394 PROCEEDINGS OF THE CALIFORNIA MEETING 



Discussion 



Prof. C. F. ToLMAN, Jr., remarked that hydrogen sulphide is now recognized 

 as a great mineralizing agent and not a precipitant alone. High temperature 

 veins carry HjS. 



Prof. J. E. Wolff stated the work on sulphides is to be revised. 



ROLE OF COLLOIDAL MIGRATION IN ORE DEPOSITS 



(Abstract) 



As a result of observations made on the action of chalcocite* in becoming 

 colloidal in mildly alkaline solutions, under the influence of hydrogen sulfide, 

 and of flocculating or precipitating in contact with calcareous or argillaceous 

 material, a series of experiments were carried on to see if other sulfides, 

 arsenides, and sulfo-salt minerals would also change from the massive to the 

 colloidal condition in alkaline solution, under the influence of hydrogen sulfide. 



Experiments showed that nearly all important primary minerals do become 

 colloidal. 



The effect of contact with limestone and alumina on such colloidal solutions 

 was investigated, and in all cases these materials caused a flocculation of the 

 colloidal mineral. 



The work suggests the possibility of all important primary minerals having 

 been carried in the ore-bearing solutions as colloids, and that escape of hydro- 

 gen sulfide or contact with limestone or argillaceous material has caused the 

 migrating colloids to gather into ore bodies. 



Presented in full extemporaneously. 



Discussion 



Dr. C. S. Bastin suggested that other substances than HjS will play a simi- 

 lar role in the accumulation of ore deposits. The presence of silica may play 

 a part in transforming minerals to the colloidal state. 



Prof. A- C. Lawson challenged the statement that alkaline solutions come 

 from magmas. 



Mr. J. D. Clark replied that he would establish Ojo Caliente experiments to 

 show colloidal migration in natural waters. 



EXAMPLES OF PROGRESSIVE CHANGE IN THE MINERAL COMPOSITION OF 



COPPER ORES 



BY C. F. TOLMAN, JR. 



{Abst7'act) 



This paper was a discussion of the occurrence of high temperature minerals 

 and of low temperature minerals in the so-called "primary" chalcocite deposits 



* Introduced by C. F. Tolman, Jr. 



* A chemical study of the enrichment of copper sulfide ores. Bulletin No. 75, Uni- 

 versity of New Mexico. 



