IX, A, 3 



Dalburg and Pratt: Iron Ores of Bulacan 



221 



mostly of lime, magnesia, 'iron, and alumina silicates of the 

 pyroxene, amphibole, and garnet groups, formed through an 

 exchange between the silica of the quartz-feldspar rocks and 

 the basic constituents of the ore formation. The wall rock in 

 Bulacan consists principally of complex silicates of the amphi- 

 bole, pyroxene, and chlorite groups. Those ore bodies which 

 are not adjacent to the granite in Bulacan appear to have lacked 

 the quarz-feldspar rocks which are involved in the origin as- 

 cribed to skarn, but if it be conceivable that the sources of 

 the interchanging constituents may be reversed — that is, the 

 silica be derived from the ore formation and the bases from the 

 inclosing rocks — ^then the analogy of the Bulacan "greenstone" 

 to skarn can be conceded. C. M. Weld," in a description of 



Fig. 4. Geologic section through ore body at Camaching, along an east-west line ; diagram- 

 matic in part: (a) Miocene shales, tuffs, and elastics; (6) altered wall rock; (c) iron ore; 

 (d) intrusives ; (e) blocks of limestone in ore; (/) limestone and elastics; (h) effusives ; 

 length of section, about 200 meters. • 



an iron-ore deposit near Hongkong, strikingly similar in some 

 respects to the Bulacan ore deposits, noted the occurrence of an 

 enveloping greenstone which he relates to skarn. 



The following descriptions of the individual ore bodies will 

 make clearer the general character of the deposits. 



THE CAMACHING ORE DEPOSIT 



The largest outcrop of iron ore in Bulacan Province is at 

 Camaching near the head of Balaong River in the northern 

 part of the district. The somewhat diagrammatic cross section 

 in fig. 4 shows the general structure and geologic relations of 

 the deposit. The iron ore is encountered between the usual 

 walls of greenstone or skarn in steeply tilted beds of tuffs and 

 fragmental rocks with a limited thickness of crystalline lime- 

 stone. The altered rock in the hanging wall grades into a clastic 

 or fragmental rock which contains a large proportion of volcanic 



"fiwZZ. Am. Inst. Min. Eng. (1914), 86, 177. 



