Tracing The Ore Forming Process With In Situ LA-(MC)-ICP-MS Techniques

This study describes the developing of several in situ laser ablation(multiple collector)inductively coupled plasma mass spectrometry techniques(LA-(MC)-ICP-MS)and their applications in tracing the ore forming processes.Besides,technical details and the the limitations of each technique are also pointed out according to the current scope of the author.Part of these techniques are brand new(e.g.in-situ S isotope mapping with LA-MC-ICP-MS and U-Pb dating of hematite),the others have already been achieved by international peers.From the site of the applications in geosciences,these techniques are effective,however,the isotopic or elemental fractionation behaviors behind each technique are not well understood at present.Hence,this study tries to reveal the unknown fractionations,as well as developing these techniques.This study reports a detailed evaluation of how key parameters of operation influence the measurement of sulfur isotopes using laser ablation multiple collector inductively coupled plasma mass spectrometry.Sulfur isotopes are observed to display a large fractionation(up to 2‰ ?34S)during analysis of pyrite with different laser parameters using a 193nm ArF excimer laser.In order to understand why the laser parameters control S isotope fractionation when measuring S isotopes in pyrite,Scanning Electron Microscopy(SEM)and Transmission Electron Microscopy(TEM)techniques were used to characterize debris formed during the ablation of pyrite,i.e.,morphology and speciation of phases.The results show that pyrite decomposes to two phases:ball-like troilite(FeS);and a sulfur-rich floc-like agglomeration surrounding the troilite.The measured ?34S varies due to the different proportions of troilite balls and the floc-like material generated under different laser parameters.The proportion of troilite and S was evaluated with a LA-(Quadrupole)-ICP-MS through direct comparison of the counts per second(CPS)ratio of 56Fe to 32S.In contrast to pyrite,natural pyrrhotite shows no decomposition process and the particle size of the debris from pyrrhotite is nearly 10 times larger than that of pyrite(?5 ?m for pyrrhotite compared to<1 ?m for pyrite).Therefore,a biased analysis of pyrite may happen using laser ablation although this problem can be minimized using high raster velocity.Last but not least,we provide a case study of S isotope mapping using high raster velocity,which extends the application of the in-situ S isotope analysis technique.The results here carry implications for the choice of settings needed to obtain accurate LA-MC-ICP-MS S-isotope maps of pyrite.U-Pb dating of the common iron-oxide hematite,using LA-ICP-MS,provides unparalleled insight into the timing and processes of mineral deposit formation.Until now,the full potential of this method has been negatively impacted by the lack of suitable matrix-matched standards.To achieve matrix-matching,we report an approach in which a U-Pb solution and ablated material from 99.99%synthetic hematite are simultaneously mixed in a nebulizer chamber and introduced to the ICP-MS.The standard solution contains fixed U-and Pb-isotope ratios,calibrated independently,and aspiration of the isotopically homogeneous solution negates the need for a matrix-matched,isotopically homogenous natural iron-oxide standard.An additional advantage of using the solution is that the individual U-Pb concentrations and isotope ratios can be adjusted to approximate that in the unknown,making the method efficient for dating hematite containing low(?10 ppm)to high(>1 wt%)U concentrations.Downhole fractionation of 206Pb/238U is observed to occur in spot analyses of hematite.The use of rasters in future studies will hopefully minimize this problem,allowing for matrix-matched data.Using the mixed-solution method in this study,we have validated a published hematite Pb-Pb age for Olympic Dam,and provide a new age(1604 ± 11 Ma)for a second deposit in the same province.These ages are further evidence that the IOCG mineralizing event is tied to large igneous province(LIP)magmatism in the region at?1.6 Ga.In-situ Sr isotope and trace elements analysis of apatite are applied to investigate the ore bearing and ore barren granodiorite in middle-lower Yangtze metallogenic belt.Distinguished differences of Sr isotope and La/Yb ratio of the two kinds granodiorite may imply the contribution of recycled continental materials.Trace elements mapping of sulfides from Chenmenshan Cu deposit suggests that the stratiform ores have suffered from hydrothermal superposition.The precious metals are enriched and migrated by the late stage hydrothermal events in Yanshannian.