Source of ore fluids at the Kalahari Goldridge deposit, Kraaipan greenstone belt, South Africa: evidence from Sr, C and O isotope signatures in carbonates

The Kalahari Goldridge deposit is located in the Archaean Kraaipan greenstone belt in the north-west province of South Africa. Gold mineralization in this deposit is hosted within banded iron formation which is flanked by a mafic schist in the footwall and clastic metasedimentary units in the hanging wall. Data from carbonate minerals from mineralized veins and bulk rock from the A and D zone ore bodies have helped to define the ultimate origin of the ore-forming fluids and their migration history. Carbon isotope ratios of carbonates from both the A and D zone ore bodies have tight clustering from −7.6 to −5.3‰ that indicates a unique origin for the ore-forming fluids associated with the mineralization at Kalahari Goldridge. The δ¹⁸O values of the carbonates have been influenced by temperature gradients and variable degrees of fluid–rock interaction promoting oxygen isotope exchange between ore fluid and host rocks. Minimum ⁸⁷Sr/⁸⁶Sr ratio values of 0.70354 in mineralized veins are most consistent with ore-forming fluids being relatively pristine with a mantle origin. Strontium and the corresponding ore-forming fluids were most likely derived from mantle-derived magmatic rocks probably represented by the meta-basaltic rocks that underlie the ferruginous package in the Kraaipan greenstone belt. Strontium isotopic composition of vein carbonates show considerable variation in ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.70354 to 0.73914. This is consistent with an ore fluid composition that has been modified by the addition of radiogenic Sr possibly during passage of fluid through siliciclastic country rock concomitant with the observed hydrothermal alteration.     

Experiments with Lithic Tools: Understanding Starch Residues from Crop Harvesting

Stone knives were used widely in Neolithic East Asia, presumably in the harvesting of grain crops, but their function has not been clearly understood due to the lack of study of residues from these tools. To address this issue, starch grain analysis was employed to study the residues on the surface of ancient stone knives and large amounts of starches were recovered. The sources of these starches, however, were not well understood, because harvesting of crops involves the cutting of stems rather than direct contact with starchy seeds. To determine whether harvesting could deposit these types of residues, we designed a simulation experiment using stone flakes to harvest ears of wheat, rice and foxtail millet, then analysed the residues on both the flakes and in the plant tissues. A large number of starch grains were found in the stems, including both typical morphotypes from seeds and newly described types that occur only in stems, which can be used as indicators of harvesting. Our study demonstrates that starch grains from residues on the surfaces of archaeological stone knives can indicate not only that the tools were used to harvest ears, but also the type of crops harvested.     

Fluid environment for preservation of pore spaces in a deep dolomite reservoir

Well TS1 reveals many uncemented pores and vugs at depths of more than 8000 m in a deep Cambrian dolomite reservoir in the Tarim Basin, northwestern China. The fluid environment and mechanism required for the preservation of reservoir spaces have yet not been well constrained. Carbon, oxygen, and strontium isotope compositions and fluid inclusion data suggest two types of fluids, meteoric water and hydrothermal fluid, affecting the Lower Paleozoic carbonate reservoirs in the Tarim Basin. Based on simulation using a thermodynamic model for H₂O‐CO₂‐NaCl‐CaCO₃ system, meteoric water has the ability to continuously dissolve carbonate minerals during downward migration from the surface to deep strata until it reaches a transition depth, below which it will begin to precipitate carbonate minerals to fill preexisting pore spaces. In contrast, hydrothermal fluid has the ability to dissolve carbonate in deep strata and precipitate carbonate in shallow strata during upward migration. Based on the dissolution–precipitation characteristics of the two types of fluids, the ideal fluid environment for the preservation of preexisting reservoir spaces occurs when carbonate reservoir is neither in the CaCO₃ precipitation domain of meteoric water nor in the CaCO₃ precipitation domain of hydrothermal fluid. Taking the Lower Paleozoic carbonate reservoirs in the north uplift area as an example, the spaces in the deep Cambrian dolomite reservoir near well TS1 were seldom filled because thick Ordovician deposits blocked meteoric water from migrating downward into the Cambrian dolomite reservoir and because the Cambrian dolomite reservoir has been in the domain of hydrothermal dissolution since the Permian. The deep carbonate layers in basins elsewhere with a similar fluid environment may have high uncemented porosity and consequently have good hydrocarbon exploration potential.     

Comments on economic change and the prehistoric fur trade in Northern Sweden

In this study, we used stable isotope analysis and radiocarbon dating to study diet, mobility and chronology in two late medieval/historical coastal populations in northern Norway. We have shown that the individuals buried at Kirkegårdsøya date between 1331 and 1953 cal AD and had a homogenous marine diet, whereas the individuals buried at Gullholmen had a more heterogeneous diet, consisting of both terrestrial and marine proteins and date between 1661 and 1953 cal AD. We have demonstrated that reindeer protein was not an important part of their diet, and also discussed the importance of correcting for the marine reservoir effect in populations with a coastal subsistence. Our interpretation is that individuals buried at Kirkegårdsøya primarily belonged to a Coastal Sámi community, although Norwegians with a similar diet (and likely comprising a minor population in the area) cannot be ruled out. The more varied diet and mobility at Gullholmen could, as predicted, indicate that these individuals may have had a more diverse cultural affinity.     

Organized industrial workers in Bangalore India

Mark Holström. South Indian Factory Workers: Their Life and Their World. Cambridge: Cambridge University Press, 1976. xi+ 158 pp. $14.95.