Saltpan impact crater, South Africa: Geochemistry of target rocks, breccias, and impact glasses, and osmium isotope systematics

The Pretoria Saltpan crater is a well-preserved 220,000 year-old, 1.13 km-diameter, simple impact crater. The crater was formed in Nebo granites of the Bushveld Complex. Some minor intrusions thought to be younger than the Nebo granite are present at the crater and have earlier been believed to support a volcanic origin of the structure, but recent geological studies showed them to be part of the regional geology and of Proterozoic age. We studied the petrology and geochemistry of fourteen target granite samples, three suevitic breccias, nine intrusive rocks, as well as melt agglutinates, handpicked impact glass fragments and sulfide spherules from the Saltpan impact crater. Unconsolidated suevitic breccias recovered from different depths in the crater were found to contain abundant evidence of shock metamorphism. The target rock granites show only limited compositional variability. The major and trace element composition of the bulk breccia is very similar to that of average basement granite. Impact glass fragments recovered from the unconsolidated suevitic breccia have a CIPW normative composition similar to that of the basement granites. No evidence for admixture from any of the minor intrusions was found. The similarity of trace element abundances and ratios, and REE patterns between impact glasses and granites favors derivation of the glasses from the granites. The impact glass fragments show considerable en~chments of Mg, Cr, Fe, Co, Ni, and Ir, compared to the basement granites. The abundances of these elements in the glasses (after correction for indigenous concentrations) can be explained by admixture of about ~10% o f a chondritic component. High Ir concentrations (= 100 ppb) have been found in sulfide spherule samples, which may complement the (lower) lr abundances in the glasses and could indicate some fractionation during impact. Re-0s isotopic studies were applied to further investigate the presence of a meteoritic component in the suevitic breccia. The target granite shows very low osmium abundances of about 7 ppt and high ‘s70s/‘880s ratios of about 0.72 that would be expected for old continental crust. In contrast, the breccia samples were found to have much higher osmium abundances (-80 ppt) and lower ix70s/18xOs ratios of about 0.205. These values can be explained by mixing of target rocks with a chondritic component. lNTRODUCI’lON AND SUMMARY (BRAND-F and REIMOLD, 1993) include lamprophyre dikes OF CRATER GEOLOGY and possibly sills, trachyte, and minor carbonatite. THE PRETORIA SALTPAN (also called “Zoutpan,” or “Tswaing”-“The Place of Salt”) structure has a diameter of I. 13 km (Fig. 1) and is located at 25”24’30” S and 28”04’59” E, about 40 km Noah-no~hwest of Pretoria (Transvaal Province, South Africa, Fig. 2). The interior of the structure consists of a flat crater floor that is partiafly covered by a highly saline lake, the extent of which depends largely on annual rainfall patterns. The maximum elevation of the crater rim over the present crater floor is 119 m, and the rim rises up to 60 m above the surrounding plains. The Saltpan crater was formed about 220 ka ago (STORZER et al., 1993; KOEBERL et al., 1994b) in the crystalline basement of the 2.05 Ga old {WAI_RAVEN et al., 1990) Nebo granite of the Bushveld Complex. Granite exposures are confined to the crestal areas of the crater rim and to isolated plateau outcrops of restricted extent in the crater environs. Other lithologies that can be sampled in limited outcrop of often not more than 50 cm exposure along the inner rim wall and in a few exposures in the region surrounding the structure About 3 km south-southeast of the Saltpan crater a perfectly circular depression with a diameter of about 400 m was recently discovered (BRANDT et al., 1993a,b, 1994). Complete lack of exposure and only limited, still inconclusive geophysical evidence (BRANDT et al., 1993a,b, 1994) have so far precluded positive identification of the origin and nature of this small structure. However, no magnetic anomaly was found at this satellite structure in a first magnetic survey, which clearly rules out the possibility that it could be a volcanic structure with a central magnetic body (such as a kimberlite pipe). The conspicuous shape, its vicinity to the Saltpan crater, and regional uniqueness suggest that this feature could represent a twin or satellite crater to the Pretoria Saltpan structure. In the past the origin of the Saltpan crater has been controversial. The first visitors to the crater in the mid1800s felt strongly that the unique presence of this crater indicated a volcanic origin (for a historical account of reports on the Pretoria Saltpan see, e.g., LEVIN, 1991). The first detailed geological study of the structure was reported by WAGNER