Rb-Sr dating of sphalerites from Mississippi Valley-type (MVT) ore deposits

Abstract

Mississippi Valley-type (MVT) ore deposits arc epigenetic carbonate-hosted Pb-Zn deposits that contain galena, sphalerite, fluorite, bar&e, dolomite, calcite, and quartz. Although they are thought to form from basinal brines, their exact origins are still unclear, partly because of the scarcity of reliable geochronological data. Rb-Sr dating of sphalerites has recently been shown to be a promising technique for the direct dating of ore minerals in MVT deposits. This paper reports the results of a reconnaissance study of sphalerites, their fluid inclusions, and associated minerals from MVT deposits of North America. Sphalerites from Immel mine, Mascot-Jefferson City district, east Tennessee, define a Rb-Sr age of 347 f 20 Ma consistent with a Rb-Sr age of 377 +-29 Ma for sphalerites from Coy mine in the same district, but inconsistent with models that ascribe their genesis to the effects of the late Paleozoic Alleghenian orogeny. Rb-Sr isotopic analyses of K-feldspar from Immel mine preclude the possibility that the Rb-Sr data reflect feldspar inclusions. Sphalerites from the main ore zone of Daniel's Harbour mine, Newfoundland, do not form a linear isochron and open behavior of the Rb-Sr system is suspected. Sphalerites from the Pine Point district, Northwest Territories, Canada, define a Rb-Sr age of 36 1 + 13 Ma, indicating that the mineralization took place shortly after the deposition of the middle Devonian host carbonate rocks. These results are not compatible with mineralization models based on regional fluid migration related to early Tertiary Cordilleran deformation. Sphalerites from northern Arkansas have very low Rb and Sr concentrations (less than 0.1 ppm). The Rb-Sr data do not form isochrons and the sphalerites have higher '%r/%r ratios than expected, given their Rb/Sr ratios and reasonable constraints on their ages. The sphalerites are suspected to contain clay inclusions; and it is likely that the Sr isotopic compositions of these sphalerites, which have very low Sr concentrations, were affected by small amounts of inherited inclusions. Except for sphalerite from northern Arkansas, SEM studies and isotope dilution trace element measurements have so far failed to identify any suitable phases other than sphalerite that might be a host for the Sr.

Incidence and Importance of Tectonics and Natural Fluid Migration on Reservoir Evolution in Foreland Fold-And-Thrust Belts

“…When saline fluids also interact with siliciclastic formations, subsequent fluid migration may contribute to the development of Mississippi Valley Type (MVT) ore deposits within more distal and shallower carbonates. MVTs are known to host fluorite, barite, galena, sphalerite, pyrite, dolomite, quartz, and of course any type of Fe-calcite (Leach and Rowan, 1986;Packard et al, 1990;Nakal et al, 1993;Muchez et al, 1994;Qing and Mountjoy, 1995). These ore deposits occur in most FFTB around the world, i.e., in the foreland of the Appalachians, Ouachitas and Rocky Mountains in North America, in the Ardèche part of the Alpine foreland in France, the frontal areas of the Variscides in the United Kingdom, Belgium and Germany, and also in the foothills of Tunisia and Pakistan.…”

Depósitos de Pb-Zn-Cu-Ba-F-Sr epigenéticos estratoligados en series sedimentarias en relación con salmueras de cuenca: depósitos de tipo “Mississippi Valley” (MVT) y similares en México

“…Entre la técnicas radiométricas, una de las que ha dado mejor resultado es el fechamiento Rb/Sr de esfalerita mediante la obtención de una isocrona a partir de las relaciones isotópicas 87 Rb/ 86 Sr y 87 Sr/ 86 Sr. Esta técnica presenta un problema fundamental, ya que para obtener la isocrona a partir de cantidades realmente mínimas de Rb y Sr se analiza el que contiene tanto la esfalerita como sus inclusiones fl uidas. Este método ha permitido obtener la edad de los depósitos de Pine Point (Nakai et al, 1993) y Upper Mississippi Valley (Brannon et al, 1992) en los EE. UU.…”

Rb-Sr isotopic dating of sphalerite from the giant Huize Zn-Pb ore field, Yunnan Province, Southwestern China

“…The Rb-Sr method was used to determine the ages of sphalerite from the Mississippi Valley-type Zn-Pb deposits (Nakai et al, 1990(Nakai et al, , 1993Brannon et al, 1992;Christensen et al, 1995a, b;Diamond, 1995, 1996) and this method was a promising technique for the direct dating of ore minerals in Zn-Pb deposits. In this study, two precise ages were obtained by means of Rb-Sr isotopic dating of sphalerite from the Huize Zn-Pb ore field.…”