陕西省华阳川铀铌铅矿床碳酸岩岩石学及地球化学特征

陕西省华阳川铀铌铅矿床是小秦岭成矿带中成矿特征最为独特的矿床,碳酸岩脉的破碎带是重要的成矿空间。未矿化的碳酸岩中矿物以方解石为主,其他矿物很少;发育铀矿化的碳酸岩脉中矿物种类繁多,大部分为方解石,其次为角闪石、金云母、榍石、褐帘石、铌钛铀矿、重晶石、磷灰石、石英、磁铁矿、碱性长石等矿物。碳酸岩的LREE含量异常高,δ13CV-PDB和δ18OV-SMOW值显示典型的火成碳酸岩特征。基于碳酸岩脉的Sr、Nd、Pb同位素比值(87Sr/86Sr-206Pb/204Pb、207Pb/204Pb-206Pb/204Pb-143Nd/144Nd-87Sr/86Sr)的关系图,初步判断华阳川铀铌铅碳酸岩脉是源于EMI的碱性硅酸盐-碳酸盐熔体-溶液结晶分异的产物。     

Geochemistry and petrogenesis of carbonatites from South Nam Xe,Lai Chau area,northwest Vietnam

This paper presents a study of the petrography, mineral chemistry, geochemistry, and Sr–Nd–Pb–C–O isotope systematics of carbonatite dykes and associated rocks from the northeastern part of the Song Da intracontinental rift in South Nam Xe (northwest Vietnam) aimed at constraining the origin of the carbonatite magmas. The carbonatites are characterized by SiO₂ < 12.18 wt.% and by wide ranges in FeO, MgO and CaO content that define them as calciocarbonatite and ferrocarbonatite. On U–Th–Pb isochron diagrams, whole rocks and mineral separates from the ferrocarbonatites form linear arrays corresponding to ages of 30.2–31.6 Ma (Rupelian, Oligocene). The South Nam Xe carbonatites are extremely enriched in Sr, Ba, and light rare earth elements (LREE), and depleted in high field strength elements (HFSE) (e.g. Ti, Nb, Ta, Zr and Hf). The age–corrected Sr–Nd–Pb isotope ratios and C isotope data are relatively uniform (⁸⁷Sr/⁸⁶Sr_(t) = 0.708193–0.708349; ¹⁴³Nd/¹⁴⁴Nd_(t) = 0.512250–0.512267; ε_Nd(t) = ?6.46 to ?6.80; ²⁰⁶Pb/²⁰⁴Pb_(t) = 18.26–18.79; ²⁰⁷Pb/²⁰⁴Pb_(t) = 15.62–15.64; ²⁰⁸Pb/²⁰⁴Pb_(t) = 38.80–39.38; δ¹³C_V-PDB = –2.7?‰ to ?4.1?‰). These isotopic compositions indicate source contamination that occurred before the production of the carbonatite magmas, and did not change noticeably during or after emplacement. The variation in oxygen isotopes is consistent with the change in mineral compositions and trace element abundances: the lower δ¹⁸O values (9.1–11.0?‰) coupled with Sr-rich, Mn-poor calcite, and igneous textures such as triple junctions among calcite grain boundaries, define a magmatic origin. However, the elevated δ¹⁸O values of the ferrocarbonatites (12.0–13.3?‰) coupled with a volatile-bearing mineral assemblages (including REE fluorcarbonates, sulfates, sulfides and fluorite) may be due to interaction with meteoric water during low-temperature alteration. High δ¹³C values and Sr–Pb ratios, and low Rb/Sr (0.00014–0.00301), Sm/Nd (0.089–0.141) and ¹⁴³Nd/¹⁴⁴Nd ratios, coupled with very high Sr-Nd concentrations, suggest the involvement of an enriched mantle component, which probably resulted from metasomatism due to the migration of subducted material. Because of the lack of tectonic data and the limited number of samples studied, this conclusion is still ambiguous and requires further study.     

Geochronology and mineralogy of the Weishan carbonatite in Shandong province,eastern China

The Weishan REE deposit is located at the eastern part of North China Craton (NCC), western Shandong Province. The REE-bearing carbonatite occur as veins associated with aegirine syenite. LA-ICP-MS bastnaesite Th-Pb ages (129 Ma) of the Weishan carbonatite show that the carbonatite formed contemporary with the aegirine syenite. Based on the petrographic and geochemical characteristics of calcite, the REE-bearing carbonatite mainly consists of Generation-1 igneous calcite (G-1 calcite) with a small amount of Generation-2 hydrothermal calcite (G-2 calcite). Furthermore, the Weishan apatite is characterized by high Sr, LREE and low Y contents, and the carbonatite is rich in Sr, Ba and LREE contents. The δ¹³C_V-PDB (−6.5‰ to −7.9‰) and δ¹³O_V-SMOW (8.48‰–9.67‰) values are similar to those of primary, mantle-derived carbonatites. The above research supports that the carbonatite of the Weishan REE deposit is igneous carbonatite. Besides, the high Sr/Y, Th/U, Sr and Ba of the apatite indicate that the magma source of the Weishan REE deposit was enriched lithospheric mantle, which have suffered the fluid metasomatism. Taken together with the Mesozoic tectono-magmatic activities, the NW and NWW subduction of Izanagi plate along with lithosphere delamination and thinning of the North China plate support the formation of the Weishan REE deposit. Accordingly, the mineralization model of the Weishan REE deposit was concluded: The spatial-temporal relationships coupled with rare and trace element characteristics for both carbonatite and syenite suggest that the carbonatite melt was separated from the CO₂-rich silicate melt by liquid immiscibility. The G-1 calcites were crystallized from the carbonatite melt, which made the residual melt rich in rare earth elements. Due to the common origin of G-1 and G-2 calcites, the REE-rich magmatic hydrothermal was subsequently separated from the melt. After that, large numbers of rare earth minerals were produced from the magmatic hydrothermal stage.     

Strontium and Lead Isotopic Study of the Carbonate‐hosted Xujiashan Antimony Deposit from Hubei Province,South China: Implications for its Origin

The Xujiashan antimony deposit is hosted by marine carbonates of the Upper Sinian Doushantuo and Dengying Formations in Hubei Province, South China. Our Sr isotopic data from pre‐ and syn‐mineralization calcites that host the mineralization show that the pre‐mineralization calcite displays a narrow range of ⁸⁷Sr/⁸⁶Sr ratios (0.7096 to 0.7097), similar to the ratios of the Sinian seawater, and high Sr concentrations (2645 to 8174 ppm). In contrast, the syn‐mineralization calcite exhibits low Sr concentrations (785 to 2563 ppm) and high ⁸⁷Sr/⁸⁶Sr ratios (0.7109 to 0.7154), which is interpreted as the result of addition of radiogenic strontium during the antimony mineralization. The study of Sr isotopes suggests that their Sr component to the pre‐mineralization calcite derived directly from the host rocks (i.e. the Sinian marine carbonates), while radiogenic ⁸⁷Sr for the syn‐mineralization calcite derived from the underlying Mesoproterozoic Lengjiaxi Group basement through hydrothermal fluid circulation along the major fault that hosts the mineralization. The Pb isotopic ratios of stibnite are subdivided into two groups (Group A and Group B), Group A is characterized by higher radiogenic lead, with ²⁰⁶Pb/²⁰⁴Pb = 18.874 to 19.288, ²⁰⁷Pb/²⁰⁴Pb = 15.708 to 15.805, and ²⁰⁸Pb/²⁰⁴Pb = 38.642 to 39.001. Group B shows lower lead isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 17.882 to 18.171, ²⁰⁷Pb/²⁰⁴Pb = 15.555 to 15.686, and ²⁰⁸Pb/²⁰⁴Pb = 37.950 to 38.340). The single‐stage model ages of Group A are mainly negative or slightly positive values (‐258 to 3 Ma), while those of Group B range from 636 to 392 Ma, with an average of 495 ± 65 Ma. In addition, there are positive linear correlations among Pb isotopic ratios. These results suggest that the lead of Group A stibnite was mainly derived from the Sinian marine carbonates, and that of Group B stibnite from the underlying Lengjiaxi Group basement. This conclusion is consistent with the results of the Sr isotopes. These results indicate that the Xujiashan deposit is not syngenetic sedimentary and in situ reworked origin as previously considered. The metal (mainly Sb) of this deposit was not only derived from the Sinian host rocks, but also partly derived from the underlying Mesoproterozoic Lengjiaxi Group basement.     

Tracing element sources of hydrothermal mineral deposits: REE and Y distribution and Sr-Nd-Pb isotopes in fluorite from MVT deposits in the Pennine Orefield,England

Fluorite from Mississippi Valley Type (MVT) deposits in the South Pennine Orefield, England, displays significantly different distributions of rare earths and yttrium (REY) compared to fluorite from similar MVT deposits in the North Pennine Orefield. Samples from the South Pennine Orefield display negative Ce and positive Gd and Y anomalies but lack any Eu anomaly, indicating that the REY were mobilized from relatively pure marine sedimentary carbonates. In marked contrast, fluorite from the North Pennine Orefield lacks any Ce and Gd anomalies but shows a pronounced positive Eu anomaly, suggesting that the REY were provided by different source rock(s), that the mineralizing hydrothermal fluid had experienced higher temperatures prior to fluorite precipitation, and that it was derived from deeper crustal levels in the north compared to the south. The isotopic composition of Sr in Blue John fluorite from the South Pennine Orefield suggests that Sr was mobilized from Lower Carboniferous (Tournaisian) limestones, whereas Pb isotopes suggest that in contrast to REY and Sr, Pb was derived from aluminosilicate rocks. Neither Nd nor Sr or Pb isotopes can be used to radiometrically date the formation of Blue John fluorite. All isotope systems studied indicate that the limestone host rock of this fluorite mineralization did not contribute to the trace element budget of the hydrothermal fluid. Our results show that different solutes in a natural water (hydrothermal fluid, groundwater, etc.) may be derived from different sources, and that the study of a small set of elements or isotope ratios may not provide full insight into the genesis or history of a mineralization or a hydrothermal fluid. Our data provide evidence for the uncoupling of Sr, Nd and Pb during fluid-rock interaction and fluid migration, and show that the use of plots such as ⁸⁷Sr/⁸⁶Sr vs. _Nd. to learn about mixing relationships (as is commonly done in igneous geochemistry) is unreliable when applied to natural waters and their precipitates.Editorial handling: B. Lehmann     

Isotope Geochemistry of the Pb-Zn-Ba(-Ag-Au) Mineralization at Triades-Galana,Milos Island,Greece

The Pb-Zn-Ba(-Ag-Au) mineralization in the Triades and Galana mine areas is hosted in 2.5–1.4 Ma pyroclastic rocks, and structurally controlled mostly by NE-SW or N-S trending brittle faults. Proximal pervasive silica and distal pervasive sericite-illite alteration are the two main alteration types present at the surface. The distribution of mineralization-alteration in the district suggests at least two hydrothermal events or that hydrothermal activity lasted longer at Galana. The Sr isotope signature of sphalerite and barite (⁸⁷Sr/⁸⁶Sr = 0.709162 to 0.710214) and calculated oxygen isotope composition of a fluid in equilibrium with barite and associated quartz at temperatures of around 230°C are suggestive of a seawater hydrothermal system and fluid/rock interaction. Lead isotope ratios of galena and sphalerite (²⁰⁶Pb/²⁰⁴Pb from 18.8384 to 18.8711; ²⁰⁷Pb/²⁰⁴Pb from 15.6695 to 15.6976; ²⁰⁸Pb/²⁰⁴Pb from 38.9158 to 39.0161) are similar to those of South Aegean Arc volcanic and Aegean Miocene plutonic rocks, and compatible with Pb derived from an igneous source. Galena and sphalerite from Triades-Galana have δ³⁴S_VCDT values ranging from +1 to +3.6‰, whereas barite sulfate shows δ³⁴S_VCDT values from +22.8 to +24.4‰. The sulfur isotope signatures of these minerals are explained by seawater sulfate reduction processes. The new analytical data are consistent with a seawater-dominated hydrothermal system and interaction of the hydrothermal fluid with the country rocks, which are the source of the ore metals.     

Geochronology and fluid source constraints of the Songligou gold‐telluride deposit,western Henan Province,China: Analysis of genetic implications

The Songligou gold‐telluride deposit, located in Songxian County, western Henan Province, China, is one of many gold‐telluride deposits in the Xiaoqinling‐Xiong'ershan district. Gold orebodies occur within the Taihua Supergroup and are controlled by the WNW F101 Fault, and the fault was cut across by a granite porphyry dike. Common minerals in gold orebodies include quartz, chlorite, epidote, K‐feldspar, calcite, fluorite, sericite, phlogopite, bastnasite, pyrite, galena, chalcopyrite, sphalerite, tellurides, gold, bismuthinite, magnetite, and hematite, and pyrite is the dominant sulfide. Four mineralization stages are recognized, including pyrite‐quartz stage (I), quartz‐pyrite stage (II), gold‐telluride stage (III), and quartz‐calcite stage (IV). This work reports the Rb–Sr age of gold‐telluride‐bearing pyrite and zircon U–Pb age of granite porphyry, as well as S isotope data of pyrite and galena. The pyrite Rb–Sr isochron age is 126.6 ± 2.3 Ma (MSWD = 1.8), and the average zircon U–Pb age of granite porphyry is 166.8 ± 4.1 Ma (MSWD = 4.9). (⁸⁷Sr/⁸⁶Sr) _i values of pyrite and δ³⁴S values of sulfides vary from 0.7104 to 0.7105 and ?11.84 to 0.28‰, respectively. The obtained Rb–Sr isochron age represents the ore formation age of the Songligou gold‐telluride deposit, which is much younger than the zircon U–Pb age of the granite porphyry. Strontium and S isotopes, together with the presence of bastnaesite, suggest that the ore‐forming fluid was derived from felsic magmas with input of a mantle component and subsequently interacted with the Taihua Supergroup. Tellurium was derived from metasomatized mantle and was related to the subduction of the Shangdan oceanic crust and Izanagi plate beneath the North China Craton (NCC). This deposit is a part of the Early Cretaceous large‐scale gold mineralization in east NCC and formed in an extensional tectonic setting.     

Tracing the metasomatic and magmatic evolution of continental mantle roots with Sr, Nd, Hf and and Pb isotopes: A case study of Middle Atlas (Morocco) peridotite xenoliths

We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr-Nd-Hf isotope data. However, we focus in particular on Pb isotope data and ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated ²³⁸U/²⁰⁴Pb, ²³²Th/²³⁸U and ²³²Th/²⁰⁴Pb in continental mantle roots and (c) the ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM).Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU.U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between ²⁰⁶Pb/²⁰⁴Pb = 19.93-20.25, ²⁰⁷Pb/²⁰⁴Pb = 15.63-15.66 and ²⁰⁸Pb/²⁰⁴Pb = 39.72-40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle.The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower ²³⁸U/²⁰⁴Pb, ²³²Th/²⁰⁴Pb and ²³²Th/²³⁸U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high ²³²Th/²³⁸U also suggest that the metasomatism was due to carbonatitic liquids.When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have ²³²Th/²⁰⁴Pb ratios exceeding those known from terrestrial basalts.     

Recurrent hydrothermal activity induced by successive extensional episodes: the case of the Berta F–(Pb–Zn) vein system (NE Spain)

A mineralogical and geochemical (fluid inclusion, stable and radiogenic isotopes) study of the Berta F–(Pb–Zn) vein system has identified the source and temperature of the fluid reservoirs involved and proved the existence of two separate hydrothermal events at the mine scale, which reflect distinct periods of regional fluid circulation. Main stage minerals (fluorite I, sulphides, calcite I and barite I) precipitated by mixing between a polysaline H₂S bearing (δ³⁴S=11‰) brine (up to 23% NaCl eq salinity) and a more dilute fluid (δ¹⁸O from −3.2‰ to 0‰), at temperatures between 80 and 150°C. The progressive increase in ⁸⁷Sr/⁸⁶Sr ratio from the early precipitated minerals (0.71242 in calcite I) to the late ones (0.71894 in fluorite II) is mainly (but not exclusively) due to a difference in age separating the two hydrothermal events. The assumed genetic model for the main stage fluorite (I) is based on a convective circulation of surficial waters leaching the crystalline basement rocks acquiring a high salinity, high ⁸⁷Sr/⁸⁶Sr ratios and a high temperature. These fluids then mixed with low salinity–low temperature waters, having a low ⁸⁷Sr/⁸⁶Sr ratio. An at least Jurassic age is suggested for the main period of vein filling, contemporaneous with the extensional regime during the Mesozoic, when fluid circulation was probably enhanced by crustal thinning. During the early Burdigalian (lower Miocene), a new period of important extension in this area took place. Hydrothermal activity related to this new and younger extensional regime is geochemically different and produced a distinctive mineralogical record, developing a set of veinlets filled with green octahedral fluorite (fluorite II), calcite (II) and barite (II). The Sr isotope compositions of these late stage vein minerals are compatible with leaching the granodiorite host-rocks during recent times. The existence of successive hydrothermal events in the same area is not surprising as geothermal systems, like La Garriga–Samalus, are still active and currently precipitating fluorite.     

U-Th-Pb and Lu-Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

We have carried out a Pb double-spike and Lu-Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U-Th-Pb and Lu-Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf (ε_Hf = +39.6 to +2586) that reflect ∼15-25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf-Sm/Nd ratios and Nd-Hf isotopes (ε_Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic (²⁰⁶Pb/²⁰⁴Pb > 19), and typically more so than the host intraplate volcanic rocks. ²³⁸U/²⁰⁴Pb ratios range from 17 to 68, and most samples have distinctively low ²³²Th/²³⁸U (<1) and ²³²Th/²⁰⁴Pb (3-22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic (ε_Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. ²³⁸U/²⁰⁴Pb (53-111) and ²³²Th/²⁰⁴Pb ratios (157-355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle ²³⁸U/²⁰⁴Pb (northern and southern FMC), ²³²Th/²³⁸U (northern FMC) and ²³²Th/²⁰⁴Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (?80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (?70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.     

Geological and C–O–S–Pb–Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb–Zn deposit,Southwest China

Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ³⁴S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.561 to 18.768, ²⁰⁷Pb/²⁰⁴Pb = 15.701 to 15.920, and ²⁰⁸Pb/²⁰⁴Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ⁸⁷Sr/⁸⁶Sr ratios of sphalerite range from 0.7107 to 0.7136 and (⁸⁷Sr/⁸⁶Sr)_200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.     

Origin and granite alteration effects of hydrothermal fluid: isotopic evidence from fluorite veins, Co. Galway, Ireland

The Costelloe Murvey Granite is a chemically evolved, high heat production, leucocratic component of the 400 Ma old Galway Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previously reported clinopyroxene ⁴⁰Ar-³⁹Ar age of 231±4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this mineralization. The hydrothermal fluid extensively altered its granite wallrocks, leading to lower Sm and Nd and higher Rb concentrations in altered granite, disturbing both its Rb-Sr and Sm-Nd isotopic systems. The ⁸⁷Sr/⁸⁶Sr ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios are very much lower than in the Costelloe Murvey Granite at the time of mineralization, precluding the granite as a source for more than 2% of the hydrothermal Sr. The initial ¹⁴³Nd/¹⁴⁴Nd ratio varies between fluorite in different veins due to Nd derivation from local wallrocks, and between fluorite of petrographically distinct growth phases within a single hand specimen, highlighting the difficulty of Sm-Nd isochron dating of fluorite in cases where there are multiple sources of hydrothermal Nd. It is proposed that fluorite and calcite precipitated where hot, dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous limestone which then overlay the granite. Received: 3 August 1995 / Accepted: 11 April 1996     

Chemical and isotopic (Pb,Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km²) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.Editorial responsibility: T.L. Grove     

Lead isotopic evidence for interaction between plume and lower crust during emplacement of peralkaline Lovozero rocks and related rare-metal deposits,East Fennoscandia,Kola Peninsula,Russia

The Lovozero alkaline massif—an agpaitic nepheline syenite layered intrusion—is located in the central part of the Kola Peninsula, Russia, and belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Associated loparite and eudialyte deposits, which contain immense resources of REE, Nb, Ta, and Zr, constitute a world class mineral district. Previous Sr, Nd, and Hf isotope investigations demonstrated that these rocks and mineral deposits were derived from a depleted mantle source. However, because the Sr, Nd, and Hf abundances in the Kola alkaline rocks are significantly elevated, their isotopic compositions were relatively insensitive to contamination by the underlying crustal rocks through which the intruding magmas passed. Pb occurring in relatively lower abundance in the KACP rocks, by contrast, would have been a more sensitive indicator of an acquired crustal component. Here, we investigate the lead isotopic signature of representative types of Lovozero rocks in order to further characterize their sources. The measured Pb isotopic composition was corrected using the determined U and Th concentrations to the age of the crystallization of the intrusion (376?±?28 Ma, based on a ²⁰⁶Pb/²⁰⁴Pb versus ²³⁸U/²⁰⁴Pb isochron and 373?±?9 Ma, from a ²⁰⁸Pb/²⁰⁴Pb versus ²³²Th/²⁰⁴Pb isochron). Unlike the previously investigated Sr, Nd, and Hf isotopes, the lead isotopic composition plot was well outside the FOZO field. The ²⁰⁶Pb/²⁰⁴Pb values fall within the depleted MORB field, with some rocks having lower ²⁰⁷Pb/²⁰⁴Pb but higher ²⁰⁸Pb/²⁰⁴Pb values. Together with other related carbonatites having both lower and higher ²⁰⁶Pb/²⁰⁴Pb values, the combined KACP rocks form an extended linear array defining either a?~2.5-Ga secondary isochron or a mixing line. The projection of this isotopic array toward the very unradiogenic composition of underlying 2.4–2.5-Ga basaltic rocks of the Matachewan superplume and associated Archean granulite facies country rock provides strong evidence that this old lower crust was the contaminant responsible for the deviation of the Lovozero rocks from a presumed original FOZO lead isotopic composition. Evaluating the presence of such a lower crustal component in the Lovozero rock samples suggests a 5–10% contamination by such rocks. Contamination by upper crustal rock is limited to only a negligible amount.     

Enigmatic association of the carbonatite and alkali-pyroxenite along the Northern Shear Zone, Purulia, West Bengal: A saga of primary magmatic carbonatite

The Purulia carbonatite, ‘carbonatite’-‘alkali-pyroxenite’-‘apatite-magnetite rock’ association, is located at Beldih area of Purulia district, West Bengal and falls within the 100 km long Northern Shear Zone (NSZ). Published literature suggests that the Purulia carbonatite was formed by the process of liquid immiscibility from under-saturated silicate parent magma. However, no silica under-saturated rocks like ijolite, nepheline-syenite etc. is known from the area. The trace element geochemistry (Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios in the present study) also does not support this view. Present study indicates that the Purulia carbonatite is enriched in ΣREE and incompatible elements but the carbonatite is also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites. The lower value of Nb is characteristics of carbo(hydro)thermal carbonatite where carbonatite is associated with alkali-pyroxenite and suggests probable origin of the carbonatite as carbothermal residua evolved from an unknown parentage. However, the field, petrographic and geochemical data indicate the genesis of this carbonatite from a primary carbonatitic magma of mantle decent. The ⁸⁷Sr/⁸⁶Sr ratio of the carbonatite and apatite separated from the carbonatite (∼0.703) implies primary magmatic derivation of the Purulia carbonatite. Close similarity of the apatite of the apatite-magnetite rock with the mantle apatite (of type Apatite B) indicates that they are also of primary magmatic origin. The present work portrays a unique example where primary magmatic carbonatite is associated with the alkali-pyroxenite.     

Enigmatic association of the carbonatite and alkali-pyroxenite along the Northern Shear Zone,Purulia, West Bengal: A saga of primary magmatic carbonatite

The Purulia carbonatite, ‘carbonatite’-‘alkali-pyroxenite’-‘apatite-magnetite rock’ association, is located at Beldih area of Purulia district, West Bengal and falls within the 100 km long Northern Shear Zone (NSZ). Published literature suggests that the Purulia carbonatite was formed by the process of liquid immiscibility from under-saturated silicate parent magma. However, no silica under-saturated rocks like ijolite, nepheline-syenite etc. is known from the area. The trace element geochemistry (Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios in the present study) also does not support this view. Present study indicates that the Purulia carbonatite is enriched in ΣREE and incompatible elements but the carbonatite is also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites. The lower value of Nb is characteristics of carbo(hydro)thermal carbonatite where carbonatite is associated with alkali-pyroxenite and suggests probable origin of the carbonatite as carbothermal residua evolved from an unknown parentage. However, the field, petrographic and geochemical data indicate the genesis of this carbonatite from a primary carbonatitic magma of mantle decent. The ⁸⁷Sr/⁸⁶Sr ratio of the carbonatite and apatite separated from the carbonatite (～0.703) implies primary magmatic derivation of the Purulia carbonatite. Close similarity of the apatite of the apatite-magnetite rock with the mantle apatite (of type Apatite B) indicates that they are also of primary magmatic origin. The present work portrays a unique example where primary magmatic carbonatite is associated with the alkali-pyroxenite.     

Carbonate alteration associated with talc-chlorite mineralization in the eastern Pyrenees, with emphasis on the St. Barthelemy Massif

Summary The eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112–97 Ma), the largest of which occur in the St. Barthelemy massif. There talc develops by hydrothermal replacement of dolostones, which were formed by alteration of calcite marbles. This alteration is progressive. Unaltered calcite marbles have oxygen isotope composition of about 25‰ (V-SMOW). The δ¹⁸O values decrease down to values of 12‰ towards the contact with dolostones. This ¹⁸O depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which vary from ⁸⁷Sr/⁸⁶Sr = 0.7087–0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with ⁸⁷Sr/⁸⁶Sr = 0.7094 near dolomitization fronts. Dolostones have δ¹⁸O values (about 9‰) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (⁸⁷Sr/⁸⁶Sr = 0.7109 to 0.7130). Hydrothermal calcites have δ¹⁸O values close to dolostones but substantially lower δ¹³C values, down to −6.5‰, which is indicative of the contribution of organic matter. The REE content of hydrothermal calcite is one order of magnitude higher than that of calcitic marbles. Its highly radiogenic Sr composition with ⁸⁷Sr/⁸⁶Sr = 0.7091 to 0.7132 suggests that these elements were derived from silicate rocks, which experienced intense chlorite alteration during mineralization. The chemical and isotopic compositions of the calcite marbles, the dolostones and the hydrothermal calcites are interpreted as products of successive stages of fluid-rock interaction with increasing fluid-rock ratios. The hydrothermal quartz, calcite, talc and chlorite are in global mutual isotopic equilibrium. This allows the calculation of the O isotope composition of the infiltrating water at 300 °C, which is in the δ¹⁸O = 2–4.5‰ range. Hydrogen isotope compositions of talc and chlorite indicate a δD = 0 to −20‰. This water probably derived from seawater, with minor contribution of evolved continental water.     

Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada,California

This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.70597–0.70653; ²⁰⁶Pb/ ²⁰⁴Pb=18.862–19.018; ²⁰⁷Pb/²⁰⁴Pb=15.640–15.686; ²⁰⁸Pb/ ²⁰⁴Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (⁸⁷Sr/⁸⁶=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high ⁸⁷Sr/⁸⁶Sr (71141–0.71240) and low ²⁰⁶Pb/²⁰⁴Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H₂O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.     

Ankerite carbonatite from Swartbooisdrif, Namibia: the first evidence for magmatic ferrocarbonatite

Although general accounts of carbonatites usually envisage Ca-Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe²⁺)(CO₃)₂ cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe₃O₄. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420-460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (⁸⁷Sr/⁸⁶Sr=0.7033; )Nd=0.2-1.0; ²⁰⁶Pb/²⁰⁴Pb=16.37; ²⁰⁷Pb/²⁰⁴Pb=15.42; ²⁰⁸Pb/²⁰⁴Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate '¹⁸O=8.0 and '¹³C=-7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)_n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.     

Reconciling fluid inclusion types,fluid processes,and fluid sources in skarns: an example from the Bismark Deposit,Mexico

The Bismark deposit (8.5 Mt at 8% Zn, 0.5% Pb, 0.2% Cu, and 50 g/t Ag) located in northern Mexico is an example of a stock-contact skarn end member of a continuum of deposit types collectively called high-temperature, carbonate-replacement deposits. The deposit is hosted by massive sulfide within altered limestone adjacent to the Bismark quartz monzonite stock (~42 Ma) and the Bismark fault. Alteration concurrently developed in both the intrusion and limestone. The former contains early potassic alteration comprising K-feldspar and biotite, which was overprinted by kaolinite-rich veins and alteration and later quartz, sericite, and pyrite with minor sphalerite and chalcopyrite. Prograde exoskarn alteration in the limestone consists of green andradite and diopside, and transitional skarn comprising red-brown andradite, green hedenbergite and minor vesuvinite, calcite, fluorite, and quartz. The main ore stage post-dates calc-silicate minerals and comprises sphalerite and galena with gangue pyrite, pyrrhotite, calcite, fluorite, and quartz. The entire hydrothermal system developed synchronously with faulting. Fluid inclusion studies reveal several distinct temporal, compositional, and thermal populations in pre-, syn- and post-ore quartz, fluorite, and calcite. The earliest primary fluid inclusions are coexisting vapor-rich (type 2A) and halite-bearing (type 3A; type 3B contain sylvite) brine inclusions (32 to >60 total wt% salts) that occur in pre-ore fluorite. Trapping temperatures are estimated to have been in excess of 400 °C under lithostatic pressures of ~450 bar (~1.5 km depth). Primary fluid inclusions trapped in syn-ore quartz display critical to near critical behavior (type 1C), have moderate salinity (8.4 to 10.9 wt% NaCl equiv.) and homogenization temperatures (Th) ranging from 351 to 438 °C. Liquid-rich type 1A and 1B (calcite-bearing) inclusions occur as primary to secondary inclusions predominantly in fluorite and show a range in Th (104–336 °C) and salinity (2.7–11.8 wt% NaCl equiv.), which at the higher Th and salinity ranges overlap with type 1C inclusions. Oxygen isotope analysis was carried out on garnet, quartz, and calcite (plus carbon isotopes) in pre-, syn-, post-ore, and peripheral veins. Pre-ore skarn related garnets have a δ¹⁸O_mineral range between 3.9 and 8.4‰. Quartz from the main ore stage range between 13.6 and 16.0‰. Calcite from the main ore stage has δ¹³C values of –2.9 to –5.1‰ and δ¹⁸O values of 12.3 to 14.1‰, which are clearly distinct from post-ore veins and peripheral prospects that have much higher δ¹⁸O (16.6–27.3‰) and δ¹³C (1.3–3.1‰) values. Despite the numerous fluid inclusion types, only two fluid sources can be inferred, namely a magmatic fluid and an external fluid that equilibrated with limestone. Furthermore, isotopic data does not indicate any significant mixing between the two fluids, although fluid inclusion data may be interpreted otherwise. Thus, the various fluid types were likely to have formed from varying pressure–temperature conditions through faulting during exsolution of magmatic fluids. Late-stage hydrothermal fluid activity was dominated by the non-magmatic fluids and was post-ore.