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Quartz-pebble conglomerates and interbedded quartzites of Late Precambrian age have been traced for more than 50 miles along strike in the Serra de Jacobina. At the Canavieiras mine these rocks appear to lie uncomformably on acid gneisses and intrusives of early Precambrian age. In a section through the mine there are 28 conglomerate beds (>0.5 m thick) totalling about 145 m in a stratigraphic thickness of 555 m. Virtually all the conglomerates contain at least a small quantity of gold in the matrix between pebbles, and economic concentrations of gold occur in a foreset system of cross beds. These facts support a placer origin for the gold.Economic concentrations of gold also occur at the top of conglomerate beds and in conglomerates that come in contact with dikes or sills of ultramafic intrusives. Occasionally, thin plates of gold are found in tiny fractures that cut through conglomerate boulders. High-grade gold has been found associated with stringers of pitchblende. These facts suggest that the original placer gold has been redistribtued within the conglomerates during periods of folding, metamorphism, and intrusion. There is no evidence at Jacobina that gold and either pyrite or uranium-bearing minerals are necessarily cogenetic.

Studies have been made of the S32/S34 ratios in four large basic sills presumably of deep‐seated origin. The average ratio for these sills was determined by weighting each sample according to sulfur content and in accordance with the thickness of rock represented by each sample. These average ratios were within 1.0 per mil of the value for meteoritic sulfur. This is comparable with a spread of about 12.0 per mil for igneous rocks and an over‐all spread of about 120.0 per mil for terrestrial samples. In the study of igneous rocks, both intrusive and extrusive, a number of granitic intrusives were found to have S32/S34 ratios completely outside the range of values for all other igneous rocks. The results suggest that these intruded granites are regenerated sediments rather than acidic differentiates that have risen from the subcrust or mantle.

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