Hydrothermal alteration and Pb/Zn mineralisation in the Allanridge formation, Ventersdorp supergroup, near Douglas, Northern Cape Province, South Africa

[摘要] English: Hydrothermal galena and sphalerite mineralisation is present in amygdales and isolated linear breccia zones within andesite lavas of the Allanridge Formation, Ventersdorp Supergroup in the Northern Cape Province, South Africa.The amygdale ores are believed to be a separate generation from the breccia zone ores and formed during burial metamorphism to greenschist facies, which was coupled with slight metasomatic action. Metal ions from the mafic Allanridge lavas were able to migrate on a small scale and precipitate as sulphides in the amygdales. A later hydrothermal eventoverprinted the pre-existing alteration with a potassic assemblage, and hydraulic fracturing created the mineralised breccia zones. Fluid inclusion studies indicate the fluids are of sedimentary brine ongm (18 wt. % equivalent NaCl). Homogenisation temperatures of fluid inclusions in late stage quartz veins (118°C) suggest higher temperatures during the main stage of ore deposition. σ34S values correlate with the Kuruman and Griquatown Banded Iron Formations, which is therefore believed to be the fluid and sulphur source. 87Rb/86Sr analyses on amygdale chlorite and highly altered whole rock samples from Kalkdam yield an age of 2014 m.y, which is similar to those obtained for other Mississippi Valley type (MVT) deposits and ore occurrences in the Transvaal dolomites of the Griqualand West basin, e.g. Bushy Park, Langrug and Balloch.Pb isotope results for galena and sphalerite also show similarities with other Pb/Zn deposits in the Griqualand West basin, suggesting similar ages of formation. Samples from Katlani are very similar to those from Pering and Geelbekdam and derivation from the Ventersdorp mafic lavas is indicated. On the other hand, Kalkdam samples are more radiogenic, show younger model ages and are similar to the Bushy Park deposit and the other occurrences around the Griquatown Fault. The Makwassie Formation and the basement are believed to be the source of the more radiogenic lead in the breccia zones and the K source which gave rise to the potassic alteration of the flow tops at Kalkdam.Basinal brines, squeezed out of the Asbestos Hills BIFs by continuing compaction and slight pressure from the west, were channelled into faults and fractures which cut deep into the basement. Heating of these brines at depth allowed them to leach metals and potassium from the basement and Makwassie Formation on their convection driven ascent. Faults and fractures provided pathways enabling migration of the metal bearing fluids. Fluids flowing along faults and fractures which cross cut lavas at Kalkdam were tapped off into the porous flow tops. Wall rock alteration and hydraulic fracturing of the less permeable massive lava and possible mixing with cold surface waters resulted in ore deposition in the breccia zones.

[发布机构] University of the Free State