Archive for the ‘Economic Geology’ Category
Gold-bearing quartz veins related to the orogenic gold type are typically ribboned as known from my research area northern California. The ribbons there are formed by vein-parallel shear planes, dividing the quartz in decimeter to micrometer thick bands. Ore-bearing fluids intruded the shear planes several times. This multi-stage mineralization is verified by microscopic investigation of gold ore. In most cases gold and galena are the last species deposited in the veins. The veins occur dominantly at intensely sheared lithological contacts.
A result of these observations is that the deposits developed in an active tectonic environment with multiple stages of fluctuating fluid influx in depths characterized by brittle deformation.
To improve my own understanding of fluid migration processes in hydrothermal systems I will try to summarize some general thoughts I worked off for myself from Cox et al. (2001) and, in part, from Yardley (1983). This is certainly not a complete overview of the topic. So donâ€™t hesitate to post critical questions, remarks or further arguments and ideas. (more…)
The son of Mother Lode? State and perspectives of the gold deposit research in the French Gulchâ€“Deadwood district, Klamath Mountains, USAWednesday, June 16th, 2010
By Lutz Geißler & Thomas Seifert
Mother Lode gold has been of scientific interest since the famous Californian gold rush began in 1848 in the Sierra Nevada foothills. Mining these clusters of low-sulfide gold-quartz veins bound to a major NW-SE trending fault zone in a tectonically active metamorphic complex yielded approx. 86 million ounces of gold (including placer deposits; Böhlke, 1999). Besides the strong impact on the economical development of the western United States of America, Mother Lode has been and is still a significant key for defining and characterizing the globally important â€śorogenic goldâ€ť deposit type. In contrast to this geologically well known gold belt, most of the gold deposits of the northerly located Klamath Mountains were never investigated with modern geoscientific methods. Researching their genesis in comparison to the Mother Lode deposits may be essential (1) for an improved geotectonic model of northern California, (2) for an enhanced understanding of orogenic gold deposits, and (3) for characterizing new exploration targets. (more…)
Geology, Mineralogy and Geochemistry of Gold-bearing Polymetallic-Sulfide Quartz Veins in the French Gulch Mine, French Gulchâ€“Deadwood District, Klamath Mountains, California, USAWednesday, June 16th, 2010
By Lutz Geißler & Thomas Seifert
The French Gulch-Deadwood district was the most productive gold mining district in the Klamath Mountains, Northern California (Au production: 800,000 to 1,500,000 oz; Hotz, 1971; Clark, 1970). It is located in the southeast of the Eastern Klamath terrane, to which a succession of terranes has been accreted by eastward directed subduction during Late Paleozoic to Early Jurassic time. (more…)
Geological setting and characteristics of gold-bearing quartz veins in the French Gulch mine, Klamath Mountains, Shasta County, Northern California, USAWednesday, June 16th, 2010
By Thomas Seifert & Lutz Geißler
The French Gulch mine is located in the French Gulch-Deadwood gold district in the south-eastern Klamath Mountains, California, as part of the Eastern Klamath terrane. With an approximated production of 800,000 oz (Hotz, 1971) to 1,500,000 oz gold (Clark, 1970) the district developed to the largest gold district of the Klamath Mountains. Three NE-SW to ENE-WSW and one N-S striking and steeply dipping gold-quartz veins crop out in the French Gulch mine. They are hosted by Middle Devonian greenstone and Lower Carboniferous black shales (Albers, 1965). These low-metamorphic units were intruded by Lower Cretaceous monzodiorite to diorite dikes (about 160 Ma) and rhyolite dikes (about 135 Ma) (Silberman and Danielson, 1991; Ross, 2006). (more…)
The Rammelsberg shale-hosted Cu-Zn-Pb sulfide and barite deposit, Germany: Linking SEDEX and Kuroko-type massive sulfidesSaturday, June 12th, 2010
Slide presentation and explanatory notes
The Rammelsberg Cu-Zn-Pb sulfide-barite deposit in the Harz mountain range, northern Germany, was mined almost continuously for more than a 1000 years (968-1988 AD). The mine, located south of the medieval town centre of Goslar, is now a UNESCO world heritage site. The Rammelsberg is a type locality for shale-hosted, sedimentary-exhalative (SEDEX) Zn-Pb-Ag deposits but is unusual because of the high grade (27 Mt at 19% Zn, 9% Pb, 160 g/t Ag) and high copper-gold content (1% Cu, 0.5-1 g/t Au) of the sulfide ore. (more…)