Metamorphic Hydrothermal Fluids Research Articles

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.

Genesis of the Mount Colin Cu-Au deposit, NW Queensland: Evidence from geology, multi-mineral U–Pb geochronology, S-, O- and C-isotope constraints

The Mount Colin Cu-Au deposit is located in the Mary Kathleen Domain of the Mount Isa Inlier, NW Queensland. The deposit is hosted in the Corella Formation, which transitions into the Burstall Granite at a depth of ~400 m. The host rocks of the deposit are overprinted by Na-Ca alteration and a Ca-Mg-Fe skarn assemblage. Na-Ca alteration yields a titanite 207Pb/206Pb weighted mean age of 1717 ± 8 Ma (MSWD = 0.32) and is composed of albite, actinolite, zircon, titanite and scapolite with retrograde calcite, prehnite and sericite. The Ca-Mg-Fe skarn yields a titanite 207Pb/206Pb weighted mean age of 1712 ± 7 Ma (MSWD = 1.2) and is composed of hastingsite, K-feldspar, clinopyroxene, albite, titanite, calcite, biotite, and zircon with retrograde quartz and chlorite. Based on temporal relationships, Na-Ca alteration and the Ca-Mg-Fe skarn is interpreted to have formed from a hydrothermal magmatic fluid during late-stage magmatism of the (1740–1710 Ma) Wonga-Burstall plutons. A second generation of titanite (denoted titanite II) is present within Na-Ca alteration and the Ca-Mg-Fe skarn assemblage. Titanite II yields a 207Pb/206Pb weighted mean age of 1576 ± 7 Ma (MSWD = 0.37), and is interpreted to have formed from an external metamorphic hydrothermal fluid during widespread deformation associated with the (1610–1500 Ma) Isan Orogeny. The Mount Colin Cu-Au deposit consists of a predominantly coarse-grained, mineralogically zoned assemblage that infills along the NW-striking Mount Colin Fault. The centre of the deposit comprises a largely unmineralized core composed of calcite and/or quartz ± apatite ± microcline. Cu-Au mineralization is positioned along the outer margins of the deposit and consists of biotite, actinolite, chalcopyrite and pyrrhotite with trace abundances of magnetite, pyrite, galena, melonite and arsenopyrite. Apatite from the core of the deposit yields a U–Pb age of 1523 ± 16 Ma (MSWD = 1.2). The δ34SCDT values of chalcopyrite and pyrrhotite range from 0.1–2.1 ‰ and 0.9–1.5 ‰ respectively. The δ13CVPDB values of calcite range from −5.54 ‰ to −4.54 ‰ with corresponding δ18OVSMOW values ranging from 8.16–11.26 ‰. Based on the temporal and isotopic constraints presented in this study, a hydrothermal magmatic origin for Cu-Au mineralization is favoured. The timing of Cu-Au mineralization is coeval with a regional-scale metamorphic/hydrothermal event throughout the Mary Kathleen Domain and widespread magmatism throughout the Eastern Fold Belt of the Mount Isa Inlier.

Fe-Si-C isotope constraints on the genesis of iron ores in Gongchangling iron deposit in northeastern China

The Gongchangling iron deposit is an oxide-facies Algoma-type banded iron formation (BIF) and one of the most representative iron deposits in northeastern China. We present new data from petrological, whole-rock geochemical, and Fe-Si-O isotope compositional data from the Gongchangling BIFs to constrain the origin of ore-forming materials and mineralization processes. The low Al2O3 and TiO2 contents in iron ores indicate that the sources of the BIFs were not contaminated by terrigenous clastic sediments. The Gongchangling BIFs are characterized by depletions in LREEs and 30Si and enrichments in HREEs, with positive Eu and Y anomalies. Given these and the Fe isotope results, we conclude that both high-temperature hydrothermal fluid and seawater contributed to the formation of the BIFs and that a mixture of seawater and ~ 0.1 % hydrothermal fluid, which percolated and dissolved the submarine volcanic rocks, provided the ore-forming materials. Combining the Ce anomalies, enrichments in heavy Fe isotopes and low δ13C compositions, we conclude that the fO2 of Archean seawater was limited. Integrated petrographic and geochemical evidence indicates that the high-grade iron ores from the Gongchangling BIFs experienced incomplete oxidation and low amounts of Fe precipitation in a marine environment and that dissimilatory iron reduction and metamorphic hydrothermal fluid played important roles.

Trace elements of sulfides in the Dengjiashan Pb–Zn deposit from West Qinling, China: Implications for mineralization conditions and genesis

Dengjiashan is a large‐scale Pb–Zn deposit discovered in the western part of the Xicheng ore field within the West Qinling metallogenic belt. Information regarding the distribution and occurrence of trace elements in ore minerals and mineralization, as well as the genesis of this deposit, remains scarce. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) analysis and elemental mapping were used to determine the distribution and occurrence of trace elements in the sulfide minerals of the Dengjiashan deposit, as well as delineate the process of its genesis. Our results showed that the trace elements enriched in the different sulfides were significantly different. Sphalerite is the main carrier mineral of the scattered elements Cd, Ge and Ga. Based on the trace element content and ratio, principal component analysis and colour of the sphalerite and pyrite, the deposit likely formed in a medium‐temperature environment. The comparative analysis of trace elements from multiple deposits of different origins, combined with the geological characteristics and distribution of sulfide trace elements, suggests that Dengjiashan is a sedimentary exhalative (SEDEX)‐type deposit. As the metallogenic process had a closer relationship to medium‐low temperature hot brine and later tectonic metamorphic hydrothermal fluid, than with magmatic hydrothermal fluid, the deposit is likely of non‐magmatic SEDEX–hot brine superimposed transition origin. This study provides additional insight on the distribution of trace elements in the sulfide minerals of the Dengjiashan deposit, as well as the processes that led to its formation, and may further facilitate ongoing and future key metal prospecting and exploration efforts.

Caledonian Tin Mineralization in the Jiuwandashan Area, Northern Guangxi, South China

The Jiangnan orogenic belt is located between the Yangtze and Cathaysia blocks in South China and is one of the largest W–Sn–Nb–Ta ore belts worldwide. Mineralization occurred from the Proterozoic to Mesozoic, but Caledonian Sn mineralization has rarely been reported. The Jialong cassiterite–sulfide deposit is located in the western Jiangnan orogenic belt. It is hosted by the Sibao Group and in contact with the northeastern part of the Yuanbaoshan granite. The deposit was overprinted by the Sirong ductile shear zone. Here, we present cassiterite U–Pb and mylonitic granite muscovite 40Ar/39Ar ages for this deposit. The cassiterite and muscovite yielded concordant U–Pb and 40Ar/39Ar ages of 422–420 Ma, indicating that Sn mineralization occurred during the early Paleozoic and was spatially and temporally related to the ductile shear zone. The cassiterite is depleted in Nb (0.51–5.46 ppm), Ta (0.01–1.09 ppm), Ti (32.84–423.15 ppm), Sc (0.02–1.45 ppm), Hf (0–1.11 ppm), and other high-field-strength elements. Elements, such as Pb (0.01–8.11 ppm) and Sb (9.92–56.45 ppm), are relatively enriched in the cassiterite, which indicate the Jialong deposit was not directly related to magmatism. Shearing along the Sirong ductile shear zone occurred at 419.6 ± 3.8 Ma, concurrent with the formation of the Jialong Sn–Cu deposit. Moreover, cassiterite in the deposit exhibits obvious shear and brittle deformation, and dissolution and regrowth, suggesting that Sn mineralization was closely related to ductile shearing. The Sirong ductile shear zone and secondary shear structures had a key role in controlling the Sn orebody. The heat generated during tectonic deformation in the ductile shear zone may have produced the ore-forming hydrothermal fluids, and NW–SE-trending fractures in the strata provided the space for mineralization. Metamorphic hydrothermal fluids generated by Caledonian shear deformation extracted Sn from Sn-rich strata, which then migrated along interlayer fractures produced by shearing. A decrease in pressure and water–rock reactions led to the mineralization of Sn and other elements. This deposit is the first example of Caledonian and shear zone-related Sn mineralization identified in the Jiuwandashan area of northern Guangxi.

Geochemical Characteristics of Fluid Inclusions and Isotopes in Zhazixi Sb-W Deposit

Zhazixi Sb–W deposit, where Sb and W coexist in different orebodies, features a unique metal association between the two minerals. This study aims to analyze the respective sources of Sb and W, through temperature measurement of fluid inclusions, laser-based Raman spectroscopy, as well as isotope test. The ore-forming fluid of Zhazixi Sb-W deposit was found to be a CO2-N2-H2O-NaCl system, which is rich in CO2 and N2. N2-rich inclusions and C, H, O isotopes were observed, indicating that the ore-forming fluid mainly comes from deep magmatic rocks, and secondarily from the metamorphic hydrothermal fluids. The detected S and Pb isotopes suggest that the ore-forming material stems from multiple sources. After comprehensive analysis of the geological features of the deposit and the local region, we believed that the Sb in Zhazixi Sb-W deposit stems from Banxi group, but W comes from the concealed rock body. The results provide a theoretical basis for the metallogenic research and prospecting direction of Xuefengshan metallogenic belt.

U-Pb and Sm-Nd Evidence for Episodic Orogenic Gold Mineralization in the Kalgoorlie Gold Camp, Yilgarn Craton, Western Australia

Abstract Different genetic and timing models for gold mineralization in the Kalgoorlie gold camp (Yilgarn craton, Western Australia) suggest either broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma or a punctuated mineralization history from ca. 2675 to 2640 Ma with the involvement of early magmatic-hydrothermal systems (represented by the Fimiston, Hidden Secret, and Oroya gold-telluride lodes) and late metamorphic fluids (represented by the Mt. Charlotte gold stockwork veins). The results of U-Pb and Sm-Nd geochronological studies of zircon, apatite, and titanite from pre-ore dikes and syn-ore dikes constrain the absolute timing of mineralization and provide new evidence to this timing controversy. Emplacement ages constrained by U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon data are interpreted to be similar for both the pre-ore dikes (n = 10) and syn-ore dikes (n = 7) at ca. 2675 Ma. An inferred emplacement age of ca. 2675 Ma for the syn-ore dikes is supported by a Sm-Nd isochron age from apatite (laser ablation-inductively coupled plasma-mass spectrometry; LA-ICP-MS) of 2678 ± 15 Ma and by a U-Pb titanite age (LA-ICP-MS) of 2679 ± 6 Ma. The results of chemical abrasion-isotope dilution-thermal ionization mass spectrometry U-Pb zircon analysis from the pre- and syn-ore dikes are complicated by multistage Pb loss, reverse discordance, and potential inheritance. However, the data are compatible with the emplacement of Fimiston/Hidden Secret gold mineralization at ca. 2675 Ma and suggest a younger age for Oroya mineralization at ca. 2665 Ma. These results contrast with models for orogenic gold deposits that invoke broadly synchronous, late-stage mineralization related to metamorphic fluids at ca. 2640 Ma. The bulk of the Kalgoorlie gold camp’s estimated 2,300 t Au endowment was emplaced at ca. 2675 Ma as Fimiston/Hidden Secret Au mineralization. This early Au mineralization was deformed and overprinted twice by subordinate Au mineralization at ca. 2665 (Oroya mineralization) and ca. 2640 Ma (Mt. Charlotte mineralization). Gold mineralization in the Kalgoorlie gold camp was protracted in nature from ca. 2675 to 2640 Ma and reflects the interplay of early magmatic (Fimiston, Hidden Secret, Oroya) and late metamorphic (Mt. Charlotte) hydrothermal fluid systems in the formation of hybrid intrusion-related and metamorphic orebodies.

Naldrettite (Pd2Sb): A new find in Brazil and comparison with worldwide occurrences

ABSTRACT Naldrettite (Pd2Sb) is a PGM discovered in 2005 in Mesamax Northwest deposit, Ungava region, Quebec, Canada. Before and after its approval, PGM with the naldrettite type composition have been reported from a number of localities worldwide. Most frequently, naldrettite has been documented in magmatic Ni–Cu–PGE sulfide deposits, hydrothermal veins in porphyry coppers of the Cu–Au type, and PGE deposits of Alaskan-type zoned intrusions. Naldrettite has been occasionally found in metasomatic Sb–As sulfide ore, metamorphic Ni–oxide ore, and podiform chromitites, although these occurrences have not been fully constrained by solid chemical analyses or paragenetic reconstruction. In this paper we report the first discovery of naldrettite in Brazil. This new finding occurs in a chromitite sample collected in the Luanga Complex, a Neo-archaean layered intrusion in the Carajás Mineral Province. Paragenetic association with alteration assemblages (ferrianchromite, Fe-hydroxides, chlorite) suggests precipitation of naldrettite from metamorphic hydrothermal fluids. The average composition of the Luanga sample (Pd1.76Pt0.24)Σ2.00(Sb0.57As0.43)Σ1.00 shows major substitution of Pt and As. These elements were derived from the breakdown of primary sperrylite, and were incorporated in naldrettite deposited by percolating fluids, at temperature below 350 °C (maximum temperature registered by the crystallization of associated chlorite). An overview of documented occurrences indicates that naldrettite can form in a variety of igneous rocks (ultramafic, mafic, felsic), even involving minimal concentrations of Pd and Sb. Crystallization of naldrettite generally occurs in the post-magmatic stage due to the activity of hydrothermal fluids containing volatile species Sb, As, Bi, Te, and Pd due to its higher mobility compared with the other PGE. A major issue concerns the origin of fluids that can be: (1) “residual”, after the main crystallization of the host magma, (2) “metamorphic”, during regional metamorphism or serpentinization, and (3) “metasomatic”, emanating from an exotic magma intrusion. The combination of two or three of these factors is the most likely process observed in the naldrettite-bearing complexes.

An in situ sulfur isotopic investigation of the origin of Carlin-type gold deposits in Youjiang Basin, southwest China

• In situ sulfur isotope analysis of pyrite and arsenopyrite in Carlin-type deposits. • Study has focused on relationships between different deposits in the Youjiang Basin. • There are two source end-members from NW to SE basin, δ 34 S ~ +0‰ and δ 34 S > +10‰ • Results lead to a differential deep metamorphic–hydrothermal genetic model. Debate over the origins of ore-forming fluids and metal accumulation in Carlin-type gold (Au) deposits has hampered the development of genetic models for such deposits. Youjiang Basin in southwest China contains Carlin-type deposits, including the Nibao, Banqi, and Yata deposits in Guizhou, and the Gaolong and Jinya deposits in Guangxi, but there has been little study of the relationships between these deposits, and the origins of the ore-forming fluids and metals remain enigmatic. We conducted in situ laser ablation multi–collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) sulfur (S) isotope analyses of different stages of pyrite and arsenopyrite to improve our understanding of the sources and genesis of the deposits. Average δ 34 S values for ore-stage pyrite in the Nibao, Banqi, Yata, Gaolong, and Jinya deposits are −0.05‰, +4.04‰, +7.02‰, +11.51‰, and −3.80‰, respectively, and those of ore-stage arsenopyrite in the Banqi, Gaolong, and Jinya deposits are +7.33‰, +10.92‰, and −3.84‰, respectively. Average δ 34 S values of sedimentary–diagenetic (i.e., pre-ore) stage pyrite in the Gaolong and Nibao deposits are −6.41‰ and −15.61‰, respectively. Two source end-members contribute δ 34 S values of ~0‰, which are deep metamorphic fluids and volcaniclastic rocks in the Nibao deposit, and δ 34 S values of greater than +10‰ are from mixed basinal fluids or marine sulfates in the Gaolong deposit. The Yata and Banqi deposits are intermediate between the other two deposits. The δ 34 S values of ore-stage pyrite increase from northwestern platforms to southeastern basins, with this trend also occurring in other Carlin-type deposits in Youjiang Basin. The sources of S and Au were deep Proterozoic metamorphic basement rocks, which were mixed with a sedimentary source. We propose a genetic model for these Carlin-type Au deposits based on a deep metamorphic–hydrothermal fluid system, with the initial ore-forming fluid generated by the Indosinian or Yanshanian orogenies. Ascending fluids mixed with deeply circulating meteoric waters and/or basinal fluids and reacted with wall-rocks in the shallow crust to form the Au deposits. This study confirms that in situ S isotope analysis is effective for investigating ore genesis and assisting in exploration for new ore fields.

Trace elements of pyrite and S, H, O isotopes from the Laowan gold deposit in Tongbai, Henan Province, China: implications for ore genesis

The Laowan deposit is a large gold deposit in the Qinling-Dabie orogenic belt where pyrite is the main Au-bearing mineral phase. We present results from the occurrences of gold, trace elements and sulfur isotopes of pyrite, and hydrogen and oxygen isotopes of quartz and calcite to elucidate the sources of ore-forming fluid; the genesis of pyrite and the ore-forming process. From field observations, five generations of pyrite are identified; one formed in a metamorphic-diagenetic epoch (PyI), and the others during four mineralization stages: 1) the coarse-grained pyrite-quartz stage (PyII), 2) the quartz and medium- to fine-grained pyrite stage (PyIII), 3) the polymetallic sulfide stage (PyIV), and 4) the carbonate-quartz stage (PyV). Gold mainly occurs in PyIII and PyIV. We find that Au, Ag, Pb, and Cu are incorporated into pyrite as micro-/nano-inclusions and that Co, Ni, As, and Se enter the pyrite lattice via isomorphous replacement. The Co/Ni values and Se concentrations indicate that PyI formed from metamorphic hydrothermal fluids and that pyrites (PyII, PyIII, and PyIV) from the ore-forming stages typically reflect a hydrothermal genesis involving magmatic fluid. The δ34S values of PyI (1.45 ‰–2.09‰) are similar to that of plagioclase amphibole schist, indicating that S was primarily derived from wall rock, while those of PyII, PyIII, and PyIV (3.10‰–5.55‰) suggest that S was derived from the Guishanyan Formation and the Laowan granite. The four mineralization stages show a systematic decrease in δD (from −77.1‰ to −82.8‰, −84.7‰, and −102.7‰), while the $${{\rm{\delta }}^{18}}{{\rm{O}}_{{{\rm{H}}_2}{\rm{O}}}}$$ values showed a gradual decrease from 5.7 to 2.7‰, 1.0‰, and −1.3‰. These data show that the ore-forming fluid was similar to a mixture of magmatic and meteoric waters. Thus, we conclude that the Laowan gold deposit is related to magmatic-hydrothermal fluid.

Geochronological constraints on the genesis of high-grade iron ore in the Gongchangling BIFs from the Anshan-Benxi area, North China Craton

The Gongchangling iron deposit located in the northeast of the North China Craton is hosted in late Neoarchean Algoma-type BIFs. It is famous for the major production of high-grade iron ore in the Gongchangling No.2 mining area in China. With regard to the genesis of high-grade iron ore, more and more evidences indicate that it was related with hydrothermal enrichment of BIFs. However, the hydrothermal nature was argued for meteoric, metamorphic or migmatitic fluid. In this study, the trace element compositions of garnet from the altered wall-rock of high-grade iron ore obtained by LA–ICP–MS show compositional zoning, indicating it is a metamorphic origin. These garnets yield a Sm–Nd isochron age of 1888 ± 77 Ma, interpreted as the time of metamorphism in this area. LA–ICP–MS U–Pb dating of zircon from the upper migmatite zone shows that the migmatitic granite was formed at 2478 ± 36 Ma. Since the REE patterns of garnet suggest that it was formed in the nearly neutral fluid rather than the acid meteoric fluid in the Paleoproterozoic. Hence, combined the ages of metamorphism and migmatization with high-grade iron ore of 1840 ± 7 or 1860 ± 7 Ma obtained by previous studies, it indicates that the genesis of the high-grade iron ore derived from the enrichment of BIFs by metamorphic hydrothermal fluid.

The giant Zaozigou Au-Sb deposit in West Qinling, China: magmatic- or metamorphic-hydrothermal origin?

Understanding the relationship between mineral occurrences and host granitic rocks can be controversial. The Zaozigou Au-Sb deposit (118 t Au, 0.12 Mt Sb), hosted in metasedimentary rocks and dacitic to granodioritic sills and dikes, is one such example of a large gold deposit argued to have formed from either magmatic or metamorphic hydrothermal processes. Two populations of monazite are identified within a mineralized dacite located along a major shear zone. Magmatic monazite commonly occurs within magmatic biotite and quartz phenocrysts and is characterized by uniform and high Th concentrations. It has a crystallization age of 238.3 ± 2.6 Ma, consistent with the zircon U-Pb age of 238.0 ± 1.8 Ma from the same dacite. Hydrothermal monazite is associated with sulfides and sericite, and has a 207Pb-corrected 206Pb/238U age of 211.1 ± 3.0 Ma. The amount of Th in hydrothermal monazite is widely variable. The low Th content of some monazite grains reflects direct precipitation from a metamorphic hydrothermal fluid. Furthermore, the elevated Th content in other hydrothermal monazite grains is likely due to the release of Th (and U) into hydrothermal fluids by dissolution of pre-existing Th-rich minerals in the country rock during ore-related alteration events. The magmatism, which overlaps Middle-Late Triassic terrane subduction-accretion in the West Qinling orogen, thus pre-dates the ore-forming event by about 30 m.y. The δ34S values of pyrite, arsenopyrite, stibnite, marcasite, and chalcopyrite from disseminated- and vein-type ores range from − 12.0 to − 5.5‰. Such negative values are distinct from those measured for other deposits in the northwestern part of the orogen that are genetically related to Triassic magmatism, including the Xiekeng-Jiangligou-Shuangpengxi Cu-Au-Fe-Mo skarn, Laodou reduced intrusion-related Au, and Gangcha epithermal Au ores. The Zaozigou deposit is best classified as an epizonal orogenic Au-Sb deposit. Our results demonstrate the usefulness of high-precision in situ geochronology on monazite for deciphering age relationships in ore deposits that have spatial associations with granitic rocks, thus aiding in the testing of the veracity of ore formation models.

The nature and sources of ore-forming fluids in the Bhukia gold deposit, Western India: constraints from chemical and boron isotopic composition of tourmaline

Gold mineralization in the newly discovered Bhukia deposit in northwestern India is hosted in Proterozoic metamorphosed volcano-sedimentary rocks of the Aravalli-Delhi Belt. Three generations of tourmaline occurring in different textural settings are recognized in the host rocks of the deposit. Fine-grained texturally early tourmalines (Tur-I) precipitated during the gold-sulfide mineralization stage. They are dravitic in composition and occur parallel to the S1 tectonic foliation as fine-grained crystals in tourmaline-albite layers within quartz-albite rock and as scattered grains in calc-silicate rocks. Coarse-grained texturally later schorls (Tur-II) are also restricted to calc-silicate and quartz-albite rocks and characterized by sector zoning. Late-stage type III tourmaline (Tur-III), also of schorl composition, replaces Tur-II along fractures and margins. They are interpreted to have formed during a phase of ore remobilization. All tourmalines, especially the schorls, are strongly enriched in Li, Ga, Mn and Zn with Ga concentrations being the highest ever reported in natural tourmalines (up to 1380 ppm). The boron isotope composition is similar in all three tourmaline types with the consistently light δ11B (−10.4‰ to −7.2‰) indicative of a continental source for B. The chemical and B-isotopic composition of tourmaline is suggestive of the involvement of two fluids, a granitic-derived hydrothermal fluid and metapelite-derived metamorphic fluid. The δ11B variations in the tourmalines can be explained by mixing between 11B-poor granitic-derived hydrothermal fluid and 11B-rich metamorphic-hydrothermal fluid. While high V (2110–4247, avg. 3339 ppm) in the early dravitic tourmalines indicate mixing of granitic-derived hydrothermal fluid with pelite-derived metamorphic hydrothermal fluids during gold-sulfide mineralization, the enrichment of Li, Mn, Zn and Ta, and depletion of V in the later schorlitic tourmalines suggest increasing influence of granitic-derived hydrothermal fluids during ore remobilization.

Episodic hydrothermal alteration recorded by microscale oxygen isotope analysis of white mica in the Larderello-Travale Geothermal Field, Italy

Microscale oxygen isotope analysis (18O/16O) of minerals can identify distinct events of fluid-rock interaction. This method is, however, still limited to a few major and accessory minerals of which most are anhydrous minerals. We present the systematic study of oxygen isotope distribution in white mica by Secondary Ion Mass Spectrometry (SIMS). Texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the Larderello-Travale geothermal field (LTGF), Italy, record stages of magmatic crystallization, metamorphic-hydrothermal replacement and fluid-rock interaction. The large range in intra- and inter-grain white mica δ18O values between 1 and 14‰ reflects varying protoliths and degrees of fluid-mineral interaction at variable temperatures (180–450°C present-day temperatures; p.d.T.). This variability reflects the large-scale circulation of both (1) magmatic, syn-intrusive to early contact metamorphic hydrothermal fluids with high-δ18O values, and (2) meteoric fluids with δ18O values of −7‰ during a post-intrusive, late hydrothermal stage.Metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity (μm-scale), including detrital grains (δ18O=12–14‰; high Na, low Mg), partially altered white mica (δ18O=8–9‰) and late hydrothermal white mica (1–6‰; low Na, mid Mg). The late hydrothermal white mica has similar δ18O values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a δ18O of −6 to 0.5‰, which circulated in the late hydrothermal stage. Downhole towards the lower reservoir, white mica from two contact metamorphic micaschist samples shows either (1) homogeneous δ18O values of ca. 9‰ likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600°C), or (2) a large spread in δ18O from 2 to 12‰ within and across grains of variable texture and chemistry in the host rock and a cross-cutting quartz-white mica vein (ca. 300°C, present day temperature; hereafter p.d.T.). This contrasting δ18O signature of white mica is also recorded in granite cored at up to 4.6km depth. The Carboli granite contains white mica with a homogeneous magmatic δ18O of 10±0.6‰, whereas older granite samples from Radicondoli have magmatic to hydrothermal white micas that vary in δ18O from 4 to 10‰. A pronounced intra-grain δ18O variability of up to 6‰ occurs in white mica domains with higher Fe-Mg-Ti halos around inclusions of chloritized biotite, as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4.6km. In the Porto Azzurro granite on Elba, Italy, altered white mica has δ18O values of 2.6‰ down from 10‰ in unaltered grains.The distribution of oxygen isotope ratios in white mica is thus firstly a result of pervasive versus selective fluid alteration (at depth, sample and grain scale). Secondly, the actual preservation of these μm-scale variabilities indicates that volume diffusion is not detectable at microscale at p.d.T at or below 350°C where most of the heterogeneous white mica is found. Selective, sample- and grain-scale fluid penetration occurs episodically and anisotropically, on micro- and megascale, along faults, fractures and cleavages, producing lower δ18O white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation.

Trace Element Analysis of Pyrite from the Zhengchong Gold Deposit, Northeast Hunan Province, China: Implications for the Ore-Forming Process

The Zhengchong gold deposit is located in the central segment of the Jiangnan Orogen in northeastern Hunan Province, South China. The host rocks of this deposit are the Neoproterozoic slates of the Lengjiaxi Group and granodiorite. The structures in the Zhengchong gold deposit are dominated by NE-trending reverse faults, which control the gold-bearing veins. The orebody consists of NE-trending laminated quartz veins and NW-trending quartz veins. The alteration styles include silicification, carbonatization, sulfidation, sericitization and chloritization. The Zhengchong gold mineralization can be divided into four stages: Quartz-pyrite (stage I), quartz-pyrite-arsenopyrite (stage II), quartz-polysulfide (stage III) and quartz-carbonate (stage IV). Three generations of hydrothermal pyrite were identified: Disseminated euhedral to subhedral cubes in altered wall-rock (PyI), euhedral to subhedral cubes inter-grown with arsenopyrite and tetrahedrite in quartz veins and wall-rock (PyII), and euhedral cubes with microinclusions (native gold, galena, sphalerite, chalcopyrite, tetrahedrite, and pyrrhotite) or metasomatic textures in sulfide-rich veins or veinlets (PyIII). PyII and PyIII are arsenian pyrite and represent the main Au-bearing minerals. PyI records the lowest concentrations of Au; PyII and PyIII record similar amounts of Au, Cu, Pb, Zn, and Bi, but PyIII is more enriched in Co, Ni, Te, and Se. The substitution of As, Se and Te for S and that of Co and Ni for Fe occurs by direct-ion exchange. Invisible gold is uniformly distributed within the arsenian pyrite, and visible gold fills microfractures in PyII or occurs as inclusions in PyIII. Co, Ni, Cu exhibit positive correlations with Au and a negative correlation between Au + Cu + Co + Ni and Fe reflect that Fe vacancies may have been a major cause of the precipitation of invisible Au and other metal elements in pyrite structure. There are systematic trace element differences between the three generations of pyrite (PyI, PyII, PyIII). The more Co, Ni and Se, Te substitution that occurred for Fe and S, respectively, the greater the increase in the Co/Ni ratio (<1) and the decrease in the Se/Te ratio (<10) in stage III, indicating that a more reduced, lower-temperature metamorphic hydrothermal fluid was present in stage III.

In situ LA‐ICP‐MS trace element analysis of magnetite from the late Neoarchean Gongchangling BIFs, NE China: Constraints on the genesis of high‐grade iron ore

The Precambrian banded iron formations (BIFs) not only relate to the evolution of life, ocean, and atmosphere but also provide important reserves of iron around the world. The Gongchangling iron ore deposit located in the Anshan‐Benxi area of Liaoning Province, China, is oxide facies Algoma‐type BIFs, and the Gongchangling No.2 mining area is famous for the production of high‐grade iron ore in China. Magnetite is the main ore mineral in the Gongchangling iron ore deposit, and the magnetite mainly exhibits three modes of occurrence: BIFs (without actinolite), actinolite‐bearing BIFs, and high‐grade iron ore. Trace elemental compositions of the magnetite with different occurrences of the Gongchangling iron ore deposit were obtained by laser ablation inductively coupled plasma mass spectrometry to constrain the genesis of the high‐grade iron ore. The magnetite from actinolite‐bearing BIFs shows relatively lower contents of Mg, Al, Mn, and Zn compared to the magnetite from BIFs (without actinolite), suggesting that coexisting minerals have played an important role in the trace element concentration in magnetite. The magnetite from high‐grade iron ore has lower contents of Ti and V and higher contents of Al and Mn than counterpart from BIFs (with/without actinolite), indicating that the high‐grade iron ore may be reformed by high temperature metamorphic hydrothermal fluid. The staurolite‐garnet‐biotite schist is the wall‐rock of high‐grade iron ore, and the garnet‐biotite geothermometry is used to evaluate the metamorphic temperature of 593 ± 17 °C. It is proposed that the metamorphic hydrothermal fluid produced during regional metamorphism reformed BIFs to generate high‐grade iron ore.

Fluid-driven destabilization of REE-bearing accessory minerals in the granitic orthogneisses of North Veporic basement (Western Carpathians, Slovakia)

A variety of rare earth elements-bearing (REE) accessory mineral breakdowns were identified in granitic orthogneisses from the pre-Alpine basement in the Veporic Unit, Central Western Carpathians, Slovakia. The Ordovician granitic rocks were subjected to Variscan metamorphic-anatectic overprint in amphibolite facies. Chemical U-Th-Pb dating of monazite-(Ce) and xenotime-(Y) reveal their primary magmatic Lower to Middle Ordovician age (monazite: 472 ± 4 to 468 ± 6 Ma and xenotime: 471 ± 13 Ma) and/or metamorphic-anatectic Variscan (Carboniferous, Visean) age (monazite: 345 ± 3 Ma). Younger fluid-rock interactions caused breakdown of primary magmatic and/or metamorphic-anatectic monazite-(Ce), xenotime-(Y), fluorapatite and allanite-(Ce). Fluid-induced breakdown of xenotime-(Y) produced numerous tiny uraninite inclusions within the altered xenotime-(Y) domains. The monazite-(Ce) breakdown produced secondary egg-shaped coronal structures of different stages with well-developed concentric mineral zones. Secondary sulphatian monazite-(Ce) (up to 0.15 apfu S) occasionally formed along fluorapatite fissures. Localized fluorapatite and monazite-(Ce) recrystallization resulted in a very fine-grained, non-stoichiometric mixture of REE-Y-Fe-Th-Ca-P-Si phases. Finally, allanite-(Ce) decomposed to secondary REE carbonate minerals (members of the bastnasite and synchysite groups) and calcite in some places. Although the xenotime alteration and formation of uraninite inclusions is believed to be the result of dissolution-reprecipitation between early magmatic xenotime and late-magmatic granitic fluids, the monazite, apatite and allanite breakdowns were driven by metamorphic hydrothermal fluids. While earlier impact of post-magmatic fluids originated probably from Permian acidic volcanic and microgranitic veins crosscutting the orthogneisses, another fluid-rock interaction event most likely occurred during Late Cretaceous metamorphism in the Veporic basement and covering rocks. This stage indicates carbon-bearing fluids precipitating the carbonate minerals.

Polygenetic Compound Mineralization of Guoluolongwa Gold Field, Qinghai

two stages. Based on microthermometic study, three types of fluid inclusions are divided, namely aqueous inclusions (type I), aqueous-CO2 inclusions (type II) and pure CO2 inclusions (type III). The majority of type I and type II inclusions of stage B and type I inclusions of stage C often appear in groups, representing their primary causes. Besides, near the sulfide veinlet of stage C, many type I inclusions of stage B are arranged in rows along the fracture, showing characteristics of secondary inclusions, these inclusions may be formed at the same time with primary type I inclusions of stage C. So, there may be two main stages of inclusion formation, related to B and C stages, respectively. Stage B develops type I, type II, and type III inclusions, while stage C mainly contains type I inclusions. Homogenization temperatures and salinities of fluids are different in different stages. Mineralization of Guoluolongwa is mainly gold bearing quartz veins and quartz sulfide veins superimposed on the milky quartz veins. The early milky quartz veins of stage A may represent the product of low-grade metamorphic hydrothermal activity. Inclusions of stage B, including all these three types, are primary causes. Coexistence of aqueous and carbonaceous inclusions illustrates the separation of H2 O and CO 2. With a medium homogenization temperature ranging from 240℃ to 300℃, and salinities varing between 18~24% and 3~10% for type I and type II inclusions, respectively, fluid of stage B may be derived from metamorphic hydrothermal fluid. Stage C is characterized by type I inclusions with high salinity, ranging from 15% to 22%, shows that fluid may originate from magmatic hydrothermal, belongs to magmatic hydrothermal period. Homogenization

Geology and fluid characteristics of the Ulu Sokor gold deposit, Kelantan, Malaysia: Implications for ore genesis and classification of the deposit

The Ulu Sokor gold deposit is one of the most famous and largest gold deposits in Malaysia and is located in the Central Gold Belt. This deposit consists of three major orebodies that are related to NS- and NE-striking fractures within fault zones in Permian-Triassic meta-sedimentary and volcanic rocks of the East Malaya Block. The faulting events represent different episodes that are related to each orebody and are correlated well with the mineralogy and paragenesis. The gold mineralization consists of quartz-dominant vein systems with sulfides and carbonates. The hydrothermal alteration and mineralization occurred during three stages that were characterized by (I) silicification and brecciation; (II) carbonatization, sericitization, and chloritization; and (III) quartz–carbonate veins.Fluid inclusions in the hydrothermal quartz and calcite of the three stages were studied. The primary CO2–CH4–H2O–NaCl fluid inclusions in stage I are mostly related to gold mineralization and display homogenization temperatures of 269–389°C, salinities of 2.77–11.89wt.% NaCl equivalent, variable CO2 contents (typically 5–29mol%), and up to 15mol% CH4. In stage II, gold was deposited at 235–398°C from a CO2±CH4–H2O–NaCl fluid with a salinity of 0.83–9.28wt.% NaCl equivalent, variable CO2 contents (typically 5–63mol%), and up to 4mol% CH4. The δ18OH2O and δD values of the ore-forming fluids from the stage II quartz veins are 4.5 to 4.8‰ and −44 to −42‰, respectively, and indicate a metamorphic–hydrothermal origin. Oxygen fugacities calculated for the entire range of T-P-XCO2 conditions yielded log fO2 values between −28.95 and −36.73 for stage I and between −28.32 and −39.18 for stage II. These values indicate reduced conditions for these fluids, which are consistent with the mineral paragenesis, fluid inclusion compositions, and isotope values.The presence of daughter mineral-bearing aqueous inclusions is interpreted to be a magmatic signature of stage IIIa. Combined with the oxygen and hydrogen isotopic compositions (δ18OH2O=6.8 to 11.9‰, δD=−77 to −62‰), these inclusions indicate that the initial fluid was likely derived from a magmatic source. In stage IIIb, the gold was deposited at 263° to 347°C from a CO2–CH4–H2O–NaCl fluid with a salinity of 5.33 to 11.05wt.% NaCl equivalent, variable CO2 contents (typically 9–15mol%), and little CH4. The oxygen and hydrogen isotopic compositions of this fluid (δ18OH2O=8.1 to 8.8‰, δD=−44 to −32‰) indicate that it was mainly derived from a metamorphic–hydrothermal source. The CO2–H2O±CH4–NaCl fluids that were responsible for gold deposition in the stage IIIc veins had a wide range of temperatures (214–483°C), salinities of 1.02 to 21.34wt.% NaCl equivalent, variable CO2 contents (typically 4–53mol%), and up to 7mol% CH4. The oxygen and hydrogen isotopic compositions (δ18OH2O=8.5 to 9.8‰, δD=−70 to −58‰) were probably acquired at the site of deposition by mixing of the metamorphic–hydrothermal fluid with deep-seated magmatic water and then evolved by degassing at the site of deposition during mineralization. The log fO2 values from −28.26 to −35.51 also indicate reduced conditions for this fluid in stage IIIc. Moreover, this fluid had a near-neutral pH and δ34S values of H2S of −2.32 to 0.83‰, which may reflect the derivation of sulfur from the subducted oceanic lithospheric materials.The three orebodies represent different gold transportation and precipitation models, and the conditions of ore formation are related to distinct events of hydrothermal alteration and gold mineralization. The gold mineralization of the Ulu Sokor deposit occurred in response to complex and concurrent processes involving fluid immiscibility, fluid–rock reactions, and fluid mixing. However, fluid immiscibility was the most important mechanism for gold deposition and occurred in these orebodies, which have corresponding fluid properties, structural controls, geologic characteristics, tectonic settings, and origins of the ore-forming matter. These characteristics of the Ulu Sokor deposit are consistent with its classification as an orogenic gold deposit, while some of the veins are genetically related to intrusions.

Reprint of "The Gongchangling BIFs from the Anshan–Benxi area, NE China: Petrological–geochemical characteristics and genesis of high-grade iron ores"

The Gongchangling BIFs are located in the Anshan–Benxi area, northeastern part of the North China Craton, of which the No. 2 mining area is the largest and most typical high-grade iron ore producing area in China. In this study, the mineralogy, petrology and chemistry of iron ores (BIFs and high-grade iron ores) and their wall-rocks (amphibolite and garnet–chlorite schist) from the Gongchangling No. 2 mining area were examined to constrain the genesis of high-grade iron ore.The BIFs are composed essentially of magnetite and quartz, in addition to actinolite in a few samples; while the high-grade iron ores are mainly composed of magnetite and the quartz is relatively rare. The low contents of Al2O3, TiO2 and HFSE in the BIFs and high-grade iron ores indicate that they are marine chemical precipitates with little input of terrigenous clastic sediments. The magnetite and quartz from BIFs and high-grade iron ores exhibit the similar chemical composition. The REY patterns of BIFs and high-grade iron ores suggest their precipitation after mixing of hydrothermal component with seawater in certain proportions under relatively low oxygen level, and the obvious positive Eu anomalies indicate that high-temperature hydrothermal fluids which percolate and dissolve the submarine volcanic rock may supply the ore-forming material to the Gongchangling iron deposit.The amphibolite as interlayer of BIFs recorded the metamorphic temperature of 567±25°C, moreover, the garnet from wall-rock of high-grade iron ore exhibits the growth zoning produced during prograde metamorphism, indicating that metamorphic hydrothermal fluids played an important role in the genesis of high-grade iron ore. It was proposed that the Gongchangling iron deposit was formed in an arc-related tectonic setting at the late Neoarchean, then it underwent lower amphibolite facies metamorphism, and the high-grade iron ore is the reformed product of BIFs by metamorphic hydrothermal fluids.