed in the syn-ore stage (Py2a and Py2b). Py2a is euhedral-subhedral granular with cavities, whilst Py2b mainly grew colloidally around Py2a, indicating that pre-existing pyrite may have been dissolved to form the porous Py2a and Py2b rim. Py2a and Py2b have similar δ34SV-CDT values of 6.98‰ to 7.54‰ and 6.18‰ to 6.85‰, respectively, which are also similar to those of the ore-hosting sediments (6.08‰ to 7.08‰). In-situ trace element analysis shows that Py2b (avg. 1.63% Au) contains more gold than Py2a (avg. 0.42% Au). Combined with petrological and geochemical data, we hypothesized that the gold was derived from the metamorphic devolatilization of the sedimentary wallrocks, and pyrite dissolution-reprecipitation has driven the gold enrichment from Py2a to Py2b. The sericite Ar-Ar date of 19.60 ± 0.66 Ma likely represents the gold mineralization age, as the dated sericite coexists with the auriferous pyrite. This age is much younger than most orogenic gold mineralization (ca. 60–41 Ma) in southern Tibet and indicates an orogenic gold mineralization event (ca. 20–15 Ma) related to extensional tectonics during the late-stage India-Asia continental collision.

Fluid inclusions, H-O-S-Pb isotopes, and Rb-Sr geochronology of the Augaro gold deposit, West Eritrea, NE Africa: Implications for ore genesis

Northeast Africa is an emerging world-class gold province due to the numerous lode gold deposits with Au resources of over 43 Moz discovered in the Nubian Shield. The Augaro gold deposit, located in the Adobha-Augaro Cu-Au belt of west Eritrea, is composed of several auriferous quartz veins. The orientations of the auriferous quartz veins in this deposit are strictly controlled by NE-trending shear zones and nearly E-trending fractures. The most common metallic mineral is pyrite, and gold has been identified as native gold, electrum, and calaverite. Fluid inclusion study, H-O-S-Pb isotope analysis, and Rb-Sr dating were carried out in the Augaro gold deposit. A petrographic investigation revealed that fluid inclusions of type W (Liquid-rich aqueous), type C (CO2-rich), and type S (halite-bearing) are prevalent in auriferous quartz veins. The H2O, NaCl, and CO2 are the main components of the metallogenic fluids, which are medium-low temperatures (150–360 °C) and medium to low salinities (<34 wt% NaCl eqv.). The physicochemical features of fluid inclusions indicate that magmatic fluids experienced phase separation earlier in their evolution and mixed with meteoric water subsequently. Rb-Sr dating of fluid inclusions in auriferous quartz suggests that the Augaro gold deposit formed during the post-orogenic stage (543 ± 14 Ma) of East African orogeny in the Late Neoproterozoic, which is a new metallogenic epoch for the lode gold deposits in Nubian Shield. Geological and Geochemical features refer to the Augaro gold deposit being an intrusion-related gold deposit (IRGD).

Orogenic gold belt overlap in the eastern Peninsular Ranges batholith: La Rumorosa mining area, Baja California, Mexico

Gold deposits in the La Rumorosa mining area (Baja California), located in the eastern Peninsular Ranges batholith (PRB), consist of auriferous quartz-(carbonate) veins hosted by brittle-ductile shear zones within metamorphic and synorogenic plutonic rocks. A combination of two geochronology methods, U–Pb zircon dating of country rocks and 40Ar/39Ar dating of hydrothermal and metamorphic white micas, along with the structural characterization of the main orebodies, has allowed the differentiation of two gold mineralization events in the study area. The oldest deposit (>93 Ma) is related to the main compressive phase of the PRB construction (∼110‒90 Ma) and is part of the yet poorly known Peninsular Ranges orogenic gold belt. Two other gold deposits were dated at ∼77 and 75 Ma, an age span that approximately corresponds to the beginning of the Laramide tectonism instead. Therefore, these deposits can be considered the oldest and westernmost constituents of the Caborca orogenic gold belt (∼69‒36 Ma), formerly recognized in the neighboring states of Sonora, Arizona, and the southeastern tip of California. Furthermore, the Laramide gold deposits at La Rumorosa are contemporaneous with the first generation (∼76‒74 Ma) of copper porphyries in the southern North American Cordillera belt, which are also situated on the opposite margin of the Gulf of California-San Andreas fault system. Both gold and copper deposits show an eastward migration during the early Upper Cretaceous, similar to the well-documented shift in magmatism and deformation in the region, albeit occurring asynchronously. The apparent continuous formation of orogenic gold deposits in southwestern North America from the middle Cretaceous to the Eocene, resulting from the strong convergence between the subducted Farallon plate and the continent, suggests a connection between the Peninsular Ranges and Caborca gold belts. We surmise that both of these orogenic gold belts may form part of a large metallogenic province in western Mexico.