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Heavy Metals or Punk Rocks?

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Science  06 Feb 2009:
Vol. 323, Issue 5915, pp. 724-725
DOI: 10.1126/science.1166394

At the recent World Copper Congress in Santiago, Chile, Rio Tinto's chief executive for copper, Bret Clayton, reported that copper consumption is expected to double over the next two decades (1); demand for other metals is expected to parallel this trend. These projected metal needs cannot be satisfied with known ore bodies. To locate new deposits, minerals exploration programs require robust genetic models for the formation of economic accumulations of metals. On page 764 of this issue, Wilkinson et al. (2) elucidate one of the least understood aspects of ore formation: the concentration of metals in hydrothermal solutions that deposited the ores.

Fluid inclusions provide the best available tool for determining the physical and chemical conditions during ore formation (3). These microscopic samples (∼5 to 50 μm in diameter) of the ore-forming fluid are trapped in ore and non-ore minerals during mineralization; they thus record the temperature, pressure, and composition of the ore-forming fluid. Advanced microanalytical techniques allow individual fluid inclusions to be analyzed to determine ore metal concentrations (46).

Wilkinson et al. now report unusually high metal contents of hydrothermal fluids from two ore districts containing sediment-hosted zinc-lead deposits, based on microanalysis of fluid inclusions. Other recent studies of fluid inclusions from copper (7) and gold (8) deposits also found much higher metal concentrations than would have been predicted on the basis of experimental data or theoretical models. If confirmed by further studies, these results have important implications for both the duration of the ore-forming process and the amounts of ore fluids needed to generate world-class ore deposits. This information is of crucial importance for understanding ore genesis, given that the duration of ore-forming systems is one of the major unknowns related to the formation of mineral deposits (9).

Average continental crust contains about 70 ppm zinc (Zn) and 12.5 ppm lead (Pb). In contrast, average ore grades in Mississippi Valley-type (MVT) Zn-Pb deposits similar to those studied by Wilkinson et al. typically are about 6% Zn and 2% Pb by weight, representing enrichment factors of about 850 and 1600, respectively. Thus, metals must be scavenged from a large volume of rock with average crustal metal values and concentrated into a much smaller rock volume to generate economic deposits. For example, large to giant MVT deposits contain on the order of 106 to 2 × 107 metric tons combined Zn + Pb (10), with an average Zn: Pb ratio of ∼3.

Garven (11) modeled the fluid-flow history associated with formation of a large MVT Zn-Pb deposit in Pine Point, Canada, and concluded that the total hydrothermal fluid discharge through the mineralized area was 5 × 106 m3 year−1. Garven assumed that 5 mg of zinc precipitated per kilogram of solution that flowed through these rocks and concluded that it would take from 0.5 to 5.0 million years to form the deposits, with Darcy flow rates (12) of 1 to 5 m year−1. Similar durations for ore formation (0.3 million years) have been estimated for the MVT deposits in the Upper Mississippi Valley district of the United States (13).

Flow rates and duration of the ore-forming process reported by Garven (11) require total hydrothermal fluid volumes ranging from 2500 to 25,000 km3 over the lifetime of the ore-forming system. Similar volumes of fluid would be required to form other large to giant MVT deposits if each kilogram of fluid only precipitates a few milligrams of metal. However, if the metal content of the ore-forming fluid is considerably higher, as suggested by Wilkinson et al., then both the amount of fluid required and the duration of the ore-forming event would be reduced by orders of magnitude (see the figure). For example, if each kilogram of hydrothermal fluid deposited 103 mg of Zn (orange dot in the figure), then Pine Point and similar deposits could have formed in about 104 years from a few cubic kilometers of hydrothermal fluid, compared to the millions of years and hundreds of cubic kilometers of fluid required assuming that each kilogram of hydrothermal fluid deposited 5 mg of Zn (green dot in the figure).

How to form an ore deposit.

This modified “Roedder diagram” (15) shows the relationship between the amount of metal precipitated per unit mass of hydrothermal fluid (y axis) and the size of the ore deposit (x axis). The time (black diagonals) and volume of fluid (red dashed diagonals) required to form the deposit are contoured onto these coordinates. The width of the shaded box represents the range in ore tonnage for large to giant MVT Zn-Pb deposits (10). Wilkinson et al. report zinc concentrations of 5000 ppm and 3000 ppm (dashed lines near the top of the box). These concentrations are higher than previously reported and suggest that economic deposits can form faster than previously suggested (green and orange dots).

The results presented by Wilkinson et al. further highlight the importance of depositional processes in the formation of economic occurrences of metals. Most ore geologists now agree that fluids with metal contents sufficient to produce economic mineralization are relatively common (14), and that it is the lack of a suitable depositional mechanism that often limits ore formation. Temperature decrease alone cannot be the dominant mechanism, because the solubility of most metals in most hydrothermal fluids decreases by only a small amount over the temperature range determined for most deposits. Thus, other processes—such as boiling or immiscibility, fluid mixing, or fluid-rock interactions—must operate to promote the precipitation of all (or most) of the dissolved metals transported by the hydrothermal fluids. The results presented by Wilkinson et al. provide important new insights into metal contents of ore-forming fluids and emphasize the need for continued research to constrain the amounts of hydrothermal fluids required to form world-class ore deposits and the duration of the ore-forming events.

References and Notes

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