IQ PKOCEEDINGS OF THE NATIOISTAL MUSEUM VOL. 74 



PARAGENESIS 



The first definite step in the formation of the ores of Cerro Mer- 

 cado was a brecciation of the country rock. This brecciation can be 

 best seen in the old workings on the north side of Picacho Norte at 

 the point called Labores de la Cueva and from which the specimen 

 illustrated on Plate 4 was obtained. The groundmass is nov\^ com- 

 pletely altered, or nearly so, to a mixture of calcite and magnetite 

 with minor amounts of diopside. One may also find here partially 

 altered fragments of wall rock coated over with a thin crust of 

 martite crystals. This brecciation may also be observed in a number 

 of places along the western contact of the ore. The effects here are 

 illustrated in the figure on Plate 3 illustrating the alteration of the 

 fragments of brecciated latite to a pyroxene (diopside) rock. 



Following this brecciation came the introduction of magnetite and 

 minor amounts of calcite. Apatite and diopside began to form later 

 in this same stage, and continued after the complete crystallization 

 of the hematite. This was followed again by brecciation leaving 

 much of the ore in a badly broken condition and with numerous 

 broken apatite crystals which were cemented by later quartz, calcite, 

 and sepiolite. 



A still further stage was hj^drothermal in character, its chief 

 effects being an introduction of silica and the alteration of some of 

 the minerals already formed. In this stage magnetite (or martite) 

 was hydrated to goethite ; colorless apatite and dalillite were formed ; 

 small amounts of quartz, calcite, and barite were introduced and in 

 places a considerable development of sepiolite took place. 



Later changes were of minor importance and only feebly devel- 

 oped. Chalcedony was introduced in small amounts, to be followed 

 later by opal, and finally, as a secondary effect due to surface weath- 

 ering, a small amount of limonite formed. 



The various steps in the paragenesis of these ore bodies may be 

 tabulated as follows: 



Brecciation of the country rock. 

 Introduction of magnetite. 



Introduction of apatite and diopside. 

 Brecciation of the ore. 

 Formation of goethite. 

 Formation of (1) sepiolite, (2) colorless apatite, dahllite, quartz, calcite, 

 and barite. 

 Formation of chalcedony. 

 Formation of opal. 

 Formation of limonite. 



Concerning one of the most important steps of the sequence — the 

 conversion of magnetite to hematite — we have as jet no data. It is 

 the vrriters' opinion that this took place after the brecciation of the 



