BASTIN AND HILL: ORE DEPOSITS OF GILPIN COUNTY 163 



of erosion since the veins were formed it is estimated that they 

 were deposited at depths of from 7000 to 11,000 feet. The tem- 

 perature of formation can not be accurately determined. A 

 minimum may be placed at 100°C., for this temperature is 

 reached under the ''normal" increment of temperature at a 

 depth of about 9000 feet. A maximum may be placed at 575°, 

 for all of the quartz associated with the veins is of the low 

 temperature variety. It is believed that the temperature of 

 formation was probably between 150 and 300 °C. The minerals of 

 the veins are those characteristic of moderate depth and pressure. 



Downward enrichment. This process has produced important 

 modifications in many of the gold-silver ore deposits. There 

 has been enrichment in one or all of the metals, gold, silver, and 

 copper; enrichment in lead and zinc is insignificant. The water 

 level, as in most mountainous regions, is irregular. In most 

 veins it originally stood 50 to 150 feet below the surface. 



Gold enrichment appears to be confined to the oxidized zone 

 and has been accomplished largely by mechanical concentration 

 during weathering. Presumably there was also solution of gold 

 in the oxidized zone, but it is believed that the dissolved gold 

 was promptly reprecipitated within the oxidized zone or imme- 

 diately below it through the action of primary sulphides or 

 of ferrous sulphate. Careful sampling in a number of the mines 

 failed to show any relation between gold content and depth 

 after the oxidized zone was passed. Gold enrichment in the 

 oxidized zone is most conspicuous in certain so-called silver 

 veins. Where unaltered these carry only about 0.1 of an ounce 

 of gold, but in the oxidized zone this has in some mines been 

 increased to 1.5 or even 3 ounces. 



In contrast with gold there has been impoverishment in silver 

 rather than enrichment, in the oxidized zone. The primary 

 silver minerals are silver alloyed with gold, silver-gold tellurides, 

 and possibly argentite. The secondary silver minerals are ce- 

 rargyrite, pearceite, polybasite, and proustite. The scarcity of 

 silver in the oxidized zone is readily understandable, for it is 

 well known that silver is more readily dissolved than gold and 

 is reprecipitated by fewer of the common sulphides. The silver 



