MINERAL RESOURCES OF AROROY DISTRICT. 415 



rocks of this series at one time had a far greater thicloaess over the greater 

 part of the area, and have been largely eroded where not protected by the 

 resistant quartz veins as is the case in Mounts Aroroy, Bagadilla, and 

 Kalakbao. A'^eins cutting the quartz diorite are to be found in the 

 northern part of the district, but as yet they are undeveloped. No 

 veins of any size have been found cutting the slates of the Kaal formation, 

 although stringers of quartz and calcite are very common. It is not to 

 be exjDccted that large fissures would be developed in rocks of this 

 character as in the resistant volcanics and pyroclastics of the Panique 

 series, and it is reasonable to suppose that if the underground workings 

 on the three hills ever reach the level of the slates, the veins will be 

 found to split up into similar small stringers. This possibility shoiild 

 be borne in mind in estimating the probable continuation of the ore 

 bodies at considerable depth, but othenvise it is merely of theoretical 

 interest as the future of the district is assured if the ore bodies now in 

 process of development continue to hold their values at least as far as 

 the limit of the zone of oxidation. As a rule, the veins are wide and 

 regular. Six meters is not an uncommon width, and some veins can be 

 traced for from 1,200 to 1,500 meters along their outcrops. Transverse 

 faulting is rare, but motion parallel to the veins is abundantly shown, 

 particularly on the footwall. All the important veins follow a north- 

 westerly direction and show very steep dips, those to the north of the 

 Gruinobatan Eiver dipping to the northeast at angles of 70° to vertical, 

 while those on the southern side dip to the southwest at angles of from 

 50° to 80°. 



Three veins, the small one on Mount Bagadilla, forming the northern 

 boundary of the Guinobatan limestone, and the Pirate and the Mabel 

 veins, on Mount Kalakbao, do not follow the dominant northwesterly 

 strike, but have a nearly east and west course. They are clearly younger 

 than the main vein system, as the Pirate vein cuts off the El Sol vein, 

 and the Mabel vein likewise appears to fault the two Nancy veins. 



The ore. — So far practically all the work has been done in the oxidized 

 zone, and it is only in a few places that the unaltered form of the ore 

 can be seen. This is, typically, a very hard, dark-blue quartz carrying 

 irregular amounts of pyrite, the dark color being due to finely divided 

 particles of manganese oxide; purple amethystine quartz, however, is 

 not found. Successive periods of mineralization are shown by the small 

 veins of more crystalline quartz cutting across the fine-grained, dark 

 quartz which makes i;p the body of the ore. Faint banding may some- 

 times be observed near the walls, especially the footwall. In many veins 

 calcite is a prominent gaiigue mineral in the sulphide zone. It is gen- 

 erally massive and dark blue or dark gray, similar to the quartz. How- 

 ever, it is curious that rhodochrosite (the carbonate of manganese) has 

 never been found in connection with those manganese-bearing calcite 



