IX, A, 3 Dalburg and Pratt: Iron Ores of Bulacan 219 



ECONOMIC GEOLOGY 

 GENERAL CHARACTER OF THE ORES 



The iron ores consist of magnetite and hematite in intimate 

 mixture but in varying proportions. Both minerals are usually 

 massive, although specularite is not uncommon. The surface 

 bowlders appear to be principally hematite, but beneath the 

 surface even in the shallow pits that have been opened magnetite 

 is encountered. Thus the ores from Hison and Montomorong 

 where pits are worked are practically pure magnetite as shown 

 by chemical analysis, while the Santol ore and other ores which 

 are obtained by breaking up bowlders contain very little mag- 

 netite. At Camaching, however, an ore high in magnetite is 

 taken from the surface. 



The predominant gangue mineral is quartz, although the ore 

 used for smelting, which is selected so as to eliminate gangue 

 as much as possible, contains an unusually small proportion of 

 silica. The slightly leaner ore which constitutes the bulk of the 

 ore reserve is typically quartzose, the quartz filling interstices 

 in the iron minerals and cutting the ore in small secondary 

 veins. Eddingfield, in a microscopic study of the Bulacan iron 

 ores,^ found quartz to be even more abundant than appears on 

 megascopic examination. According to his results, some of the 

 quartz crystallized simultaneously with the iron oxides, but the 

 deposition of quartz was also renewed subsequently as is evident 

 from the presence of the later veinlets through the ore. Edding- 

 field and Rowley agree that both magnetite and hematite occur 

 as primary minerals in quartz. 



Next to quartz, complex silicates are most prominent gangue 

 minerals. They occur most abundantly in and near the walls 

 of the deposits. These minerals appear to be alteration prod- 

 ucts of wall rocks caused by the action of the mineralizing solu- 

 tions and, according to the determinations of Eddingfield, include 

 fibrous amphibole (tremolite) , pyroxene, chlorite, epidote, etc. 

 ■ Pyrite occurs in the ore in varying proportions, but is usually 

 conspicuous in or near the walls. Eddingfield and Rowley both 

 found primary pyrite with the iron oxides in the ore, but pyrite, 

 like the quartz, also occurs secondary along cracks in the ore. 

 Chalcopyrite is found sparingly with the pyrite at several places. 



The average specific gravity of the ores is 4.7; a metric ton 

 of ore, therefore, occupies 0.21 cubic meter or 7.4 cubic feet. 

 In the present practice all the ore is broken to nut size in mining. 

 If it were desired, the hematite could be mined so as to produce 



'This Journal, Sec. A (1914), 9, 263. 



