Coupled Plasma Mass Spectrometry Trace Research Articles

Genesis of the Hongtuling Au-Mo deposit in the southern North China Craton revealed from pyrite trace element compositions and S-Pb isotopes

The Hongtuling Au-Mo deposit on the southern margin of the North China Craton consists of Au veins and minor Mo veins that formed at ~130 Ma and ~204 Ma, respectively. Mo mineralization is dominated by molybdenite that is associated with K-feldspar, quartz and calcite, whereas Au mineralization occurs mostly as auriferous quartz-sulfides veins associated with alteration assemblages consisting of pyrite ± sericite ± quartz. Laser ablation inductively coupled plasma mass spectrometry trace element analysis shows that pyrite from Mo veins (Py-Mo) is relatively enriched in Mo and Pb, whereas pyrite from Au veins (Py-Au) contains much higher Au, Ag, Te, Cu, Pb, and Ni. Sulfide minerals and celestine from Mo veins range in δ34S values from −13.4 to −8.89‰ (n = 24) and 5.26 to 7.76‰ (n = 8), respectively. Extrapolation of sulfur isotopes of coexisting sulfides and celestine to zero sulfide-celestine fractionation on Δ34SClt-Py versus δ34S diagram shows that the hydrothermal fluid from which molybdenite precipitated has δ34S of 2.49 ± 3.93‰. In comparison, Py-Au yields δ34S values of −6.23 ~ 0.16‰, with a mean of −2.60‰ (n = 20). The Py-Mo and Py-Au have significantly different lead isotopic compositions, which form two distinct groups on the 207Pb/204Pb versus 206Pb/204Pb diagram, with Py-Au plotting close to the lead growth curve of the subcontinental lithospheric mantle of the North China Craton at ~130 Ma.Combined with previous studies, our new trace element and S-Pb isotope data suggest that Mo mineralization at Hongtuling is most likely related to magmas derived from partial melting of Late Archean to Paleoproterozoic metamorphic basement rocks under a post-collisional setting related to the collision between the North China Craton and Yangtze Craton, whereas Au mineralization is genetically associated with mantle-derived magmas generated in an extensional setting due to thinning and destruction of subcontinental lithospheric mantle keel beneath the North China Craton. As such, the Triassic Mo and Early Cretaceous Au veins can both be classified as magmatic-hydrothermal mineralization but are affiliated with two distinct systems. This study highlights the utilization of pyrite trace element and isotopic compositions in distinguishing multiple mineralization events in a single deposit.

Pyrite textures and compositions in Jiangshan gold deposit, Bengbu Uplift, southeastern North China Craton: Implications for ore genesis

Many Phanerozoic gold deposits in East China, e.g., those in the Jiaodong Peninsula, hosted either in granitoids of the same age or Precambrian basement rocks of the North China Craton, have been well documented. The Bengbu Uplift along the southeastern margin of the North China Craton is believed to be the western extension of the Jiaodong gold province, separated by the large-scale sinistral Tan-Lu Fault Zone. However, no deposits with more than 10 t gold resources have been discovered in the Bengbu Uplift, and it is unclear whether the timing and genesis of gold deposits in the Bengbu Uplift are similar to those in the Jiaodong gold province.In this study, laser ablation inductively coupled plasma mass spectrometry trace element analyses, mapping, and in situ sulfur isotope analyses were used to determine the source and process of gold mineralization in Jiangshan gold deposit, the largest in the region. The wall rocks in the Jiangshan gold deposit are metamorphic rocks of the Neo-Archaean Wuhe Group, and the ore bodies are controlled by the eastern part of the Guangou–Xiajijia Fault. Four stages were identified from cross-cutting relationships and textural and mineralogical characteristics: pre-ore stage (stage I), quartz–pyrite–muscovite alteration (stage II), pyrite–quartz veins (stage III), and quartz–pyrite veinlets (stage IV). Each of the four stages is characterized by a different type of pyrite, here named Py1, Py2, Py3 and Py4. Py1 formes the core of Py2, has elliptical and irregular shapes, contains low gold and arsenic concentrations, and has δ34S values of 0.86–2.86‰. Py2 is hosted in quartz–pyrite–muscovite-altered rocks with disseminated and aggregated euhedral to subhedral textures. It has elevated arsenic content and the lowest gold content, with some rhythmic zones and fractures. Stage II was crosscut by stage III. Py3 has anhedral textures with brittle deformation and occupies fractures in stage III pyrite–quartz veins. The gold content of Py3 is slightly higher than that of Py1 and Py2, but its arsenic content is extremely low. Py4 in the quartz–pyrite veinlets has cataclastic textures with the highest gold and chalcophile element contents. Native gold and electrum also formed in stage IV, which is the main gold stage in the Jiangshan deposit. The δ34S values of Py2–Py4 are broadly similar (5.79–7.98‰), suggesting a common sulfur source. Based on previous studies and higher δ34S values of hydrothermal pyrite, the fluid sulfur isotopes are interpreted to relate to degassing of the mantle wedge metasomatized by slab fluids during the westward subduction of the palaeo-Pacific plate beneath the North China Craton during the early Cretaceous. Based on the geological characteristics of the deposit, we classify the Jiangshan gold deposit as a Jiaodong fracture-disseminated-type gold deposit which occurs as disseminated veinlets and disseminated mineralization in wallrocks along fracture zones. Tectonic events may be the most important factor in gold mineralization in fracture-disseminated type gold deposits.

Distinguishing Ore Deposit Type and Barren Sedimentary Pyrite Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Trace Element Data and Statistical Analysis of Large Data Sets

Faced with ongoing depletion of near-surface ore deposits, geologists are increasingly required to explore for deep deposits or those lying beneath surface cover. The result is increased drilling costs and a need to maximize the value of the drill hole samples collected. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of pyrite is one tool that is showing promise in deep exploration. Since the trace element content of pyrite approximates the composition of the fluid from which it precipitated and the crystallization mechanism, the trace element characteristics can be used to predict the type of deposit with which a pyritic sample is associated. This possibility, however, is complicated by overlapping trace element abundances for many deposit types. The solution lies with simultaneous comparison of multiple trace elements through rigorous statistical analysis. Specifically, we used LA-ICP-MS pyrite trace element data and Random Forests, an ensemble machine learning supervised classifier, to distinguish barren sedimentary pyrite and five ore deposit categories: iron oxide copper-gold (IOCG), orogenic Au, porphyry Cu, sedimentary exhalative (SEDEX), and volcanic-hosted massive sulfide (VHMS) deposits. The preferred classifier utilizes in situ Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Tl, and Pb measurements to train the Random Forests. Testing of the Random Forests classifier using additional data from the same deposits and sedimentary basins (test data set) yielded an overall accuracy of 91.4% (94.9% for IOCG, 78.8% for orogenic Au, 81.1% for porphyry Cu, 93.6% for SEDEX, 97.2% for sedimentary pyrite, 91.8% for VHMS). Similarly, testing of the Random Forests classifier using data from deposits and sedimentary basins that did not have analyses in the training data set yielded an overall accuracy of 88.0% (81.4% for orogenic Au, 95.5% for SEDEX, 90.0% for sedimentary pyrite, 73.9% for VHMS; insufficient data was available to perform a blind test on porphyry Cu and IOCG). The performance of the classifier was further improved by instituting criteria (at least 40% of total votes from the Random Forests needed for a conclusive identification) to remove uncertain or inconclusive classifications, increasing the classifier’s accuracy to 94.5% for the test data (94.6% for IOCG, 85.8% for orogenic Au, 87.8% for porphyry Cu, 95.4% for SEDEX, 98.5% for sedimentary pyrite, 94.6% for VHMS) and 93.9% for the blind test data (85.5% for orogenic Au, 96.9% for SEDEX, 96.7% for sedimentary pyrite, 84.6% for VHMS). The Random Forests classification models for pyrite trace element data can be used as a predictive modeling tool in greenfield terrains by providing an accurate indication of ore deposit type. This advance will assist mineral explorers by allowing early implementation of predictive ore deposit models when prospecting for ore deposits. Furthermore, the ability of the classifier to accurately identify pyrite of sedimentary origin will allow researchers interested in paleoenvironmental conditions of ancient oceans to effectively screen prospective samples that are affected by a hydrothermal overprint.

Timing and evolution of Middle Triassic magmatism in the Southern Alps (northern Italy)

Middle Triassic magmatism in the Southern Alps (northern Italy) consists of widespread volcanoclastic deposits, basaltic lava flows and intrusive complexes. Despite their importance in understanding the geodynamic evolution of the westernmost Tethys, the timing of magmatic activity and the links between the different igneous products remain poorly understood. We present a comprehensive high-precision zircon U–Pb geochronology dataset for the major intrusive complexes and several volcanic ash layers and integrate this with a high-resolution stratigraphic framework of Middle Triassic volcano-sedimentary successions. The main interval of Middle Triassic magmatism lasted at least 5.07 ± 0.06 myr. Magmatic activity started with silicic eruptions between 242.653 ± 0.036 and 238.646 ± 0.037 Ma, followed by a <1 myr eruptive interval of voluminous basaltic lava flows. Coeval mafic to intermediate intrusions dated at 238.190 ± 0.055 to 238.075 ± 0.087 Ma may represent feeder and subvolcanic complexes related to the basalt flows. The youngest products are silicic tuffs from latest Ladinian to early Carnian sequences dated at 237.680 ± 0.047 and 237.579 ± 0.042 Ma. Complemented by zircon trace element data, our high-resolution temporal framework places tight constraints on the link between silicic and mafic igneous products in a complex geodynamic setting. Supplementary material: Isotope dilution thermal ionization mass spectrometry U–Pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables, sample coordinates, supplementary geochemical data, cathodoluminescence images of isotope dilution thermal ionization mass spectrometry dated zircons and supplementary field documentation are available at https://doi.org/10.6084/m9.figshare.c.4287506

High-resolution stratigraphy and zircon U–Pb geochronology of the Middle Triassic Buchenstein Formation (Dolomites, northern Italy): precession-forcing of hemipelagic carbonate sedimentation and calibration of the Anisian–Ladinian boundary interval

Orbitally forced cyclic variations in sedimentary sequences provide evidence for short-term fluctuations of Earth climate and a tool for high-resolution timescale calibration. We here present stratigraphic and geochronological evidence for precession-forcing in Middle Triassic hemipelagic limestones of the Buchenstein Formation (Dolomites, northern Italy). High-resolution stratigraphy of several correlative sections of the Buchenstein Formation documents a coherent cycle pattern. Isotope dilution thermal ionization mass spectrometry zircon U–Pb geochronology of tuffs bracketing the cyclic interval reveals an average cycle duration of 18.5 ± 2.1 kyr, consistent with a shorter climatic precession cycle in the Middle Triassic compared with today. This suggests a predominantly precession-controlled climate in this low-latitude setting of the western Tethys and allows high-precision calibration of the Anisian–Ladinian boundary interval. From integrating cyclostratigraphic and U–Pb geochronological constraints, our best estimate for the age of the Anisian–Ladinian boundary is 241.464 ± 0.064/0.097/0.28 Ma. We also provide precise estimates for lithostratigraphic boundaries, biostratigraphic markers and magnetic reversals within the boundary interval. Stratigraphic intervals with elevated sedimentation rate record a sub-Milankovitch signal that may be equivalent to patterns in adjacent carbonate platforms such as the Latemar platform. The origin of this sub-Milankovitch signal remains unknown but highlights the potential to investigate shorter-term climatic variations in Mesozoic sedimentary sequences. Supplementary material: Isotope dilution thermal ionization mass spectrometry U–Pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables as well as the results of the Bayesian age modelling are available at https://doi.org/10.6084/m9.figshare.c.3861817.

Zircon record of the plutonic-volcanic connection and protracted rhyolite melt evolution

The potential petrogenetic link between a crystal-poor rhyolite (the Rhyolite Canyon Tuff) and its associated subvolcanic intrusion and crystal-rich post-caldera lavas from Turkey Creek, Arizona (USA), is examined using zircon chemical abrasion–thermal ionization mass spectrometry U-Pb geochronology and inductively coupled plasma mass spectrometry trace element analyses. U-Pb ages indicate that zircon growth within the rhyolite and the dacite-monzonite porphyry magmas was coeval over ∼300 k.y. prior to the large eruptive event. Trends in zircon trace elements (Hf, Y/Dy, Sm/Yb, Eu/Eu*) through time in the dacitic-monzonitic units and rhyolite reflect melt evolution dominated by crystal fractionation. Importantly, the Y/Dy ratio in zircons in both units remains mostly similar for the first ∼150 k.y. of the system’s evolution, but the dominant population in the rhyolitic unit diverges from that of the dacite-monzonite porphyry ∼150 k.y. before eruption. We interpret this divergence in trace element composition to record the assembly time of the melt-rich cap within its intermediate mush zone in the upper crustal reservoir. These results are consistent with (1) a connection between plutonic and volcanic realms in the upper crust, (2) a protracted time scale for constructing an intermediate mush large enough to hold 500 km 3 of rhyolite, and (3) the prolonged extraction of that melt prior to eruption.

Major- and trace-element composition of REE-rich turkestanite from peralkaline granites of the Morro Redondo Complex, Graciosa Province, south Brazil

AbstractTurkestanite, a rare Th- and REE-bearing cyclosilicate in the ekanite–steacyite group was found in evolved peralkaline granitesfrom the Morro Redondo Complex, south Brazil. It occurswith quartz, alkali feldspar and an unnamed Y-bearing silicate. Electron microprobe analysis indicates relatively homogeneous compositions with maximum ThO2, Na2O and K2O contentsof 22.4%, 2.93% and 3.15 wt.%, respectively, and significant REE2O3 abundances(5.21 to 11.04 wt.%). The REE patterns show enrichment of LREE over HREE, a strong negative Eu anomaly and positive Ce anomaly, the latter in the most transformed crystals. Laser ablation inductively coupled plasma mass spectrometry trace element patterns display considerable depletions in Nb, Zr, Hf, Ti and Li relative to whole-rock sample compositions. Observed compositional variations suggest the influence of coupled substitution mechanisms involving steacyite, a Na-dominant analogue of turkestanite, iraqite, a REE-bearing end-member in the ekanite–steacyite group, ekanite and some theoretical end-members. Turkestanite crystals were interpreted as having precipitated during post-magmatic stages in the presence of residual HFSE-rich fluidscarrying Ca, the circulation of which wasenhanced by deformational events.

Petrology, Geochemistry and Geochronology of Kaua‘i Lavas over 4·5 Myr: Implications for the Origin of Rejuvenated Volcanism and the Evolution of the Hawaiian Plume

Kaua‘i lavas provide a unique opportunity to examine over 4·5 Myr of magmatic history at one location along the Hawaiian chain. New field, geochronological, petrological and geochemical results for a large suite of shield, post-shield and rejuvenated lavas are used to examine models for the origin of rejuvenated volcanism, and to evaluate the composition and structure of the Hawaiian plume. Kaua‘i has the most voluminous (∼58 km3 based on new field and water well interpretations) and longest-lived suite of rejuvenated lavas (∼2·5 Myr) in Hawai‘i. New K–Ar ages and field work reveal an ∼1 Myr gap (3·6–2·6 Ma) in volcanism between post-shield and rejuvenated volcanism. Isotopic and trace element ratios, and modeling of major elements of Kaua‘i’s rejuvenated lavas require low-degree melting (0·02–2·6%) at ∼1525 ± 10°C and 3·5–4·0 GPa of a heterogeneous, peridotitic plume source. High-precision Pb, Sr, Nd and Hf isotopic, and inductively coupled plasma mass spectrometry trace element data show substantial source variations with a dramatic increase in the depleted component in younger lavas. Some shield, post-shield and rejuvenated lavas (4·3–0·7 Ma) have high 208Pb*/206Pb* (radiogenic Pb produced since the formation of the Earth) values (>0·947) indicative of Loa-type compositions, the first reported Loa values in rocks >3 Ma, questioning previous models for the emergence of the Loa component in Hawaiian lavas. The timing, long duration, temporal variation in rock types and voluminous pulse of rejuvenated volcanism (58 km3), and the synchronous eruption of compositionally similar rejuvenated lavas, indicating tapping of common components along 350 km of the Hawaiian chain, are inconsistent with current models for this volcanism. Combining the lithospheric flexure and secondary zone of melting models provides a physical mechanism to initiate and focus the melting at shallower levels within the plume (flexural uplift) with a means to extend the duration of Kōloa volcanism at higher degrees of partial melting.

U-series Isotope Data on Lau Basin Glasses: the Role of Subduction-related Fluids during Melt Generation in Back-arc Basins

New thermal ionization mass spectrometry U-series disequilibrium data are presented for 24 basaltic to dacitic glasses from active spreading centres in the back-arc Lau Basin (SW Pacific), together with additional inductively coupled plasma mass spectrometry trace element analyses and Sr–Nd–Pb isotope data. Valu Fa Ridge samples, adjacent to the arc front, have high U/Th and (230Th/238U) <1, implying a recent (≪350 ka) addition of a U-rich slab-derived fluid. The Valu Fa data can be combined with existing 230Th–238U data for the Central Tonga arc to infer a fluid addition event at ∼50 ka. The similar sources and time scales for fluid transfer beneath the Valu Fa Ridge and beneath the arc itself suggest that the Valu Fa Ridge is propagating into the arc-front region. Central Lau Basin samples, further behind the arc, have lower U/Th and (230Th/238U) ≥1, similar to typical mid-ocean ridge basalts (MORB). Within the Central Lau Basin, a water-rich subduction component is seen only in samples closest to the arc, and this fluid does not have the high-U/Th composition of the fluid at Valu Fa. Melt generation in the Central Lau Basin appears to be dominated by normal ridge-type processes, but the relatively low (230Th/238U) for these shallow ridges compared with global MORB could be a consequence of increased melt productivity as a result of the elevated water contents. The transition from 238U to 230Th excesses within the back-arc basin is not a smooth function of decreasing addition of a U-rich fluid moving away from the arc front, but also reflects the complex dynamics between two major mantle domains within the mantle wedge (‘Pacific’ beneath Valu Fa Ridge, ‘Indian’ beneath the Central Lau Basin).

A micro-scale mercury cathode electrolysis procedure for on-line flow injection inductively coupled plasma mass spectrometry trace elements analysis in steel samples

An on-line matrix-analyte separation technique was developed for flow injection inductively coupled plasma mass spectrometry (FI–ICP-MS) trace analysis. A micro electrolytic cell was designed to be inserted in the FI manifold. The technique was used to separate Zr, Hf, Y, rare earth elements (REEs), Th and U from a steel-matrix (Fe, Cr, Ni, Co, Mn and Mo). A microwave-assisted HNO 3–HCl–HF–H 2SO 4 digestion procedure, with temperature/pressure regulation, was used for sample dissolution. Obtained solutions were evaporated to SO 3 fumes, and 2 ml of this diluted sulphuric solution were introduced in the electrolytic cell through the manifold circuit. After matrix removal, the electrolyte was conducted to load a 300 μl sample loop to be injected into the plasma torch. Direct multielement standard solutions in diluted sulphuric acid (without matrix matching and sample pretreatment) were applied for external calibration. The determination limits, with reference to the solid, were improved by a factor of about 10 compared with that obtained from direct measurements of 0.1% (m/v) sample solutions. The relative standard deviations for all the analytes were better than 3.5% for concentrations above 10 times the limit of quantification. The developed method was applied in the determination of certified elements in Steel Reference Materials: NIST 363 and NIST 364. Recoveries from 0.200 g test portions of high-purity iron spiked at two different concentration levels were found better than 97%.