2020-12-02_PERMIT FILE - M2020060

Home Browse Search

l GUIDES AVAILABLE AND THEIR SIGNIFICANCE 9 productive. It has been explored by shafts as much as 800 feet deep, which were sunk before the presence and dimensions of the stock were realized and were intended to reach limestone beneath the porphyry; also by the Yak tunnel, which crosses it in a northeast- ward direction at an average depth of 1,200 feet, where it is walled by pre-Cambrian granite. The only ore mined within the stock has been taken from the upper portions of a few small siliceous pyritic veins enriched by chalcocite and gold. These veins strike north-northeast and are walled by pyritic porphyry. They have been presumably located by following float and trenching. The character of the float and outcrops has not been subjected to syste- matic study, which might result in the discovery of more veins and perhaps bodies of low-grade rock of sufficient size to justify milling. At the old Antioch open cut which is in pyritic porphyry close to the southeast margin of the stock, oxidized gold ore was mined from a stockwork, which, according to Irving, was formed in shattered rock at an intersection of fissures trending north-northeast and east- northeast. Whether or not there are similar stockworks or bodies of "brecciated ore" within the Breece Hill stock can not be deter- mined from the meager evidence at hand. The discovery of such bodies, as well as lodes, is dependent upon intensive study of out- crops and accessible mine workings. In the search for large veins and replacement ore bodies, how- ever, the Breece Hill stock is a negative guide. The larger veins are found in fissures and minor faults that were formed in the surrounding sedimentary rocks and intrusive sheets of porphyry as an after-effect either of folding and reverse faulting or of the intrusion of the stock. Adjustments in the surrounding rocks were greater than within the stock, and the fissures in them were therefore more continuous. The irregular replacement bodies in the vicinity are almost entirely in limestone, but most of those nearest the stock are of little or no commercial value. Some blocks of limestone are inclosed in the stock but have been replaced by silicate minerals and the iron oxides, magnetite and specularite. The Blue limestone in the Penn mine, on the north side of the stock, and in the Ibex mine; on the east side of the stock, has also been replaced by magnet- ite and specularite. These minerals were deposited at an earlier stage than the pyritic gold-copper ores and the silver-lead-zinc ores, and they only served to destroy the limestone before the valuable ores could reach it. The magnetite-specularite masses have been productive only at the old Breece iron mine (part of the Penn group), where a large body has been quarried in an open cut for flux, and in parts of the Ibex mine, where they have been fissured or shattered and recemented by pyritic gold ore.