EL 357 Mineralogical Report Interpretations based on DDH 5, 8, 12A 12

Abstract

By and large, the observations which have already been made on the cores by the Resident Geologist, have established the major features of rock alteration and vertical zonation. This Report simply confirms these, with some additional data; and also draws some comparisons with orebodies such as Peko. It should be recognised that the petrographic-mineragraphic data may have some shortcomings because of inadequate sampling at certain critical contacts, but if necessary these can be eliminated by a supplementary sampling programme. The country rocks - The cores include examples of slate, siltstone and tuffaceous sandstones, but only the one sample from DDH 12 at 904 ft can be regarded as entirely free from alteration. This has the separate peculiar feature of a content of disseminated chalcopyrite. Using this unaltered rock as a reference, the initial and subsequent stages of wall rock alteration can be established in the following terms; Gradations in rock alteration near, and within the mineralised zones. Stage 1 - chloritic alteration-clinochlore. In each of the four drill holes, outside of the mineralised zone, there are examples of sediments which consist predominantly of perfectly oriented clinochlore. This clinochlore is postulated to have replaced entirely, the randomly-oriented clastic sericite and chlorite components of the country rock sediments. This feature is especially well-manifest in those facies which contain silt or sand grade clastic components. This postulation implies magnesia metasomatism. In most examples of this type, there are several %small porphyroblasts of penninite which are forming in place, and which do not conform to the strongly established clinochlore cleavage structure. Stage 2 - chloritic alteration - penninite. In two drill holes viz., 5 and 12, but especially in 5, completely recrystallised, wholly penninitic schist, exists closer to the mineralised zone than does the clinochlore schist. This therefore indicates the extended growth and merging of the porphyroblasts of Stage 1; and the ultimate replacement of all clinochlore by penninite. It is furthermore significant that incipient mineralisation by magnetite, quartz and carbonates accompanied the formation of the penninite. Stage 3 - talc-magnesite development. This was not observed in the highest levels, i.e in DDH 5, but it is present in all intersections below this. The textural intergrowth relationships in the relevant samples, clearly indicate that the talc formed from penninite schist which was contained within the mineralised areas, vide descriptive section. This simply implies more advanced magnesia metasomatism by the removal of Al, Fe from the silicate minerals. In general, large amounts of magnesian carbonates are associated with the talc, thus implying carbonatisation, as well as increased magnesia metasomatism. Stage 4 - amphibole development. The formation of a non-aluminous amphibole of the cummingtonitegrunerite series was noted only at one site within the talcose-magnesite zone in DDH 12A. This is interpreted to be the result of a localised higher temperature-stress condition in this part of the highly altered and mineralised zone, but it is important since it establishes a parallel with the incidence of anthophyllite at depths in excess of 900 ft in the Peko orebody. Stage ? - silicification. The relative status of this type of alteration is not fully apparent from the present investigation. Only one example is included in this suite of cores, viz. from the hanging wall zone in DDH 12. Since this is sited on the country rock side of a Stage 2 alteration at this contact, it can be presently regarded as an outer zone of rock alteration. This postulation is in accord with observations at Peko, and elsewhere at Tennant Creek, where marginal silicic zones exist beyond the chloritised, and other silicate zones.