Bulletin - United States Geological Survey (No. 502) (Paperback) - Livres de poche

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: BERNERS FORMATION. 15 fissure veins. Nearly all the ore bodies lie within the belt of slates and graywackes?a fact of distribution which seems to have determined their character as stringer lodes. Except for sporadic shoots of rich ore, the stringer lodes are of low grade. They range in width from a few feet to 100 feet and apparently, at a few prospects, to 300 or 400 feet. The greatest depth attained anywhere in the district hardly exceeds 200 feet. Some of the mineralized dikes are of economic importance; all are of considerable interest in that the profound alterations which they have undergone throw light on the character of the solutions that brought in the gold. A common change is a large introduction of albite, which is shown to be a regional feature of the Juneau gold belt. Other changes consist in the introduction of apatite into the altered wall rocks and the conversion of amphibole into biotite. From these and other features the vein-forming waters are believed to have been hot, ascending solutions of deep-seated origin, probably connected with the intrusion of the diorite magma. Whatever oxidized products may have existed in the past have been largely removed by the recent glacial erosion; this fact, together with the similarity in origin of the deposits to those at Juneau, whose behavior in depth is known, renders it highly probable that the gold content found near the surface will persist downward without essential change of value. GENERAL GEOLOGY. BERNERS FORMATION (UPPER JURASSIC OR LOWER CRETACEOUS). GENERAL CHARACTER. The most important rocks in the region are an interstratified series of clay slates and graywackes which have been named the Berners formation because they are typically exposed at Berners Bay.1 Kocks of volcanic origin, usuall...