LUFILFLOW · Pan-African Fluid Flow Reconstruction in the Lufilian Arc (Democratic Republic of Congo and Zambia): A study Using Quantitative Fluid Inclusion Analysis and Isotope Modelling

Final Activity Report Summary - LUFILFLOW (Pan-African fluid flow reconstruction in the Lufilian Arc (Democratic Republic of Congo and Zambia): A study using quantitative fluid inclusion analysis ...)

The mineral wealth of Central Africa can play an important role in future sustainable economic development of the region. A key factor in successful and responsible exploitation of these natural resources is the understanding of the genesis and geological relationships of ore deposits. This project aimed at unravelling the nature and evolution of the mineralising fluids that formed post-orogenic vein-type base metal sulphide deposits by investigating the traces of the fluids trapped as micrometre-sized fluid inclusions in ore and gangue minerals. To do this, two known ore deposits were selected for detailed investigation, one (Kipushi) situated in the Lufilian fold-and-thrust belt (known as the "copperbelt") and one in the Lufilian foreland (Dikulushi). The historically important mine of Kipushi (now largely abandoned) was digitally reconstructed between mine levels 0 and -700m based on archives and historical sample collections. Subsequently, renewed sampling of the historical collections based on the 3D deposit model enabled the selection of representative samples for further study. Petrographic and microthermometric study of primary fluid inclusions in ore and gangue minerals showed that the Kipushi mineralisation was formed at app.300°C at confining pressures of app.2kbar. The mineralising fluids were highly saline brines (app.35 eq. wt% NaCl) that resulted from an immiscibility process of a primary aqueous-carbonic saline fluid. The elemental composition of the fluid inclusions was analysed with bulk crush-leach techniques, and by analysis of individual inclusions using laser ablation ICP-MS. Optimisation of the latter technique formed the major analytical challenge of the project, and initiated the development of a new approach for analysis, which involves the laser ablation sampling of frozen fluid inclusions in a cryogenic ablation cell. The major, minor and trace element content of the mineralising fluids showed that they were very highly evolved in terms of water-rock interaction. This is suggestive of their derivation from a deep crystalline basement, or could even be indicative for the involvement of magmatic fluids in the mineralising process. Younger fluids that flowed through the Lufilian fold-and-thrust belt were also saline brines, but gained their salinity mainly from dissolution of salts in the sedimentary strata. The results obtained require revision of current metallogenic models of post-orogenic ore formation in the region. Furthermore, the research led to the development of a new cryogenic laser ablation technique for the analysis of fluid inclusions in minerals.

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