Hydrous Silicate of Manganese and Calcium. 



-±93 



even where it could not be distinguished in hand 

 specimens. 



The Costa, Manuel and A^icente claims/^ aggregating 

 403 hectares (995 acres), cover an area of alternating 

 bedded tuffs and breccias of latitic andesitic type, latite 

 flows and thin limestones. These rocks dip northward at 

 10° to 15° and are broken by two systems of faults. 

 Several andesite dikes occur in faults. The fine tuffs 

 that have been studied under the microscope contain 

 oligoclase, orthoclase, diopside, hornblende, and rock frag- 

 ments which are largely glass, here and there reddish in 

 color, due to included ferric oxide grains and containing 

 laths of feldspar. The matrix of the tuffs contains glass 

 which is generally altered near the ore bodies. The ore- 

 bodies of these claims contain psilomelane, manganite, 

 pyrolusite and orientite and are wholly in the fine tuffs. 

 Where coarse breccias are adjacent to ore-bodies, some 

 manganese oxides may occur in the fine matrix of the 

 breccias. The manganese oxides and silicate were depos- 

 ited in the tuffs largely by replacing the glassy portion of 

 the rock fragments. 



Associated Minerals. — Psilomelane is present in most 

 of the deposits and both the variety, harder as well as 

 that softer than a steel knife, were recognized. It forms 

 fibrous masses that commonly range from 1 to 50 mm. in 

 maximum size. Some plumose aggregates having similar 

 associations are probably manganite. Commonly, these 

 masses occur in layers roughly parallel to layers of tuff; 

 in some places they are sporadically distributed through 

 relatively unaltered tuft's. The available thin sections of 

 the larger masses indicate that the manganese minerals 

 have locally completely replaced all of the minerals and 

 rock fragments that previously were present, feldspar, 

 diopside and glass. In some sections of material that 

 shows disseminated minute particles of psilomelane, 

 largely found on the borders of the deposits, the manga- 

 nese minerals fill the space between the residual feldspar 

 grains, which clearly have survived the process of replace- 

 ment. In other thin sections, there are minute opaque 

 grains in the crystals and other particles of orientite 

 {fig. 1) that are probably, psilomelane. Psilomelane and 

 the plumose manganite were clearly the first minerals to 

 be deposited in the tuffs. Veinlets of orientite in fig. 2 



" Burchard, E. F., loc. cit., p. 80. 



