Geochemistry and Evolution of Mississippi Valley-Type Mineralizing Brines from the Tri-State and Northern Arkansas Districts Determined by LA-ICP-MS Microanalysis of Fluid Inclusions

Abstract

The Tri-State and Northern Arkansas districts of the Ozark plateau, North America, are both classic examples of Mississippi Valley-type (MVT) mineralization, formed by continent-scale basinal brine migration as a result of the uplift of the Arkoma foreland basin in response to the Early Permian Ouachita orogeny. The chemistry of the fluids responsible for both sulfide mineralization and gangue precipitation in these districts was studied by quantitative microanalysis of individual fluid inclusions in quartz and sphalerite using 213-nm laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Using halogen systematics, an evaporative seawater origin for the brines was determined, but higher Br concentrations suggest that the sphalerite-hosted "ore fluids" underwent a significantly greater degree of evaporation in the initial stages of fluid evolution compared to brines hosted by gangue phases. Metal contents of the brines responsible for quartz and dolomite precipitation are low compared to modern basinal brines, but many of the fluid inclusions trapped in sphalerite in both districts contained anomalously high metal concentrations, suggesting that mineralization involved incursion of a metal-rich fluid of distinct geochemistry. Examination of the multicomponent chemical characteristics revealed that dolomitization was probably an important process in the early chemical evolution of fluids that infiltrated both districts. In the Tri-State district, precipitation of sulfides was most likely driven by mixing of the metalliferous fluid with another brine, possibly rich in reduced sulfur. In northern Arkansas the compositional variations observed are best explained by local dissolution of the carbonate host rock. This may have been the process that ultimately drove sulfide deposition through fluid neutralization and reduction. Alternatively, the digestion of the host rock may have been the result of locally generated acidity produced by the deposition of sulfides. The discovery of anomalously metal-rich fluids linked to mineralization suggests that these deposits are not simply the product of typical basin evolution, helping to explain the abundance of MVT mineralization in some forelands, whereas others are barren. It is likely that a significant portion of the history of the hydrothermal flow system was characterized by the precipitation of barren gangue assemblages from metal-poor brines, with metalliferous fluids only being expelled from a specific stratigraphic package at a distinct stage of basin evolution.

Source of diagenetic fluids from fluid-inclusion gas ratios

“…This standard from the Hansonburg Mississippi-Valley type deposit (MVT) in New Mexico was originally collected, described, and analyzed using the thermal decrepitation method of gas extraction by Norman et al (1985). The origin of MVT deposits is known to be amagmatic but from a basinal brine fluid and/or highly modified seawater component (Chi and Savard, 1997;Stoffel et al, 2008). By reanalyzing the HF-1 standard using the newer incremental crush fast scan method, we can certainly compare the diagenetic carbonate data to material that hosts basinal fluids acquired by a similar mechanism.…”

Uma revisão dos fundamentos do estudo de inclusões fluidas aquosas e de petróleo

“…Fluidos com composição química do sistema CaCl 2 -NaCl-H 2 O são abundantes em diversos ambientes geológicos, sejam em bacias sedimentares (e.g., Collins 1975, Hanor & McIntosh 2007, em ambientes hidrotermais submarinos (e.g., Vanko 1988) e em diversas classes de depósitos minerais, a exemplo: depósitos de Pb-Zn do tipo Mississippi Valley (e.g., Haynes & Kesler 1987, Basuki & Spooner 2002, Stoffell et al 2008, depósitos de ouro orogênico (e.g., Robert & Kelly 1987), depósitos de óxido de ferro-Cu-Au, também denominados pela sigla IOCG (e.g., Xu 2000).…”

Fluids and Ore Formation in the Earth's Crust

“…Stoffell et al (2008) and redox reactions in hydrothermal systems, to the point that great variability of δ 34 S among sulfides can be taken as first-order evidence for significant involvement of sulfate in an oreforming process (Ohmoto, 1986).…”

“…Note that many low-temperature brines have Pb concentrations well above the rock-buffered solubility line (red). These include 'anomalously' Pb-rich solutions in fluid inclusions hosted by sphalerite in Mississippi Valley-type (MVT) and Irish Pb-Zn deposits, which contrast with relatively metal-poor fluids in associated gangue minerals of the same deposits (Stoffell et al, 2008;). Similar to some recent basin brines (Kharaka et al, 1987), these ore-forming fluids must be sulfide-deficient, either due to sulfate predominance under unusually oxidizing conditions at their source (Fig.…”