234 The Philippine Journal of Science 1914 



at the border of the magnetite veins in it contains no minerals 

 other than calcite. 



On the other hand, some of the characteristics of the ores that 

 have been recorded are strongly significant of contact metamor- 

 phic deposits. The association of contemporaneous magnetite 

 and pyrite, for instance, according to the widely quoted state- 

 ment of Waldemar Lindgren,^* is the one unique feature of such 

 deposits. 



It is probable that the intrusions which are involved in the 

 formation of the Bulacan ores cooled at no great distance from 

 the surface; the fine-grained or porphyritic texture of all the 

 intrusives observed would support such an idea, and it appears 

 that the overlying sedimentary rocks, in the vicinity of the ore 

 bodies, were never very thick in their overlap on the older rocks. 

 If this supposition is correct, the phenomena of mineralization 

 characteristic of deeply buried contacts would have been modified 

 by the proximity of the surface and the absence of certain contact 

 features so explained. Typical thermal effects from the actual 

 contact of molten rock are subordinate in Bulacan to the more 

 widespread effect of the solutions associated with the cooling 

 magma. Whether the solutions were juvenile waters expelled 

 from the magma or were heated meteoric waters circulating 

 along contacts with the intrusions and along openings formed 

 by the cooling and contraction of the intrusions is undetermined. 



If the genesis which has been outlined in this paper for the 

 Bulacan ores is correct, there is reason to believe that the deposits 

 will continue in depth in proportion to their outcrop dimensions. 

 The suggested manner of origin has a further economic bearing 

 in that it involves no necessary increase in the proportion of 

 pyrite in the ore with increasing depth, such as would be ex- 

 pected if the ores had resulted from surface alteration of iron- 

 bearing minerals. 



QUANTITY OF ORE AVAILABLE 



In the absence of all development work it is not possible to 

 estimate closely the ore reserves. In an attempt to arrive at 

 some idea of the extent of the ore bodies beneath the surface, 

 a magnetic survey was made at each deposit. This method failed 

 to yield reliable information because of the deflections of the hori- 

 zontal and vertical needles caused by the large bowlders of iron 

 ore scattered over the surface in the vicinity of each ore body. 

 Horizontal deflections of as much as 17° and corresponding in- 



"^ Trans. Am. Inst. Eng. (1901), 31, 227. 



