Paleoredox conditions, hydrothermal history, and target vectoring in the MacMillan Pass base-metal district, Yukon, Canada: 1 – Lithogeochemistry of proximal and distal shales

ABSTRACT The MacMillan Pass District in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Bulk geochemical paleoredox proxies (Eu/Eu*, Ce/Ce*, Mo, Re/Mo, and Ni/Co) indicate anoxic–dysoxic water column and sulfidic porewater conditions persisted during the Late Devonian deposition of the Lower Earn Group host rocks. Positive Eu/Eu* anomalies (up to 3.31) in sulfide mineralization at the Tom deposit are consistent with relatively high temperature (probably >250 °C), reducing, acidic hydrothermal fluids that infiltrated laterally through unconsolidated sediments proximal to the hydrothermal upflow zone and/or exhaled at the seafloor as moderate- to high-density brines. Molybdenum and U enrichment factors (relative to upper continental crust) and Mo/organic C values are consistent with a moderately restricted basin; Mo/C values fall between those of the Black Sea (highly restricted) and the Framvaren Inlet (moderately restricted). A Ba-rich shale was identified in rocks that are distal and time-equivalent units to the Pb-Zn mineralization; based on the bulk chemical compositions and on previous S and Sr isotope studies, we interpret the baryte in this unit to be largely hydrothermal in origin and perhaps remobilized and reprecipitated during hydrothermal base-metal mineralization.

Base metal mineralization of the Kolyma terrain in Northeast Russia: Overview and genetic classification

Research subject. The Prikolyma terrain located in the Northeastern part ofRussia constitutes a long-lived Precambrian thrust-faulted structure hosting numerous Cu, Pb and Zn deposits of different types.Materials and methods. The mineralization of the terrain was examined during a course of research and exploration works over the 2007–2012. The rock geochemistry was studied using ICP-OES analysis at the Stuart Geochemistry and Essay laboratory (Moscow). The microprobe analysis of minerals was carried out at the facilities of the Far Eastern Branch of the Russian Academy of Sciences (Magadan) using a Camebax X-ray microanalyzer. The isotopic ratios of sulphur in sulphides were measured using a Finnigan MAT 253 isotope mass spectrometer.Results. The porphyry-copper deposit Nevidimka is represented by skarns and sulphide-quartz stockworks embedded in porphyry granites. The vein deposits Opyt and Glukhoye constitute sulphide-carbonate-quartz veins, the composition of which corresponds to copper-polymetallic ores of the peripheral parts of the copper-porphyry formation. These deposits feature a similar geochemistry and composition of sulphides and sulphur isotopes, which is characteristic of the Riphean complexes of the Prikolyma terrain. The stratiform Pb-Zn veins Nadezhda-3 and Khaya enclosed in Proterozoic dolomites represent parallel-bedding disseminated sulphides. The composition of these ores indicates their diagenetic origin. Tne stratiform copper deposit Oroyok is embedded in Proterozoic shales and can be referred to sediment-hosted copper deposits of a transgressive type.Conclusions. The diversity of Cu-Pb-Zn mineralization types in the Prikolyma terrain is established to have resulted from multiple cyclic changes of the geodynamic ore formation regime. During each such cycle, syngenetic mineralization was followed first by epigenetic and then by vein mineralization. The mobile, thrust-faulted structure caused repeated rejuvenation of ores, which inherited the geochemical features of hosting rocks.

Geochronological Study of the Jiashengpan Zn–Pb Deposit in Northern China: Implications for Base Metal Mineralization in Collisional Orogens

The Jiashengpan Zn–Pb deposit is located in the Langshan-Zhaertai region of the North China Craton. Zinc-lead mineralization at the Jiashengpan shows characteristics of shear-zone controlled syn-metamorphic mineralization. The 39Ar/40Ar ages of syn-ore hydrothermal muscovite averages at ~380 Ma, suggesting that the Zn–Pb mineralization took place in the Devonian. These results agree with zircon U–Pb ages of post-ore granite, which constrain the ore formation to be older than 277 ± 2 Ma. Ore formation was coeval with the emplacement of regional orogenic belts that formed as result of subduction associated with the closure of the eastern Paleo-Asian Ocean (~400 Ma). The Jiashengpan deposit provides evidence for base metal mineralization associated with metamorphogenic and shear-zone controlled characteristics during continental-continental collision, stressing the significance of orogenic activities for enrichment of base metals.