Postorogenic Origin of the Stratiform Cu Mineralization at Lufukwe, Lufilian Foreland, Democratic Republic of Congo

摘要

The Central African Copperbelt, which stretches across the border between Zambia and the Democratic Republic of Congo, is one of the largest sediment-hosted stratiform Cu-Co provinces in the world. The triangular-shaped Lufilian foreland is located to the northeast of the Copperbelt. Recently sediment-hosted stratiform copper occurrences have been explored in the foreland, including the Kinkumbi prospect in the Lufukwe anticline. Although the stratiform copper mineralization in the Copperbelt is mainly concentrated in the Roan Group sedimentary rocks, where it is commonly associated with cobalt, the Lufilian foreland copper mineralization occurs in sedimentary rock units of the Nguba and Kundelungu Groups where it is mainly associated with silver. This paper examines the metallogenesis of the stratiform copper mineralization at Lufukwe. The Lufukwe anticline is situated in the eastern part of the Lufilian foreland and is composed of Neopro-terozoic sedimentary rocks belonging to the Katanga Supergroup. The Kinkumbi prospect, a sediment-hosted stratiform copper-silver occurrence, is located in the northern part of the western flank of the anticline. This part of the anticline is characterized by disseminated copper-silver mineralization hosted in the lower 10 to 15 m of the Monwezi Sandstone (Nguba Group). A comparison between the location of high copper grades in surface samples and boreholes and the location of structural lineaments visible on ASTER images indicates that the mineralization is spatially related to northeast-southwest to east-northeast-west-southwest strike-slip faults. These faults are nearly perpendicular to the strike of the host rock and postdate both the Lufilian folding and deposition of the entire Katanga Supergroup. The Monwezi Sandstone was subjected to strong compaction and silica cementation (authigenic quartz overgrowths), followed by intense feldspar dissolution, which resulted in a well-developed secondary porosity represented by dissolution cavities. Copper sulfides are mainly concentrated in these cavities and partially replace the detrital grains. The copper mineralization is both hypogene (chalcopyrite, bornite, and chalcocite) and su-pergene (digenite, covellite, and minor native copper), with malachite and chrysocolla as the main oxidation products. Point counting and grain size measurements demonstrate that the sandstone horizons with high copper content (>1.25 percent Cu) are those with a detrital grain size larger than 175 mu m, more than 35 percent altered feldspars and little or no fine-grained matrix. Microthermometry of fluid inclusions indicates that the authigenic quartz overgrowths precipitated from a moderate-temperature (80 deg -130 deg C), high-salinity (18.8-23.4 wt percent CaCl_2 equiv) H_2O-NaCl-CaCl_2 fluid. The hypogene copper-silver mineralization was deposited from a hot (120 deg-180 deg C) and low- to moderate-salinity (1.9-7.7 wt percent NaCl equiv) H_2O-NaCl fluid with a general trend of increasing homogenization temperatures with increasing salinities. The interpretation of the available structural, stratigraphic, petrographic, and fluid inclusion microthermometric data constrain the timing of the mineralization to a time after the Lufilian folding and deposition of the entire Katanga Supergroup. The data presented support a postorogenic fluid-mixing model in which the mineralization is related to the mixing of a copper-rich mineralizing fluid with a temperature >= 180 deg C and salinity >= 7.7 wt percent NaCl equiv, likely migrating upward along northeast-southwest- to east-northeast-west-southwest-oriented strike-slip faults, with a colder, low-salinity, reducing fluid in the Monwezi Sandstone. The location and distance between the northeast-southwest to east-northeast-west-southwest faults strongly influenced the spatial distribution of the copper mineralization in the anticline, and the variability in grain size and composition of the Monwezi Sandstone caused the