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.
The most convenient location for that research is the French Gulchâ€“Deadwood district, 30 km northwest of Redding, southern Klamath Mountains, because (1) the district has been the most productive one in the Klamath Mountains (800,000 to 1,500,000 ounces; Hotz, 1971; Clark, 1970); (2) the deposit is geographically positioned between the Mother Lode belt and the sole modernly investigated gold deposit in the Klamath Mountains, Quartz Hill (Oro Fino district), and plays, therefore, an essential role for attaining a possible synthesis of both, the Klamath and the Mother Lode genetic models; (3) the deposit is very well exposed (French Gulch mine) and (4) has a significant economic potential (Geißler and Seifert, in press).
The host rocks of the ENE-WSW to NE-SW and minor N-S trending and steeply dipping gold-quartz veins in the French Gulchâ€“Deadwood district are Lower Devonian meta-basalts and meta-andesites (Copley greenstone), which are separated from overlying Upper Devonian and Lower Carboniferous graphitic meta-shales and meta-graywackes (Bragdon Formation) by a thrust fault (Albers, 1964). This thrust is locally occupied by Lower Cretaceous rhyolitic sills and dikes (135 Ma; Silberman and Danielson, 1991). The district is characterized by large scale folds with axes striking NE-SW and WNW-ESE. The investigated French Gulch mine is located at the southern flank of a WNW striking anticline, within the prominent French Gulch fault zone. This fault system extends over 14 km in length and 3 km in width, strikes nearly E, westward to ESE, and dips steeply to the S. Upper Jurassic dacitic dikes and sills (Birdseye porphyry, 160 Ma; Silberman and Danielson, 1991) intruded the fault zone. The youngest intrusives are intensely altered lamprophyric dikes and sills with elevated concentrations of Cr (mean 725 ppm, n = 2), Ni (175 ppm), Co (35 ppm) and MgO (8.2 wt.-%) indicating a mantle-derived magma. Strong negative Nb-Ta-anomalies of the dacitic, rhyolitic, and lamprophyric intrusives suggest subduction-related magmatism in a volcanic arc milieu (Geißler and Seifert, 2009).
The pinching and swelling and typically ribboned gold-quartz veins are the youngest geological units in the French Gulch mine with a thickness of few decimeters to 2-3 m. They developed in an active tectonic environment and are characterized by at least five mineralization stages with (I) quartz Â± carbonate Â± albite, (II) coarse pyrite, (III) fine pyrite + arsenopyrite + sphalerite Â± chalcopyrite Â± pyrrhotite, (IV) galena Â± gold Â± tellurides (hessite, petzite, altaite), and (V) carbonate Â± quartz Â± plagioclase. Stringer zones in the wall rocks, mineralized macroscopic and microscopic shear planes, and angular wall rock fragments are characteristic features of the veins. Lithological contacts are the favored positions of ore, especially near or in contact to the dacitic dikes, which acted as structural control for the vein-forming fluids. In average, the ore contains 20 ppm Au (nÂ =Â 32), 16 ppm Ag, 59 ppm Cu, 1.95 ppm Pb, 0.23 wt.-% Zn, and 0.5 wt.-% As (Geißler and Seifert, 2009). Wall rocks in contact to the veins show slightly elevated Au contents. The bulk ore Au/Ag ratio (mean: 4.2; n = 25) is typical for lode gold deposits (Boyle, 1979).
The host rocks of the Mother Lode veins (mainly greenstones and graphitic meta-shales) form the Foothills metamorphic belt which consists of several accreted terranes (mostly oceanic and island arcs and associated sediments) comparable to the geotectonic construction of the Klamath Mountains. The gold deposits are concentrated along NW-SE trending structures. The most productive veins occur along the Melones fault system (Lindgren, 1895). An active tectonic environment is marked by brecciation, open-space vug fillings, ribbon structures and crinkly banding (pressure-solution). The maximal extent of the veins is 1 to 2 km in length and depth. In comparison, the vein system of the French Gulchâ€“Deadwood district extents at least 0.6 km in length and 0.9 km in depth (Geißler and Seifert, in press). Besides gold, also pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, tetrahedrite and various tellurides (e.g., hessite, petzite, altaite, melonite) occur within millimeter thin veinlets to more than 10 m thick quartz veins (Böhlke, 1999). Fahlore seems to be a characteristic element of the paragenesis, but has not yet been found in the French Gulchâ€“Deadwood district. Mother Lode gold was deposited relatively late in the sequence of vein-filling phases and is associated with galena (Böhlke, 1999). A further similarity between the Mother Lode and the French Gulch deposits is their equal average amount of Ag in gold (17 wt.-%; Geißler and Seifert, in press).
Hydrothermal micas (mariposite, Cr-bearing mica) were used by several authors for dating the metasomatic overprint of the mainly sericitized and carbonatized Mother Lode wall rocks and, therefore, the mineralization event. Thus, the vein emplacement occurred over a period of 44 m.y. (Ar/Ar-ages; Böhlke and Kistler, 1986; Snow et al., 2008). The oldest mineralization was found in the north of the Mother Lode belt (152 Ma), the youngest in the south (108 Ma).
In comparison to Mother Lode and the French Gulch area, the Quartz Hill deposit in the central Klamath Mountains shows similar characteristics in the tectonic and petrologic setting of the veins, their paragenesis and macroscopical and microscopical features (cf. Elder and Cashman, 1992). The deposit is much older (147 Ma; K/Ar, hydrothermal sericite; Elder and Cashman, 1992) than any other mineralization age in the Klamath Mountains (indicated by geological relationships), but lies in the Mother Lode age range. The maximum temperatureâ€“pressure conditions of the gold-bearing fluids from Quartz Hill were estimated by Elder and Cashman (1992) to Â± 375Â°C and Â± 1.35 kbar. The Mother Lode vein formation was characterized by comparable temperatures of 250-400Â°C and pressures of 0.5-3 kbar (Coveney, 1981; Böhlke and Kistler, 1986; Weir and Kerrick, 1987).
The presented data of the French Gulchâ€“Deadwood district and the summarized information about Quartz Hill and Mother Lode suggest a strong spatial and temporal genetic relationship between the gold-quartz veins of the Klamath Mountains and the Sierra Nevada foothills. This is supported by the joint geotectonical development until the Lower Cretaceous resulting, for instance, in similar accretionary complexes and comparable ages of batholithic intrusions in both geomorphological units (cf. Irwin, 2003). The subsequent separation of the Klamath Mountains block from the Sierra Nevada westward along a NE-SW trend seems to have occurred exactly during the period of gold mineralization, which is thought to be far younger than 135 Ma in the French Gulchâ€“Deadwood district (Geißler and Seifert, 2009; Geißler and Seifert, in press). The planned comprehensive research on the gold ores and associated intrusions in this district using multiple dating methods combined with stable and radiogenic isotope and fluid inclusion investigations may provide detailed and reliable answers to the following questions: 1) Did the gold vein systems of the Klamath Mountains and the Sierra Nevada foothills develop jointly during late Upper Jurassic to late Lower Cretaceous or (how) did the mineralizing regime change with the separation of both geomorphologic units? 2) Did the vein formation occur as a single continuous event (cf. Snow et al., 2008) or as multiple events (e.g., Elder and Cashman, 1992)? 3) If there is a unifying geological model to explain the genesis of the Mother Lode and the Klamath Mountains gold deposits, why did the Sierra Nevada foothills get mineralized more intense than the Klamath Mountains (with respect to the gold production rates)?
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- Geißler, L., Seifert, Th., in press, Geology, Mineralogy and Geochemistry of Gold-bearing Polymetallic Sulfide-Quartz Veins and Associated Intrusions in the French Gulch-Deadwood District, California: Freiberger Forschungshefte C533 Geowissenschaften, 361 p.
- Geißler, L., Seifert, Th., 2009, Geology, Mineralogy and Geochemistry of Gold-bearing Polymetallic-Sulfide Quartz Veins in the French Gulch Mine, French Gulch-Deadwood District, Klamath Mountains, California, USA: Hallesches Jahrbuch für Geowissenschaften, 31, p. 75.
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- Map showing accreted terranes and plutons of the Klamath Mountains and Sierra Nevada, scale 1:1,000,000: Sheet 2 â€“ Successive accretionary episodes of the Klamath Mountains and northern part of the Sierra Nevada: U.S. Geological Survey, Open-File Report, 02-490, 2 sheets.
- Lindgren, W., 1895, Characteristic features of California gold-quartz veins: Geological Society of America Bulletin, v. 6, p. 221â€“240.
- Silberman, M.L., and Danielson, J., 1991, Geologic setting, characteristics, and geochemistry of goldbearing quartz veins in the Klamath Mountains in the Redding 1 X 2 degree quadrangle, northern California: U.S. Geological Survey, Open-File Report, 91-595, 27 p.
- Snow, C.A., Bird, D.K., Metcalf, J., McWilliams, M., 2008, Chronology of Gold Mineralization in the Sierra Nevada Foothills from 40Ar/39Ar Dating of Mariposite: International Geology Review, Vol. 50, 2008, p. 503â€“518.
- Weir, R.H., and Kerrick, D.M., 1987, Mineralogic, fluid inclusion, and stable isotope studies of several gold mines in the Mother Lode, Tuolumne and Mariposa Counties, California: Economic Geology, v. 82, p. 328â€“344.
Both the abstract and presentation were published at the International Forum-Competition of Young Researchers “Topical Issues of Subsoil Usages” in St. Petersburg 2010:
- Geißler, L. & Seifert, Th. (2010): The son of Mother Lode? State and perspectives of the gold deposit research in the French Gulch-Deadwood district, Klamath Mountains, USA. – International Forum-Competition of Young Researchers “Topical Issues of Subsoil Usages”, 21.-23. April 2010, St. Petersburg State Mining Institute.
Tags: abstract, California, Deadwood, DMG, DMG meeting, French Gulch, French Gulch-Deadwood district, Gold, gold-quartz vein, Klamath Mountains, Melones fault zone, Mother Lode, orogenic gold, Poster, Shasta, Sierra Nevada, Sierra Nevada foothills, Washington mine