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

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Date
1998-10
Authors
Whitelaw, Hannah Taig
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University of the Free State
Abstract
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 event overprinted 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.
Afrikaans: Hidrotermale galeniet en sfalerietmineralisasie kom in amandels asook geïsoleerde liniêre breksiesones in andesitiese lawa van die Allanridge Formasie, Ventersdorp Supergroep in die Noord Kaap Provinsie, Suid Afrika voor. Die erts in die amandels is moontlik van 'n ander ouderdom as dié van die breksiesones en het gedurende groenskis beladingsmetamorfose gevorm. Metasomatose het ook tot 'n mate 'n rol gespeel tydens die proses van ertsvorming. Metaalione van die Allanridge lawas het oor relatief kort afstande migreer en is as sulfiedes in die amandels gepresipiteer. 'n Jonger hidrotermale gebeurtenis het die bestaande verandering gesuperponeer met 'n kaliurnryke paragenese terwyl hidroliese breking die gemineraliseerde breksiesones tot gevolg gehad het. Vloeistofinsluitselstudies dui daarop dat die vloeistowwe van sedimentêre pekeloorsprong was (18 gewigs % ekwivalent NaCl). Homogeniseringstemperature van vloeistofinsluitsels in laatgevormde kwartsare (lI8°C) dui op hoër temperature gedurende die hoofstadium van ertsvorming. σ34S waardes korreleer met die van die Kuruman en Griekwastad Gebande Ysterformasies, wat moontlik die bron van die swawel was. Rb/Sr analises van chloriet in amandels en van baie veranderde lavamonsters van Kalkdam lewer 'n ouderdom van 2014 m.j. wat ooreenstem met ouderdomme van ander Mississippi Vallei tipe (MVT) afsettings en ertsvoorkomste in die Transvaal dolomiete van Griekwaland-Wes soos bv. Bushy Park, Langrug en Balloch, verkry is. Pb-isotoopstudies van galeniet en sfaleriet dui ook op ooreenkomste met ander Pb/Zn afsettings in die Griekwaland-Weskorn, asook op soortgelyke ouderdomme van vorming. Monsters van Katlani is soortgelyk aan die van Pering en Geelbekdam en dui op 'n oorsprong vanaf mafiese Ventersdorplawas. Die monsters vanaf Kalkdam is egter meer radiogeen, toon jonger model-ouderdomme en is soortgelyk aan Bushy Park en ander voorkomste in die omgewing van die Griekwastadverskuiwing. Die Makwassie Formasie en die vloergesteentes word as moontlike bron van die meer radioaktiewe lood in die breksiesones beskou asook die K-bron wat die kaliumverandering van die vloeitoppe by Kalkdam veroorsaak het. Vloerpekels wat uit die Asbesheuwel Gebande Ysterformasie gepers is deur volgehoue kompaksie en druk vanuit die weste, is in verskuiwings en nate diep in die vloer gekanaliseer. Verhitting van die vloeistowwe in diepte het loging van metale sowel as kalium vanuit die vloergesteentes en die Makwassie Formasie tydens opwaartse konveksie veroorsaak. Verskuiwings en naatstelsels het as vervoerkanale vir die migrasie van die metaaldraende vloeistowwe gedien. Vloeistowwe wat langs geskikte kanale beweeg het, het in die vloeitoppe van lawas by Kalkdam gekonsentreer. Wandgesteenteverandering en hidroliese breking van ondeurlatende massiewe lawa, asook moontlike vermenging met koue oppervlaktewater, het die afsetting van erts in die breksiesones tot gevolg gehad.
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Dissertation (M.Sc. (Geology))--University of the Free State, 1998, Mineralogy, South Africa, Douglas, Effect of temperature on metals, Hydrothermal deposits
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