234 W. LINDGREN CALIFORNIA GOLD-QUARTZ VEINS. 



found in wall-rocks originally very poor in this metal, it is probable that 

 some potassium was also added. In silicious rocks the quartz is often 

 attacked, but never completely carried away. The iron ores and partly 

 also the bisilicates of the original rock appear to have been converted 

 into pyrites,* while the titanium in it was transformed to leucoxene.t 



In the case of clean-cut fissures, with well defined quartz-veins, it is 

 usual to find by far the largest amount of gold in the quartz and in the 

 sulphides associated with it. The altered country-rock is not entirely 

 barren, but it does not often contain native gold, and its sulphides are 

 much poorer than those in the quartz. J This is not entirely without ex- 

 ceptions, for in several places, usually adjoining rich chutes, the altered 

 countrj^-rock will pay for milling, and may, in isolated cases, go as high 

 as $12 per ton. And again, there are cases in which the altered country- 

 rock is traversed by a great number of minute quartz seams, in which 

 the gold is concentrated. Such a case is the Rawhide mine, Tuolumne 

 county, in which this altered and fissured country-rock is far richer than 

 the main quartz-vein. At the same place the gold sometimes also pene- 

 trates and coats the cleavage faces of the adjoining talcose or serpentinoid 

 schistose rock. One frequently hears of native gold in talc, slate or other 

 rocks. I have always found such occurrences to be more or less altered 

 rocks from the immediate vicinity of some vein. The gold occurs on 

 minute, sometimes hardly visible, seams traversing them. Indeed, many 

 fissures are absolutely microscopic. 



It has been stated above that serpentine § is peculiarly liable to altera- 

 tion by the auriferous solutions. The zones of altered rock are in this 

 case often very large and always very characteristic. They may be 

 twenty or thirty feet wide, or in the case of branching veins a whole area, 

 several hundred feet across, may be more or less completely altered. 

 The serpentine is converted into a mixture of magnesic and calcic car- 

 bonates, a green micaceous mineral containing potassium and colored by 

 chromium, to which the name of mariposite has been given by Professor 

 Silliman, together with more or less iron pyrites. The altered mass is 

 frequently shattered and traversed by seams of mixed quartz and car- 

 bonates. It has a rather coarse, crystalline structure, and a bright green 

 color from the disseminated mariposite. The carbonates referred to as 



* A similar alteration has been shown to have taken place in the country-rock of the Comstoek 

 lode by G. F. Becker, Monograph III, U. S. Geol. Survey, p. 210. 



fThe alteration and replacement of the wali-roeks has been emphasized by S. F. Emmons in 

 regard to the fissure-veins of Colorado and Montana, and he points out that, especially where ex- 

 tensive sheeting has taken place, the fillings of open spaces are often small compared with the 

 alteration and impregnation of the adjoining country-rock. "Structural relations of ore-deposits," 

 Trans. Am. Inst. Min. Eng., xvi, p. 808. 



I This fact, as well as many others, of course, speaks strongly against the derivation of the gold 

 in the vein from the decomposed zone adjoining it. 



§ As well as talc-schist and other slaty magnesian roeks derived from serpentine. 



