Arizona citizen., 1879-05-23
Gro 6idnco and ArYvacG

EDITOR CrTIzEN:-During a recent visit to the mining districts of Orc
 Blanco and Arivaca, I was much inter ested in the geological formation, and the mineralogical character of that section. The Arivaca district is about sixty miles south, southwest from Tue. son and has an elevation of 4OOO feet The formation is clay slate, and quartz porphyry; the slate predominates in the valleys and low hills; and is strewn in many directions, produced doubt. less by lateral pressure, during the Plutonic porphyry period. This slate carries no fossil rests and appears to belong to the aZoic, or primitive formation. It is evident the slate is intersected by the quartz porphyry, This quartz porphyry seldom inter. sects younger formations, than the palaezOic grawacre formation. and therefore this slate is of older date. The mineral veins are in this charac- ter of porphyry and run in various directions, forming a net work. Many of them crop at the surface, and can be traced for long distances, and ap- pear to be well defined. Wherever shafts have been sunk the walls ap- pear solid and the veins increase in width. The matriX of the veins is generally quartz. In few cases how ever, calc and brown spar was found with quartz. This may be regarded as favorable for high grade ores. The character of the ores at or near the surface, are oxides of iron, manganese, and copper stains, and in some cases chlorides and bromides of silver. Galena, iron and copper pyrites are found at greater depths, and give to the ores their base character. In some instances free ores are found. From the general character of the ores there can be no doubt but that they will have to be treated by smeltlng or lixiviatiOn, and as they do not con- tain now sufficient lead for smelling. and as lead mines are not found in the district, the only safe and econo- mical method for their treatment in my judgment in the lixiviation pro- cess.
 The Gro Bianco District joins the Arivaca on the south. The elevation is about 5500 feet. The country is well wooded with sufficient water for all mining purposes. The formation is porphyry, two varieties of which are plainly distinguishable, one is call ed the Orthoklas," quartz porphyry, and consists of felsitic elementary dense matter (quartz and feldspar) with quartz and feldspar crystals. The other is syenitic pOrphyry. The dark greenish elementary matter con- siStS of hOrnblende and chlorite, with large feldspar crystals, which German authors call porphyrite. The syenitic pOrphyry frequently intersects the quartz, porphyry, granite, gneisS, states, &c., in which large bodies of ore are found. Another variety of rock found there is felsite, or petrosileX, or what the miners in Sweden call halle flinta:" it is hard stone matter of a grayish color, quite as hard as telalspar with a couchOidal fracture. The min eral veins in this district are frequent ly found between these varieties of They are, as a rule, large and we'll defined and show good ore to the extent of their present develop ment. At or near the surface the char acter of the ore is oxides of iron and copper and carbonates of lead. At a greater depth, they show their respective sulphurets, the pyrites and galenas. There are, however, some mines which give free ores, but the general features of the ores are base, and need to be treated in the same way as those of the Arivaca District. Besides the aZoic or primitive gneiss and slate formations, with their con- eruptive intersections of granites, diorites and sven ties. are the transition formations with their Graycdcse slates and sandstOnes, silica slates and quartZites, are those in which all kinds of ore are found. The porphyry eruptions during the lzoic and transition periods seem to- have produced great influence in the formation of the multifariOuS veins, md there is no doubt that the aZOic hills in which the mineral veins are found in Gro Blanco, now only wait inc sturdy hand of the miner to bring 10 the surface immense treasure. In his formation are found the rich mines of Mexico, Peru and Chile, which have attracted the attention of he civilized world for their immense amounts of gold, silver, copper, lead, Sc.. &c. The same may be said of all he rich mines of the world.
 B. SALAZAR.