Characteristics and depositional environments of the source rocks of the ore-bearing strata in the Dahenglu Cu-Co deposit:Constraints on the cobalt enrichment
The Dahenglu Cu-Co deposit,located in the northern part of the Paleoproterozoic Jiao-Liao-Ji orogenic belt,is a large metasedimentary-hosted Cu-Co deposit.Currently,there is a lack of research on the relationship between the sedimentary source feature and depositional redox condition of the ore-bearing strata and the cobalt mineralization.This study selects the representative borehole samples that are minimally affected by deformation,hydrothermal and epigenetic oxidation alteration,as well as the surface meta-siliciclastic and carbonate rocks of the Laoling Group,as key research objects.Detailed petrographic,whole-rock geochemistry and stable isotope studies are conducted to compare the differences between the ore-bearing and barren samples.Geochemical characteristics reveal that the ore-bearing samples of the Dalizi Formation have relatively higher Zr/Sc ratios,and Fe2O3T and Sc contents compared to meta-siliciclastic samples from other formations of the Laoling Group,indicating more detritus derived from the Late Neoarchean and Middle Paleoproterozoic mafic rocks in the ore-bearing strata.This is consistent with the fact that a higher proportion of detrital zircons with Hf content less than 8300 × 10-6 and εHf(t)values greater than+2.3 occurred in the Dalizi Formation.When compared to the Co-poor samples,the Co-rich ones exhibit higher corrected CIA(chemical index of alteration;＞83),CIW(Chemical Index of Weathering;＞89)and PIA(Palgioclase Index of Alteration;＞85),indicating a higher degree of chemical weathering in the source rocks,which leads to higher clay mineral contents in the ore-bearing rocks.Additionally,the Co-rich samples display lower Fe3+/Fe2+ratios,suggesting a more reducing depositional environment.In summary,the deposition of sericite phyllites of the Dalizi Formation which is also the host rock of the Dahenglu Cu-Co deposit was characterized by higher mafic source compositions with higher clay mineral contents resulting from a higher degree of chemical weathering in the source rocks and a relatively reduced depositional environment,and thus provided important metallogenic conditions for their more colloidal pyrites in the sedimentary stage and precipitation of Co-bearing sulfides in the post-sedimentary deformation and hydrothermal stages.
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