Details and Characterstics of Seikyu Molybdeunm Deposits and Some Consideration on their Genesis

Study on molybdenum deposits in Japan (3)

Abstract

The Seikyu molybdenum deposits, consisting of fracture-filling molybdenite-quartz veins, are embedded in various granitic rocks. The veins are all gently dipping ones. The host granitic rocks are irregularly shaped but almost flat-lying in megascopscal view. Some of the veins are exactly parallel to the rock boundaries where disinct physical and chemical differences exist, e.g., aplite vs. hornblende-biotite hybrid.
Important veins are either a large scale vein or a vein associated with branchveins. The large vein can be considered as a series of echeolonsveins. A small size (50 by 70 m) vein is followed by thee other with similar strike and dip. The two veins get as close as one meter, giving their look a large single unit vein. The branchveins show a horse-tail structure not throughout the deposits but in limited places.
The veins are locally and weakly controlled by kind of the host rocks, small faults, and variation. in dip of the veins. However, no distinct features to control molybdenum minerallization is so far observed. In most cases branched parts are more widely and more or less intensively minerallized.
Ore can be devided into types as follows : massive ore (flaky molybdenite with little quartz), flaky ore (flaky molybdenite and quartz), streak ore (finer molybdenite in quartz), film-ore (thin molybdenite bands in quartz, very low grade), molybdenite-clay ore, molybdenite-sulfide ore, disseminated ore, molybdenite-paints (molybdenite film along cracks in the host rocks), and brecciated ore. Ratio of these. ores is roughly 3, 20, 45, 15, 10, 1, and 1 percent, respectively. The Seikyu ore is mainly flaky molybdenite accompanied by quartz as well as the other productive molybdenum deposits in Japan. Because of simple mineral combination, no much work has been investigated on the ore and alteration products.
Considering the genesis of the Seikyu deposits, it has been controversial from which granitic masses, the ore solution would have been derived. As mentioned, there is no prominant structure control throughout the deposits, although there is a grid pattern of fault system. Location of the deposits is megascopically controlled by rock boundaries, and not by the faults but cooling joints or tension fractures of the host rocks. This environment could be happened just after solidification of the host rocks.Therefore it is feasible to judge that the ore solution may have been brought from one or some granitic masses which are very close to the veins, i, e., the present host rocks.