MICROSCOPIC STUDY OF THE BULACAN IRON ORES 



By F. T. Eddingfield 

 {From, the Division of Mines, Bureau of Science, Manila, P. I.) 



Thin sections of ores and wall rocks of the iron deposits of 

 Bulacan were prepared in the usual way. No attelmpt was made 

 to polish the sections since the only opaques present were mag- 

 netite, hematite, pyrite, and chalcopyrite. Magnetite and hema- 

 tite were differentiated by their magnetic properties and streaks, 

 both of which were determined by tests made directly on thin 

 sections. It was found to be very easy to obtain the streak by 

 scratching the section with a knife point and to determine the 

 magnetic properties by digging out a small grain with a magne- 

 tized point or knife blade. 



The iron ore was found in the following forms and associa- 

 tions : 



1. Original crystalline magnetite in quartz veins. 



(a.) Massive. 



(6) Rounded, granular, in some cases with pyrite, and rarely with 



pyrite and chalcoipyrite. 

 (c) Crystals in needlelike and fan-shaped groups. 



2. Original crystalline hematite in quartz veins. 



(a) In masses with needle and fan- shaped groups attached. 

 (6) Needle and fan-shaped groups. 



3. Masses of magnetite and hematite intimately mixed, usually with some 



pyrite, magnetite predominating. 



4. Hematite granular and tabular with pyroxene, quartz, and usually with 



chlorite. 



5. Magnetite granular and tabular with amphibole, quartz, and usually with 



chlorite. 



6. Magnetite granular, in silicified holocrystalline igneous rock, containing 



pyroxene, amphibole, plagioclase, epidote, and in some cases titanite 

 and chlorite. 



7. Magnetite in small scattered grains in different rocks of all kinds found 



in the region. 



ORIGIN OF THE ORES 



There are many sources given for the origin of iron-ore de- 

 posits. Of these may be eliminated all except those relating to 



263 



