Constraints on the genesis of gold in the Tati Greenstone Belt of Northeastern Botswana: insights from integrative geochronology and Pb isotope compositions

The NW trending Archean Tati Greenstone Belt is located in the southwestern part of the

Zimbabwe Craton, in northeastern Botswana and consists of mafic to felsic volcanic rocks and

metasediments. All rocks in the TGB have undergone greenschist facies metamorphism at 2630 ±

70 – 2570 ± 70 Ma and intrusion of granitoids that comprises of granodiorite, tonalites, quartz

diorite and monzonite occurred at 2.65- 2.73 Ga. The TGB is host to numerous Au deposits

including from NW to SE: Map Nora, Golden Eagle, Mupane and Signal Hill. In order to unravel

the genetic relationships between the different Au mineralization areas and to track the sources of

gold, Ar/Ar geochronology, U/Pb geochronology, Pb/Pb geochronology and Pb isotope analyses

were carried out.

The Pb/Pb dating of sulfides from the Mupane Au deposit yielded a Pb/Pb errorchron age of 2873

± 140 Ma, pointing to a pre-metamorphic mineralization event, whereas Shashe sulfides (Map

Nora and Golden Eagle), all sulfides and all arsenopyrite from both Mupane and Shashe deposits

yielded overlapping and post-metamorphic Pb/Pb errorchron dates of 2250 ± 110 Ma, 2226 ± 66

Ma and 2220 ± 73 Ma, respectively. The U/Pb dating of hydrothermal almandine garnet that is

spatially associated with the Mupane Au mineralization yielded overlapping concordia ages and a

Tera-wasserburg lower intercept 206Pb/238U age of 2105 ± 24 Ma and 2119 ± 18 Ma, respectively.

The ages also correspond to a post-metamorphic type of Au mineralization. Furthermore, phengite

spatially associated with the Signal Hill Au mineralization yielded an Ar/Ar plateau age of 1987 ±

24 Ma and a weighted mean Ar/Ar ages of 1976 ± 31 Ma and 1987 ± 13 Ma, also suggesting a

post-metamorphic (2630 ± 70 - 2570 ± 70 Ma) mineralization event. The 2873 ± 140 Ma Pb/Pb

age of Mupane Au mineralization coincides within error with the age of BIF deposition (2.73 ±

0.15 Ga), intrusion of granitoids (2.65 - 2.73 Ga) in the TGB and also correlates within error with

the ca 2.7 Ga first Limpopo-Orogeny (Limpopo-Liberian tectogenetic cycle). The phengite Ar/Ar

ages coincide within error with the 1.95 - 2.05 Ga second Limpopo-Liberian tectogenetic cycle,

which is here considered to have possibly triggered the Au mineralization in the Signal Hill Au

deposit. The garnet ages do not correspond to any geotectonic event known both locally and

regionally. The results clearly indicate that the Au mineralization across the TGB is of various ages

and corresponds to at least three temporally distinct gold mineralization events, with two of them

triggered by regional geotectonic events.iii

Lead isotope compositions of both sulfides and whole rocks are heterogeneous. Sulfides show,

206Pb/204Pb ratios of 14.274 - 29.711, 207Pb/204Pb of 14.957 - 17.179 and 208Pb/204Pb ratios ranging

from 33.962 - 65.243, whereas the whole rocks have 206Pb/204Pb ratiosrange from 13.491 - 17.739,

207Pb/204Pb from 14.550 - 15.538 and 208Pb/204Pb from 33.744 - 38.261. Lead isotope compositions

of both whole rocks and sulfides from Golden Eagle, Mupane and Signal Hill Au deposits overlap

and the possible sources of Pb are granodiorite from Signal Hill, schist from Mupane and Map

Nora Au deposits. At Map Nora however, arsenopyrite has a wide range of Pb isotopic

compositions,

206Pb/204Pb range from 29.011 - 29.711, 207Pb/204Pb from 17.118 - 17.179 and the

208Pb/204Pb range from 59.921 - 65.243, suggesting that the Au source rock is likely the black shale

because is the more radiogenic lithology that is known to occur in the Map Nora area.