No abstract

Hydrothermal ore deposits form when metals, often as sulfides, precipitate in abundance from aqueous solutions in Earth's crust. Much of our knowledge of the fluids involved comes from studies of fluid inclusions trapped in silicates or carbonates that are believed to represent aliquots of the same solutions that precipitated the ores. We used laser ablation inductively coupled plasma mass spectrometry to test this paradigm by analysis of fluid inclusions in sphalerite from two contrasting zinc-lead ore systems. Metal contents in these inclusions are up to two orders of magnitude greater than those in quartz-hosted inclusions and are much higher than previously thought, suggesting that ore formation is linked to influx of anomalously metal-rich fluids into systems dominated by barren fluids for much of their life.

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.