IX, A, 3 Edding field: Study of Bulacan Iron Ores 265 



conditions where concentration could be more easily effected. 

 The magnetite was the first to deposit, but at approximately the 

 same period the pyroxene and amphibole began forming; the 

 quartz crystallized later, filling all interstices, replacing some of 

 the nonmetals already formed, and attacking the wall rocks and 

 capping. 



CHARACTER OF THE ORES 



A most striking feature is the abundance of magnetite and 

 apparent scarcity of hematite in the vein ores and the apparent 

 absence of hematite in the wall rocks. In only two cases is hema- 

 tite found alone as a primary mineral in the ore. In one case 

 it occurs in vein quartz as needlelike crystals arranged generally 

 in fan-shaped groups, and in the other case it occurs as tabular 

 grains with pyroxene and a little quartz. On the surface are 

 found numerous bowlders of hematite, which has led to the belief 

 that most of the ores were hematite. These bowlders for the 

 most part must have been the results of the oxidizing of magne- 

 tite. Hematite is also found in very small quantity mixed in 

 with grains of magnetite in a few of the samples examined. It 

 is apparently crystalline, but could not be identified easily. 



Another prominent feature is the association of magnetite 

 with asbestiform minerals. This class of ore is very common 

 in the district, and is well represented in the samples examined. 

 It comes under the head of class 5, listed on page 263, magnetite 

 and amphibole. The asbestiform mineral appears to be the am- 

 phibole variety. It is well developed in some cases, but for the 

 most part it represents a stage of transition and is not completely 

 changed over to the fibrous state. The magnetite is usually 

 granular or tabular, and exhibits a tendency for the longer axes 

 to be parallel. The asbestiform mineral surrounds the magne- 

 tite, and has adjusted itself by bending to the shape of the 

 magnetite. 



Some specimens show augite and hematite and some horn- 

 blende and magnetite. These, too, have a general parallel ar- 

 rangement of crystals. 



These forms suggest pressure schistosity, but the character 

 of the wall rock and the quartz-magnetite ore does not support 

 this idea. It would seem that the parallelism was caused by 

 crystallization related to the walls of the fissure, influenced by 

 the pressure caused by the solution entering the fissure or 

 by movement along the fissure fault. Several specimens taken 

 from the walls of the veins have slickensides, showing that 

 movement has taken place. 



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