Petrochemical study on the rocks in the Hokuroku district, Akita Prefecture, northeastern Japan
Over 1900 rock samples from drill core of Hokuroku district were chemically analysed and subjected to Principal Component Analysis and Cluster Analysis hoping to delineate the interrelationship among the major and minor elements in relation to the alteration and mineralization associated with the kuroko deposits. The principal component analysis on all of the analytical data (N=1917) showed that factor 1 governed the major oxide chemistry of the rock, i.e., the contribution of Fe, Ti, Mg and Ca
... f Fe, Ti, Mg and Ca were significant while Si showed very significant negative contribution. Basic rocks thus showed higher factor scores while acidic varieties showed lower scores. In factor 2 on the other hand, Cu, Pb, Zn and Ba had large contribution while Na and Al showed large negative contribution. This combination and behaviour of the elements were thought to be of prime interest since this factor might have a possibility to suggest the existence of mineralization with associated alteration. The fact that Na does not show high contribution in factor lwhile it does in factor 2 is another very interesting finding. This is more apparent in the case of the subset of igneous rocks (N=1047). Younger dacitic intrusives are thought to be the least affected by alteration associating mineralization. They were subjected to a principal component analysis and were found that the contribution of Na in factor 1 also negligible. Here factor 2 was thought to be controlling the diagenetic zeolitic alterations. The cluster analysis showed several interesting clusters of which further investigation should be carried on in association with the data on geology, X-ray diffraction and others.