GC–MS analysis of crocetane,phytane and some of their stereoisomers using cyclodextrin-based stationary phases

The detection and complete separation of crocetane (2,6,11,15-tetramethylhexadecane) and phytane (2,6,10,14-tetramethylhexadecane), two widely investigated hydrocarbon biomarkers for archaea in geological environments, are reported. The essential components of the analysis are a series of derivatized cyclodextrin capillary GC stationary phases. In addition, the further separation of stereoisomers of both compounds was observed on the highly selective permethyl-2-hydroxypropyl-β-cyclodextrin stationary phase. In Nature, as in the laboratory, the synthesis (diagenesis) of these biomarkers determines their structures and stereochemistry. Hence, methods that allow their characterization and quantitation provide useful geochemical information.     

LA–ICP–MS U–Pb Geochronology of Detrital Zircon in the Guanzhong Basin, China and Its Tectonic Response

正Objective The Guanzhong Basin in the transitional zone of the Qinling orogenic belt and the southern margin of the Ordos Basin has been extensively studied in recent years.Although some results have been obtained,some problems such as whether the materials from the North China craton and the Qinling orogenic belt are detrital sedimentary rocks of the Guanzhong Basin still remain unresolved.     

Cadmium and sulfur isotopic compositions of the Tianbaoshan Zn–Pb–Cd deposit,Sichuan Province,China

Although Zn–Pb deposits are one of the most important Cd reservoirs in the earth, few studies have focused on the Cd isotopic fractionation in Zn–Pb hydrothermal systems. This study investigates the causes and consequences of cadmium and sulfur isotope fractionation in a large hydrothermal system at the Tianbaoshan Zn–Pb–Cd deposit from the Sichuan–Yunnan–Guizhou (SYG) metallogenic province, SW China. Moderate variations in Cd and S isotope compositions have been measured in sphalerite cover a distance of about 78 m. Sphalerite has δ^114/110Cd values ranging from 0.01 to 0.57‰, and sulfides (sphalerite, galena and chalcopyrite) have δ³⁴S_CDT values ranging from 0.2 to 5.0‰. Although δ³⁴S_CDT and δ^114/110Cd values in sphalerites have no regular spatial variations, the δ³⁴S_CDT values in galena and calculated ore-forming fluid temperatures decreased from 2.1 to 0.2‰ and from about 290 to 130 °C, respectively, from the bottom to the top of the deposit. Heavy Cd isotopes are enriched in early precipitated sphalerite in contrast to previous studies. We suggest that Cd isotopic compositions in ore-forming fluids are heterogeneous, which result in heavy Cd isotope enrichment in early precipitated sphalerite. In comparison with other Zn–Pb deposits in the SYG area, the Tianbaoshan deposit has moderate Cd contents and small isotope fractionation, suggesting differences in origin to other Zn–Pb deposits in the SYG province.In the Tianbaoshan deposit, the calculated δ³⁴S∑_S-fluids value is 4.2‰, which is not only higher than the mantle-derived magmatic sulfur (0 ± 3‰), but also quite lower than those of Ediacaran marine sulfates (about 30 to 35‰). Thus, we suggest that reduced sulfur of ore-forming fluids in the deposit was mainly derived from the leaching of the basement, which contains large amount of volcanic or intrusive rocks. Based upon a combination of Cd and S isotopic systems, the Tianbaoshan deposit has different geochemical characteristics from typical Zn–Pb deposits (e.g., the Huize deposit) in SYG area, indicating the unique origin of this deposit.     

Sources of pollution and distribution of Pb,Cd and Hg in Wrocław soils: Insight from chemical and Pb isotope composition

Human activities in cities affect properties of urban soils. In particular, urban soils often contain high contents of harmful metals even in deeper horizons added to them from diverse sources over centuries of the city development. This is reflected in complex distribution of metals in bulk soils with depth and the complex metal fractionation, but the exact sources of the metals are difficult to identify. This is also the situation in soils from Wroclaw, one of the largest cities in Poland. Potentially harmful elements Pb, Cd and Hg were examined in six profiles located along the major communication route and compared to two non-urban soils profiles located close to the same route. In all of the urban profiles, Pb and Cd exceeded the element contents observed in non-urban profiles and showed an erratic distribution compared to the more predictable one in non-urban soils. The differences between urban and non-urban profiles were explained as the result of contamination coming from more pollution sources in the case of urban soils, the conclusion supported by Pb isotope analyses. In fact, Pb isotopes showed that the contamination sources in urban soils included leaded petrol, coal combustion, smelting and possibly old pre-industrial ore processing, whereas leaded petrol and pre-industrial lead were the only possible anthropogenic sources in non-urban soils. The comparison of Wrocöaw soils with those from cities of comparable size Kraków and Poznań show similar ranges of metal contents with implication that urban pollution oversteps diverse geogenic chemical background. On the other hand, the comparison with other European cities show large variability in metal contents and suggest that urban soils contamination is time integrated and reflects long-term industrial evolution of each country.     

The geological significance of Pb–Bi- and Pb–Sb-sulphosalts in the Damajianshan tungsten polymetallic deposit,Yunnan Province,China

The Sanjiang Tethyan domain in SE Asia is one of the most important mineral belts in China. Cu, Pb–Zn, Ag, Au and Sn are the most important resources in this domain, while the tungsten mineralization is poorly reported. In this study, we report on mineralogy in recent discovered Damajianshan (DMJS) tungsten (–Cu–As–Mo–Bi) polymetallic deposit in the southern part of Sanjiang Tethyan domain related to Triassic quartz porphyry. Studies have shown that besides common ore minerals, such as native bismuth, bismuthinite, ikunolite, some specific minerals of Pb–Bi- and Pb–Sb-sulphosalts (e.g. izoklakeite, bournonite, cosalite, and boulangerite) have also been found. Based on paragenetic mineral assemblages, fluid inclusions, and thermodynamic studies, the physicochemical conditions were evaluated for the entire metallogenic process. The sulfur fugacity (logf_S2) ranges from − 9.7 to − 37 with ore-forming temperatures between 190 °C and 330 °C, and the oxygen fugacity (logf_O2) ranges from − 37.5 to − 38.5 when the temperature is 250 °C. The sulfur fugacity and oxygen fugacity show strong fluctuations with broadly negative correlation, indicating that these variations in physicochemical conditions should be responsible for mineral assemblages, and are one of the most significant factors leading to the formation of the DMJS deposit. Our mineralogical studies provide new information for tungsten mineralization and further exploration of tungsten resources in the Sanjiang Tethyan mineralization domain.     

Metallogeny of precious and base metal mineralization in the Murchison Greenstone Belt, South Africa: indications from U–Pb and Pb–Pb geochronology

The 3.09 to 2.97 Ga Murchison Greenstone Belt is an important metallotect in the northern Kaapvaal Craton (South Africa), hosting several precious and base metal deposits. Central to the metallotect is the Antimony Line, striking ENE for over 35?km, which hosts a series of structurally controlled Sb–Au deposits. To the north of the Antimony Line, hosted within felsic volcanic rocks, is the Copper–Zinc Line where a series of small, ca. 2.97 Ga Cu–Zn volcanogenic massive sulfide (VMS)-type deposits occur. New data are provided for the Malati Pump gold mine, located at the eastern end of the Antimony Line. Crystallizations of a granodiorite in the Malati Pump Mine and of the Baderoukwe granodiorite are dated at 2,964?±?7 and 2,970?±?7?Ma, respectively (zircon U–Pb), while pyrite associated with gold mineralization yielded a Pb–Pb age of 2,967?±?48?Ma. Therefore, granodiorite emplacement, sulfide mineral deposition and gold mineralization all happened at ca. 2.97?Ga. It is, thus, suggested that the major styles of orogenic Au–Sb and the Cu–Zn VMS mineralization in the Murchison Greenstone Belt are contemporaneous and that the formation of meso- to epithermal Au–Sb mineralization at fairly shallow levels was accompanied by submarine extrusion of felsic volcanic rocks to form associated Cu–Zn VMS mineralization.     

Mantle and crustal sources of Archean anorthosite: a combined in situ isotopic study of Pb–Pb in plagioclase and Lu–Hf in zircon

Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth’s mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase (An_75–89) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenæsset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U–Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenæsset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ε_Hf units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenæsset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenæsset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the surface to date, could still be present within the Fiskenæsset and Nunataarsuk regions.     

Enrichment of Cd and Pb and associations with grain size in sediments in Nanjing alluvial island of Yangtze River

Intermittence belts of metal contaminated sediment have been formed along the urban riverside of the Yangtze River because of rapid urbanization and industrial development during the last three decades. Baguazhou Island, the third alluvial island in the lower reaches of the Yangtze River, was chosen to assess the contaminations level of Cd, Pb, Ni, Cr, Cu and Zn in suspended particulate matter （SPM）, recent sediments and sediment-derived soils. The relations between the metal concentrations and the grain size distributions in sediments were analyzed in order to explore the association of heavy metal settling with sediment deposition. The results showed that significantly high levels of Cd and Pb in SPM dominated enrichment factors （EFs） of recent sediments in the area of Baguazhou Island. High levels of Cd and Pb were found both in the riverine sediments and in the soils. However, the average levels of Cd and Pb in sediments were almost as twice as those in soil. About 43% of all sediment samples have EFCd≥1.5 and 14% with EFCd≥2. There are about 21% of the samples with EFPb≥1.5 and none of them has EFPb≥2. Meanwhile, soil samples with EFCd≥1.5 account for 39% of all 64 samples and no one with EFPb≥1.0. Most of the sediment samples contain more than 80% of fine grains lower than 60 μm in size, and few samples located in the site where flow eroding process dominates contain fine grains lower than 30%. Correlation analysis indicated that the concentrations of Cd and Pb in the sediments have significant relationships with grain size.     

S,Pb, and Sr isotope geochemistry and genesis of Pb–Zn mineralization in the Huangshaping polymetallic ore deposit of southern Hunan Province,China

The Huangshaping Pb–Zn–W–Mo polymetallic deposit, located in southern Hunan Province, China, is one of the largest deposits in the region and is unique for its metals combination of Pb–Zn–W–Mo and the occurrence of significant reserves of all these metals. The deposit contains disseminated scheelite and molybdenite within a skarn zone located between Jurassic granitoids and Carboniferous sedimentary carbonate, and sulfide ores located within distal carbonate-hosted stratiform orebodies. The metals and fluids that formed the W–Mo mineralization were derived from granitoids, as indicated by their close spatial and temporal relationships. However, the source of the Pb–Zn mineralization in this deposit remains controversial.Here, we present new sulfur, lead, and strontium isotope data of sulfide minerals (pyrrhotite, sphalerite, galena, and pyrite) from the Pb–Zn mineralization within the deposit, and these data are compared with those of granitoids and sedimentary carbonate in the Huangshaping deposit, thereby providing insights into the genesis of the Pb–Zn mineralization. These data indicate that the sulfide ores from deep levels in the Huangshaping deposit have lower and more consistent δ³⁴S values (− 96 m level: + 4.4‰ to + 6.6‰, n = 13) than sulfides within the shallow part of the deposit (20 m level: + 8.3‰ to + 16.3‰, n = 19). The δ³⁴S values of deep sulfides are compositionally similar to those of magmatic sulfur within southern Hunan Province, whereas the shallower sulfides most likely contain reduced sulfur derived from evaporite sediments. The sulfide ores in the Huangshaping deposit have initial ⁸⁷Sr/⁸⁶Sr ratios (0.707662–0.709846) that lie between the values of granitoids (0.709654–0.718271) and sedimentary carbonate (0.707484–0.708034) in the Huangshaping deposit, but the ratios decreased with time, indicating that the ore-forming fluids were a combination of magmatic and formation-derived fluids, with the influence of the latter increasing over time. The lead isotopic compositions of sulfide ores do not correlate with sulfide type and define a linear trend in a ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagram that is distinct from the composition of the disseminated pyrite within sedimentary carbonates and granitoids in the Huangshaping deposit, but is similar to the lead isotopic composition of sulfides within coeval skarn Pb–Zn deposits in southern Hunan Province. In addition, the sulfide ores have old signatures with relative high ²⁰⁷Pb/²⁰⁶Pb ratios, suggesting that the underlying Paleoproterozoic basement within southern Hunan Province may be the source of metals within the Huangshaping deposit.The isotope geochemistry of sulfide ores in the Huangshaping deposit shows a remarkable mixed source of sulfur and ore-forming fluids, and the metals were derived from the basement. These features are not found in representative skarn-type Pb–Zn mineralization located elsewhere. The ore-forming elements (S, Pb, and Zn) from the granitoids made an insignificant contribution to sulfide precipitation in this deposit. However, the emplacement of granitoids did provide large amounts of heat and fluids to the hydrothermal system in this area and extracted metals from the basement rocks, indicating that the Jurassic magmatism associated with the Huangshaping deposit was crucial to the Pb–Zn mineralization.     

In situ LA–MC–ICP–MS boron isotope and zircon U–Pb age determinations of Paleoproterozoic borate deposits in Liaoning Province,northeastern China

A large number of Paleoproterozoic borate deposits are hosted by the lower units of a volcanic-sedimentary sequence in Liaoning Province, northeastern China, and are a major source of boron in China. The ore-bearing wall rocks in the deposits are serpentinized ultrabasic rocks and carbonates, with layered leptynites, leptites, amphibolites, and migmatites adjacent to the ore. Both the borate ores and country rocks contain tourmaline, although the country rocks have much lower abundances of the mineral. Based on in situ boron isotope measurements using laser ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS), boron isotope data show that: (1) δ¹¹B values of borate ores range from + 6.8‰ to + 13.9‰ (mean + 10.8‰); (2) tourmalines from the borate ores have δ¹¹B values from + 9.5‰ to + 12.7‰; and (3) the wall rocks within the borate ores yield slightly lower δ¹¹B values ranging from + 5.7‰ to + 7.6‰, and those outside the deposits from − 9.9‰ to − 5.9‰. Positive δ¹¹B values in borates as well as in tourmalines inside the mining area indicate that boron in these Paleoproterozoic borate deposits was derived from marine evaporites. δ³⁴S_V-CDT (where V-CDT is Vienna Canyon Diablo Troilite) values of borate ores, serpentinized marbles, and anhydrites range from + 16.1‰ to + 24.7‰, whereas δ¹³C_V-PDB (where V-PDB is Vienna Pee Dee Belemnite) values of marbles range from + 3.2‰ to + 5.9‰. These isotopic characteristics are interpreted to reflect formation in a marine evaporative environment. LA–MC–ICP–MS zircon weighted²⁰⁷Pb/²⁰⁶Pb ages of leptite and serpentinized olivine basalt from the hanging wall of the borate deposits are 2139 ± 13 Ma and 2130 ± 19 Ma, respectively. Therefore, the (~ 2.2 Ga) borate deposits may have originated from marine evaporative boron-bearing sediments, which were interbedded within bimodal volcanic rocks during the early stages of development of the Liaoji rift.     

Tracing the ore-formation history of the shear-zone-controlled Huogeqi Cu–Pb–Zn deposit in Inner Mongolia,northern China,using H,O, S,and Fe isotopes

The original ore-fluid of the Huogeqi Cu–Pb–Zn deposit in Inner Mongolia, northern China, was enriched in heavy oxygen isotopes with δ¹⁸O values ranging from 9.9 to 11.4 per mil, which is characteristic of the metamorphic devolatilization of pelitic rocks. The δD values determined by direct measurement of syn-ore hydrothermal tremolite range from − 116 to − 82 per mil, lying between the domains of typical metamorphic fluid and meteoric water, which is in equilibrium with organic matter. Oxygen and hydrogen isotope ratios indicate that the ore-fluid was derived from deep-sourced metamorphic fluid and interacted with organic-rich shale during fluid migration, which is consistent with the fluid evolution history revealed by a previous fluid inclusion study. Sulfides in the deposit are characteristically enriched in heavy S isotopes, with an average δ³⁴S value of 13.4 ± 6.2 per mil (1σ, n = 103). The S-isotope ratios are identical to stratabound sulfides generated through the non-bacterial reduction of Neoproterozoic marine sulfate (with δ³⁴S values of ~ 17 per mil). Previous studies on lead isotopes of sulfides revealed that the ore-forming metals (Cu, Pb, and Zn) at the Huogeqi deposit were also remobilized from a stratabound source. This source was syngenetically elevated in its Cu-, Pb-, and Zn-sulfide content as a result of submarine hydrothermal activities forming sulfide-rich layers within a rift tectonic setting. The Fe isotope ratios for sulfides are consistent with those of an intercalated iron-formation within the ore-hosting rocks, suggesting that the Fe in the sulfides was derived from local host rocks during sulfide precipitation and the Fe-rich rocks are favorable lithological units for high-grade mineralization. The heterogeneous sources of ore-fluid, S, ore-forming metals, and Fe are explained by a multistage genetic model, which is supported by the geological characteristic of the deposit. The enriched sulfides were subsequently remobilized and enriched by metamorphic devolatilization during the Permian and Triassic periods. The metamorphic ore-fluid ascended along a shear zone and interacted with organic-rich shale. Sulfides eventually precipitated within the shear zone at a shallower crustal level, especially where the shear zone intersected Fe-rich host rocks. This multistage genetic model has implications for mineral exploration. Greenschist to amphibolite facies terranes containing thick Neoproterozoic rift sequences are ideal regions for potential Cu–Pb–Zn mineralization. In particular, intercalated volcanic rocks within the rift sequences are indicative of high heat-flow and are ideal for the development of submarine hydrothermal systems. The primary structures hosting mineralization and ore shoots in the Huogeqi area are jogs in the shear zones. In addition, Fe-rich lithological units, such as iron-formations, are ideal hosts for high-grade ore.     

Tracing sources of pollution in soils from the Jinding Pb–Zn mining district in China using cadmium and lead isotopes

Systematic variations in the Cd and Pb isotope ratios in polluted topsoils surrounding the Jinding Pb–Zn mine in China were measured so that the sources of the metals could be traced. The average δ^114/110Cd value and ²⁰⁶Pb/²⁰⁷Pb isotope ratio in background soils from the region were +0.41‰ and 1.1902, respectively, whereas the contaminated soil samples had different values, with the δ^114/110Cd values varying between −0.59‰ and +0.33‰ and the ²⁰⁶Pb/²⁰⁷Pb isotope ratios varying between 1.1764 and 1.1896. We also measured the Cd and Pb isotopic compositions in oxide ores, sulfide ores, and slags, and found that binary mixing between ores and background soils could explain almost all of the variations in the Cd and Pb isotope ratios in the contaminated soils. This suggests that Cd and Pb pollution in the soils was mainly caused by the deposition of dust emitted during anthropogenic activities (mining and refining). The Pb and Cd isotope ratios clearly showed that contamination in soils in the northeastern part of the area was caused by surface mines and zinc smelters and their slagheaps, while contamination in soils in the southwestern part of the area also came from tailing ponds and underground mines. The main area of soil polluted by dust from Pb–Zn mining processes roughly extended for up to 5 km from the mine itself.     

Estimation of the rupture velocity and fault length of the 2004 Sumatra–Andaman earthquake using a dense broadband seismic array in Taiwan

The rupture process of the disastrous Sumatra–Andaman earthquake of 26 December 2004 was analyzed by array processes for teleseismic P-waves recorded by a dense broadband seismic array in Taiwan with epicentral distances of close to 31°. The azimuthal variation from the BATS array center to both ends of the rupture fault is approximately 21°, which is larger than that reported previously for seismic arrays used to image the rupture process of this earthquake, thereby providing a high spatial resolution in studying the source rupture behavior. Two array-processing methods were used to analyze teleseismic P-wave trains. Both analyses were based on data recorded by a broadband network, covering a region of 200 × 400 km, with the aim of evaluating the rupture behavior of the earthquake. Consistent results from both analyses indicate that the earthquake had a rupture duration exceeding 500 s, with major asperities encountered at 80, 260, and 330 s after the initiation of rupturing. We traced the ruptured fault for more than 1200 km from the point of initial rupture. The average rupture velocity was approximately 3.0 km/s and the major northward rupture propagation began at 80 s after the initiation of rupturing.     

U–Pb zircon ages,geochemical and Sr–Nd–Pb isotopic constraints on the dating and origin of intrusive complexes in the Sulu orogen,eastern China

TPost-orogenic intrusive complexes from the Sulu belt of eastern China consist of pyroxene monzonites and dioritic porphyrites. We report new U–Pb zircon ages, geochemical data, and Sr–Nd–Pb isotopic data for these rocks. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 127.4 ± 1.2 Ma for dioritic porphyrites, consistent with crystallization ages (126 Ma) of the associated pyroxene monzonites. The intrusive complexes are characterized by enrichment in light rare earth elements and large ion lithophile elements (i.e. Rb, Ba, Pb, and Th) and depletion in heavy rare earth elements and high field strength elements (i.e. Nb, Ta, P, and Ti), high (⁸⁷Sr/⁸⁶Sr)_i ranging from 0.7083 to 0.7093, low ?_Nd(t) values from ?14.6 to ? 19.2, ²⁰⁶Pb/²⁰⁴Pb = 16.65–17.18, ²⁰⁷Pb/²⁰⁴Pb = 15.33–15.54, and ²⁰⁸Pb/²⁰⁴Pb = 36.83–38.29. Results suggest that these intermediate plutons were derived from different sources. The primary magma-derived pyroxene monzonites resulted from partial melting of enriched mantle hybridized by melts of foundered lower crustal eclogitic materials before magma generation. In contrast, the parental magma of the dioritic porphyrites was derived from partial melting of mafic lower crust beneath the Wulian region induced by the underplating of basaltic magmas. The intrusive complexes may have been generated by subsequent fractionation of clinopyroxene, potassium feldspar, plagioclase, biotite, hornblende, ilmenite, and rutile. Neither was affected by crustal contamination. Combined with previous studies, these findings provide evidence that a Neoproterozoic batholith lies beneath the Wulian region.     

Isotopic composition of Pb and Th in interplinian volcanics from Somma–Vesuvius volcano, Italy

Summary We present a detailed isotopic study of volcanic rocks emitted from Somma–Vesuvius volcano during three periods of interplinian activity: Protohistoric (3550 y B.P. to 79 A.D.), Ancient Historic (79 to 472 A.D.) and Medieval (472 to 1631 A.D.). Pb isotopic compositions of two acid leached fractions and whole rock residues of 37 whole rock samples (determined by Somma et al., 2001) show that each of the three interplinian periods is distinguished by small, systematic, and unique uranogenic and thorogenic Pb isotopic trends. This key and novel feature is compatible with the notion that the Pb isotopic data reflect small-scale source heterogeneity operating over relatively short periods of time. From this representative group of samples, a selected set of nine whole rocks were analysed for Th isotopes. ²³²Th/²³⁸U ratios in the source can be obtained independently from Pb and from Th isotopes. Those obtained from Pb isotopes represent source ratios, time-integrated over the whole age of the Earth; they range from 3.9 to 4.1. ²³²Th/²³⁸U obtained from Th isotopes are those of the present source. They are lower, and cluster around 3.5; this difference probably indicates recent U enrichment of the present source.The behaviour of Pb, as inferred by its isotopic ratios, is quite distinct from that of Sr and Nd isotopes: Pb isotope variations are not correlated to Sr or Nd isotope variations. The isotopic contrast is compatible with the idea that the isotopes were decoupled during magmatic production, evolution, and ascent through the crust. Thus, the Pb isotopes do not reflect the effects of the same processes as in the case of the Sr and Nd isotopes, or, as we also favor, they do not necessarily reflect the same source contributions into the magmas. Moreover, the Pb isotopic evolution of the interplinian rocks chiefly reflects mixing, driven by processes that are superimposed on, and independent of, other source contributions that determine the isotopic compositions of Sr and Nd. We suggest that reactions between magmas and fluids transported Pb and U, but not Sr. These data show that isotope mixing in the mantle is active at different times and scales.     

Zircon U–Pb and molybdenite Re–Os geochronology of copper–molybdenum deposits in southeast Liaoning Province,China

ABSTRACT

Liaoning Province in China is an area known for the occurrence of numerous copper and/or molybdenum deposits of variable size. However, the age of mineralization and tectonic setting in this region are still a subject of debate. In this study we describe the geology of these deposits and apply zircon U–Pb and molybdenite Re–Os isotopic dating to constrain their ages and define the metallogenic epochs of this province. The Huatong Cu–Mo deposit yields molybdenite Re–Os model ages of 127.6–126.3 Ma and an isochron age of 127.4 ± 0.7 Ma. The Dongbeigou Mo deposit yields molybdenite Re–Os model ages of 132.6–127.1 Ma, an isochron age of 128.1 ± 5.1 Ma, and a zircon U–Pb age of 129.4 ± 0.3 Ma for the associated monzogranite. The granodiorite associated with the Wanbaoyuan Cu–Mo deposit yields a zircon U–Pb age of 128.4 ± 1.1 Ma; the plagiogranite associated with the Yaojiagou Mo deposit yields an age of 167.5 ± 0.9 Ma; and the biotite–plagioclase gneiss from the Shujigou Cu deposit yields an age of 2549.4 ± 5.6 Ma. These results, together with previous geochronology data, show that intense Cu–Mo porphyry and skarn mineralization were coeval with Early–Middle Jurassic and Early Cretaceous granitic magmatism. The former was associated with the orogeny that followed the collision of the Siberian and North China plates and the resulting closure of the palaeo-Asian Ocean, and the latter with rifting that followed the subduction of the palaeo-Pacific Plate and associated lithospheric thinning. Volcanogenic massive sulfide Cu deposit. mineralization took place much earlier, in the late Archaean, and was related to continent–continent collision, palaeo-ocean closure, the formation of a united continental landmass, bimodal volcanism, magma emplacement, and subsequent metamorphism and deformation of syn-collisional granites.     

Origin of epithermal Ag–Au–Cu–Pb–Zn mineralization in Guanajuato, Mexico

The Guanajuato epithermal district is one of the largest silver producers in Mexico. Mineralization occurs along three main vein systems trending dominantly northwest–southeast: the central Veta Madre, the La Luz system to the northwest, and the Sierra system to the east. Mineralization consists dominantly of silver sulfides and sulfosalts, base metal sulfides (mostly chalcopyrite, galena, sphalerite, and pyrite), and electrum. There is a broad zonation of metal distribution, with up to 10 % Cu+Pb+Zn in the deeper mines along the northern and central portions of the Veta Madre. Ore occurs in banded veins and breccias and as stockworks, with gangue composed dominantly of quartz and calcite. Host rocks are Mesozoic sedimentary and intrusive igneous rocks and Tertiary volcanic rocks. Most fluid inclusion homogenization temperatures are between 200 and 300 °C, with salinities below 4 wt.% NaCl equivalent. Fluid temperature and salinity decreased with time, from 290 to 240 °C and from 2.5 to 1.1 wt.% NaCl equivalent. Relatively constant fluid inclusion liquid-to-vapor ratios and a trend of decreasing salinity with decreasing temperature and with increasing time suggest dilution of the hydrothermal solutions. However, evidence of boiling (such as quartz and calcite textures and the presence of adularia) is noted along the Veta Madre, particularly at higher elevations. Fluid inclusion and mineralogical evidence for boiling of metal-bearing solutions is found in gold-rich portions of the eastern Sierra system; this part of the system is interpreted as the least eroded part of the district. Oxygen, carbon, and sulfur isotope analysis of host rocks, ore, and gangue minerals and fluid inclusion contents indicate a hydrothermal fluid, with an initial magmatic component that mixed over time with infiltrating meteoric water and underwent exchange with host rocks. Mineral deposition was a result of decreasing activities of sulfur and oxygen, decreasing temperature, increasing pH, and, in places, boiling.     

In-situ LA–ICP–MS U–Pb geochronology and trace elements analysis of polygenetic titanite from the giant Beiya gold–polymetallic deposit in Yunnan Province,Southwest China

The Beiya gold–polymetallic deposit, located in the middle of the Jinshajiang–Ailaoshan alkaline porphyry metallogenic belt, is one of the largest gold deposits in China. The mineralization mainly occurs in skarn along the intrusive contacts between the alkaline porphyries and Middle Triassic limestone. In this paper, we present U–Pb age as well as major and trace element geochemistry of titanite from the Beiya deposit, and distinguish the titanite into a magmatic- and a hydrothermal suite. Our study indicates that the titanite from the ore-related porphyry and from the mineralized skarn is texturally and geochemically very different. The euhedral, envelope-shaped titanite from the ore-related porphyry has lower FeO, F, HFSEs, Nb/Ta and Lu/Hf, together with higher TiO₂ and Th/U than the subhedral titanite from the mineralized skarn. The titanite from the porphyry also displays higher LREE/HREE and more subtle negative Eu anomaly than its mineralized skarn counterpart. This suggests a magmatic- and a hydrothermal origin for, respectively, the titanite from the ore-related porphyry and from the mineralized skarn. In-situ magmatic titanite U–Pb dating has yielded an Eocene age of 36.0 ± 5.9 Ma, consistent with the porphyry zircon U–Pb age (36.07 ± 0.43 Ma) obtained in previous studies. Hydrothermal titanite has yielded a weighted average ²⁰⁶Pb/²³⁸U age of 33.1 ± 1.0 Ma (MSWD = 2.0), which represents the age of the retrograde skarn alteration and the maximum age for the gold mineralization. Together with the previous molybdenite Re–Os age, we have further constrained the Beiya gold–polymetallic metallogeny to 33.1–34.1 Ma. The mineralization age is slightly younger than the porphyry emplacement, indicating that the Beiya metallogeny was likely to be a post-magmatic hydrothermal product of the Himalayan orogenic event. The REE characteristics of hydrothermal titanite also reveal that the ore forming fluids may have been derived from a highly oxidized magma.     

Zircon U–Pb Ages and Lu–Hf Isotope of the Dongchuan Group in Central Yunnan,China, and their Geological Significance

On the southwestern margin of the Yangtze Block, the Dongchuan Group consists of slightly metamorphosed sedimentary rocks, including silty slate, argillaceous slate, clayey slate, arkose, dolomite, and minor volcanic rocks. To date, it is still a controversy over the depositional age and stratigraphic sequence of the Dongchuan Group. In this study, we analyzed five samples of meta-sedimentary rocks and one sample of meta-tuff from the Yinmin, Luoxue and Etouchang Formations of the Dongchuan Grou...