1975 Volume 25 Issue 132 Pages 293-301
It is one of the important problems in explorating the Kuroko ore deposit to determine the prospecting target having an area from several to about ten times as wide as ore extent. In order to attack this problem, sulphur contents and magnetic susceptibilities of the drill cores from acid lavas widely underlying the ore deposits, have been measured. In the area surveyed is the Hanaoka-Shakanai deposit, one of the typical Kuroko deposits in Hokuroku district, Japan.
Reasons of selecting the acid lavas are as follows; (1) The stratiformed Kuroko deposit is of submarine volcanic sedimentary origin, and the followings are expected; Hanging wall rocks were altered only after the ore deposition, whereas foot wall rocks were altered through all stages of it. To catch the most widespread extent of the alteration, it is preferable to select the latter. (2) To check the variation of magnetic susceptibility of rocks, relating to the alteration, it is desirable that the primary distribution offerrimagnetic minerals in the rocks is homogeneous. It is probable that the rocks except lavas and intrusives have heterogeneous distribution of them. So that tuffs and mudstones are cut out. (3) When the prospecting method based on peculiar rocks found in an ore field has been established, the applicability of this method tends to be restricted within the ore field. Therefore, it is desirable to select the rocks associated commonly with this type of ore deposits. In many Kuroko fields, it is reported that acid lavas exist beneath the ore deposits.
By the measurements of sulphur content and magnetic susceptibility on 299 cores from 20 drill holes, the wide haloes of them have been recognized. In the limited extent surrounding the ore deposits, it is found that almost all of the drill cores from the acid lavas indicate high sulphur contents and low magnetic susceptibilities. Their bordering numbers to restrict the haloes are seemed to be S=0.3% and k=30×10-6c.g.s./cm3, and the geometric mean values for each halo are S=0.74% and k=6×10-6c.g.s./cm3. Outside of the haloes, the drill cores indicate low sulphur contents and high magnetic susceptibilities. Their geometric mean values are S=0.04% and k=105× 10-6c.g.s./cm3, respectively. For cheapness and saving time for prospecting, it might be considered to use magnetic susceptibility only. But, in this case, there is possibility to catch the ghost halo of it. The best method to use the two indicators is magnetic susceptibility measurement of all cores as the first step, and the sulphur analyses of the cores indicating low magnetic susceptibilities as the second.
As the results of mineral separation tests by magnetism and specific gravity and of chemical analyses, it is considered that the formation of haloes of sulphur content and magnetic susceptibility is due to pyritization of magnetite included in the acid lavas.
