Magmatic Hydrothermal Solution Research Articles

Mineralogy, Geochemistry, and Genesis of Agates from Chihuahua, Northern Mexico

The present study aimed to investigate the genesis and characteristics of some of the world-famous agate deposits in the state of Chihuahua, Mexico (Rancho Coyamito, Ojo Laguna, Moctezuma, Huevos del Diablo, Agua Nueva). Geochemical and textural studies of host rocks showed that all the studied deposits are related to the same rock type within the geological unit of Rancho el Agate andesite, a quartz-free latite that shows clear indications of magma mixing. As a result of their large-scale distribution and various differentiation processes, as well as transport separation, different textures and local chemical differences between rocks of different localities can be observed. These differences have also influenced the properties of SiO2 mineralization in the rocks. The mixing of near-surface fluids from rock alterations with magmatic hydrothermal solutions led to the accumulation of various elements in the SiO2 matrix of the agates, which were, on the one hand, mobilized during secondary rock alteration (Fe, U, Ca, K, Al, Si) and, on the other hand, transported with magmatic fluids (Zn, Sb, Si, Zr, Cr). Different generations of chalcedony indicate a multi-stage formation as well as multiple cycles of filling the cavities with fluids. The hydrothermal fluids are presumably related to the residual solutions of a rhyolitic volcanism, which followed the latitic extrusions in the area and probably caused the formation of polymetallic ore deposits in the Chihuahua area. The enrichment of highly immobile elements indicates the involvement of volatile fluids in the agate formation. The vivid colors of the agates are almost exclusively due to various mineral inclusions, which consist mainly of iron compounds.

Mineralization Based on CSAMT and SIP Sounding Data: A Case Study on the Hadamengou Gold Deposit in Inner Mongolia

The Hadamengou deposit is the largest gold deposit in Inner Mongolia. However, given that the sources of ore-forming alkaline magmatic hydrothermal solutions and ore-controlling structures are still controversial, the theories behind the genesis of the deposit have been controversial. In this study, four controlled-source audio magnetotellurics (CSAMT) and spectral induced polarization (SIP) profiles in the mining area were used to obtain the underground resistivity model and the pseudo section map of the apparent frequency dispersivity based on fine inversion. In the resistivity model, there are two high-resistivity blocks with resistivity greater than 3000 Ω m and three low-resistivity channels with resistivity less than 50 Ω m. Combined with the regional geological and drilling data, it is inferred that the high-resistance bodies, R4 and R5, may be alkaline magmatic intrusions related to multiple stages of magmatic hydrothermal activities, ranging from the Precambrian to Yanshanian periods. The highly conductive channels, C3, C5, and C4, may represent the Baotou-Hohhot fault, secondary faults, and ductile shear zone, respectively, which were formed in the Precambrian era and underwent multiple activations during the Hercynian to Yanshanian period. According to the spatial relationship, it is inferred that the ductile shear zone is an important ore-controlling and ore-hosting structure. However, the Baotou–Hohhot fault may be a pre-metallogenic fault rather than an ore-controlling fault. By comparing the resistivity model with the pseudo section of the apparent frequency dispersivity, it was found that all the known gold veins are located in the superimposed area of low resistivity and high-frequency dispersivity. It is speculated that the ductile shear zone outside the alkaline magmatic rock with the superimposed characteristics of low resistivity and high-frequency dispersivity is the favorable area for mineralization.

Metallogenesis of the Bawang Sn-Zn Polymetallic Deposit, Wuxu Ore Field, Guangxi, South China: U-Pb Dating and C-O-S-Pb Isotopic Constraints

The Bawang deposit, located in the west of Wuxu ore field, southern section of the Nandan-Hechi metallogenic belt, is a medium-sized tin–zinc polymetallic deposit. Its genesis has been a matter of debate because of lacking constraints from isotope geology. In this study, LA-MC-ICP-MS U-Pb dating of cassiterite and C-O-S-Pb isotope analyses of monominerals were used to investigate the mineralization age and source of the ore-forming minerals in the Bawang deposit. LA-ICP-MS U–Pb dating of cassiterite yielded ages of 93.1 ± 4.8 Ma and 85.3 ± 6.3 Ma, indicating that the mineralization occurred in the early Late Cretaceous. The δ13CPDB and δ18OSMOW values of calcites at the ore stage range between −0.41‰ and 0.44‰ (average = −11‰) and between 13.8‰ and 15.40‰ (average = 14.59‰), respectively. This shows that ore fluid sources were a mixture of those derived from magma and stratum. Pyrite and sphalerite have uniform δ34S values (−4.45‰~−2.20‰), indicating that sulfur is also derived from the mixing of magmatic hydrothermal and stratum fluids. The Pb isotopic composition of sulfide (206Pb/204Pb = 18.4055~18.7625, 207Pb/204Pb = 15.6745~15.7209, 208Pb/204Pb = 38.6232~39.0370) is consistent with the granite of the same age, indicating that ore-forming metals are mainly derived from magmatic hydrothermal solution. The Bawang deposit is a hydrothermal vein-type deposit in the external contact zone of Late Cretaceous granite, controlled by tectonic fractures, and formed by the interaction of magmatic hydrothermal fluid and carbonate rock. There may be large skarn-type ore bodies in the deep part. The results of this study provide insights into the research and exploration of similar deposits in Nandan-Hechi metallogenic belt and in the Youjiang Basin.

Micro-Nanoscale Characteristics of Pyrite and Its Implications for Gold Mineralization: Two Cases of Gold Deposits in the Youjiang Basin and Southwestern Tianshan Mountains.

The mineralogical and compositional characteristics of gold-bearing minerals and the occurrence of gold are not only of great significance to exploring the sources of ore-forming materials and their formation mechanisms but also helpful for designing reasonable beneficiations and smelting schemes and achieving remarkable economic benefits. This paper presents an integrated study on the crystal characteristics, elemental composition and distribution of pyrite (the main gold-bearing minerals), on the basis of electron probe microanalysis (EPMA), scanning electron microscopy (SEM), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and nano-secondary ion mass spectrometry (NanoSIMS). The occurrence of gold in the Shuiyindong gold deposit and Ashawayi gold deposit has been studied by means of microscopy, SEM, and EPMA images, elemental correlations, S-Fe-As ternary diagrams, logAs-logAu diagrams and Au/As ratios. The gold in pyrite of the Shuiyindong deposit is in the form of nano gold inclusions and lattice gold. The gold in pyrite of the Ashawayi deposit dominantly exists in the form of nano gold inclusions or is present as micro-nano gold particles in the cracks or edges of pyrite, some of which can exist as lattice gold. The ore-forming hydrothermal solution of the Shuiyindong gold deposit is mainly underground hot brine, but it may be reformed by a deep magmatic hydrothermal solution or volcanic-subvolcanic hydrothermal solution. The ore-forming hydrothermal solution of the Ashawayi gold deposit is mainly derived from the metamorphic hydrothermal solution formed during the orogenic process, and the ore-forming process or post-mineralization process may be reformed by the leaching of underground hot brine. Finally, the characteristics of ore-forming fluids and evolution of the two types of deposits are determined via pyrite element surface scanning. This paper shows that micro-nanoscale study of gold-bearing pyrite is of great significance to understanding the gold mineralization process and is worth further study.

Mineralogy, isotope geochemistry and ore genesis of the miocene Cuonadong leucogranite-related Be-W-Sn skarn deposit in Southern Tibet

The Himalayan orogenic belt is characterized by widespread leucogranites, which are mostly peraluminous with less than 5% mafic minerals. Previous studies mainly focused on the petrological characteristic, genetic mechanism, magmatic evolution and tectonic significance of these leucogranites. However, recently, a skarn-type Be–W–Sn polymetallic deposit, which is spatially related to the Cuonadong leucogranites of the Himalayan leucogranites belt, was discovered. The orebodies occur in the inner and outer contact zones between the Cuonadong leucogranites and their surrounding marble. This study focuses on the the characteristics of the skarn mineral assemblages. The deposit shows a typical skarn mineral zonation, from inside to outside, including dark pyroxene-garnet skarn zone, red garnet zone, dark green amphibole-specularite zone, light-green vesuvianite-epidote ribbon, and vesuvianite-chlorite zone. Zircon U-Pb dating shows that the ore-related leucogranite was emplaced at ca. 18 Ma. Geochemically, the leucogranite has high SiO2, Al2O3, Na2O, K2O and low CaO contents belonging to peraluminous granite. The Cuonadong leucogranite is characterized by right-dipping chondrite-normalized REE patterns with distinct negative Eu anomalies. Importantly, all samples show Lanthanide tetrad effect on chondrite-normalized REE patterns, indicating significant melt-fluid interaction has occurred due to strong fractional crystallization. Additionally, the Cuonadong leucogranite is enriched in large ion lithophile elements (such as Rb) and relatively depleted in high field strength elements (such as Nb, Zr, Hf), suggesting a crustal source region. High F and B contents in the magma could have significantly increased the solubility of water in the magma, while the magmatic-hydrothermal solution enriched in F and B should have remarkably enhanced the metal-carrying capacity and increased the permeability of surrounding rocks, which in turn effectively facilitated water–rock reaction.

The origin of the mineralizing fluids in different type mineralizations associated with the Upper Cretaceous Elazig Magmatic Complex, Turkey; an isotopic approach

This study examined the origin and properties of mineralized fluids by using C, O and S isotopes in different type mineralizations associated with the Upper Cretaceous Elazig Magmatic Complex. The isotopic compositions of vein type mineralizations show that the thrust zone affects the formation of the Karakas iron mineralization by meteoric and magmatic hydrothermal solution mixtures due to the average δ18OH2O value 6.40‰. The calculated δ18OH2O composition values is 5.20‰ in biotite from the Kızıldag vein type Cu–Pb–Zn mineralizations, which is consistent with a magmatic origin of the fluids. The calculated δ18OH2O composition is 4.30‰ that indicates a medium and low temperature magmatic hydrothermal fluid effect. The skarn type mineralizations isotopic compositions indicate that the calculated δ13CCO2 values are between −12.70‰ and −36.39‰ that could be late magmatic fluids that were modified by interaction with the host meta-sedimentary rocks and with meteoric water at the Birvan and Asvan iron mineralizations. Also the δ18OH2Ovalues in quartz of the Meseli iron mineralization are between 0.70‰ and 1.30‰. The lower δ18OH2O oxygen isotope composition compared to magmatic origins must be hydrothermal solutions mixing with meteoric waters. In the massive sulfide type Kavallı and Derince pyrite samples, δ34SH2S values are between 17.73‰ and 20.63‰. These values clearly indicate the volcano-sedimentary effect on hydrothermal solutions, which form the mineralization. The first findings of this study present information that all of the measured isotopic composition was modified by mixing metamorphic, magmatic and meteoric waters in the final stages of the hydrothermal solutions circulation.

REEQUILIBRATION PROCESSES IN MAGNETITE FROM IRON SKARN DEPOSITS

Research Article| January 01, 2015 REEQUILIBRATION PROCESSES IN MAGNETITE FROM IRON SKARN DEPOSITS* Hao Hu; Hao Hu 1State Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China Search for other works by this author on: GSW Google Scholar David Lentz; David Lentz 2Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada Search for other works by this author on: GSW Google Scholar Jian-Wei Li; Jian-Wei Li † 1State Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China †Corresponding author: e-mail, jwli@cug.edu.cn Search for other works by this author on: GSW Google Scholar Travis McCarron; Travis McCarron 2Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada Search for other works by this author on: GSW Google Scholar Xing-Fu Zhao; Xing-Fu Zhao 1State Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China Search for other works by this author on: GSW Google Scholar Douglas Hall Douglas Hall 3Microscopy and Microanalytical Facilities, University of New Brunswick, Fredericton, NB E3B 5A3, Canada Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (1): 1–8. https://doi.org/10.2113/econgeo.110.1.1 Article history received: 07 Aug 2014 accepted: 06 Oct 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Hao Hu, David Lentz, Jian-Wei Li, Travis McCarron, Xing-Fu Zhao, Douglas Hall; REEQUILIBRATION PROCESSES IN MAGNETITE FROM IRON SKARN DEPOSITS. Economic Geology 2015;; 110 (1): 1–8. doi: https://doi.org/10.2113/econgeo.110.1.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract Textural and compositional data for magnetite from nine iron skarn deposits in Canada, Romania, and China show that most samples have reequilibrated by dissolution and reprecipitation, oxy-exsolution, and/or recrystallization. The dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined, whereas the oxy-exsolution occurs only in high-Ti magnetite, forming exsolution lamellae of Fe-Ti-Al oxides. Electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite, notably Si, Mg, Ca, Al, Mn, and Ti, whereas oxy-exsolution is effective in decreasing the Ti content of high-Ti magnetite. Many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations (BIF), iron oxide–copper-gold (IOCG), or porphyry Cu fields using the Ti + V versus Ca + Al + Mn discrimination diagram. This pattern suggests that trace element data for magnetite that has unusual composition and/or reequilibrated cannot be reliably used as a petrogenetic indicator. Mixing of externally derived saline fluids with Fe-rich magmatic-hydrothermal solutions, an increase in temperature, and local decreasing pressure and fO2 are considered the most important causes for the dissolution and reprecipitation, or recrystallization, of the magnetite; increasing fO2 and decreasing temperature may facilitate oxy-exsolution of Fe-Ti-Al oxides in high-Ti magnetite. Results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geological and Geochemical Characteristics of the Huangshilao Stratabound Gold Deposit in the Tongguanshan Orefield, Tongling, East‐Central China

AbstractThe Huangshilao gold deposit (>13.5 t Au) is comprised of stratabound pyrite‐dominant massive sulfide ores, and is distinguished from the skarn Cu, Au, and Cu–Au deposits that are dominant in the Tongguanshan orefield, Tongling, east‐central China. The stratabound orebodies are situated along flexural slip faults along the unconformity between the Upper Devonian Wutong and the Upper Carboniferous Huanglong Formations. The ores, dominated by crystallized pyrite, colloform pyrite, and pyrrhotite, are systematically sampled from the underground stopes along strike drifts. The δ34S values of ore sulfides yield a wide variation from −11.3 to 11.4‰, but mostly within 4–8‰, corresponding to the δ34S range (3.4–8.7‰) of the Yanshanian Tongguanshan and Tianshan quartz diorite intrusions in the Tongguanshan orefield, suggesting a magmatic dominated sulfur source. Few obvious negative δ34S values are induced by an involvement of sedimentation‐related biogenic sulfur. The wide δ34S variation denotes an incongruent physical and chemical interaction of the two sources. Combined analysis of gold contents and sulfur isotopes of the sulfides show that the magmatic hydrothermal solution provides primary metals despite a small quantity that may have been contributed by the sedimentary pyrites. The hydrothermal alteration, thermal metamorphism, trace element concentration in pyrites, and existing aeromagnetic data jointly suggest that the hydrothermal fluid migrated vertically from an intrusion below, along the flexural slip faults, but not laterally from the nearby outcrop of Tianshan stock.

Preliminary Study on HFSE Mineralization in the Peralkaline Granites of Nusab El Balgum Area, South Western Desert, Egypt

Nusab El Balgum mass complex represents one of peralkaline volcanic activity phenomena in the south Western Desert of Egypt, which is typical for within-plate event, which formed in Mesozoic period. It consists of acidic volcanic (rhyolite and their pyroclastics) and sub-volcanic granitic rocks (incomplete ring, small stock and dyke of a peralkaline aphanites) as well as dykes (trachyte, bostonite, rhyodacite, rhyolite and porphyritic rhyolite) variable in thickness and the most of run in NNE-SSW trend. The peralkaline granitic rocks, especially those located at the southwestern part of this mass are characterized by extreme enrichments in HFSE (rare metals such as Zr, Nb, U and Th and REEs) which are the highest concentrations (e.g., >1% Zr, 0.5% Nb and 2.6% total REEs, Y up to 1%, eU up to 300 ppm and eTh up to 1100 ppm). The rare metal bearing minerals are thorite, uranothorite, autunite, amorphous secondary uranium, zircon and ferrocolumbite, while the REEs bearing minerals are bastnaesite, monazite and xenotime. The positive relations in all of the binary diagrams of Zr versus Nb, Y, eU and eTh, Nb versus Y, eU and eTh, Y versus eU and eTh in post magmatic intensely hematised peralkaline granites indicated that, this process is responsible for the enrichment in these HFSE. The chondrite-normalized pattern of high-altered peralkaline granites indicates: 1) higher LREEs enriched pattern (La/Gd = 11.34 and 12.25) means the alteration processes taking place under open system and these rocks evolved from magma of lithospheric rifting, 2) ΔCe 2O, and thus very low viscosity, despite its low temperature (ments, as indicated by strong negative Eu anomalies; and c) it had abundances of HFSE cations. Redistribution of elements took place by post magmatic hydrothermal solutions.

Gold in Magmatic Hydrothermal Solutions and the Rapid Formation of a Giant Ore Deposit

The Ladolam hydrothermal system, on Lihir Island, Papua New Guinea, hosts one of the youngest and largest gold deposits in the world. Several deep (more than 1 kilometer) geothermal wells were drilled beneath the ore bodies to extract water at >275 degrees C and to facilitate open-pit mining. Using a titanium down-hole sampler, we determined that the deep geothermal brine of magmatic origin contains approximately 15 parts per billion gold. At the current gold flux of 24 kilograms per year, this deposit could have formed within approximately 55,000 years. The combination of sustained metal flux and efficient metal precipitation led to the formation of a giant hydrothermal gold deposit in a short period.

Thermodynamic properties of copper chloride complexes and copper transport in magmatic-hydrothermal solutions

The behaviour of copper in hydrothermal waters and brines is poorly known at the pressure–temperature–salinity conditions typical of magmatic-hydrothermal systems, severely limiting our understanding of how much copper can be transported and deposited in such environments. We need to know the identity of relevant copper complexes and have reliable thermodynamic properties for them in order to understand and predict the solubilities of copper-bearing minerals, the partitioning of copper between liquid and vapour and which physico-chemical factors and processes control copper deposition in magmatic-hydrothermal systems. Under saline conditions, copper chloride complexes are likely to be the most important aqueous species of copper and recent experimental studies have shown good agreement for their derived properties up to approximately 300 °C, vapour-saturated pressure and up to approximately 9 m total chloride. There is still a need, however, to have reliable properties for higher temperature and pressure conditions. In this paper we present new equation-of-state parameters and partial molal properties for aqueous Cu(I) chloride complexes (CuCl (aq), CuCl 2 −, CuCl 3 2− and CuCl 4 3−) regressed from experimentally derived log K values derived between 25 and 350 °C and vapour-saturated pressure. The results are used to calculate formation constants for a wide range of temperature and pressure (0–1000 °C and 1–5000 bar). The extrapolation of the properties is tested by calculating chalcopyrite solubilities and comparing them with measured values from Seyfried and Ding (2003) [Seyfried, W.E., Ding, K., 1993. The effect of redox on the relative solubilities of copper and iron in Cl-bearing aqueous fluids at elevated temperatures and pressures: an experimental study with application to subseafloor hydrothermal systems. Geochimca et Cosmochimca Acta 57, 1905–1917]; 400 °C, 500 bars, 0.4–2 m chloride and Hemley et al. (1992) [Hemley, J.J., Cygan, G.L., Fein, J.B., Robinson, G.R., Jr., D'Angelo, W.M., 1992. Hydrothermal ore-forming processes in the light of studies in rock-buffered systems. I, Iron–copper–zinc–lead sulfide solubility relations. Econ. Geol. 87, 1–22]; 300–500 °C, 500–2000 bars, 1 m chloride. There is good agreement with these two experimental datasets, which indicates that our extrapolated thermodynamic properties are reliable at least over these ranges of pressure, temperature and chloride concentration. The new properties are used to calculate chalcopyrite solubilities under similar conditions of two magmatic-hydrothermal copper deposits for which copper concentrations have been measured in individual fluid inclusions: the Starra iron oxide–Au–Cu deposit, Australia; and the Bajo de la Alumbrera porphyry copper deposit, Argentina. In both cases our calculated copper concentrations are consistent with the measured values in inclusions that trapped pre-mineralising and mineralising fluids. More generally, calculated chalcopyrite solubility at different temperature, pressure, pH, chloride concentrations and oxidation states indicates that hypersaline, neutral-weak acidic, and intermediate-reduced brines can transport thousands of ppm copper at 400 °C and above, raising interesting questions about the interpretation and importance of liquid-vapour partitioning of copper. Although cooling is probably the major factor responsible for copper deposition in magmatic-hydrothermal environments, fluid mixing, boiling and fluid–water interaction may be more important in other geological environments, e.g., iron–oxide–copper–gold and epithermal ore deposits.

Isotope geochemistry of gold ore deposits in the Gezhen shear zone, Qiongxi, Hainan Island

Gold deposits hosted in the Gezhen shear zone at Qingxi, Hainan Island occur in the Precambrian metamorphic rock series and are regionally developed in the N-E direction along the tectonic zone. From northeast to southwest are distributed the Tuwaishan-Baoban gold mining district, the Erjia gold mining district and the Bumo gold mining district, making up the most industrially important gold metallogenesis zone on the Hainan Island. Isotope geochemical studies of the typical gold deposits in this metallogenesis zone indicate that their ore-forming materials stemmed largely from the Baoban Group migmatite series, though the involvement of some plutonic materials could not be ruled out. The ore fluids are the mixture of migmatitized hydrothermal solutions and meteoric waters in addition to the involvement of local magmatic hydrothermal solutions. The superimposition of plutonic materials and magmatic hydrothermal solutions is controlled by the deformation environment of the shear zone and later magmatic activities. Obvious variations are noticed in isotopic composition in the region studied, probably related to tectonic deformation, metamorphism and other evolutionary characteristics. This study is of great significance in understanding the relationship between the shear zone and gold metallogenesis, the rules of gold metallogenesis and gold ore prognosis.

Precious-metal distribution and fluid-inclusion petrography of the Elatsite porphyry copper deposit, Bulgaria

The Elatsite porphyry copper deposit occurs in an island-arc setting hosted by Late Cretaceous monzonitic-monzodioritic porphyry stocks which were emplaced into Precambrian-Cambrian phyllites. Trace element data of the Late Cretaceous intrusive rocks suggest that they are I-type volcanic arc granitoids. Two main ore mineral assemblages are distinguished: (1) magnetite–bornite–chalcopyrite, and (2) chalcopyrite–pyrite. The first one is linked to potassic-propylitic, and the second to phyllic-argillic alteration. Minor ore minerals are hematite, molybdenite, sphalerite, pyrrhotite, marcasite, hessite, and solid solutions of linnaeite–siegenite–carrollite, tetrahedrite–tennantite, clausthalite–galena, gold–electrum and merenskyite–moncheite. Precious-metal contents are relatively high throughout the deposit but Au, Pd and Pt are concentrated more strongly in the magnetite–bornite–chalcopyrite assemblage. Average grades of Au, Ag, Pd and Pt calculated for the 0.33% Cu ore body are 0.96, 0.19, 0.007 and 0.002 g/t respectively. Analyses of flotation concentrates revealed 25.6% Cu, and Ag, Au, Pd and Pt contents of 33.0, 13.6, 0.72 and 0.15 g/t respectively. The copper mineralisation at Elatsite took place at pressures of 120 to 300 bar, corresponding to depths of formation of 1 to 3 km under hydrostatic conditions. The precious metals were probably transported jointly as chloride complexes in highly saline magmatic-hydrothermal solutions. The fluids had temperatures of 340 to >700 °C and salinities of 28 to 64% NaCl, and mixed with meteoric water.

Geochemical characteristics of gold polymetallic deposits as exemplified by the Laozuoshan deposit, Heilongjiang, China

From the studies of ore deposit geologic settings, sulfur isotopes, lead isotopes, carbon isotopes and oxygen isotopes, fluid inclusions and petrochemistry in this paper, the authors have drawn a conclusion that the ore-forming hydrothermal solutions are the high-temperature magmatic hydrothermal solutions for the gold ore deposit, and at the same time, the involvement of crustal materials can not be ruled out. It is the first time that the authors have proposed that the Laozuoshan gold-polymetallic ore deposit in Heilongjiang Province was formed in the calc-alkaline series environment at the margin of an active continent.

Geochronology of Gold Deposits and Its Implication for Metallogenesis in the Fengxian‐Lixian Area, Qinling Orogenic Belt, China

Abstract: A series of super large‐scale and large‐scale Pb and Zn, and Au deposits are distributed in the Qinling orogenic belt, China. Gold deposits were generally ascribed to Carlin‐type originated from circular meteoric water. Visible and coarse‐grained gold (up to over 3mm in grain size) was recently identified in some gold deposits in the Fengxian‐Lixian area, Qinling. Au‐bearing quartz lodes related to magmatism were discovered in the Xiaogouli gold deposit. Two types of Au‐bearing quartz veins, i.e., NW‐trending quartz veins and NE‐trending quartz veins cutting strata are widely present in the Baguamiao gold deposit. Both are spatially associated with each other. The former is generally snake–like, S‐shape or zigzag, which was resulted from plastic deformation by ductile shearing, being generally cut by the latter. The latter is generally linear with widely developed bleaching alteration zones in its adjacent wall rocks, which symbolizes the superimposition of brittle deformation and filling and metasomatism of magmatic hydrothermal solution in ductile shear zones after uplifting of the shear zones near the surface. The NW‐trending quartz veins contain Au of lower than 3ppm. The NE‐trending quartz veins contain Au of more than 3 ppm, so that NE‐trending quartz veins and the adjoining altered rocks are important ores. The NW‐trending Au–bearing quartz vein was dated as 210.61.26 to 232.581.59 Ma by 40Ar–39Ar method, i.e., late Indosinian epoch (Triassic). The NE‐trending Au–bearing quartz vein was dated as 131.910.89 to 197.451.13 Ma by 40Ar–39Ar method, i.e., Yanshanian epoch (Jurassic). The 40Ar–39Ar age of the NW‐trending Au–bearing quartz veins represents the age of the ductile shear formation. The isotope data of the NE‐trending quartz veins indicate that gold mineralization was closely related to Indosinian and Yanshanian granite intrusives not only in time and space, but also in origin.

A Preliminary Study on Exhalative Mineralization in Permian Basins, the Southern Segment of the Da Hinggan Mountains, China – Case Studies of the Huanggang and Dajing Deposits–

Abstract: The southern segment of the Da Hinggan Mountains is a well‐known tin–polymetallic metallogenic belt of North China with Jurassic‐Cretaceous volcanic–plutonic rocks widespread. Principally because of this, most of the deposits are regarded as epigenetic hydrothermal deposits in genetic connection with the Mesozoic magmatism. But nearly 90 % of the deposits occur in Permian strata, and show concordant stratiform mineralization with a spatial distribution constrained by sedimentary facies of the Permian strata. A close association between mineralization and Permian strata is recognizable.The Huanggang Fe‐Sn deposit was regarded as a standard skarn‐type deposit formed by magmatic hydrothermal solutions in connection with Mesozoic granites. But there are abundant fabrics indicating submarine hydrothermal exhalation both in magnetite ores and in skarns, including bedding/lamination, soft–deformation, synsedimentary brecciation, and collo‐form fabrics. The magnetite orebodies and skarn‐bodies are predominantly concordant stratiform, and extend nearly 20 km along certain stratigraphic horizon, that is, the upper section of the Lower‐Permian submarine volcanic rocks. The Mesozoic granitic rocks crosscut the magnetite and skarn zone. Instead of skarnization, they show strong greisenization associated with cassiterite‐quartz veins, distinct from the magnetite skarn‐ore with disseminated tin in the Permian rocks.The Dajing Sn‐polymetallic deposit is generally regarded as subvolcanic‐hydrothermal origin, principally because of the close spatial association between ores and some of the Mesozoic subvolcanic dikes (called rhyolitic porphyry). Detailed geological, fabric, petrographical and mineralogical study demonstrates that this very kind of subvolcanic rocks is actually a new type of exhalites (called ‘siderite‐sericite chert’ according to its mineral assemblage), formed by hydrothermal sedimentation during the evolution of the Later‐Permian lacustrine basin. There are, however, indeed some rhyolitic porphyry dikes that crosscut orebod–ies. The orebodies and their associated exhalite predate, and thus have no genetic relation, to the Mesozoic magmatic process.We thus conclude that subaqueous exhalative mineralization did occur during the basin evolution at the Permian time in the southern segment of the Da Hinggan Mountains, which is ignored and poorly understood, but might be as important as the hydrothermal mineralization connected with the Mesozoic magmatism.

Dolomite Formation in Caribbean Islands: Driven by Plate Tectonics?!

ABSTRACT Postdepositional dolomite occurs in the shallow subsurface of several Caribbean Islands in Tertiary and Pleistocene carbonates, e.g., on Barbados, Grand Cayman, Jamaica, and St. Croix. Recent investigations have interpreted these dolomites as relatively early-diagenetic formations from chemically modified mainly on the basis of petrography, stratigraphic association, and stable isotope values. Interpretations of the hydrologic regimes during dolomitization vary and usually include freshwater-seawater mixing, thermal or density-driven convection of seawater, or a combination thereof. This paper offers two alternative interpretations based on the relatively low Sr isotope ratios of the dolomites (0.7085 to 0.7092) and their proximity to active plate margins. Dolomitization may have been driven by plate tectonics, i.e., either by thermal convection of pore fluids at active spreading centers or by tectonic expulsion of fluids from accretionary prisms. The dolomitizing solution was seawater that was enriched in 86Sr by magmatic-hydrothermal solutions or by interaction with siliciclastics in an accretionary prism, respectively.

Research on fluid inclusions of the Yinshan polymetallic deposit, Jiangxi Province, China

In this paper some new advances in the study of fluid inclusions of the Yinshan deposit in Jiangxi Province, China, are presented. The fluid inclusions of the deposit can be divided into four types: 1) gas-rich inclusions; 2) liquid-rich inclusion; 3) salt-saturated H2O inclusion; and 4) single-phase or multiphase CO2-rich inclusion. Homogenization temperatures of the fluid inclusions range from 100 to 520°C, mainly in the range 120–400°C. The daughter crystal-bearing multiphase inclusions present in quartz porphyries (including Nos. 13 and 4 dikes) have been observed. In ore veins, some daughter crystal-bearing multiphase inclusions have been observed, too. On the basis of the results of fluid inclusion research, the authors suggest that the ore-forming fluid must have a close genetic relation to magmatic hydrothermal solution. The ore-forming fluid was dominantly originated from the depth and the mixing of magmatic solution with meteoric water was enhanced during ascending of the fluid, accompanying immiscibility and boiling of fluid. There may be a concealed igneous body at the depth of the Yinshan deposit, which furnishes the scientific basis for metallogenic prognosis at the depth of the Yinshan deposit.

Petrogenetic evolution of felsic volcanic sequences associated with Phanerozoic volcanic-hosted massive sulphide systems: the role of extensional geodynamics

Felsic-volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension. However, empirical geochemical links between felsic volcanism, geotectonic environment, and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic-hosted massive sulphide (VHMS) mineralization, which is important considering most VHMS deposits are hosted in felsic volcanic-bearing sequences. Using felsic geochemical data from six different VHMS mineralized Phanerozoic geotectonic settings, it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas, which in turn helps in stratigraphic and geotectonic interpretations for detailed VHMS exploration. In extensional geodynamic settings, bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral (high-stress) regimes in calc–alkaline, arc-building environments. The empirical geochemical–metallogenic (VHMS) relationships evident in the Archean felsic volcanic rocks, in particular, the low Zr/Y (<7) and low La N/Yb N (<6), are also apparent in the Proterozoic and Phanerozoic felsic volcanic rocks. These low ratios are consistent with minimal fractional crystallization of hornblende (±clinopyroxene) through much of the lower and middle crust, (i.e., except the subvolcanic environment) and are related to low crustal residence times of magmas in extensional environments (i.e., rapid adiabatic ascent of magmas), low oxygen fugacities and low water fugacities of the magmas. These elemental ratios should closely reflect partial melting reactions that are a function of P, T, p(H 2O) conditions in different source areas and geotectonic environments. They seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings, because of the limited degree of assimilation and fractional crystallization (AFC) affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment. Within-plate felsic (or A-type/`Anorogenic' type) and anomalous ocean ridge (OR-type) felsic magmas that have tholeiitic affinities (relatively low alkali elements) are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting, in part, a relationship to extension. The high contents of high-field-strength elements (Zr, Ga, LREE, HREE, Y, Nb, etc.) and other lines of evidence indicate they are relatively high-temperature, relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic (simatic) or continental (sialic) crust (low P granulites or amphibolites), but also in some cases possibly mantle, rocks such that they are significantly water undersaturated during melting (i.e., a(H 2O)≪1). Reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels (diking vs. diapirism). Higher rates of heat advection from mantle mafic magmas into the thickened oceanic and/or continental crust during lithospheric extension are needed to facilitate melting of these rock to form A-type and/or OR-type partial melts (tholeiitic felsic). This usually results in bimodal magmatism as magma mixing is not favored in extensional settings, especially at high magma production, intrusion, and extrusion rates, i.e., high heat flow. This would inhibit ferromagnesian phase, in particular hornblende, and magnetite fractionation (especially at low fO 2) in the felsic magmas that would deplete the melt in Y, HREE, as well as ore-related elements, particularly Zn, which is 2 to 3 times higher in A-type and OR-type rocks compared to other magma types. Therefore, the empirical felsic igneous geochemical–metallogenic relationships observed by others [low Zr/Y (<7) and La N/Yb N (<5) with VHMS mineralization] may simply represent the degree of fractional crystallization of ferromagnesian phases including magnetite and residence time in the deep to middle crust (i.e., rapid ascent rates) in extensional environments. The relationship between extensional geodynamic settings in intra-continental and intra-oceanic intra-arc, inter-arc, and back-arc rift environments and VHMS formation is simply related to extensional faulting, subsidence and submergence, high rates of heat advection to the near-surface environment and resultant seawater convection/reaction, and, in some cases, ocean basin redox conditions and possible contributions of magmatic–hydrothermal solutions.

Geological and Geochemical Characteristics of the Hetai Gold Deposit, South China: Gold Mineralization in an Auriferous Shear Zone

The Hetai gold deposit is contained within a ductile shear zone in low-rank metamorphosed rocks of the late Proterozoic Yunkai Group. The ores occur as veins or as lenticular bodies within altered mylonite and ultramylonite host rocks. Native gold is disseminated in silicified, pyritized, and sericitized phyllonites. The auriferous shear zone has undergone two stages of evolution. The early stage was characterized by ductile deformation during which auriferous mylonite containing n x 106 Au was formed. The later stage was developed in brittle-ductile and brittle environments in which magmatic hydrothermal mineralization was superimposed, resulting in local gold enrichments. Studies of fluid inclusions show that the ore-forming temperature varied from 200° C to 280° C, the pressure was 57.4 MPa, and f(O2) varied from 10-37 to 10-39. The ore-forming fluids are rich in HCO3− and CO2, and have a relative proportion of Na+ > K+ > Ca2+ and HCO3− > Cl− > F−> ΣS. The δDH2O of inclusion water ranges from −57 %o to −82 %o and the from 5.71 ‰ to 4.28 ‰, implying multiple sources for the ore-forming fluids. The Pb-isotope composition of ores suggests that the Pb came from the upper crust-i.e., mainly from metamorphosed rocks and granites. The δ34S has a range of −3.06‰ to −1.73 ‰ and is in agreement with the δ34S value of the wall-rock mylonite, indicating a wall-rock source. On the basis of the above characteristics, the authors propose the following metallogenic model for the Hetai gold deposit: (1) early ductile shear deformation and metamorphism in the Hercynian-Indosinian epoch formed altered mylonite-type gold deposits; (2) the superimposition of a magmatic hydrothermal solution occurred in the late Indosinian epoch and resulted in local gold enrichment.