Geology and Orc Deposits of the Shakanai Mine(5)
On the Elongated Pumices of the Miocene Pyroclastic Rocks
1972 Volume 22 Issue 116 Pages 421-435
Details
1972 Volume 22 Issue 116 Pages 421-435
A great number of pumices occurs in the submarine acidic pyroclastic rocks of Sainokami and Shakanai formations which develop around the Kuroko deposits of the Shakanai Mine. These pumices are generally found showing an elongated outline, as clearly as that of welded tuff, which is approximately parallel to the bedding plane. The writers measured the ratio of the maximum length to the minimum one of the pumices. We call the ratio "degree of flattening" in this report. The flattening seems to be due to one or more of the following reasons; compaction during diagenesis, Kuroko mineralizations, post-mineralization activities, environment of the pyroclastic rocks piled up, and nature of the pumice.
The maximum/minimum ratios in the barren zone, which is several hundred meters from the ore body, varies from 2 at the upper formation, to 32 at the lower formation. We consider this is simply caused by the load pressure of rocks deposited after the individual formation. The ratio around the mineralized zone is higher than that in the barren zone, even within the same horizon. We consider this to be due to the argillization related to the mineralization and post-mineralization activities.
In some places degree of the flattening in the lower formation is smaller than that of the upper formation. In such cases, the pumice of the lower formation occurs in the pyroclastic rocks intercalated with mudstone, or in a transitional phase of the pyroclastic rocks to rhyolite. Amount of pumice and degree of the vesiculation decrease toward the xhyolite side. Therefore, we consider this phenomenon depends on place where the pyroclastic rocks piled up, and degree of the vesiculation of the pumice. But pressure of the sea water.in which sedimentation of the pumice occurred and composition of the pumice are needed to be considered in further studies.
As compared with pumices of welded tuff, the elongated pumices of the submarine pyroclastic rocks are different in retaining vesicles and having higher degree of the fattening that generally increases downwards.
