Blogs

A lode is a zone of fissuring that contains roughly parallel mineral masses of the general type of fissure veins, usually connected by cross veins and mineralized breccias to such a degree that over certain portions the whole width constitutes a single ore-body.

A ledge is an irregular mass of altered rock, containing ore bodies, the alteration of which is due to and characteristic of the action of mineralizing solutions.

These veins are divided by Mr. Lindgren into four classes: (a) Contact deposits, which are discussed elsewhere.
(b) Cassiterite veins. The characteristic minerals of this type are cassiterite, pyrite, arsenopyrite, specularite, quartz, tourmaline, topaz, lepidolite, muscovite, apatite, fluorite, and wolframite, with subordinate calcite and siderite. These veins are commonly poor in gold and silver, and the metasomatic alteration along their walls is likely to be intense.

Primary mineral deposits formed at the surface by hot waters are rarely of economic importance; their primary condition is commonly obscured by the action of surface agencies. The sinters characteristic of surface-formed deposits are commonly made up of silica, as opal or chalcedony, and earthy carbonates. Calcite, fiuorite, celestite, barite, and many other gangue minerals may also develop in crystallized form. Stibnite, pyrite, marcasite and cinnabar are known in crystallized form, and many other sulphides have been detected chemically in such deposits.

The only primary ore of zinc of importance is zincblende; in characteristic occurrences it is associated with pyrite, and occasionally with galena and chalcopyrite. The Depth of Primary Ore Deposition.—An important factor in the consideration of the probable behavior of a primary deposit in advance of exploration is the depth at which it formed.

At Lake City, Colorado, the primary ore minerals are galena, tetrahedrite, chalcopyrite, sphalerite and pyrite, associated with quartz, rhodonite, rhodochrosite, and barite. The silver is contained in the galena to the extent of 22 to 30 oz. per ton, and in the tetrahedrite, which is probably related to freibergite, in much larger quantity.

The primary ore is quartz that carries free gold in both fine and coarse particles, with from 2 per cent, to 3 per cent, of sulphides which also carry gold. Pyrite is the predominant sulphide; associated with it are galena, zincblende, chalcopyrite, and arsenopyrite. Subordinate accessory minerals are tetrahedrite and molybdenite. The quartz carries a little calcite. Fluid inclusions are abundant, and in many specimens are distributed in a manner dependent upon the distribution of the sulphides through the quartz.

Tetrahedrite is a guide to high silver and gold values in most deposits in which it occurs. In quartz veins, the presence of finely disseminated galena or chalcopyrite ("sulphurets ") , or the presence of fluorite, are often indicative of high gold or silver values. In quartz veins that carry gold and silver it is frequent that quartz of a certain texture carries high values, while associated quartz of other textures is low grade or barren.

The minerals present in an ore frequently afford a basis upon which to judge its origin. Some minerals are distinctively primary, some distinctively secondary, others, and among them are some of the most important ore minerals, are in some instances primary and in others secondary. The presence of a mineral of secondary origin proves the action of surface agencies; a mineral that is known to be sometimes of secondary origin indicates that surface agencies may have enriched the ore under consideration, and so casts doubt upon the primary character of the ore containing it.

In the investigation of any ore-shoot the first consideration is whether the ore is primary, secondary, or residual, as upon this rests all conclusions in regard to its persistency in depth. A primary ore is an ore that has undergone no change since deposition. A secondary ore is an ore formed by secondan, or surface, agencies. A residual ore is an ore that has remained after the solution and removal of associated minerals by secondary processes.

There is a persistent connection between ore-deposits and monzonitic rocks throughout the Cordilleran region; the examples of this relation include many of the most important districts. That the converse of this relation—that ore-deposits may be expected where monzonitic stocks are found—is not true, as is illustrated by the numerous monzonite masses through New Mexico that are not connected with any important mineralization.