Intrusion-related Gold Deposits Research Articles

Timing and Evolution of Gold Mineralization in the Maljavr Occurrence (NW Russia), NE Part of the Fennoscandian Shield

Gold occurrence Maljavr is the first Archean conglomerate-hosted gold mineralization found in the Fennoscandian Shield. Gold-mineralized metasomatic rocks form a set of lenses within a 10 m thick linear zone, conformable to the bedding of host conglomerates. The lenses are up to 10 m long and up to 1 m thick and they clearly exhibit three alteration envelopes: the rock in the central part consists of garnet and quartz or garnet-only; biotite, garnet, and quartz make the intermediate biotite–garnet envelope; hornblende, hedenbergite, and quartz are the principal rock-forming minerals in the outer zone of the lenses. All metasomatic rocks contain sulfide mineralization up to 15–20 vol.% and up to 0.6 g/t Au. The main ore mineral is pyrrhotite, and the minor minerals are arsenopyrite, chalcopyrite, pentlandite, löllingite, and troilite. The age of zircon from biotite gneiss in the zone of alteration is 2664 ± 18 Ma, this is considered as the time of formation of lenses of metasomatic rocks. Biotite gneiss-conglomerate and metasomatic rocks were later intruded by tourmaline granite pegmatite 2508 ± 7 Ma. The injection of pegmatite caused re-crystallization of sulfides (mainly arsenopyrite and löllingite) and redistribution of gold. Visible gold in association with Bi minerals native bismuth, ehrigite, maldonite, bismuthinite, joseite-B, and hedleyite was found in inclusions in recrystallized arsenopyrite and löllingite. Au content in the rocks with recrystallized arsenopyrite and löllingite is >1 g/t, up to 30 g/t in hand samples. The 2508 Ma pegmatite is interpreted as synchronous with formation of gold mineralization in its present form. The linkage of gold mineralization with pegmatite and geochemical association Au-As-Se-Te-Bi in the mineralized rocks agree with characteristics of intrusion-related gold deposits worldwide. Biotite gneiss–metaconglomerate, hosting the mineralized altered rocks, was the probable primary source of arsenic and gold for mineralization.

Characteristics and evolution of quartz-calcite-sulfide veins in the Nazca-Ocoña belt, Peru

The Nazca-Ocoña metallogenic belt in southern Peru host about 70 recognized Au-Ag and base metal (Pb-Cu-Zn) deposits such as Orión, Caravelí, Ishihuica, Calpa, Arirahua, Mollehuaca, and San Juan de Chorunga. These deposits are spatially associated with Cretaceous intrusive rocks of the coastal Batholith in a zone that extends more than 350 km along the Central to Southern part of Peru. Gold and silver are structurally controlled in mineralized NW-SE, N-S and W-E-trending fault-fill veins that are less than one-meter wide, crosscutting granodiorite and diorite bodies. The origin of the veins is still debated, and they may be classified as intrusion-related gold deposits (IRGD), orogenic or mesothermal deposits. In this study, we focus on two representative deposits in the Nazca-Ocoña metallogenic belt: Mollehuaca and San Juan de Chorunga. At both sites, the mineralization is composed of quartz-calcite sulfide veins hosted in Cretaceous Coastal Batholith intrusions. Petrographic and chemical analyses on pyrite were performed with field-emission scanning electron microscopy (FE-SEM), electron microprobe analysis (EMPA), nano-scale secondary ion mass spectrometry (NanoSIMS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).Native gold and Au-rich silver are disseminated throughout the veins as micro to sub-micron inclusions in pyrite and chalcopyrite, and in pyrite as cavity and fracture fill. The are three pyrite generations and these have complex internal zonation and are host to precious (Au, Ag) and base metals (Co, Ni, Cu) metals, and also metalloids (As, Sb) as lattice-bound impurities and micron inclusions. The San Juan de Chorunga pyrite has δ34S values ranging from 0.4 to 1.4 ‰, and the Mollehuaca pyrite δ34S from 5.8 to 7.4 ‰. In both San Juan de Chorunga and Mollehuaca, the highest Au concentrations occur in pyrite zones with the heaviest δ34S values. Fluid inclusion petrography of these veins shows recrystallized, wispy quartz with implosion and explosion fluid inclusion textures, indicative of pressure variations during quartz vein formation. The presence of CO2 in these inclusions requires mid-crustal pressure conditions. However, the main stage of gold mineralization occurs with carbonate, chlorite, and euhedral quartz crystals lacking post-entrapment recrystallization textures or CO2. Our observations suggest that most of the gold in these veins formed in association with a shallower hydrothermal event, potentially in association with the emplacement of batholiths and shallower intrusions in the Late Cretaceous.

Devonian intrusions of southwestern New Brunswick, Canada: U-Pb geochronology, petrogenesis and genetic constraints on gold mineralization

ABSTRACT The Clarence Stream intrusion-related gold deposit is hosted within metasedimentary and metavolcanic sequences of the Ordovician Cookson and Silurian Mascarene groups that were intruded by several phases of the Late Silurian to Early Devonian Saint George Plutonic Suite. Based on geochemical and geochronological data, the intrusive rocks in the deposit are categorized into three distinct suites, including gabbroic, granitoid, and Whittier Ridge Granite (WRG). The gabbroic suite is the same as the East Branch Brook metagabbro intrusion, which exhibits a subalkaline mafic composition fractionated from a depleted mantle in a suprasubduction zone setting. The granitoid suite shows the same geochemical characteristics and age as the metaluminous to slightly peraluminous Magaguadavic Granite and its fractionated phases. The WRG suite reflects characteristics of calc-alkaline, metaluminous to peraluminous I-type granites in the area, such as Sorrel Ridge and Mount Douglas granitic plutons. In situ U-Pb zircon geochronology yielded an age of 419.5 ± 3.8 Ma for a monzodiorite from the gabbroic suite. Porphyritic granite and alkali granite from granitoid suite yielded U-Pb zircon age of 402.4 ± 1.8 Ma and 394.3 ± 3.8 Ma, respectively. Geochronologic data from the WRG suite reflect two different crystallization ages of 383.8 Ma (382.4 ± 3.1 to 385.1 ± 4) and 372.4 Ma (367.4 ± 1.3 to 377.1 ± 1.9). Several pieces of evidence demonstrate that the gold mineralizing events occurred after the emplacement of the East Branch Brook metagabbroic dykes and are linked to the various intrusive phases of the nearby syntectonic Early Devonian Magaguadavic Granite. The absence of gold and sulphide mineralization in felsic dykes related to the WRG confirms that this plutonic body formed adjacent to the gold zones after the main gold mineralizing event.

Hydrothermal monazite and xenotime chemistry as genetic discriminators for intrusion-related and orogenic gold deposits: implications for an orogenic origin of the Pogo gold deposit, Alaska

Attempts to geochemically distinguish between metamorphic-hydrothermal systems that form orogenic gold deposits and both reduced and oxidized magmatic-hydrothermal systems using isotopes or metal associations have proven ambiguous, particularly for orogenic gold and reduced intrusion-related gold systems. The absence of conclusive geochemical discriminators and the overlap in geologic characteristics have led to gold deposit models being potentially incorrectly applied, which in turn negatively affect regional mineral exploration and mine planning. In this study, in situ electron microprobe geochemical analyses of hydrothermal monazite and xenotime crystals associated with different types of gold-bearing deposits are shown to be effective geochemical discriminators. There are notable differences in mineral chemistry such as rare earth element (REE) profiles, total light REE, Dy, Er, Pr, Y, Nd/Sm, and La/Sm that distinguish monazite precipitated from metamorphic-hydrothermal fluids that form orogenic gold deposits and those precipitated from magmatic-hydrothermal fluids that form both porphyry Cu-Mo-Au and reduced intrusion-related gold deposits. Notable differences in overall xenotime abundances and concentrations of heavy REEs, Ca, and Sc are distinctive between the different deposit classes for xenotime. The origin of the controversially classified Pogo gold deposit, Tintina gold province, Alaska, which has been characterized as both a reduced intrusion-related and an orogenic gold deposit, is tested based upon the noted chemical differences associated with these hydrothermal phosphates. The findings of this study have implications for exploration and mine development in the Tintina gold province and other areas that contain deposits that are controversially classified as either orogenic or as magmatic-hydrothermal gold deposits.

Noble and base metal geochemistry of late- to post-orogenic mafic dykes from central Spain

The post-tectonic and post-orogenic mafic rocks from the Spanish Central System (SCS) (Iberian Massif) include dyke swarms of shoshonitic (microgabbros) and alkaline (lamprophyres and diabases) geochemical affinity, which register the nature of the metasomatic lithospheric mantle under central Spain. Such magmas sometimes show a direct (or indirect) relationship with the formation of orogenic and intrusion-related gold deposits, which are relatively abundant in the Iberian Massif. The noble and base metal composition of these intrusions shows Primitive Mantle-normalized patterns characterized by positive Au and Co anomalies and fractionated platinum group elements (PGE): from lower Ir-group PGE (IPGE; Ir–Ru) to higher Pd-group PGE (PPGE; Rh–Pt–Pd). The low contents of PGE, together with the base metal contents of pyrite (which is the dominant sulphide phase in the alkaline dykes), is in accordance with low degrees of mantle partial melting and the early segregation of sulphides during magma differentiation. The scarcity of PGE mineral deposits in the Iberian Massif could be explained in part by the apparent lack of PGE enrichment in the Iberian lithospheric mantle. On the contrary, the positive Au anomaly of the SCS mafic dykes represents relatively high Au contents, similar to (and higher than) those of mafic rocks derived from metasomatized subcontinental lithospheric mantle underlying Au-endowed cratons. Several geochemical features point to subduction-related metasomatism of either oceanic or continental nature as the main source of Au enrichment. The Au re-fertilization of the lithospheric mantle under central Spain makes it a potential source in the formation of gold mineralizations.

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).

Metallogeny of the Xiaotongjiapuzi gold deposit, Liaodong Peninsula (Eastern China): Perspective from sulfide trace element geochemistry and sulfur isotopes

Most gold ores have variable sulfur isotopic and trace elemental compositions at micro-/nano-scale, which makes the understanding of the gold formation process more challenging. The advancement of in-situ analytical techniques offer opportunity to gain insight into gold mineralization processes and ore-material source. The Xiaotongjiapuzi is a representative intrusion-related gold deposit in the Qingchengzi orefield of the Liaodong Peninsula, Eastern China. Alteration and gold mineralization comprise four stages: (I) quartz-pyrite-Au, (II) quartz-pyrite-biotite-Au, (III) Au-barren quartz-pyrite, and (IV) post-ore quartz-calcite. Detailed scanning electron microscope (SEM) backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) elemental and LA–multi-collector (MC)–ICP–MS sulfur isotope analyses were conducted to reveal the ore-material source and mineralization process. Three texturally distinct auriferous pyrite generations were identified: Py1 and Py2 (in both stage I and II) and Py3 (in stage II and III). Py1 is porous and weakly fragmented, poorly crystallized, and overgrown by Py2. Py2 is commonly intergrown with arsenopyrite and marcasite, whilst Py3 overgrew around Py2 or replace it.Gold exists mainly as solid solution (Au+) in the pyrite lattice and nanoparticles (Au0), as well as (rare) micron-scale inclusions along the Py2-Py3 interface. Py2 has the highest gold content (mean 47 ppm, max 344 ppm), especially when intergrown with marcasite (Mrc1). The δ34S values of pyrite (5.71–15.31‰, n = 18) and marcasite (-19.59 to 7.45‰. n = 6) are the highest and lowest, respectively, and the values increase steadily from Py1 to Py3 (Py1: 5.71–7.54‰ → Py2: 7.44–9.55‰ → Py2 + Mrc1: 12.88–13.92‰ → Py3: 15.17–15.31‰). The Paleoproterozoic Dashiqiao and Gaixian formation host rocks contain large amount of sulfate minerals (e.g., barite and gypsum), and the coeval global sulfate has heavy sulfur isotope enrichment, suggesting that sulfate reduction may have provided H2S-rich fluids [Au(HS)2- and/or Au(HS)0] for the Mesozoic mineralization. For the multistage gold enrichment at Xiaotongjiapuzi, we suggest that the Paleoproterozoic sedimentary and metamorphic rocks may have formed the initial sulfur reservoir, which was then reactivated by Mesozoic magmatic-hydrothermal events.

Mineralogy and Mineral Chemistry of the Au-Ag-Te-(Bi-Se) San Luis Alta Deposit, Mid-South Peru

A mineralogical and mineral chemistry study was carried out in the San Luis Alta telluride-rich gold deposit, mid-south Peru, to contribute towards determining its formation and improving the ore processing. The San Luis mineralization is considered an intrusion-related gold deposit located in the Arequipa segment of the Coastal Batholith. The mineralization occurs in quartz veins hosted in diorites and granodiorites from the Tiabaya Super-Unit. These veins are sulfide-rich in the deep areas and contain abundant iron oxides. Sulfides are mainly pyrite with minor chalcopyrite and galena. Native gold and telluride minerals are abundant. Mineral chemistry was determined using an electron microprobe. The mineralogy of veins was classified into four stages. Gold occurs in the three last stages either in large grains, visible to the naked eye, or, more frequently, in grains of less than 10 µm. Gold appears as grains encapsulated in pyrite, Fe oxides, quartz and filling fractures. The first stage is characterized by the deposition of quartz and massive pyrite, which does not contain gold. During the second stage, hessite, calaverite, petzite and altaite are formed. Additionally, Bi-tellurides, mainly volynskite, rucklidgeite, kochkarite and tellurobusmuthine, are formed. Some of these minerals occur as blebs encapsulated in pyrite, suggesting that a Bi-Te-rich melt was formed from the ore-forming hydrothermal fluid and transported the Au and Ag elements. This stage was followed by a fracturing event and tellurobismuthite, tetradymite and montbrayite precipitated. In the last stage, a supergene replacement formed covellite, bornite and goethite. Te-Bi minerals do not appear in this stage, but selenium minerals occur in minor amounts. Chlorargyrite and iodargyrite occur and are associated with gold.

Geology, geochemistry, fluid inclusion data, stable isotope characteristics, and ore genesis of the Barout Aghaji gold deposit, NW Zanjan, Iran

The Barout Aghaji gold deposit is located ∼90 km northwest of Zanjan, within the Takab-Takht-e-Soleyman subzone of the Sanandaj-Sirjan metamorphosed-deformed zone. Ore-bearing quartz veins are hosted by Neoproterozoic amphibolite and Eocene to Oligocene granitic gneisses. Oligo-Miocene Upper Red Formation unconformably overlies the amphibolite and granitic gneisses. Field observations and petrographic studies show that two deformation stages occurred in this area. The first deformation stage was ductile, producing mylonitic and proto-mylonitic microstructures, but the second one was brittle, represented by sheeted quartz veins and veinlets. In the first stage, barren milky quartz veins occurred containing minor sulfide minerals, but dark to light gray ore-bearing quartz veins and veinlets are formed in the latter stage. The mineralized veins appear as massive microcrystalline quartz cut by sheeted quartz veins with comb, druse, and crustiform textures. The gold-bearing quartz veins contain as much as 3% sulfide minerals. Pyrite is the main sulfide mineral and is associated with minor chalcopyrite. Sulfides are commonly altered to hematite, goethite, and rarely malachite. Hydrothermal alteration around the quartz veins consists of silicification, pyritization, and sericitization. The whole-rock geochemistry of the collected samples from the granitic gneisses and quartz veins shows that Au is enriched in the quartz veins (average of 114 ppb) relative to host rocks (average of 22.5 ppb). Au shows strong positive correlations with As, Ba, Mo, Pb, Sc, Tl, Ag, and negative correlations with Cu, Bi, Se, and Te in the granitic gneisses. It also shows strong positive correlations with S, Hg, Th, Co, Bi, Pb, and Ag and negative correlations with P, V, Te, W, Sc, Zn in quartz veins. Four types of primary fluid inclusions were identified, including type I, two-phase aqueous-rich fluid inclusions (liquid > vapor; LV); type II, two-phase vapor-rich fluid inclusions (gas > liquid; VL); type III, three-phase fluid inclusions containing CO2 with clathrate formation (L1L2V); and type IV three-phase fluid inclusions (aqueous, vapor, and solid; LVS). The homogenization temperatures of fluid inclusions in auriferous quartz veins range from 199 −446 with a mode of 270–300 °C. Salinities range from 0.8 to 49.02 wt% NaCl Equiv. with two distinct populations at 0.8–8.5 and 31.1–49.02 wt% NaCl Equiv. The large variations in the homogenization temperatures and salinities can be attributed to the cooling and isothermal mixing of fluids. The δ34S values for four pyrites separated from auriferous quartz veins range from +2.9 to +7.1‰, with an average of 4.5‰. δ34S values of fluids in equilibrium with pyrite were calculated from +3.5 to +7.3‰, with an average of 5.4‰, indicating a metamorphic source for the sulfur using temperatures estimated from the fluid inclusion study. The Field observations, vein textures, mineralogy, ore geochemistry, fluid inclusion studies, and sulfur isotope data indicate that gold mineralization in the Barout Aghaji area has many similarities to orogenic and intrusion-related gold deposits, such that low salinity fluids derived from metamorphic rocks are mixed with high salinity fluid inclusions possibly derived from granitic gneisses during syn to post tectonic magmatism.

Tourmaline as an indicator of ore-forming processes: Evidence from the Laodou gold deposit, Northwest China

A detailed textual and geochemical study of tourmaline is presented to trace fluid evolution and ore genesis of the Laodou gold deposit in West Qinling Orogen, Northwest China. Based on petrographic observations and relationships to gold mineralization, three generations and five types of tourmaline were identified. The pre-ore tourmaline generation (Tur A) occurs as phenocryst replacement within quartz diorite porphyry (Tur A1) and metasomatic filling in hydrothermal breccia (Tur A2). It shows intense oscillatory zoning and variable Mg/(Mg + Fe) ratios (0.30–0.67), implying rapid compositional changes controlled by host-rock composition and fluid-rock interaction. The syn-ore tourmaline generation (Tur B) forms as open-space filling (Tur B1) and peripheral overgrowth (Tur B2) of Tur B1. It occurs in quartz-tourmaline and quartz-stibnite-sphalerite-tourmaline veins. Narrow Mg/(Mg + Fe) range (0.48–0.76), poor chemical zoning and more homogeneous compositions of Tur B reflect a higher fluid flux. The post-ore tourmaline generation (Tur C) occurs as monopolar overgrowths developed at the − c pole of previous tourmaline within granitoids and hydrothermal veins. This has a composition between dravite and foitite with Mg/(Mg + Fe) from 0.65 to 0.95 and no zoning, which reflect formation under low-temperature conditions. Relatively high Sn, Nb and Ta concentrations, LREE enrichment accompanied by strong positive Eu anomalies of Tur A mostly support a magmatic-hydrothermal origin. Slight LREE enrichment and low abundances of ΣREE of Tur B are caused by progressive input of meteoric waters. Tur C shows relative enrichment in Ba, depletions in V and Ni, and a flat REE pattern with negative Eu anomalies, suggesting a metamorphic source. Boron isotope compositions suggest that boron of Tur A (δ11B average − 6.49 ‰) is sourced from an I-type melt involved with the Laodou quartz diorite porphyry, while the lighter δ11B values in paragenetically younger tourmaline (Tur C with δ11B average − 9.10 ‰) support cooling and a metamorphic input sourced from argillaceous slate of the Lower Permian Daguanshan Group. The variations in textural characteristics, chemical and boron isotope compositions from Tur A to Tur C support a fluid mixing between a magmatic-hydrothermal fluid and meteoric waters leaching metasedimentary rocks. Typical morphology, core-to-rim decrease in Mg/(Mg + Fe), Fe3+Al−1, (□XAl)(NaR2+)−1 and (AlO)(R2+OH)−1 vectors, decreasing Na, Ca concentrations and Eu anomalies of tourmaline may record cooling, oxidation and dilution during fluid evolution. This process is possibly caused by fluid mixing and results in gold precipitation. Boron sources, as well as fluid provenance and evolution interpreted by tourmaline provide insight into the Laodou deposit as an intrusion-related gold deposit.

Geology, U-Pb geochronology and stable isotope geochemistry of the Heihaibei gold deposit in the southern part of the Eastern Kunlun Orogenic Belt, China: A granitic intrusion-related gold deposit?

The Heihaibei gold deposit is a newly discovered gold deposit in the southern part of the Eastern Kunlun Orogenic Belt. Its most distinctive features are that the gold mineralization is hosted in monzogranite, and that the presence of pre-ore (possibly syn-ore) monzogranite and post-ore gabbro allows to constrain the mineralization’s formation age. Zircons from the monzogranites yield U-Pb ages of 454 ± 3 Ma, while zircons separated from the gabbro dikes cutting the monzogranites and gold mineralized body yield U-Pb ages of 439 ± 3 Ma, which is interpreted to be the minimum age of the Au mineralizing event. Combined with the regional geological background, we proposed that the Heihaibei Au mineralization occurred during the subduction stage of the Early Paleozoic Proto-Tethys ocean.The ore assemblage is dominated by pyrite, arsenopyrite and native gold. The hydrothermal alteration that has led to the peculiar enrichment of Au is not systematically distributed and displays no clear concentric zoning pattern. The main mineralization formed during three stages: the K-feldspar-quartz-pyrite (Py1)-arsenopyrite-sericite-epidote stage (I), the quartz-pyrite (Py2)-native gold-chlorite stage (Ⅱ), and the quartz-carbonate stage (III). The main gold mineralization occurred during stage Ⅱ. Fluid inclusion homogenization temperature and salinities decrease from stage I (Th., 268–412 °C; W., 6.87–16.63 wt% NaCl equiv.) to stage Ⅱ (Th., 183–288 °C; W., 3.69–14.84 wt% NaCl equiv.). The δ18O and δD values (δ18OH2O = 4.9 to 9.7‰; δDV-SMOW = -84.1‰ to −81.1‰) of quartz samples from stage I and stage Ⅱ are comparable to a magmatic-hydrothermal ore-forming fluid that possibly underwent fluid-rock interaction with the Nachitai Group metamorphic rocks during the early ore-forming stage. The relatively uniform δ34S values (δ34SV-CDT = 7.7 to 8.5‰) are slightly elevated compared to magmatic δ34S values, but could be derived from a magma if a significant crustal melt component is present. Moreover, the δ34S values are within the S isotopic composition range of a granitic reservoir, suggesting that they are probably inherited from the Heihaibei monzogranites. The Pb and Hf isotope compositions imply a close genetic association between the gold mineralization and granitic magmatism, which are both the products of the mixing of crustal and mantle sources. The trace element compositions of pyrite provide additional evidence that the gold mineralization in the Heihaibei deposit was related to the magmatism. Compared with the typical characteristics of orogenic gold and intrusion-related gold systems (IRGS) deposits, the Heihaibei gold deposit may instead be classified as a granitic intrusion-related gold deposit.

Genesis of the Maoling gold deposit in the Liaodong Peninsula: Constraints from a combined fluid inclusion, C-H-O-S-Pb-He-Ar isotopic and geochronological studies

The large tonnage Maoling gold deposit (25 t @ 3.2 g/t) is located in the southwest Liaodong Peninsula, North China Craton. The deposit is hosted in the Paleoproterozoic metamorphic rocks. Four stages of mineralization were identified in the deposit: (stage I) quartz-arsenopyrite ± pyrite, (stage II) quartz-gold- arsenopyrite-pyrrhotite, (stage III) quartz-gold- polymetallic sulfide, and (stage IV) quartz-calcite-pyrrhotite. In this paper, we present fluid inclusion, C-H-O-S-Pb-He-Ar isotope data, zircon U-Pb, and gold-bearing sulfide (i.e. arsenopyrite and pyrrhotite) Rb-Sr age of the Maoling gold deposit to constrain its genesis and ore-forming mechanism. Three types of fluid inclusions were distinguished in quartz-bearing veins, including liquid-rich two-phase (WL type), gas-rich two-phase (GL type), and daughter mineral-bearing fluid inclusions (S type). Fluid inclusions data show that the homogenization at temperatures 197 to 372 °C for stage I, 126 to 319 °C for stage II, 119 to 189 °C for stage III, and 115 to 183 °C for stage IV, with corresponding salinities of 3.7 to 22.6 wt.%, 4.7 to 23.2 wt.%, 5.3 to 23.2 wt.%, and 1.7 to 14.9 wt.% NaCl equiv., respectively. Fluid boiling was the critical factor controlling the gold and associated sulfide precipitation at Maoling. Hydrogen and oxygen stable isotopic analyses for quartz yielded δ18O = −5.0‰ to 9.8‰ and δ D = −133.5‰ to −77.0‰. Carbon stable isotopic analyses for calcite and ankerite yielded δ13C = −2.3‰ to −1.2‰ and O = 7.9‰ to 14.1‰. The C-H-O isotope data show that the ore-forming fluids were originated from magmatic water with meteoric water input during mineralization. Hydrothermal inclusions in arsenopyrite have 3He/4He ratios of 0.002 Ra to 0.054 Ra, and 40Ar/36Ar rations of 1225 to 3930, indicating that the ore-forming fluids were dominantly derived from crustal sources almost no mantle input. Sulfur isotopic values of Maoling fine-grained granite range from 6.‰1 to 9.8‰, with a mean of 7.7‰, δ34S values of arsenopyrite from the mineralized phyllite (host rock) range from 8.9‰ to 10.6‰, with a mean of 10.0‰, by contrast, δ34S values of sulfides from ore vary between 4.3‰ and 10.6‰, with a mean of 6.8‰, suggesting that sulfur was mainly originated from both the host rock and magma. Lead radioactiveisotopic analyses for sulfides yielded 206Pb/204Pb = 15.830–17.103, 207Pb/204Pb = 13.397–15.548, 208Pb/204Pb = 35.478–36.683, and for Maoling fine-grained granite yielded 206Pb/204Pb = 18.757–19.053, 207Pb/204Pb = 15.596–15.612, and 208Pb/204Pb = 38.184–39.309, also suggesting that the ore-forming materials were mainly originated from the host rocks and magma. Zircon U-Pb dating demonstrates that the Maoling fine-grained granite was emplaced at 192.7 ± 1.8 Ma, and the host rock (mineralized phyllite) was emplaced at some time after 2065.0 ± 27.0 Ma. Arsenopyrite and pyrrhotite give Rb–Sr isochron age of 188.7 ± 4.5 Ma, indicating that both magmatism and mineralization occurred during the Early Jurassic. Geochronological and geochemical data, together with the regional geological history, indicate that Early Jurassic magmatism and mineralization of the Maoling gold deposit occurred during the subducting Paleo-Pacific Plate beneath Eurasia, and the Maoling gold deposit is of the intrusion-related gold deposit type.

Tetrahedrite group minerals of the Kekura reduced intrusion-related gold deposit, Western Chukotka, Russia

The Kekura gold deposit in Western Chukotka, Russia is spatially related to the granitic rocks of the Early Cretaceous Gvardeisky Complex. The deposit was formed during five mineralization stages: (1) cassiterite-scheelite-chalcopyrite-bornite-arsenopyrite, (2) molybdenite, (3) chalcopyrite-gold-bismuth tellurides, (4) arsenopyrite-gold-scheelite, and (5) stibnite-gold-silver. The tetrahedrite group minerals (Ttgs) were identified in stages 2, 4, and 5. In the molybdenite stage Ttgs are associated with digenite, bornite, chalcopyrite, molybdenite, and arsenopyrite. Ttgs from the arsenopyrite-gold-scheelite stage are associated with scheelite, chalcopyrite, sphalerite, galena, and native gold with fineness of 800–850. Along with tetrahedrite, stibnite-gold-silver mineralization includes Ag-Au and Au-Ag-Hg alloys, native silver, sphalerite, stibnite, boulangerite. The molybdenite stage Ttgs display a wide range of the Sb/(Sb + As + Bi) ratio (sb) 0.06–0.85 and a narrow range of the Fe/(Fe + Zn) ratio (fe) 0.78–1.00, and are enriched in Cu (up to 5.767 apfu Cu at the M(1) site); the Ag content does not exceed 0.080 apfu. Ttgs of the arsenopyrite-gold-scheelite stage have the Cu-free at M(1) site, but are highly variable in the sb and fe values, ranging from 0.24 to 1.00, and from 0.14 to 1.00, respectively; the Ag concentration is higher than that in the molybdenite stage Ttgs, reaching 0.365 apfu. The stibnite-gold-silver tetrahedrite has high Ag (up to 3.442 apfu) and sb and fe ranging from 0.68 to 1.00 and from 0.10 to 0.84, respectively. Some Ttgs compositions of the molybdenite and stibnite-gold-silver stages are Bi-bearing that is caused by the replacement of previous Bi-minerals.

Application of portable X-ray and micro-X-ray fluorescence spectrometry to characterize alteration and mineralization within various gold deposits hosted in southern New Brunswick, Canada

New Brunswick hosts multiple types of gold deposits related to various stages of Appalachian orogenesis. Some of these structurally controlled deposits are considered intrusion-related; however, the majority have characteristics of orogenic deposits. The geochronology and genesis of the gold and antimony deposits in southern New Brunswick have been studied to varying degrees, but the relationship between mineralization and tectonism remains unresolved. This study involves the examination and comparison of auriferous quartz (-carbonate) veins associated with orogenic and intrusion-related gold mineralization of southern New Brunswick. The Belleisle Bay and Annidale groups (Annidale area) are host to mainly orogenic type gold mineralization, whereas in the Cookson and Mascarene groups (Clarence Stream area), gold mineralization is characterized as intrusion-related. For this study, the Devil Pike Brook deposit and Sheba Pit, East Scotch Settlement BP gold occurrences and the Bald Hill antimony deposit represent the Annidale area, whereas the South (Central and East subzones), George Murphy, Richard, and Jubilee zones represent gold mineralized areas in the Clarence Stream region.To facilitate the geochemical investigation of the various deposits and occurrences, an Olympus Vanta™ portable X-ray fluorescence spectrometer (pXRF) was used to rapidly analyse the composition of the different auriferous quartz (-carbonate) veins and characterize the alteration assemblages related to gold mineralization. The precision and accuracy of the pXRF data was monitored through QA/QC procedures, with the resulting corrected pXRF data undergoing principal component analysis (PCA) to fully interrogate the large data set.The PCA results highlight hydrothermal alteration and mineralization geochemical trends in the studied orogenic and intrusion-related gold deposits and occurrences. Some gold pathfinder elements reflect these different trends on the PC analysis on the studied samples. From the different PCs, various information about alteration and mineralization can be deduced. In Annidale area PC1 retrieves positive loadings of S, Cu, As, Pb, Zn, Fe, and Sb consistent with mineralization, PC2 displays positive loadings of Ca, Sr, Y, Mn and Ti, and PC3 extracts information about K, Rb, Ba, Zr, and Al consistent with alteration. In the Clarence Stream area, PC1 retrieved information about Al, K, Zr, Ba, Nb, Th and Rb consistent with alteration, PC2 displays positive loadings of As, Sb, Fe, Co, and S consistent with mineralization and PC3 extracts information about Zn, Pb, Mn, Fe, Cu, S and Si.Data obtained from the micro-X-Ray fluorescence–energy dispersive spectrometer (μ-XRF) analyses of the same sample suite concur with the alteration and mineralization characteristics identified using pXRF; these data indicate that chloritic and sericitic alteration with sulphides are associated with quartz (-carbonate) veins. The μ-XRF analysis is also helpful to further investigate sulphide and gold mineralization, as well as to locate potentially datable minerals, such as titanite, rutile, zircon, apatite, and monazite, in the gold-bearing quartz (-carbonate) veins; these data identified rutile and titanite in the Annidale samples and apatite in Clarence Stream samples that are being followed up geochronologically.

Paleoproterozoic gold events in the southern West African Craton: review and synopsis

The southern part of the West African Craton includes the Baoulé-Mossi Domain, the world’s premier Paleoproterozoic gold province (~10,000 metric ton gold endowment). Structural, metamorphic, and geochronological data suggest gold mineralisation occurred during three episodes that span much of the Eoeburnean and Eburnean orogenic cycles. Eoeburnean orogenic and rare skarn-hosted gold deposits formed between ca. 2200 and 2135 Ma during repeated episodes of volcanism, plutonism, and shortening, which thickened the Paleoproterozoic crust. Early Eburnean orogenic and placer gold deposits formed between ca. 2110 and 2095 Ma during inversion, metamorphism, and subsequent oblique shortening of intra-orogenic basins filled after ca. 2135 Ma. This episode of mineralisation terminated when the Baoulé-Mossi Domain docked with the Archean Kénéma-Man Domain at ca. 2095 Ma. Late Eburnean orogenic and less common intrusion-related gold deposits formed between ca. 2095 and 2060 Ma during strike-slip to oblique-slip tectonics, post-collisional high-K plutonism and crustal reworking across the western and southern Baoulé-Mossi Domain. Eoeburnean gold deposits include ca. 10 % of the gold endowment of the Baoulé-Mossi Domain, whereas the Early Eburnean and Late Eburnean deposits include ca. 50–70% and 20–40%, respectively. Here, we highlight the favourable confluence of accretion-collision tectonics, involving juvenile crust formation as well as protracted magmatic, metamorphic, and deformation histories that resulted in diachronous gold events spread over at least 100 myr throughout the Baoulé-Mossi Domain.

Geochemical signatures in native gold derived from Au-bearing ore deposits

Native gold is a significant indicator mineral for Au-bearing deposits. The chemical composition of gold in diverse geological settings is a function of fluid composition under various physicochemical conditions. Native gold grains from various Au-bearing deposits and districts, including orogenic, volcanogenic massive sulfide (VMS), porphyry Cu-Au deposits, epithermal Au-Ag, and reduced intrusion-related gold (RIRG) deposits, were investigated by electron probe microanalyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to establish discriminant chemical signatures to apply indicator mineral surveys for exploration for gold-bearing deposits. Detailed textural observation of gold identifies compositional heterogeneities in gold grains from epithermal and VMS deposits. Partial least square-discriminant analysis (PLS-DA) identifies that Pd, Ag, Sb, Pb, Cu, Hg, and Te contents in gold grains have a high potential to discriminate gold from different deposit types. Comparison of gold compositional data from different deposit types using Random Forests classification models can be used as a predictive modeling tool to provide an accurate indication of gold-bearing deposit types.

Age and Origin of the Dongping Au-Te Deposit in the North China Craton Revisited: Evidence from Paragenesis, Geochemistry, and In Situ U-Pb Geochronology of Garnet

Abstract Dongping is the largest Au-Te vein deposit (~120 t Au) in the North China craton, but its age, origin, and setting remain unsolved. Here, we integrate paragenesis, geochemistry, and in situ U-Pb geochronology of garnet to constrain the timing and possible origin of the Dongping Au-Te deposit. Gold mineralization at Dongping is hosted in the Shuiquangou alkaline complex (ca. 401–390 Ma) and dominated by quartz-sulfide veins with minor ores in adjacent alteration envelopes. Andradite to grossular garnets are recognized in pre-, syn-, and post-ore quartz veins as well as mineralized alteration envelopes and are closely associated with a variety of ore and gangue minerals, mainly including K-feldspar, quartz, specularite, magnetite, pyrite, tellurides, epidote, and calcite. The paragenetic, textural, fluid inclusion, and compositional data suggest that garnets precipitated directly from a low-salinity fluid at 302° to 383°C and 90 to 330 bar. Garnets from various veins and alteration envelopes have similar U contents ranging from 0.80 to 13.89 mg/kg and yield reproducible U-Pb dates of 142 ± 5 to 139 ± 6 Ma (1σ) by laser ablation-inductively coupled plasmamass spectrometry. The dating results suggest that gold mineralization at Dongping occurred in the Early Cretaceous and thus preclude a genetic link between Au-Te mineralization and the ore-hosting alkaline intrusion as commonly suggested. When combined with independent geologic, geochemical, and geochronological studies, the new garnet U-Pb dates allow us to classify the Dongping Au-Te deposit as an oxidized intrusion-related gold deposit, with the causative magma likely derived from melting of an ancient enriched lithospheric mantle source due to destruction of the subcontinental lithospheric keel beneath the North China craton—a catastrophic event induced by the westward subduction of the Paleo-Pacific plate. This study highlights garnet U-Pb dating as a potential robust geochronometer for gold vein deposits elsewhere.

Zircon U--Pb ages of Au-bearing host rocks in Northwest Iran as evidence for gold mineralization in the Carboniferous, not Cenozoic time!

A few gold deposits occur in northwest Iran which are hosted by mylonitic granitoids and phyllonite. New U--Pb ages yield ages of 350–311 Ma for the mylonitic granitoid and 354–348 Ma for the phyllonite, which indicate Carboniferous magmatic activity in northwest Iran. The host granitoid and phyllonite have high contents of Zr (212, 246 ppm) and Y (26.8, 38.7 ppm). High 10,000 × Ga/Al ratios (2.1–5.1) and fractionation of the rare earth elements (REEs) pattern with depletion of Eu, Sr, and P may infer an A-type (anorogenic) granite affinity for the host rocks. Correspondingly, these rocks are characterized by high contents of Nb (33.1 to 39.2 ppm), Ta (1.68 to 2.40 ppm), Tb (0.84 to 1.17 ppm), and Th/Yb ratios with some similarity to OIB-like rocks. Whole rock compositions of these host rocks are consistent with both A1 and A2 type granites, which infer magmatic activity in an extensional tectonic regime. Perhaps the extension may have occurred during rifting of Neo-Tethys in northern Gondwana. Upwelling of the hot OIB-like melts during the opening of Neo-Tethys ocean was the main mechanism of this extension. The OIB signatures of the A-type granitoids controlled the gold mineralization during this process. Accordingly, we classify the gold mineralization in NW Iran as intrusion-related gold deposits. These mantle-derived magmas from the asthenosphere may support a source of both sulfur and gold. These elements were concentrated in hydrothermal fluids in the late stages of magma crystallization. Also, hot magma injection increased the geothermal gradient and caused extensive dehydration of the sedimentary rocks inin the roof of the magma chamber. Circulation of these hot fluids with injection into the hot crystallizing magmatic rocks and mixing with late-stage magmatic-hydrothermal fluids and meteoric water (?) seem to all have essential roles for transporting the gold and precipitating it within fractures in the semi-brittle to brittle zone. The gold precipitated as native or nanoparticles in pyrite in the inner part of the shear zone. This research reveals that mineralization occurred in the Late Paleozoic in an extensional tectonic regime and not above the subduction zone or collision zone which had been suggested before.

The tungsten-gold veins of Bonnac (French Massif central): new constraints for a Variscan granite-related genesis

In the Brioude-Massiac district (French Massif Central: FMC), a network of W-As-Bi-Au quartz veins constitutes the Bonnac deposit, where tungsten is the major economic element, together with high-grade gold (up to 15 g/t Au). The evolution of this mineralization has been divided into 3 stages: (i) an early deep-seated wolframite-löllingite stage formed between 12 to 9 km, at up to 400 °C; (ii) a ductile/brittle deformation stage associated with scheelite and arsenopyrite deposition, with an estimated temperature of 480–300 °C; (iii) a late stage controlled by fluid-overpressure potentially triggered by fault-valve mechanism, at a depth of 7 to 5 km, and a temperature estimated between 266 to 240 °C, is marked by micro-fracturing infilled by native bismuth, bismuthinite, hedleyite, electrum, pyrite and base-metals. Structural analysis and apatite LA-ICP-MS U/Pb dating demonstrate a spatial and temporal link between the emplacement of the peraluminous leucogranitic dykes and the Bonnac mineralization. In more details, the mineralization was deposited between 321–316 Ma, during, or just after, the emplacement of the peraluminous dykes estimated around 329–315 Ma, suggesting a magmatic-hydrothermal transition for the ore-forming process. In the proposed model, the cooling of a hidden two-mica granitic pluton could have generated a magmatic fluid, and acted as the heat source responsible for fluid flow towards inherited permeability zones. The magmatic fluid could have then re-equilibrated at high temperature by fluid-rocks interaction. The sharp changes in pressure, associated with the decrease of the temperature, and sulfide-fugacity generated by a late input of meteoric fluid were responsible for the deposition of the late gold-stage. At the regional scale, the tungsten-gold event is ascribed to an early hydrothermal stage, dissociated from the formation of the antimony event in the district. The leucogranitic dykes and Bonnac quartz veins are controlled by a NW-SE stretching direction, interpreted as an expression of the Serpukhovian-Bashkirian syn-orogenic extension (D4 event of the FMC). These new data provide evidence for an early tungsten and gold metallogenic event in the FMC, prior the “Or300” event. The genetic classification of the Bonnac mineralization is equivocal. The W-As-Bi-Au-quartz veins exhibit the features of both an “orogenic gold” deposit at a relatively deep emplacement level (mesozonal), and an Intrusion-Related-Gold-Deposit (IRGD) type with a spatial-temporal link with the peraluminous intrusion emplacement. We propose that the Bonnac deposits represent an intermediate type between a typical orogenic-gold deposit and an IRGD. We argue that the presence of economic high-grade gold content in tungsten vein-type, and more generally the IRGD deposits, have been underestimated in the Variscan French Massif Central.

Super-enrichment mechanisms of precious metals by low-melting point copper-philic element (LMCE) melts

低熔点亲铜元素（LMCE）As、Sb、Bi、Hg、Pb、Se、Te、Tl、Sn等，均具有亲铜性、低熔点、半金属的特性，在成矿过程中可以形成LMCE熔体，并对Au、Ag、PGE等贵金属的高效富集沉淀起到一种重要的桥梁作用。作者对前人研究资料与LMCE热力学相图进行了分析，并结合浅成低温热液型、造山型、卡林-类卡林型、碱性-偏碱性侵入岩型金矿床的研究成果，探讨了LMCE熔体形成、类型及其对Au、Ag、PGE等贵金属富集成矿的机理，并提出了LMCE熔体参与成矿的矿物组合与结构特征标志。LMCE熔体可以在岩浆过程、（岩浆）热液过程及变质过程中形成，是贵金属矿床重要的成矿机制之一。LMCE熔体中存在大量原子团簇，团簇间的聚集生长会使熔体难以达到相平衡，形成许多非平衡矿物组合，如包含LMCE的自然元素、金属互化物及含LMCE的多相矿物。Au在LMCE熔体中也可以团簇存在，金团簇聚集形成球状或片状，并形成巨富的金矿体。LMCE熔体形成的矿物常以浑圆状、近浑圆状、不规则状的单个或群体组合的乳滴、珠滴、气泡的微粒包体产在硫化物、硒化物、碲化物、氧化物和硅酸盐矿物内或沿矿物裂隙线形排列，这些LMCE微粒包体是熔体扰动导致熔-熔或熔-液间发生乳化所致，流体沸腾是引起熔体扰动的主要机制。LMCE熔体不能快速淬火结晶，通常在低温下缓慢冷却达到相平衡，形成复杂的矿物组合，该特点即使在微米到纳米级的矿物微粒中也显著存在。熔体-流体包裹体是LMCE熔体参与成矿作用最为直接的证据。固溶体分解结构、熔体退火结构、矿物-熔体二面角结构、溶解-再沉淀结构等也是LMCE熔体参与成矿的标志性结构。;The low-melting point chalcophile elements (LMCE), including As, Sb, Bi, Hg, Pb, Se, Te, Tl, Sn and so on, have characteristics of chalcophile behavior, low melting point and semi-metallic properties, which can form LMCE melt during mineralization process and play an important role for the efficient enrichment and precipitation of Au, Ag, PGE and other precious metals. In this paper, the previous research data and the LMCE thermodynamic phase diagrams were analyzed. Combining research results of epithermal, orogenic, Carlin to Carlin-like and alkaline to meta-alkaline intrusion-related gold deposits, the authors discussed the formation and type of LMCE melts and their mechanisms for the enrichment of Au, Ag, PGE and other precious metals, and summarized mineral compositions and characteristic textures of mineralization that benefited by LMCE melts. The LMCE melts can be formed during magmatic, (magmatic-) hydrothermal and metamorphic processes, and belong to one of the important metallogenic mechanisms for precious metal deposits. There are many ion clusters in the LMCE melts, and aggregation between the clusters precludes the melt to reach phase equilibrium, which results in many non-equilibrium mineral combinations, including the coexistence of native LMCE, intermetallic compounds and multiphase minerals containing LMCE. Gold could also exist as ion clusters in the LMCE melt that gather to form spherical or flakes native gold, and form the super-rich ore bodies. The minerals formed by LMCE melt often exist as single or group combinations of emulsion droplets, beads, and bubbles in round, nearly round, and irregular particle inclusions in sulfides, selenides, tellurides, oxides and silicates, or distribute along fractures of minerals. These LMCE micro-inclusions are derived from melt disturbance that result in melt-melt or melt-liquid emulsification. Fluid boiling should be the main mechanism that causes the melt disturbance since the LMCE melt cannot be quenched and crystallized quickly at this process. It is usually cooled slowly at low temperature to achieve phase equilibrium and form complex mineral compositions. This feature is significant even in micro-to nano-scale mineral particles. Melt-fluid inclusions are the most direct evidence for the involvement of LMCE melts during mineralization. Solid solution decomposition texture, melt annealing texture, mineral-melt dihedral texture, dissolution-reprecipitation texture are also characteristic textures of LMCE melts involved in mineralization.