Genesis of the Huangshannan high-Ni tenor magmatic sulfide deposit in the Eastern Tianshan,northwest China: Constraints from PGE geochemistry and Os–S isotopes

The Huangshannan magmatic Ni-Cu sulfide deposit is one of a group of Permian magmatic Ni-Cu deposits located in the southern Central Asian Orogenic belt in the Eastern Tianshan, northwest China. It is characterized by elevated Ni tenor (concentrations in recalculated 100% sulfide) in sulfide within ultramafic rocks (9–19 wt%), with values much higher than other deposits in the region. Sulfides of the Huangshannan deposit are composed of pentlandite, chalcopyrite, and pyrrhotite and the host rock is relatively fresh, indicating that the high-Ni tenor is a primary magmatic feature rather than formed by alteration processes. It is shown that sulfides with high-Ni tenor can be generated by sulfide-olivine equilibrium at an oxygen fugacity of QFM +0.5, for magmas containing 450 ppm Ni and 20% olivine. Ores with >10 wt% sulfur have relatively low PGE and Ni tenors compared to other ores, R factor (mass ratio of silicate to sulfide liquid) modeling of Ni indicates that they formed at moderate R values (150–600). Based on this constraint on R values, ores with <10 wt% sulfides in the Huangshannan deposit can be segregated from a similar parental magma with 0.05 ppb Os, 0.023 ppb Ir, and 0.5 ppb Pd at R values between 600 and 3000. This, coupled with the supra-cotectic proportions of sulfide liquid to cumulus silicates in the Huangshannan ores imply mechanical transport and deposition of sulfide liquid in a magma pathway or conduit, in which sulfides must have interacted with large volumes of silicate magma. Platinum and Pd depletion relative to other platinum group elements (PGEs) are observed in fresh and sulfide-rich samples (S > 4.5 wt%). As sulfide-rich samples are also depleted in Cu, and as interstitial sulfides in those samples are physically interconnected at a scale of several cms, the low Pt and Pd anomalies are attributed to solid Pt and Pd phases crystallization and retention with the monosulfide solid solution (MSS) and Cu-rich sulfide liquid percolation during MSS fractionation. This finding indicates that Pt anomalies in sulfide-rich rocks from magmatic Ni-Cu deposits in the Eastern Tianshan are the result of sulfide fractionation rather than a hydrothermal effect. ¹⁸⁷Os/¹⁸⁸Os_(278Ma) values of the lherzolite samples vary from 0.27 to 0.37 and γOs_(278Ma) values vary from 110 to 189, indicating significant magma interaction with crustal sulfides, rich in radiogenic Os. Well constrained γOs values and δ³⁴S values (−0.4 to 0.8‰) indicate that crustal contamination occurred at depth before the arrival of the magma in the Huangshannan chamber. Regionally, deposits with high-Ni tenor have not been reported other than the Huangshannan deposit; however, many intrusions with high-Ni contents in olivine are present in NW China, such as the Erhongwa, Poyi and Poshi intrusions. Those intrusions are capable of forming high-Ni tenor sulfides due to olivine-sulfide-silicate equilibrium and relative high-Ni content in parent magma, making them attractive exploration targets.     

东昆仑夏日哈木镍铜硫化物矿床成矿岩浆条件:流体挥发份化学组成与碳同位素组成制约

夏日哈木镍铜硫化物矿床是我国东昆仑造山带新发现的全球岛弧环境最大的岩浆镍铜硫化物矿床。早泥盆世镁铁-超镁铁质成矿岩体中不同类型岩石的辉石中流体挥发份化学组成和碳同位素组成表明,流体挥发份总量较低,平均含量为888mm~3.STP/g(STP:标准温度和大气压)。流体挥发份组成以H_2O为主(48mol%),其次为H_2(26%)、CO_2(15%)和CO(5%)。H_2S含量从二辉橄榄岩(10mm~3/g)、橄榄辉石岩(18mm~3/g)到二辉辉石岩(28mm~3/g)逐渐增加,表明岩浆就位结晶演化过程中含硫组分的加入。CO_2的δ~(13)C值(-23.5‰~-5.0‰)位于地壳和地幔值范围之间。CH_4同系物碳同位素组成随碳数具有总体正序、局部反序的分布特征,显示大洋沉积有机质热裂解成因特征。夏日哈木成矿镁铁-超镁铁质岩体流体组分中较低的H2_O含量和较高的H_2含量表明,岩浆演化处于还原性挥发份条件,地幔岩浆源区存在俯冲洋壳沉积有机质,岩浆房围岩的混染和富S流体的加入可能是夏日哈木岩浆镍铜硫化物矿床硫饱和的主要机制。     

黄河北煤田岩浆岩侵入区内矿井地温分布规律研究—以潘店井田为例

通常情况下,地温会随深度增加而呈线性增加,地温影响着井下采掘工作面的环境温度。随着矿井深度的变化,空气所受到的压力状态也随之发生改变。当风流沿井巷向下流动时,空气的压力值增大。空气的压缩会出现放热(或吸热),从而使矿井温度升高。随着矿井向深部开采,井下作业环境条件恶化,岩层温度将达到摄氏几十度的高温。新汶孙村煤矿-800 m水平部分工作面温度高达30~33℃,巨野矿区龙固矿井-850 m水平所有工作面温度高达34~36℃,已严重影响了工人劳动效率。黄河北煤田潘店井田东部有燕山期岩浆岩活动,含煤地层的上中下部均有岩浆岩侵入,针对岩浆岩侵蚀地区采用钻孔简易测温和近似稳态测温等手段,详细查明地温分布规律,通过区内地温资料对比发现,该区有岩浆岩侵蚀影响的地段比没有岩浆岩侵蚀的地段地温高2.6~7.2℃。     

Mineral chemistry of Plio-Quaternary subvolcanic rocks,southwest Yazd Province,Iran

The NW–SE-trendingLate Cretaceous–Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA) in southwest Iran hosts numerous Plio-Quaternary subvolcanic porphyritic andesitic to rhyodacitic domes intruded into a variety of rock sequences. Bulk-rock geochemical data show that the calc-alkaline dacitic to rhyodacitic subvolcanic rocks share compositional affinities with high-silica adakites, including high ratios of Na₂O/K₂O >1, Sr/Y (most >70), and La/Yb (>35), high Al₂O₃ (>15 wt.%), low Yb (<1.8 ppm) and Y (<18 ppm) contents, no significant Eu anomalies, and flat to gently upward-sloping chondrite-normalized heavy rare-earth element (HREE) patterns. All analysed rocks are characterized by enrichment in large-ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). They also display typical features of subduction-related calc-alkaline magmas. In chondrite-normalized rare-earth element patterns, the light rare-earth elements (LREEs) are enriched ((La/Sm) _N = 3.49–7.89) in comparison to those of the HREE ((Gd/Yb) _N = 1.52–2.38). Except for the G-Aliabad Dome, plagioclase crystals in the Shamsabad, Ostaj, Abdollah, and Bouragh Domes are mostly oligoclase to andesine (An_19–49). Amphibole and biotite are abundant ferromagnesian minerals in the subvolcanic rocks. Calcic amphiboles are dominantly magnesiohornblende, magnesiohastingsite, and tschermakite with Mg/(Mg + Fe_tot) ratios ranging from 0.58 to 0.78. In all the studied domes, amphiboles are typically ferric iron-rich, but that those the Shamsabad Dome have the highest Fe³⁺/(Fe³⁺ + Fe²⁺) ratios, between 0.69 and 0.98. Amphiboles from the Ostaj and Shamsabad Domes are relatively rich in F (0.39–1.01 wt.%) in comparison to the other studied domes. This phase commonly shows pargasitic and hastingsitic substitutions with a combination of tschermakitic and edenitic types.

Temperature-corrected Al-in-hornblende data show that amphibole phenocrysts from the Ostaj, Abdollah, and G-Aliabad Domes crystallized at pressures ranging from 2.14 to 3.42 kbar, 3.49 to 3.96 kbar, and 2.02 to 3.47 kbar, respectively. Temperatures of crystallization calculated with the amphibole–plagioclase thermometer for the Ostaj, Abdollah, and G-Aliabad subvolcanic domes range from 735°C to 826°C (mean = 786 ± 29), 778°C to 808°C (mean = 791 ± 13), and 866°C to 908°C (mean = 885 ± 12), respectively. In the annite–siderophyllite–phlogopite–eastonite quadrilateral, biotite from the G-Aliabad, Bouragh, and Ostaj Domes are characterized by relatively low total Al contents with variable Fe_tot/(Fe_tot + Mg) values from 0.26 to 0.43. All biotite analyses define a nearly straight line in the X _Mg versus Fe_tot plot, with r = –0.96 correlation coefficient. In comparison to other domes, the F content of biotite from the G-Aliabad Dome shows high concentrations in the range of 1.80–2.57 wt.% (mean = 2.20). Inferred pre-eruptive conditions based on the calcic amphibole thermobarometric calculations for the Shamsabad, Abdollah, and Ostaj Domes show that the calc-alkaline subvolcanic magma chamber, on average, was characterized by a water content of 6.10 wt.%, a relatively high oxygen fugacity of 10^–10.66 (ΔNNO + 1.28), a temperature of 896°C, and a pressure of 2.75 kbar.     

Petrogenetic modeling of rock variety in the Khalkhab–Neshveh pluton,NW of Saveh,Iran

The Khalkhab–Neshveh (KN) pluton is a part of Urumieh–Dokhtar Magmatic Arc and was intruded into a covering of basalt and andesite of Eocene to early Miocene age. It is a medium to high‐K, metaluminous and I‐type pluton ranging in composition from quartz monzogabbro, through quartz monzodiorite, granodiorite, and granite. The KN rocks show subtle differentiation trends strongly controlled by clinopyroxene, plagioclase, hornblende, apatite, and titanite, where most major elements (except K₂O) are negatively correlated with SiO₂; and Al₂O₃, Na₂O, Sr, Eu, and Y define curvilinear trends. Considering three processes of magmatic differentiation including mixing and/or mingling between basaltic and dacitic magmas, gravitational fractional crystallization and in situ crystallization revealed that the latter is the most likely process for the evolution of KN magma. This is supported by the occurrence of all rock types at the same level, the lack of mafic enclaves in the granitoid rocks, the curvilinear trends of Na₂O, Sr, and Eu, and the constant ratios of (⁸⁷Sr/⁸⁶Sr)_i from quartz monzodiorite to granite (0.70475 and 0.70471, respectively). In situ crystallization took place via accumulation of plagioclase and clinopyroxene phenocrysts and concentration of these phases in the quartz monzogabbro and quartz monzodiorite at the margins of the intrusion at T ≥ 1050°C, and by filter pressing and fractionation of hornblende, plagioclase, and later biotite in the granitoids at T = ～880°C.     

Paleoproterozoic crustal evolution of the Tarim Craton: Constrained by zircon U–Pb and Hf isotopes of meta-igneous rocks from Korla and Dunhuang

The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4 Ga and ∼1.85 Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0 Ga, ∼1.85 Ga and ∼1.75 Ga. The ∼1.85 Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4 Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0 Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75 Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.     

Fluid sources for the La Guitarra epithermal deposit (Temascaltepec district, Mexico): Volatile and helium isotope analyses in fluid inclusions

The La Guitarra deposit (Temascaltepec district, South-Central Mexico), belongs to the low/intermediate sulfidation epithermal type, has a polymetallic character although it is currently being mined for Ag and Au. The mineralization shows a polyphasic character and formed through several stages and sub-stages (named I, IIA, IIB, IIC, IID, and III). The previous structural, mineralogical, fluid inclusion and stable isotope studies were used to constrain the selection of samples for volatile and helium isotope analyses portrayed in this study. The N₂/Ar overall range obtained from analytical runs on fluid inclusion volatiles, by means of Quadrupole Mass Spectrometry (QMS), is 0 to 2526, and it ranges 0 to 2526 for stage I, 0 to 1264 for stage IIA, 0 to 1369 for stage IIB, 11 to 2401 for stage IIC, 19 to 324 for stage IID, and 0 to 2526 for stage III. These values, combined with the CO₂/CH₄ ratios, and N₂-He-Ar and N₂-CH₄-Ar relationships, suggest the occurrence of fluids from magmatic, crustal, and shallow meteoric sources in the forming epithermal vein deposit. The helium isotope analyses, obtained by means of Noble Gas Mass Spectrometry, display R/Ra average values between 0.5 and 2, pointing to the occurrence of mantle-derived helium that was relatively diluted or “contaminated” by crustal helium. These volatile analyses, when correlated with the stable isotope data from previous works and He isotope data, show the same distribution of data concerning sources for mineralizing fluids, especially those corresponding to magmatic and crustal sources. Thus, the overall geochemical data from mineralizing fluids are revealed as intrinsically consistent when compared to each other.The three main sources for mineralizing fluids (magmatic, and both deep and shallow meteoric fluids) are accountable at any scale, from stages of mineralization down to specific mineral associations. The volatile and helium isotope data obtained in this paper suggest that the precious metal-bearing mineral associations formed after hydrothermal pulses of predominantly oxidized magmatic fluids, and thus it is likely that precious metals were carried by fluids with such origin. Minerals from base-metal sulfide associations record both crustal and magmatic sources for mineralizing fluids, thus suggesting that base metals could be derived from deep leaching of crustal rocks. At the La Guitarra epithermal deposit there is no evidence for an evolution of mineralizing fluids towards any dominant source. Rather than that, volatile analyses in fluid inclusions suggest that this deposit formed as a pulsing hydrothermal system where each pulse or set of pulses accounts for different compositions of mineralizing fluids.The positive correlation between the relative content of magmatic fluids (high N₂/Ar ratios) and H₂S suggests that the necessary sulfur to carry mostly gold as bisulfide complexes came essentially from magmatic sources. Chlorine necessary to carry silver and base metals was found to be abundant in inclusion fluids and although there is no evidence about its source, it is plausible that it may come from magmatic sources as well.     

塔里木盆地西北缘奥陶系碳酸盐岩中两种来源热流体的地质与地球化学证据

塔里木盆地西北缘西克尔-巴楚-柯坪地区处于巴楚-柯坪断隆带活动区,构造活动强烈.该区出露的中下奥陶统鹰山组发育大量的热液蚀变现象,既有溶蚀形成的大小孔洞、疏松灰岩层,也有各类充填的脉体,如萤石矿、硫磺石膏矿等.通过野外露头观察和采样,并对受热流体作用的样品进行了包裹体测温测盐、微量元素和硫同位素分析.结果显示研究区主要有两种流体来源:岩浆热液和地层热液.研究表明:(1)岩浆热液主要影响研究区南西段,形成萤石、重晶石、方铅矿等矿物,流体包裹体盐度低(0.5％ ～7.5％ NaCleqv)、均一化温度相对较高(142 ～ 283℃);地层热液作用范围则主要位于研究区北东段,生成白云石、方解石、石英等矿物,流体包裹体盐度高(16％ ～ 23.3％ NaCleqv)、均一化温度低(70～ 138℃);而研究区中段同时受到两种来源热液的混合作用,包裹体盐度(7.5％～18.3％ NaCleqv)和均一化温度(142 ～ 163℃)介于前两者之间;(2)热液硬石膏、硫磺的硫同位素富含32S,δ4S平均为-9.57％,说明硬石膏是H2S气体与灰岩反应的产物,硫化氢很可能来自有机物的热裂解.     

The Ordovician Sierras Pampeanas–Puna basin connection: Basement thinning and basin formation in the Proto-Andean back-arc

The Ordovician Sierras Pampeanas, located in a continental back-arc position at the Proto-Andean margin of southwest Gondwana, experienced substantial mantle heat transfer during the Ordovician Famatina orogeny, converting Neoproterozoic and Early Cambrian metasediments to migmatites and granites. The high-grade metamorphic basement underwent intense extensional shearing during the Early and Middle Ordovician. Contemporaneously, up to 7000 m marine sediments were deposited in extensional back-arc basins covering the pre-Ordovician basement. Extensional Ordovician tectonics were more effective in mid- and lower crustal migmatites than in higher levels of the crust. At a depth of about 13 km the separating boundary between low-strain solid upper and high-strain lower migmatitic crust evolved to an intra-crustal detachment. The detachment zone varies in thickness but does not exceed about 500 m. The formation of anatectic melt at the metamorphic peak, and the resulting drop in shear strength, initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies P–T conditions. P–T–d–t data in combination with field evidence suggest significant (ca. 52%) crustal thinning below the detachment corresponding to a thinning factor of 2.1. Ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25% and 44%, constituting total crustal thinning factors of 1.7–2.0. While the migmatites experienced retrograde decompression during the Ordovician, rocks along and above the detachment show isobaric cooling. This suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface. Upper crustal extension and thinning is compensated by newly deposited sediments, maintaining constant pressure at detachment level. Thinning of the migmatitic lower crust is compensated by elevation of the crust–mantle boundary. The degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension, basin formation, and sediment thickness in the back-arc basin. The initiation of crustal extension in the back-arc, however, crucially depends on the presence of anatectic melt in the middle and lower crust. Consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension.     

A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

 This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo_88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20  °C) and similar volatile abundances. The H₂O, S and Cl contents are relatively high, whereas magmatic CO₂ concentrations are very low, probably due to CO₂ loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K₂O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs. Received: 11 March 1998 / Accepted: 14 July 1998