2020-12-02_PERMIT FILE - M2020060

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10 GUIDES TO ORE IN LEADVILI.E DISTRICT, COLD. Farther from the stock the magnetite-specularite disappears. In the Penn mine it is bordered and underlain by mixed sulphide and siliceous ores or their oxidized equivalents, and other bodies of it may be similarly bordered, especially where'they are cut by veins of quartz and pyrite with or without appreciable quantities of other sulphides. These veins may expand into replacement sulphide bodies beyond the limits of the magnetite. The magnetite-specularite masses, in short, are guides in that they show that the miner is too near the intrusive stock, but they are not a guaranty that ore will be found at any place along their outer margins. The surer guide is a sulphide vein trending through the magnetite away from the intrusive stock. As the magnetite-specularite masses were formed at an early stage, they served in some places as impervious caps, beneath which sulphide ore was deposited at a later stage. Information regarding the western and southern sides of the stock is very meager. Much ground that may have been mineralized on the west side was destroyed by a mass of rhyolitic agglomerate that narrows southwestward and terminates along the Mike fault. Considerable mining was done just west of this mass in the early days, but records of it have been largely lost. Farther south the contact of the stock has not been explored. . Such explorations as have been made have disclosed thick sills of Gray porphyry, but there is little information regarding mineralization of the limestone. There is a smaller stocklike mass at the west end of Printer Boy Hill and another in the vicinity of Adelaide, but there is no evidence that mineralization around them was especially intense. The por- phyry mass on the north side of Evans Gulch, east of the Silver Spoon shaft,'may also be a stock, but it is nearly all covered by glacial debris, and its structural relations are not definitely known. OTHER SILICEOUS ROCKS Ore bodies in other siliceous rocks of the district, including por- phyry, "Weber grits," quartzite, and granite, are mostly lodes that consist of one or more parallel veins in fissure or fault zones. These lodes are present chiefly in the eastern part of the district, where the proportion of siliceous rock cut by numerous premineral faults and fissures is greater than elsewhere. Stockworks have been pro- ductive in "Weber grits " in the South Ibex mine, on Breece Hill, and in Cambrian quartzite and Gray porphyry in the Cord mine, in the southern part of Iron Hill. A few blanket replacement de- posits in "Weber grits " have been worked adjacent to lodes. Surface indications of these ores are similar to those over the Breece Hill stock—debris of silicified and pyritized rock, consider- ably leached and whitened by weathering. A considerable area is