Page 52 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 51
of the deposition of primary sulfides in Sierra County occurred under physical conditions of temperature and pressure that were intermediate between those of the true mesothermal and epither- mal (low temperature) zones, and this intermediate zone has recently been named the leptothermal zone by Graton. 22 The third and only other group worth consideration is that of the secondary sulfides which are found near water level (present or past) as a result of the deposition of minerals oxidized and dissolved near the surface and carried down by descending sur- face waters to the zone of reducing conditions, the sulfide-enrich- ment zone of the table.
Probably half of all Sierra County shipments have con- tained the minerals of the oxidized zone, and these shipments have included the richest ores of the county. It is estimated that one-fourth of the total shipments was produced from the zone of sulfide enrichment. The remaining one-fourth has come from the primary portions of the veins and deposits, but in no case has mining been prosecuted to such a depth that there has not been a noticeable amount of secondary sulfide minerals in these so- called primary sulfide ores.
PRACTICAL FEATURES OF PROSPECTING AND MINING
To those interested in finding new ore bodies and the suc- cessful exploitation of those already discovered in Sierra County, the answers to certain questions are of 'great importarfce. Some of these questions are as follows : Where are the most favorable areas for prospecting? What are the most favorable rocks or horizons in which to prospect? Will ore deposits be found at a definite depth below surface, and is there a chance of finding others at greater depths than those now known? Will there be changes in the size of veins and ore shoots at greater depths? Will the grade or character of ore change with depth due to pro- cesses of oxidation, secondary enrichment, or impoverishment of primary bodies? What average costs for mining, milling, over- head expense, etc., may be expected within the county? Do the ore deposits of the region in general warrant the construction of a mill at each mine, or should a district customs plant be erected on a co-operative agreement, and under what conditions is milling a practical solution of local problems? It is unfortunate that at many mines and prospects lack of development does not permit definite answers to many of these questions, but it is possible to discuss most of them in a general way, and to some extent at least the risks and limitations of mining in the area may be noted.
22Graton, L. C., The depth-zones in ore deposition, Econ. Geology, Vol. 28, No. 6, p. 513, 1933.