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Opaque minerals constitute an important part of the basalts under study. Analysis of opaque minerals can provide a fuller understanding of some processes during congelation of basalts, as well as on the peculiarities of low-temperature alteration of basalts. Although this investigation is limited to consideration of 27 specimens (Table 1), its results reflect main features of the ore mineralogy at Sites 417 and 418. The assemblages of opaque minerals in basalts, the composition of individual minerals, and their grain sizes depend chiefly on two factors: (1) the rate of cooling which determines the grain size of rock-forming and opaque minerals, and the texture of the rocks; and (2) the degree of secondary alteration. There is distinct correlation between degree of alteration and grain size. The coarser grained basalts are found in fresher rocks; fine-grained basalts usually are quite altered. The primary and secondary opaque mineral assemblages distribution also depends upon this regularity. We have found the following primary opaque minerals in basalts: titanomagnetite, ilmenite, chromite, pyrrhotite, chalcopyrite, and pentlandite. The most widespread secondary opaque mineral is pyrite. Chalcopyrite, marcasite, and iron hydroxides were also noted. We have observed primary sulfides only in less-altered coarse-grained basalts (Samples 417D-69-1, 142 cm and 418A-85-1, 33 cm); altered basalts contain only pyrite and chalcopyrite.

Opaque Minerals in Basalts from Holes 417D and 418A