•Achieving the combination of the micro-LA technique and chemical preconcentration•Finding a sucrose–H3PO4 medium and perfluoroalkoxy film to prepare tiny LA-objects•Obtaining high sensitivity and low memory effect for Os determination by LA-ICP-MSA method was developed for the determination of trace Os in geological samples by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with the combination of chemical separation and preconcentration. Samples are digested using aqua regia in Carius tubes, and the Os analyte is converted into volatile OsO4, which is distilled and absorbed with HBr. The HBr solution is concentrated for further Os purification using the microdistillation technique. The purified Os is dissolved in 10 μl of 0.02% sucrose–0.005% H3PO4 solution and then evaporated on pieces of perfluoroalkoxy (PFA) film, resulting in the formation of a tiny object (< 3 × 104 μm2 superficial area). Using LA-ICP-MS measurements, the object can give Os signals at least 100 times higher than those provided by routine solution-ICP-MS while successfully avoiding the memory effect. The procedural blank and detection limit in the developed technique are 3.0 pg and 1.8 pg for Os, respectively when 1 g of samples is taken. Reference materials (RM) are analyzed, and their Os concentrations obtained by isotope dilution are comparable to reference or literature values. Based on the individual RM results, the precision is estimated within the range of 0.6 to 9.4% relative standard deviation (RSD), revealing that this method is applicable to the determination of trace Os in geological samples.

A method for quasi-simultaneous determination of U-Pb ages and Hf isotope compositions on a single zircon spot is developed, by use of a multiple-collector ICPMS coupled with an excimer laser-ablation system. The ICPMS used in this study is equipped with variable zoom lens to change the dispersion of the analyzer and bring ion beams into the collector coincidentally. This feature allows rapid switching between U-Pb and Hf collector configuration, which is used to quasi-simultaneously determine the zircon U-Pb and Hf isotope compositions. The analytical results on three reference zircon standards, 91500, CZ3 and Plesovice and one nature zircon sample, agree with literature/known values, thereby demonstrating feasibility of the method.

•At Marcona Fe-Cu deposit, ore-forming fluids of sulfide-bearing stages experienced from primarily magmatic hydrothermal (magnetite-sulfide stage), to non-magmatic dominated (probably seawater, sulfide stage). Organic matter may have been involved at late stage.•Ag and Zn are concentrated in chalcopyrite but not other sulfides.•The sulfide mineralization at Marcona deposit share similarities on ore genesis with the nearby Mina Justa IOCG deposit.The Marcona magnetite deposit, located in the iron oxide copper-gold (IOCG) province of southern Perú, is an important Fe deposit with sub-economic Cu and Zn mineralization. Three generations of sulfides were identified at the Marcona alteration/mineralization Stages IV, V and VII. Magnetite, pyrrhotite, pyrite and minor chalcopyrite started to form at Stage IV. Sulfides in Stage V are dominated by chalcopyrite and pyrite, whereas only pyrite was present in Stage VII. In-situ SIMS sulfur isotope and LA-ICP-MS trace element analyses were conducted on the sulfides of these stages. Stage IV sulfides contain δ34S largely in the range of 0 to +4.0‰, indicating a dominantly magmatic origin. In particular, δ34S values of Stage IV pyrite increase from core to rim, and the corresponding trace elements concentrations vary along with δ34S. Positive δ34S vs. Se/S correlation of Stage IV pyrite indicates that the ore-forming system was open. The Stage V pyrite are rich in Mn, Cu, Ag, Sb, Te, Pb and Bi, and all Stage V sulfides have also elevated δ34S (up to +7.8‰), indicative of possible seawater incursion during its formation. The δ34S values of Stage VII pyrite are highly variable from −13.4 to 5.4‰. This, together with ubiquitous disseminated voids, low Co/Ni ratios and enrichments of Mn, Se, Sb, Te, Tl and Pb, suggests that Stage VII pyrite may have formed by sulfate reduction associated with organic materials. We propose that the ore-forming fluids of Stage IV are initially magmatic with involvement of external (e.g., seawater) at late, where the fluids of Stage V are likely mainly exotic. The external fluids, together with organic matter, may have contributed to the ore-forming fluids of Stage VII, indicating a complex fluid evolution history for this giant Fe (-Cu) deposit.Download high-res image (336KB)Download full-size image

•The Cambrian to Upper Devonian western Yangtze sandstones contain a diagnostic detrital zircon age peak at 730–900 Ma•The Silurian–Devonian sandstones–siltstone samples from the Mojiang–Lvchun area of the Ailaoshan Belt display two distinctive detrital zircon age peaks at ca. 450 Ma and 950 Ma.•The Yangtze/Indochina–Simao suture may lie along or in close proximity to the Ailaoshan–Tengtiaohe fault.•The area may have developed a wide continental rift since the Early Silurian.The Indochina–Simao and Yangtze blocks were separated by a branch of the Paleo-Tethys Ocean, commonly referred as the Ailaoshan Ocean in the Paleozoic. Remnants of this Ailaoshan Ocean have been variably suggested to locate along (from east to west) the Ailaoshan fault, Jiujia–Anding fault and Amojiang–Lixianjiang fault. In order to test these models, we have carried out comprehensive detrital zircon U–Pb dating and Hf isotope analyses on the Cambrian to Devonian sedimentary units in the Ailaoshan Belt and its adjacent western margin of the Yangtze Block. Our results indicate marked detrital zircon provenance variation on different sides of the Ailaoshan–Tengtiaohe fault: detrital zircons from east of the fault display a diagnostic age peak at 730–900 Ma, which is characterized by both positive and negative εHf(t) values with a Hf model age (TDMC) peak at ~ 1.8 Ga, whereas detrital zircons from west of the fault display two major age populations of 400–500 and 900–1000 Ma, both characterized by mainly negative εHf(t) values with a Hf model age (TDMC) peak at ~ 2.1 Ga. Our new data indicate that detritus from east and west of the Ailaoshan–Tengtiaohe fault may have been mainly derived from, respectively, the Yangtze Block and Indochina–Simao blocks, thus suggesting the fault may represent the actual suture between the two blocks. Our study also reveals that the Ailaoshan Ocean may have started its early continental rift in the Early Silurian.Download high-res image (150KB)Download full-size image

A method for quasi-simultaneous determination of U-Pb ages and Hf isotope compositions on a single zircon spot is developed, by use of a multiple-collector ICPMS coupled with an excimer laser-ablation system. The ICPMS used in this study is equipped with variable zoom lens to change the dispersion of the analyzer and bring ion beams into the collector coincidentally. This feature allows rapid switching between U-Pb and Hf collector configuration, which is used to quasi-simultaneously determine the zircon U-Pb and Hf isotope compositions. The analytical results on three reference zircon standards, 91500, CZ3 and Plesovice and one nature zircon sample, agree with literature/known values, thereby demonstrating feasibility of the method.