230 The Philippine Journal of Science i9i4 



GENESIS OF THE ORES 



The Bulacan iron ores have a sufficient number of features in 

 common to lead to the belief that they are related in origin, 

 although local conditions have undoubtedly been effective in 

 modifying the character of the individual deposits. Concerning 

 Macatalinga, Mayapo, and Tagpis, information is insufficient to 

 justify further statement. The titaniferous character of the 

 Tumotulo ore sets it apart from the others, perhaps, because 

 of the frequently observed association of titanium with a distinct 

 class of magnetites ascribed to the action of magmatic segrega- 

 tion. For Tumotulo, also, the lack of information prevents more 

 than this generalization. 



Among the features which are significant as to the origin of 

 the other and more important ores may be noted the intimate 

 and general association of the ore minerals, magnetite and 

 hematite, with quartz; the occurrence of ore in sedimentary 

 rocks, as at Camaching, and the conformity of such ore bodies 

 with the sedimentary rocks in strike and dip ; the evident replace- 

 ment of sedimentary rocks, especially limestone and breccia, by 

 ore and the presence of veins carrying the ore minerals, as, for 

 example, the small quartz veins adjacent to and parallel with 

 the main ore bodies at Montamorong and Hison; the presence 

 and character of the altered wall rock with the ore; and the 

 presence of original pyrite with magnetite in the ore. 



With these features in mind, it will scarcely be suggested 

 that the ores are original beds deposited as sediments or as 

 bog ores, nor can they be explained as the result solely of 

 concentration or metamorphism of ferruginous beds or bog ores 

 unless a separate origin be assigned to the small quartz veins. 

 In this connection it should be remembered that not all the ore 

 bodies are certainly within sedimentary rocks. Moreover, there 

 is no evidence of regional metamorphism which is usually in- 

 volved in the change of original limonite beds to magnetite and 

 hematite ; schistose rocks are not of general occurrence, and the 

 granite which is believed to be older than the ores shows no 

 trace of gneissic structure. 



A theory advanced by Adams to explain the occurrence of 

 iron ore at Santa Inez, Rizal Province, may be quoted here as 

 equally applicable to the Bulacan deposits since the Santa Inez 

 ores have features in common with those of Bulacan, namely, 

 they consist of magnetite and hematite as ore minerals, with 

 quartz, pyrite, and chalcopyrite, and they occur at or near the con- 

 tact of igneous rocks (andesites) with sedimentaries, including 



