Lower Intercept Age Research Articles

A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes

The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa. Although previous workers have noted the existence of a dolerite dyke at the Hilton Zn-Pb-Ag deposit (Valenta, 1994b; Valenta, 1994a), a dolerite dyke at the George Fisher Zn-Pb-Ag deposit has not been previously documented. Progressive underground expansion at the George Fisher mine has resulted in the discovery of a dolerite dyke in the southernmost portion of the deposit. The temporal and petrological relationship between the dolerite dykes and the adjacent Zn-Pb-Ag mineralisation at the George Fisher and Hilton Zn-Pb-Ag deposits has not been considered by previous studies, and has significant implications for the currently accepted syn-diagenetic metallogenic model. In assay data, the dolerite dykes correspond with a large spike in TiO2 and V values. In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation. In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 ± 21 Ma (MSWD = 0.93) and 1619 ± 22 Ma (MSWD = 0.53) for the dolerite dykes at the George Fisher and Hilton deposits, respectively. This age is consistent with dyke intrusion during the earliest phases of deformation during the (1620–1500 Ma) Isan Orogeny. The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining, which have paragenetic equivalents in the adjacent Zn-Pb-Ag orebodies. The hydrothermal stages include: (1) Quartz ± albite ± K-feldspar alteration/veining; (2) Dolomite alteration/veining; (3) Biotite-chlorite-sulphide alteration; (4) Paragenetically late calcite, dolomite and/or sulphide veining. Monazite from a quartz ± albite ± K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 ± 16 Ma (MSWD = 1.1), constraining the maximum age of alteration within the dolerite dykes. To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein. Calcite from the pre-mineralisation alteration assemblage produces a Lu-Hf age of 1501 ± 32 Ma (MSWD = 1.04), which is interpreted to constrain the maximum age of stratabound Zn-Pb-Ag mineralisation. Calcite from the late cross-cutting vein produced a Lu-Hf age of 1289 ± 26 Ma (MSWD = 1.00), which is interpreted to constrain the age of post-mineralisation faulting throughout the deposit. The evidence presented in this study indicates that the dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralisation.

Ion-probe (SIMS) U-Pb geochronology and geochemistry of the Upper Cretaceous Kızıldağ (Hatay) ophiolite: Implications for supra-subduction zone spreading in the Southern Neotethys

The Kızıldağ (Hatay) ophiolite in southern Turkey represents a complete, although fault-dissected, remnant of oceanic lithosphere that formed within the South Tethyan ocean during the late Cretaceous. The Kızıldağ forms part of the Upper Cretaceous belt which includes the Troodos (Cyprus), Baer-Bassit (Syria), Amanos (S Tukey), S Iran and Semail (Oman) ophiolites. Ion-probe (SIMS) dating of seven samples of crustal rocks (cumulate gabbro, isotropic gabbro and isolated dykes in mantle tectonite), and a plagiogranite intrusion provides important clues concerning the temporal development of the emplaced oceanic crust. Single grain 206Pb/238U dates that overlap within analytical uncertainty for four samples, including the plagiogranite (93.83 ± 0.46 Ma), the isotropic gabbro (92.9 ± 0.52 Ma) and the isolated dykes (92.54 ± 0.44 Ma to 93.6 ± 0.75 Ma), are interpreted as magmatic crystallisation ages, and suggest that the Kızıldağ ophiolite formed within 1-2 Ma. Three other samples with single grain 206Pb/238U dates that are outside the range of analytical uncertainty yielded nearly identical lower intercept ages of 94.2 ± 2.5 Ma to 94.4 ± 0.97 Ma for two cumulate gabbros and 90.0 ± 6.4 Ma for an isotropic gabbro. Comparison of the new and published radiometric ages of the Kızıldağ suggest that this ophiolite is ∼1.5 Ma older than previously believed, and is similar to the crystallisation ages of plagiogranites from the Troodos (Cyprus) and the Semail (Oman) ophiolites. The new age data emphasise the value of dating a range of ophiolitic rocks. Geochemically, the crustal rocks of the Kızıldağ ophiolite formed from boninitic magmas (cumulate gabbros and isolated dykes) and from island arc tholeiitic magmas (isotropic gabbro). The new whole-rock chemical data support a subduction-initiation (fore-arc) setting for the Kızıldağ ophiolite, in common with the Troodos, Semail, Baër-Bassit and other Upper Cretaceous ophiolites of the South-Tethyan region.

Using apatite to differentiate metallogenic potential and environment of granitic rocks: A case study from the Tongshanling W-Sn-Cu-Pb-Zn ore field, Nanling Range (South China)

The Tongshanling ore field is one of the most representative W–Sn–Cu–Pb–Zn polymetallic deposits in the central-western Nanling Ore Belt, which developed multiple mineralization types, including Cu–Pb–Zn (Tongshanling), W–Sn (Weijia), Pb–Zn (Jiangyong), and Mo (Yulong) deposits. Here, we analyzed apatite from the Tongshanling granodiorite and the Weijia granite porphyry, integrating major and trace elements, U–Pb dating, and Nd isotopic analyses to compare the difference in geochronology, magma source, redox environment, and evolution between the Tongshanling and Weijia deposit. The U–Pb dating of the Tongshanling and Weijia apatite samples yield a lower intercept age of 166.2 ± 11.3 Ma (1σ, MSWD = 0.22) and 160.87 ± 8.92 Ma (1σ, MSWD = 0.44) on Tera-Wasserburg diagrams, respectively. The 147Sm/144Nd and 143Nd/144Nd ratios of the Tongshanling apatite samples are 0.136769–0.182878 and 0.512159–0.512246, respectively, with corresponding εNd (t = 166 Ma) ranging from −8.32 to −6.60, suggesting that the Tongshanling granodiorite was produced by partial melting of metaigneous rocks in the deep crust. The Tongshanling apatite shows a right-inclined REE pattern, while the Weijia apatite displays a “V” type REE pattern. The Tongshanling apatite shows higher Eu/Eu* values, and lower Mn and Ce/Ce* values than the Weijia apatite, indicating that the former formed in higher magmatic oxidation state while the latter formed in more reduced conditions. The granodiorite in the Tongshanling deposit is characterized by a lower degree of fractionation than the granite porphyry in the Weijia deposit, of which the Weijia apatite has lower Sr, Mg contents, and higher Y contents. Furthermore, apatite from Cu-bearing granites exhibits lower F/Cl ratios compared to those from W-bearing granites in the South China Block. In the Tongshanling area, apatite from the Tongshanling granodiorite has higher Cl contents than the Weijia apatite, indicating that the crustal basement of the Tongshanling area was affected by the high Cl fluid derived from the Proterozoic oceanic subduction and remelted to form granite in the Mesozoic, while the Weijia granite porphyry formed from remelting of a sedimentary source. The Tongshanling granodiorite and Weijia granite porphyry with different degrees of fractionation, magma source, and halogens contents can be effectively distinguished by the geochemistry of apatite. The difference in trace elements (Sr, Y, Mg, and REE) and isotopes of apatite can be applied to the identification of different types of ore-bearing granites in the Nanling Range.

Trace-element signatures of pegmatite and post-pegmatite minerals from the Govedarnika Pb-Zn deposit, Laki district, Central Rhodopes

The current study presents mineralogical and geochemical characteristics of pegmatite bodies from the Govedarnika Pb-Zn deposit, situated in Laki district, South Bulgaria. The studied pegmatites are composed mainly of K-feldspar, plagioclase, quartz and accessories of titanite, apatite, allanite and zircon. Intensive hydrothermal alteration resulted in specific mineral assemblage of chlorite, hematite, epidote and carbonates. According to the mineral relationships, it preceded or coincided with the time of base metal ore formation and affected both pegmatites and host metamorphic rocks. Both pegmatite accessories and post-pegmatite hydrothermal silicates often show enhanced contents of some strategic elements such as Sr, Li, V, Ga, Ni and REE. Pilot U-Pb geochronology of titanite from the studied pegmatites yielded a lower intercept age of 39.9±2.3 Ma.

Temporal evolution and origin of the Yumugou Mo-W deposit, East Qinling, China: Evidence from molybdenite Re-Os age and U-Pb dating and geochemistry of titanite

The Yumugou Mo-W deposit is an important porphyry-skarn deposit hosted in the world-class Luanchuan Mo polymetallic cluster, East Qinling, China. However, the origin and age of skarn Mo mineralization, as well as the temporal evolution, redox conditions and magmatic-hydrothermal processes of the Yumugou porphyry-skarn system remain enigmatic. Hence, we conducted molybdenite Re-Os and titanite U-Pb dating together with titanite geochemical studies on porphyry and skarn from the Yumugou deposit, with major view to resolve the life-cycle and evolution of the Yumugou deposit and to provide insights into the magmatic-hydrothermal evolution of regional Luanchuan porphyry-skarn system. Titanite U-Pb dating of Mo-hosting monzogranite yielded a lower intercept age of 145.0 Ma, consistent with reported zircon (147–141 Ma) and apatite (153–141 Ma) U-Pb ages of the Huangbeiling intrusion, interpreted as rapid post-magma cooling in response to shallow emplacement of granitic porphyries. Titanite U-Pb dating of skarns yielded titanite U-Pb lower intercept ages of 149.0–144.7 Ma, representing the timing of multiple hydrothermal events. Molybdenite Re-Os dating of Mo-hosting skarns yielded a weighted mean age of 147.2 Ma, which we interpret as the age of skarn Mo mineralization. Together with published age data of the Yumugou deposit, we suggest that the Yumugou porphyry-skarn system underwent magma emplacement at ca. 147–141 Ma, contemporaneous skarnization, Mo mineralization and rapid post-magma cooling at ca. 153–141 Ma, and multiple post-mineralization cooling and exhumation at ca. 114–58 Ma. Compiled molybdenite Re-Os isotopic data indicate crust-mantle mixed Mo source for all Mo-W deposits, and coupled with crust-mantle mixed magma and ore-forming materials of granitic porphyries and skarn-hydrothermal Pb-Zn-Ag deposits in the Luanchuan region, we propose that the Luanchuan porphyry-skarn system shares a single source. In addition, titanite textures and geochemical data suggest most titanites from the Yumugou deposit are of hydrothermal origin and mainly formed under reduced conditions. In conjuction with tectonic evolution of the East Qinling Orogen, we argue that the Yumugou Mo-W deposit formed during the tectonic transition from syn- to post-collisional settings (160–130 Ma), via multiple interactions of melt and hydrothermal fluids, Mo4+ aggregates S2– under reduced conditions to precipitate as molybdenites (MoS2) in granitic porphyries and skarns. Further, we also suggest that similar Mo enrichment processes likely occurred throughout the whole East Qinling Orogen.

IN SITU U-Pb MONAZITE GEOCHRONOLOGY RECORDS MULTIPLE EVENTS AT THE MOUNT ISA Cu (± Zn-Pb-Ag) DEPOSIT, NORTHERN AUSTRALIA

Abstract The Mount Isa Cu (± Zn-Pb-Ag) deposit is the largest Cu deposit in the Western fold belt of the Mount Isa inlier. Previous geochronological studies on the deposit have produced a large range (>150 m.y.) in ages for Cu mineralization and associated hydrothermal alteration. This study combines detailed petrology with in situ monazite U-Pb geochronology on four monazite-bearing samples in order to constrain the age of hydrothermal and tectonic events experienced by the Mount Isa Cu (± Zn-Pb-Ag) deposit and enclosing host shale. Samples EY108402 and EX102476 contain singular subangular monazite grains included in dolomite and siderite, which are associated with premineralization silica-dolomite alteration. Monazite from these samples yields mean weighted 207Pb/206Pb ages of 1587 ± 43 (mean square of weighted deviates [MSWD] = 0.57) and 1623 ± 25 Ma (MSWD = 0.61), respectively. These ages constrain the maximum age of silica-dolomite alteration and Cu mineralization, reflecting monazite growth during periods of peak metamorphism and early basin inversion, respectively. A sample from the 1100 Cu orebody (DDR012-2) contains two clusters of fine-grained monazite that replace siderite associated with silica-dolomite alteration, envelop chalcopyrite, and are crosscut by chlorite-quartz-orthoclase microveins. Monazite from these clusters produces 207Pb/206Pb ages ranging from ca. 1620 to ca. 1360 Ma. The large variation in ages is attributed to variable radiogenic Pb loss from a precursor monazite due to (1) continuous coupled dissolution-reprecipitation reactions over ca. 260 m.y. or (2) partial recrystallization by a ca. 1360 Ma fluid event. As monazite from this sample envelops chalcopyrite, the ca. 1360 Ma age can be used to infer the minimum age of Cu mineralization. Sample 1758-1 is from a highly silicified and fractured section of the Eastern Creek Volcanics located adjacent the deposit. The sampled fracture plane bears a chlorite-illite-rutile infill assemblage with fine-grained irregular-shaped monazite. Monazite from this sample produces a lower intercept age of 1376 ± 32 Ma (MSWD = 1.3) and is interpreted to represent the age of a major fluid flow event coeval with uplift along the Mount Isa fault. The monazite U-Pb geochronology presented in this study brackets the age of Cu mineralization and records the presence of multiple tectonic/hydrothermal events over the history of the deposit and enclosing host rocks.

Structural analysis, mineralogy, and cassiterite U–Pb ages of the Wuxu Sb-Zn-polymetallic district, Danchi Fold-and-Thrust belt, South China

The Danchi fold-and-thrust belt, Guangxi, South China, contains the Mangchang, Dachang, and Wuxu large and giant Sn-polymetallic districts. It hosts more than 1.2 Mt of ore grading 1 % Sn, 6.23 Mt Zn at 2.4 %, and 1.13 Mt Sb at 1.3 %. Wuxu district is one of the Sb-Zn dominated hydrothermal districts without magmatic rocks exposing in the south of the Danchi belt. Recent drilling and exploring mining have identified a partially Sn mineralization associated with the Zn-Sb mineralization. These works provide an opportunity to evaluate the potential of tin mineralization in the south of Danchi belt and relationship among Sn, Zn, and Sb mineralization. Structural analysis suggests that the Wuxu district underwent at least three deformational phases since the Triassic. The first NE-SW shortening deformation controlled the structural framework of the Wuxu district and is expressed by NW-verging thrusts and folds, axial-plane cleavages around the core of the Wuxu anticline, transverse joint sets on the limbs of the Wuxu anticline, and dilational zones in the bedding-fractures. Afterward, an E-W extension stress field produced NS-striking normal faults and reactivated pre-existing NNW-striking faults which were expressed by sinistral top-to-the north sense of shear. During this deformation, some low order dilational spaces of the NW-striking faults and bedding-fractures formed and a syn-kinematic deposition of minerals in these hosting structures to form the ore shoots. The NNW-striking faults, axial-plane cleavages, and large dilational jogs controlled the subvertical large veins in the Jianzhupo deposit. The conjugate joints and inter-bedding fractures in the limb of the Wuxu anticlinorium controlled the branching veinlets and stratiform, paleokarst infilled ores in the Bawang deposit. The post-ore EW-striking faults are not obvious and displaced preceding two-phase structures and ores. Three primary paragenetic stages of hydrothermal process have been recognized: (I) Sphalerite dominated + massive pyrrhotite with euhedral pyrite porphyroblasts, (II) Cassiterite + siderite + arsenopyrite dominated with minor scheelite + quartz + fluorite, and (III) Jamesonite + stibnite + sulfosalt + carbonates. Cassiterite U-Pb dating provided a lower intercept age of 90.2 ± 3.1 Ma and 90.6 ± 4.6 Ma for the Jianzhupo and Bawang deposits, respectively. These two ages are consistent with the chronological data of tin-sulfides and ore-related felsic intrusions in the Danchi fold-and-thrust belt, South China. Our studies coupled with previous gravity data suggests a distal magmatic-hydrothermal infilling tin-polymetallic district and indicates that the Wuxu district has great potential for additional tin mineralization.

Hydrothermal fluid signatures of the Yulong porphyry Cu-Mo deposit: Clues from the composition and U-Pb dating of W-bearing rutile

AbstractHydrothermal rutile (TiO2) is a widely distributed accessory mineral in hydrothermal veins or alteration assemblages of porphyry deposits and provides important information for further understanding hydrothermal fluid signatures. This study determines the geochemical composition and U-Pb dates of hydrothermal rutile from the Yulong porphyry Cu-Mo deposit in east Tibet, China. Three types of TiO2 polymorphs have been identified based on their Raman spectroscopic, textural, and chemical characteristics. (1) Brookite and anatase pseudomorphs after titanite in a fine-grained matrix, indicating low-temperature hydrothermal fluids destabilizing primary Ti-bearing minerals during argillic alteration (type-I). (2) Elongated and prismatic rutile present in hydrothermal veins or in clusters in accompanying alteration envelope characterized by weak zoning (type-II). And (3) rutile intergrown with sulfides in hydrothermal veins, characterized by well-developed patchy and sector zoning (type-III). In contrast to the type-I and type-II TiO2 polymorphs, tungsten is enriched in backscattered bright patches and sector zones in type-III rutile, which is due to the substitution of W6+ in the Ti4+ octahedral site. The mechanism of the enrichment of tungsten is effectively driven by the halogen-rich (F, Cl) aqueous fluids during hydrothermal mineralization. In situ U-Pb dating of the type-III rutile yields a lower intercept age of 41.8 ± 1.2 Ma, which brackets the timing of the Cu-Mo mineralization. The relationship between rutile textures and composition indicates that W-bearing rutile can serve as a recorder of hydrothermal processes in porphyry Cu deposits.

Emplacement ages of diamondiferous kimberlites in the Wafangdian District, North China Craton: New evidence from LA-ICP-MS U-Pb geochronology of andradite-rich garnet

This contribution presents new U-Pb geochronological data and attempts to elucidate the complex evolution history of various garnet types identified from two kimberlite pipes in the Wafangdian diamond mining district, southern Liaoning Province. These calcic garnets are dominated by andradite with relatively low proportions of schorlomite, grossular and pyrope. Abundant euhedral to subhedral, highly brecciated andradite phenocrysts hosted by LN30 “carbonatite-like” kimberlite samples yield a lower-intercept age of 459.3 ± 3.4 Ma, which is in excellent agreement with the previously reported phlogopite Ar-Ar (463.9 ± 0.9 Ma) and Rb-Sr ages (461.7 ± 4.8 Ma). Based on their trace element and C-O isotopic compositions of associated groundmass carbonate, we infer that these primary magmatic andradites probably originated from kimberlitic magmas. By comparison, three compositionally and texturally distinct groups of Ti-bearing andradites from LN42 hypabyssal kimberlites separately define three well-fitted regression lines with lower intercept ages at 581 ± 12 Ma, 414.9 ± 9.3 Ma and 292.0 ± 5.7 Ma, respectively. Relict andradite xenocrysts implies that ancient lower crust of the North China Craton (NCC) might have been affected by a significant but less-known tectonothermal event to varying degrees at ∼ 0.6 Ga. By contrast, fresh grains of magmatic Ti-andradites with chemical zoning produce a relatively young age of ∼ 415 Ma, which can still provide minimum age estimates for the most recent pulses of Paleozoic kimberlite magmatism in this study area. Noteworthily, a yet unrecognized local-scale hydrothermal alteration event at ∼ 292 Ma has been recorded in the texturally distinct population of secondary hydroandradites, whose age reported here for the first time is geologically meaningful. To sum up, this study further highlights andradite U-Pb dating as a potential robust geochronometer for constraining the late-stage evolution of kimberlite magmas as well as post emplacement hydrothermal alteration.

Ediacaran - Cambrian fluid flow in Archean orogenic gold deposits: Evidence from U–Pb SHRIMP hydrothermal monazite ages of the metaturbidite-hosted Córrego do Sítio and Pilar deposits, Quadrilátero Ferrífero, Brazil

Archean orogenic gold deposits are hosted in the Rio das Velhas greenstone belt in the eastern sector of the Quadrilátero Ferrífero, Southern São Francisco Craton region, southeastern Brazil. These include the gold deposits along the NE-trending Córrego do Sítio lineament, stretching for some 15 km, and the Pilar deposit, object of the present study. These deposits occur in an area close to the boundary of the São Francisco craton, which is commonly interpreted to have preserved the Archean and Paleoproterozoic crust from the effects of Ediacaran-Cambrian Araçuaí orogenic front. The gold deposits were formerly interpreted as exclusively Archean (ca. 2.7 Ga). However, recent geochronological data suggest the imprint of the late Ediacaran-Cambrian Brasiliano Orogeny in host rocks of these deposits, as structural modification or hydrothermal alteration assemblages that postdates Archean mineralization. To elucidate those issues, contextual (thin section) in-situ U–Pb SHRIMP dating was conducted on hydrothermal monazite crystals from the Carvoaria and Cachorro Bravo deposits of the Córrego do Sítio lineament and from the Pilar gold deposit. Hydrothermal monazite in mineralized metapelites from Carvoaria yielded a U–Pb Discordia with intercepts at 2,514 ± 22 Ma and 555 ± 19 Ma. Three younger, age-equivalent crystals are concordant and yielded a U–Pb Concordia age of 539 ± 9 Ma, identical within uncertainties to the lower intercept age. Monazite from the Cachorro Bravo deposit yielded U–Pb Concordia ages of 551 ± 10 Ma and 510 ± 11 Ma. Monazite from the veined (silicified) sulfide-schist from the Pilar deposit is depleted in U, precluding the calculation of robust U–Pb ages, and disclosed a mean 208Pb/232Th average age of 508.2 ± 6.4 Ma. These results reinforce the proposal of a strong Ediacaran-Cambrian imprint related to the final stages of the Brasiliano orogenic event, affecting Archean gold deposits throughout the Quadrilátero Ferrífero. Consequently, the results point to the importance of mapping Brasiliano-related structures that control the spatial arrangement of the gold deposits, such as the Córrego do Sítio lineament, consisting of an important exploration target. This major NE-SW trending strike-slip shear zone hosts several gold deposits and might represent an Archean structure reactivated during the Brasiliano Orogeny that possibly led the large volume of Ediacaran-Cambrian post-collisional hydrothermal fluids among the east sector of the Quadrilátero Ferrífero.

LA-(MC)-ICP-MS U-Th-Pb dating and Nd isotopes of allanite in NYF pegmatite from lesser qingling orogenic belt, central China

U-Th-Pb dating and Sm-Nd isotopes of magmatic allanite from the Huangjiagou NYF pegmatites (lesser Qinling orogenic belt, China) were precisely measured and compared with zircon U-Pb dating and Lu-Hf isotopes to demonstrate the suitability and robustness of the U-Th-Pb and Sm-Nd system in allanite as a note-taker of formation age and magma sources for pegmatites. Allanite crystals from the Huangjiagou NYF pegmatites exhibit complex chemical zoning with a homogeneous primary internal domain, which has a typical magmatic origin with high La/Sm, Th/U, and low Eu/Eu*. The allanite has low La/Ce ratios (0.48–0.62) and thus is classified as allanite-(Ce). We used time-depended element fractionation and the changing of NAI with ablating different matrices to test the matrix differences between allanite and zircon standard 91500, and achieved excellent allanite U-Th-Pb date using zircon 91500 as external standard. Magmatic allanite-(Ce) yields a lower-intercept age of 1829 ± 23 Ma on the Tera-Wasserburg plot and a 208Pb/232Th weighted average age of 1840 ± 17 Ma, both of which are consistent with the zircon 207Pb/206Pb weighted mean age (1826 ± 11 Ma) within uncertainty, supporting the reliability and accuracy of non-matrix matched allanite U-Th-Pb dating. We used titanite standard (MKED1) to correct the time-drift in allanite Nd isotope analysis. And by integrating the zircon Lu-Hf isotopes and allanite Sm-Nd isotopes, together with field investigation and mineralogy, we suggest that the Huangjiagou NYF pegmatites belong to an abyssal class, formed in the late Paleoproterozoic, and likely related to the partial melting of the dehydrated Taihua Group (amphibolite and TTG granites) in the post-collisional tectonic setting between the Eastern and Western blocks along the trans-North China Orogen (TNCO). This study confirms the talent of allanite serving as a good indicator for the crystallization age and magma sources of NYF pegmatites using non-matrix matched U-Th-Pb dating and in situ Sm-Nd isotope analysis.

The Los Angeles martian diabase: Phosphate U-Th-Pb geochronology and mantle source constraints

In situ laser ablation inductively coupled mass spectrometry (LA ICP-MS) is used to determine the U-Th-Pb age of the phosphates ferromerrillite and apatite in the Los Angeles shergottitic meteorite. The initial 207Pb/206Pb was refined by analyzing K-rich diaplectic glass. LA ICP-MS mapping was used to document zones of elevated U and Th content and to establish textural controls on isotope ages. By critically assessing dispersion in the U-Th-Pb dataset due to Pb-diffusion in phosphates during high-temperature shock metamorphism, and as a result of subsequent terrestrial contamination, we obtain a best-estimate U-Pb age of 169 ± 5 Ma anchored at an initial 207Pb/206Pb of 0.98390 ± 0.00018. This is statistically indistinguishable from a joint-isochron age of 179 ± 6 with initial 208Pb/206Pb of 2.5151 ± 0.0028. These results complement previously determined Rb-Sr and Sm-Nd isotope ages and provide independent evidence for LA having crystallized as a medium-grained basic rock from a thick lava flow or high-level intrusion in the late Amazonian at ∼170 Ma. In the context of martian mantle evolution, the initial common-Pb values suggest that Los Angeles originated from a source (µ2 ∼ 3.2) that is similar to enriched members of the shergottite meteorite clan. The U-Th-Pb systematics of both ferromerrillite and apatite were locally affected by diffusive Pb-loss in thin U-enriched marginal domains and more profoundly in shock-induced melt pockets where temperatures briefly exceeded 2000 °C. The results reveal: (1) how precise U-Pb ages can be attained from phosphates; (2) the importance of microtextural contextualization of isotope data; (3) that the timescales of cooling from shock conditions were sufficient to promote local diffusive re-equilibration of Pb over 10 s of microns; and (4) that LA ICP-MS mapping can be used to locate domains with the highest U/Pb and Th/Pb, which increases precision on lower intercept ages and isochron regression lines.

Mobilization and fractionation of HFSE and REE by high fluorine fluid of magmatic origin during the alteration of amphibolite

High field strength elements (HFSE) and rare earth elements (REE) are normally immobile in hydrothermal fluids whereas experimental studies indicate some anionic ligands can significantly enhance their solubility in hydrothermal fluids. However, the role of fluorine (F) in hydrothermal fluids revealed by experiments and natural samples is different or even contradictory, especially in the mechanism of elements mobilization and fractionation. Here, we report a field-based geochemical study of amphibolites with extremely high Nb/Ta ratios (up to 98.7) and enriched HREE concentrations. The Paleoproterozoic amphibolites are intruded by a granitic dike with zircon UPb age of 132 ± 3 Ma. Felsic veinlets are observed in thin-sections of the amphibolite. Biotites with high F contents occur as aggregates along the boundary between the felsic veinlets and amphiboles, which indicates they are formed by reaction between amphibole and the granitic melt. The amphiboles have core-mantle-rim structure with high F actinolitic mantle surrounding low F magnesio-hastingsitic core. Some actinolites intergrow with biotites, titanites and magnetites. These evidences indicate that the actinolites are produced by hydrothermal metasomatism or interface-coupled dissolution–precipitation. The metasomatism occurred at 0.3–0.9 kbar and 500–650 °C within the hornfels facies. Hydrothermal titanites from amphibolites yield a lower intercept age of 131 ± 4.3 Ma, which is consistent with the crystallization age of the granitic dike. These observations indicate that the fluids are derived from fractional crystallization of a felsic magma for diking, and the released fluids are channeled along the interface between dike and amphibolite. The metasomatic minerals show high F and no Cl contents, indicating the hydrothermal fluids are high in fluorine (0.04–0.12 wt%) and low in chlorine. The F contents of minerals negatively correlate with HFSE, REE contents and (La/Yb)N ratios, but positively with Nb/Ta ratios. These reveal the progressive leaching and fractionation of HFSE and REE by the hydrothermal fluids. Mass balance estimates that most trace elements lose 20–60% during the hydrothermal alternation of amphibole. The calculation results also reveal that more Ta and LREE are lost than Nb and HREE, indicating Ta and LREE are more soluble than Nb and HREE, respectively, in the hydrothermal fluids. All these observations suggest that HFSE and REE are not only mobilized but also fractionated because of different degrees of mobilization by fluoride-bearing fluids.

U-Pb zircon and titanite age of the Christiansø granite, Ertholmene, Denmark, and correlation with other Bornholm granitoids

A granitic sample from the Danish island of Christiansø in the Ertholmene island group north of Bornholm is described petrographically and geochemically, and dated using U-Pb in zircon and titanite. Zircon systematics in the sample are complicated by abundant Pb loss and a large population of zircons interpreted as being inherited. Removal of highly disturbed zircons, imprecise analyses, and assumed inherited zircons yield an upper intercept date of 1500 ± 18 Ma (MSWD = 13, n = 58). Removal of zircons with high common Pb from this population yields an identical result of 1500 ± 22 Ma (MSWD = 8, n = 34). Zircons that are ≤3% discordant give a weighted average 206Pb/238U age of 1458 ± 12 Ma (MSWD = 3.0, n = 18), and a weighted average 207Pb/206Pb age of 1495 ± 14 Ma (MSWD = 4.7, n = 19). Titanites from the sample yield a lower intercept age of 1448 ± 15 Ma (MSWD = 6.8, n = 45). The sample contains a significant number of inherited grains indicative of ages around 1.7–1.8 Ga. The relatively large MSWDs for these age determinations indicate geological complexity, likely reflecting Pb loss, and the possible presence of inherited zircons which suffered major Pb loss during incorporation in the granitic magma. The zircon and titanite dates agree reasonably well with previous age determinations on felsic lithologies from the Bornholm mainland, as well as from the Blekinge Province of southern Sweden. Petrographically and geochemically, the Christiansø granite is indistinguishable from, and can be correlated with, the A-type granites and gneisses which occur on Bornholm. The high abundance of disturbed and inherited zircons (c. 1.7–1.8 Ga) may indicate that the granite was intruded into and assimilated a nearby region of unexposed Transscandinavian Igneous Belt rocks. The somewhat altered nature of the rock, and overall disturbance of U-Pb zircon systematics, suggest alteration associated with fluid-flow along nearby faults defining the northern margin of the Sorgenfrei–Tornquist Zone.

Apatite U-Pb Dating and Composition Constraints for Magmatic–Hydrothermal Evolution in the Giant Renli Nb-Ta Deposit, South China

Apatite is a nearly ubiquitous accessory phase in igneous rocks that crystallizes during the entire magma evolution process and has great implications for geochronology and petrogenesis. Previous studies suggested that Nb-Ta mineralization in the giant Renli deposit was genetically related to Late Jurassic two-mica monzogranite or Early Cretaceous muscovite monzogranite. Moreover, the magmatic–hydrothermal evolution of these two stages is poorly understood. In our study, we confirm that the muscovite monzogranite, biotite monzogranite, and two-mica monzogranite are all spatially associated with Nb-Ta pegmatites. We present new apatite U-Pb ages to constrain the timing of Nb-Ta mineralization and related magmatism. The results show that apatite from the two-mica pegmatite yield a lower intercept age of 130 ± 2 Ma (2σ), and apatite grains from two two-mica pegmatite samples yield a lower intercept age of 135 ± 8 Ma (2σ) and 134 ± 3 Ma (2σ), respectively. Apatite and whole-rock geochemistry suggest the oxidation degree of the Nb-Ta mineralization increases from north (RL-6) to south (RL-16) in the giant Renli deposit. This study demonstrates that a combination of apatite composition and U-Pb ages can be used to constrain the magmatic–hydrothermal evolution of granite and pegmatite-type Nb-Ta deposits.

Ta-Nb mineralization in the shallow-level highly-evolved P-poor Shihuiyao granite, Northeast China

The Shihuiyao granite district in the southern Great Xing'an Range, Northeast China, is a newly discovered area of rare-metal-bearing granites (RMG). The latest Jurassic Shihuiyao granitic pluton, emplaced into the Lower Permian Linxi Formation, comprises porphyritic granite, leucogranite, and amazonite-bearing pegmatite, all enriched in rare metals (mainly Ta, Nb, Rb). The Shihuiyao granites are peraluminous (A/CNK = 1.02–1.27), characterized by high contents of SiO2 (69.4–78.0 wt%), Al2O3 (12.7–16.9 wt%) and total alkalis (6.8–12.8 wt%), enriched in Ta, Nb, Rb, Cs, and Sn, but depleted in Ba and Sr. They have strong negative Eu anomalies (Eu/Eu⁎ = 0.02–0.42) and show the M-type REE tetrad effect. Low Nb/Ta (<4) and Zr/Hf (<8) values indicate high degree of magmatic fractionation. Whole-rock and mineral compositions variations indicate that the Shihuiyao granites are a typical low pressure and P-poor RMG, which experienced intensive magmatic fractionation followed by intensive hydrothermal reworking. The narrow range of δ7Li values (−0.86‰ to 0.88‰) and Rayleigh distillation modeling suggest that the porphyritic granite and leucogranite represent products of the same magma source, but evolved independently at depth. LA-ICP-MS UPb dating of monazite from the porphyritic granite and leucogranite yield lower-intercept ages of 150.2 ± 0.6 Ma and 147.0 ± 1.7 Ma, respectively. The 40Ar39Ar plateau ages of mica from leucogranite (146.3 ± 1.1 Ma) and an ore-bearing quartz vein (145.6 ± 1.0 Ma), and the UPb age of columbite (146.7 ± 4.7 Ma) further identify this as a latest Jurassic magmatic-hydrothermal mineralization event in NE China. Rapid crystallization and chemical quenching of magma resulted in neutralization of the acid fluoride solution, and a loss of the fluoride ligand during fluid evolution. This further triggered the subsequent crystallization of NbTa oxides. As revealed from geochemical and textural characteristics of the columbite group minerals, the high-degree of fractional crystallization and Ta-rich hydrosilicate fluid metasomatism are the main factors leading to the enrichment of TaNb within Shihuiyao RMG. In general, we conclude that rare metal-hosted magmatism took place in two episodes in NE China, namely, (i) Latest Jurassic to Early Cretaceous (150–130 Ma) peraluminous to metaluminous Ta-Nb-Sn-W RMG, and (ii) Early Cretaceous (130–120 Ma) alkaline Nb-Ta-Be-REE-Zr RMG, which highlight the prospecting fertility for Nb-Ta-REE (SnW) mineralization in NE China.

Exhumation of plutons controlled by boundary faults: Insights from the kinematics, microfabric, and geochronology of the Taibai shear zone, Qinling Orogen, China

Abstract The Taibai granitic plutons lie between the Taibai ductile shear zone to the north and the Shangdan suture to the south. The deformation mechanism of the ductile shearing is crucial to understanding the exhumation processes of the multiple plutons that formed after the Late Mesozoic period. Geological investigations, microstructures, and kinematic vorticity calculations indicate that the Taibai shear zone deformed in response to pure shear-dominated (54%–65%) transpression and top-to-NW shear sense as a result of NE–SW oblique contractional tectonics. The quartz crystallographic preferred orientations of the prism &amp;lt;a&amp;gt; slip system, the grain boundary migration, and sub-grain rotation dynamic recrystallization of quartz—combined with the plagioclase–hornblende thermometer—constrain the main deformation temperatures to a range of 400–650 °C, which suggests amphibolite to greenschist facies conditions. In addition, it is extremely likely that the mylonites experienced late-stage, lower temperature deformation as demonstrated by the sporadic bulging recrystallization, the quartz basal &amp;lt;a&amp;gt; slip system, and the two-feldspar geothermometer calculation. The samples collected from the weakly deformed mylonitic granite pluton and the undeformed quartz-feldspathic dike that intruded into the mylonites yield zircon U–Pb ages of 129 ± 1 Ma and 115 ± 1 Ma, respectively. This information, with the lower intercept ages of ca. 120 Ma obtained from the mylonite samples, suggests that the ductile shearing probably occurred from ca. 129 Ma to 115 Ma. Combined with the regional geological data, these findings suggest that the Taibai shear zone and the Shangdan suture accommodated the oblique upward extrusion of the Taibai plutons during Early Cretaceous time.

A newly identified Permian distal skarn deposit in the Western Tianshan, China: New evidence from geology, garnet U[sbnd]Pb geochronology and S[sbnd]Pb[sbnd]C[sbnd]H[sbnd]O isotopes of the Arqiale Pb[sbnd]Zn[sbnd]Cu deposit

The Arqiale PbZnCu deposit is located in the Wusun Range, Western Tianshan, China. Prograde and retrograde skarn minerals are abundant and associated with the PbZnCu mineralization. Given that the orebodies are stratabound and hosted in limestone and causative intrusions were not identified, the genetic relationship between mineralization and magmatic hydrothermal activity remains controversial. Geology, in-situ garnet LA-ICP-MS UPb dating and SPbCHO isotopes were carried out in this study, aiming to constrain the mineralization age, source of ore-forming components, and examine the link between mineralization and magmatism. In-situ garnet LA-ICP-MS UPb dating yielded a well-defined lower-intercept age of 268.6 ± 5.9 Ma, reflecting that the mineralization occurred much younger than that of ore hosting strata and formed in a post-collisional setting. In-situ δ34S values of sulfides (chalcopyrite, sphalerite and pyrite) range from −5.81‰ to −7.57‰ with an average of −6.37‰. The δ34S values of sulfide separates published previously vary from −7.1‰ to 1.3‰ (average −1.5‰). The in-situ206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of galena vary from 18.262 to 18.269, 15.561 to 15.567 and 38.049 to 38.065, respectively. The aforementioned S-Pb isotopic compositions indicate that the metals and sulfur were sourced partially from causative intrusion and partially from hosing sedimentary rocks. The δ13C (-2.6 – −0.9‰), δ18OV-SMOW (3.2 – 7.7‰), δ18OH2O (-5.2 – −0.7‰) and δDH2O (-128.0 – −109.1‰) values of ore-bearing calcite suggest that the ore-forming fluids were initially sourced from magmatic water exsolved from the hidden causative intrusions, with increasing input of meteoric water in later time. These geological, geochemical and isotopic data confirm that Arqiale is a typical distal skarn PbZnCu deposit. Considering the chalcopyrite-garnet-ilvaite contents increasing and Zn/Pb rations decreasing from the shallow part in the south to the deeper part in the north, it is inferred that the hidden causative intrusions and Cu mineralization within contact zone may occur at depth in the northern part of Arqiale.

U-Pb VEIN CALCITE DATING REVEALS THE AGE OF CARLIN-TYPE GOLD DEPOSITS OF CENTRAL YUKON AND A CONTEMPORANEITY WITH A REGIONAL INTRUSION-RELATED METALLOGENIC EVENT

AbstractLaser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of hydrothermal calcite associated with the waning stage of mineralization is used to constrain the age of Neoproterozoic-hosted Carlin-type gold deposits in central Yukon, Canada. Samples exhibiting minor textural complexity show low dispersion of individual spot analyses and define Tera-Wasserburg lower intercept ages between 75.1 ± 2.5 and 71.2 ± 4.3 Ma. Samples with U-rich microfractures and/or a second generation of calcite associated with vein reopening record the primary stage of calcite growth related to hydrothermal activity at ca. 74 Ma and a younger ca. 48 Ma event related to fluid-assisted U mobility. The minimum age for mineralization at ca. 74 Ma overlaps with a maximum age based on a zircon U-Pb date of 74.1 ± 1.0 Ma for a gabbro dike that is locally altered and mineralized. These time constraints indicate that mineralization is ca. 74 Ma and penecontemporaneous with dike emplacement. Furthermore, mineralization is contemporaneous with a regional gold-bearing, intrusion-related, metallogenic event in eastern Alaska and central Yukon. This temporal link between Carlin-type mineralization and other mineralization types bears similarities with world-class sedimentary rock-hosted gold deposits of Nevada. This study demonstrates the potential of using LA-ICP-MS U-Pb dating on calcite to constrain the age of mineral deposits lacking synmineralization minerals amenable to classic dating techniques.

Evolution of the Neoarchean lower crust beneath Western Liaoning of the North China Craton: Evidence from U-Pb-Hf-O isotopes in zircon

A comprehensive analysis of U-Pb geochronology and Hf-O isotopes of zircons from granulite xenoliths hosted in the Cretaceous basalt and zircon xenocrysts in the Jurassic andesitic volcanic rocks of Western Liaoning within the North China Craton (NCC) provides an opportunity to understand the evolution of the lower crust in the region. Zircons from these granulites yield 207Pb/206Pb ages of 1678–2646 Ma and consistent upper intercept ages of 2514–2544 Ma. In contrast, zircon xenocrysts yield two periods of 207Pb/206Pb ages of Neoarchean (2500–2591 Ma) and Paleoproterozoic (1733–2499 Ma), which produce consistent upper intercept ages of 2543–2549 Ma and lower intercept ages of 1761–1789 Ma, respectively. The Neoarchean zircons from the granulites and xenocrysts generally have similar positive εHf(t) values resulting in Hf depleted mantle model ages (TDM) of 2.5–2.9 Ga with a mode of 2.7 Ga and variable δ18O (5.05‰ to 8.81‰) values. However, the δ18O values of zircons from the xenocrysts are a little bit higher than those of zircons from granulites (peak of 8.00‰ for the xenocrysts and 6.00‰ for the xenoliths). Some Paleoproterozoic zircon xenocrysts have negative εHf(t) values (−12.8 to −9.39) and similar 176Hf/177Hf ratios and δ18O values to those of the Neoarchean zircons, whereas the other Paleoproterozoic zircons show chondritic εHf(t) (−4.34 to 1.16) and higher δ18O (7.70‰ to 10.19‰) values. All these data demonstrate that the primordial lower crust beneath Western Liaoning originally formed in the Neoarchean, perhaps around 2.6–2.7 Ga. The ubiquitous presence of ca. 2.5 Ga magmatic zircons illustrates that an important melting event indeed happened in the lower crust beneath Western Liaoning, similar to the thermal event throughout the entire NCC. In addition, the Neoarchean lower crust underwent another important thermal event during the Paleoproterozoic, which was considered to be associated with the final amalgamation of the Western and Eastern blocks of the NCC. Thus, we can conclude that there indeed existed an Archean lower crust beneath the Western Liaoning until to the Late Mesozoic, and this Archean lower crust underwent a similar multiple-stage (Neoarchean and Paleoproterozoic) evolutionary history as those throughout the NCC.