STUDIES IN HYDROTHERMAL ALTERATION 181 



spars and the ferromagnesian minerals. Calcite and quartz are 

 also prominent products of these reactions. Further, the relations 

 of these minerals to the ores indicate that the processes of altera- 

 tion and ore deposition have gone on contemporaneously. This is 

 shown by the progressive decrease in the intensity of the alteration 

 laterally from the veins, and by changes in the character of the 

 alteration in the same direction. For example, the following 

 extract from the report by F. L. Ransome 1 on the "Economic 

 Geology of the Silverton Quadrangle" illustrates these points. 



At points 150 feet east of the lode the country rock is fine-grained and 

 faintly mottled, showing only a few pale phenocrysts of feldspar and an occa- 

 sional tiny grain of quartz. Under the microscope the rock reveals the char- 

 acter of a much-altered andesitic tuff or fine breccia. The feldspars have been 

 completely altered to aggregates of sericite and calcite, while areas of calcite 

 and chlorite probably represent former phenocrysts of augite. The ground- 

 mass is a rather indistinct aggregate of secondary quartz, sericite, and chlorite 

 with a little apatite and rutile. The rock is wholly recrystallized into a sec- 

 ondary aggregate while retaining the gross structure of the original. 



At a distance of 100 feet from the vein the .... chlorite and calcite are 

 abundant, but much of the plagioclase is still recognizable. Sericite and 

 quartz are not such prominent constituents 



At 50 feet from the vein .... the feldspar phenocrysts have been 

 changed to aggregates of calcite and sericite, while areas of chlorite and calcite 

 with sometimes rutile are all that remain of the phenocrysts of augite or 

 biotite. The groundmass also, while preserving the outlines and in small part 

 the substance of former lath-shaped feldspars, is now an aggregate consisting 

 chiefly of quartz, chlorite, sericite, and a little rutile and apatite 



At 2 feet from the vein .... it is seen that alteration has been more 



thorough The forms of the phenocrysts are preserved by pseudomor- 



phous aggregates of sericite with some chlorite, calcite, and rutile apparently 

 after biotite, and quartz, sericite, and chlorite in varying proportions after 

 augite and plagioclase. The groundmass is entirely recrystallized .... and 

 the dominant minerals are quartz and sericite. 



A specimen taken from the wall of the vein showed more evident alteration 

 .... and the rock is wholly recrystallized. The former phenocrysts of feld- 

 spar are replaced by pseudomorphus aggregates'of quartz and sericite 



Of the augite no trace remains, but some sericite inclosing rutile is apparently 

 pseudomorphous after biotite. The groundmass is a finely crystalline mosaic 

 of quartz and sericite. The notable feature of this wall rock is the absence 

 of calcite and chlorite 



1 F. L. Ransome, Bull. 182, U.S. Geol. Survey, pp. 1 16-18. 



