Ore Genesis and Geodynamic Setting of Laochang Ag-Pb-Zn-Cu Deposit, Southern Sanjiang Tethys Metallogenic Belt, China: Constraints from Whole Rock Geochemistry, Trace Elements in Sphalerite, Zircon U-Pb Dating and Pb Isotopes

The Laochang Ag-Pb-Zn-Cu deposit, located in the southern margin of the Sanjiang Tethys Metallogenic Belt (STMB), is the typical Ag-Pb-Zn-Cu deposit in this region. Its orebodies are hosted in the Carboniferous Yiliu Formation volcanic-sedimentary cycle and occur as stratiform, stratoid and lenticular. Whether or not the stratabound ore belong to the volcanogenic massive sulfide (VMS) deposit remains unclear and controversial. In this paper, the whole rock geochemistry, trace elements in sphalerite, U-Pb zircon chronology and Pb isotopes were investigated, aiming to provide significant insights into the genesis and geodynamic setting of the Laochang deposit. Lead isotope ratios of pyrite and sphalerite from the stratabound ore are 18.341 to 18.915 for 206Pb/204Pb; 15.376 to 15.770 for 207Pb/204Pb; and 38.159 to 39.200 for 208Pb/204Pb—which display a steep linear trend on Pb-Pb diagrams. This indicates a binary mixing of lead components derived from leaching between the host volcanic rock and mantle reservoir. Sphalerite from stratabound ores is relatively enriched in Fe, Mn, In, Sn, and Ga—similar to typical VMS deposits. Moreover, the Carboniferous volcanic rock hosting the stratabound Ag-Pb-Zn-Cu ores has a zircon U-Pb age of 312 ± 4 Ma; together with previous geochronological and geological evidences, thus, we consider that the stratabound mineralization occur in the Late Paleozoic (~323–308 Ma). Collectively, these geologic, geochemical, and isotopic data confirm that the stratabound ores should be assigned to Carboniferous VMS mineralization. In addition, volcanic rocks hosting the stratabound ore exhibit elevated high field strength elements (HFSEs, Nb, Ta, Zr and Hf) abundance, slight enrichment of light rare earth element (LREE), and depletion of Ba and Sr with obvious Nb-Ta anomalies. Such characteristics suggest that their magma is similar to typical oceanic island basalt. In addition, the oceanic island basalt (OIB)-like volcanic rocks were formed at Late Paleozoic, which could be approximately synchronous with the VMS mineralization at Laochang. Thus, it is suggested that the Laochang VMS mineralization was generated in the oceanic island setting prior to the initial subduction of the Changning-Menglian Paleo-Tethys Ocean.

Igneous Rock Associations 18. Transition Metals in Oceanic Island Basalt: Relationships with the Mantle Components

Incompatible elements and isotopic ratios identify three end-member mantle components in oceanic island basalt (OIB); EM1, EM2, and HIMU. We estimate compatible to mildly incompatible transition metal abundance trends (Ni, Co, Fe, Cu, Cr, V, Mn, Sc, and Zn) in “primitive” basalt suites (Mg# = Mg/(Mg + 0.9*Fe) atomic = 0.72) from 12 end-member oceanic islands by regressing metals against Fe/Mg ratios in sample suites, and solving for concentrations at Mg/Fe = 1 (Mg# = 0.72). Using the transition metal estimates, exploratory statistics reveal that islands ‘group’ based on mantle component type even when La/Yb ratios are used to compensate metal concentrations for percentage melting. Higher chalcophile Zn (and Pb, earlier work) in EM1 and EM2 compared to HIMU, and higher Cr (3+) and Sc in HIMU relative to EM1, support views that HIMU represents subduction-processed ocean floor basalt. Incompatible elements, ratios and isotopes indicate that EM1 is Archean, EM2 is Proterozoic or younger, and both are related to sediment subduction. As found with incompatible elements, EM1 and EM2 show similar ‘compatible’ element concentrations, but lower (multi-valence) Cr, Fe and Mn in EM1 could indirectly reflect increasing oxidation of subducted sediment between the Archean and Proterozoic. Alternatively, changes in subduction processes that yielded peak continental formation in the Neoarchean, and craton-suturing in the Paleoproterozoic may account for EM1–EM2 differences. EM1 shows similar or lower Cr, Ni and Co compared to HIMU and EM2 suggesting that economic viability of layered intrusions, which have extreme EM1-like signatures, is unrelated to high metals in EM1 mantle sources, but that high % melting appears important. Because core-concentrated transition metals correlate with mantle component type, lithospheric recycling apparently controls their concentrations in OIB and core-mantle interaction may be unimportant.RÉSUMÉLes éléments incompatibles et les rapports isotopiques permettent de délimiter trois termes extrêmes de composants mantéliques dans des basaltes insulaires océaniques (OIB), soit EM1, EM2, et HIMU. Nous estimons les tendances d’abondance de métaux de transition (Ni, Co, Fe, Cu, Cr, V, Mn, Sc, and Zn) compatibles à modérément incompatibles dans des suites de basaltes « primitifs » (Mg# = Mg/(Mg + 0,9*Fe) rayon atomique = 0,72) sur 12 termes extrêmes de matériaux insulaires océaniques, par régression des concentrations des métaux sur les rapports Fe/Mg dans des échantillons des suites, la détermination étant définie au rapport Mg/Fe = 1 (Mg# = 0,72). L’utilisation d’une approche statistique exploratoire sur les estimations de métaux de transition montre que la composition des îles se « regroupent » en fonction du type de composition du manteau, cela même lorsque les ratios La/Yb sont utilisés pour compenser les concentrations de métaux pour déterminer le pourcentuel de fusion. Le caractère plus chalcophile du Zn (et Pb, travail antérieur) dans EM1 et EM2 comparé à HIMU, et la plus grande teneur en Cr (3+) et Sc dans HIMU par rapport à EM1, accréditent l’idée que HIMU représente le basalte de subduction des fonds océaniques. Les éléments incompatibles, les ratios et les isotopes montrent que EM1 est archéen, que EM2 est protérozoïque ou plus jeune, et que les deux sont liés à la subduction sédimentaire. Comme constaté pour les éléments incompatibles, EMI et EM2 affichent une compatibilité similaire des concentrations en éléments « compatibles », toutefois une concentration inférieure en Cr (multivalent), Fe et Mn dans EM1 pourrait refléter indirectement une oxydation croissante des sédiments subduits entre l’Archéen et le Protérozoïque. Par ailleurs, les changements dans les mécanismes de subduction qui ont mené à un maximum de formation continentale au Néoarchéen et à des épisodes de sutures cratoniques au Paléoprotérozoïque, peuvent expliquer les différences entre EM1 et EM2. La teneur similaire ou inférieure en Cr, Ni et Co de EM1 par rapport à HIMU et EM2 permet de croire que la viabilité économique des intrusions stratifiées – lesquelles montrent des signatures extrêmes EM1 – est sans rapport avec les sources mantéliques à fortes teneurs en métaux, mais que le fort pourcentuel de fusion qui importerait. Parce que la concentration du noyau en métaux de transition correspond avec le type de composant du manteau, c’est le recyclage lithosphérique qui contrôle apparemment leurs concentrations dans l'OIB, et l'interaction noyau-manteau pourrait être sans importance.