Provenance and geochemistry of sedimentary components in the Volcano-Sedimentary Complex, Iberian Pyrite Belt: discrimination between the sill-sediment complex and volcanic-pile models

Two highly contrasting models have been proposed for the palaeovolcanological setting of the massive sulphide deposits in the Iberian Pyrite Belt. The long-standing view of the host rocks is that they are a pile of effusive and pyroclastic rocks but this position has now been challenged by the proposal that high-level peperitic sills predominate. Discrimination between the volcanic-pile and sill–sediment-complex models is important because they lead to very different conclusions about such key metallogenic features as the timing of mineralization, the nature of the ore-forming convective system and the source of the metals. Sedimentary geochemistry, particularly REE and Ti/Nb, shows that there is no correspondence between the chemistries of mafic igneous sheets and intercalated stratified-volcaniclastic rocks that range from andesite to rhyodacite in composition. Therefore none of the mafic sheets supplied detritus to the sedimentary environment. Sedimentary rocks of continental provenance persist throughout the host-rock sequence, especially in mineralized regions, implying confinement of the majority of primary volcanic facies in the form of high-level intrusions. Some andesitic and felsic intrusions created a minor, stratified volcaniclastic component via hydrovolcanic eruptions. The volcanic-pile model is invalidated because the expected provenance patterns for this volcanic style are not present and the sill–sediment-complex setting of the sulphide deposits is confirme

3-D Mohr circle analysis of vein opening, Indarama Iode-gold deposit, Zimbabwe: implications for exploration

The Indarama lode gold deposit is hosted by vertically-dipping basalt in the Late Archaean Midlands Greenstone Belt of Zimbabwe. Major deformation events at 2.68 and 2.58 Ga established a complex array of fractures. A limited range of orientations of this fracture network opened towards the end of the younger deformation event, creating a lode pattern where 92% of mineralised veins dip at less than 50°, mainly to the E and W, and most strike directions are represented. A clustered distribution of poles to the quartz–carbonate veins indicates a constrictional stress field at the time of vein opening where 1 and 2 were near horizontal, (directed NNW–SSE and ENE–WSW, respectively), and 3 was near vertical. 3-D Mohr circle analysis demonstrates that 2 was approximately 67% of 1 (the stress ratio) and that the driving pressure ratio (R?) was approximately 0.4, reflecting the role of fluid pressure, mean stress, and the maximum shear stress in controlling conditions of fracture opening