Hydrothermal geochemistry and ore genesis of sea-floor volcanogenic copper-bearing oxide ores

Abstract

The term copper-bearing mineralization is proposed to describe exhaled sea-floor chemical sediments, which are dominated by magnetite, quartz, and hematite, with subordinate chalcopyrite, pyrite, and in some cases, gold; they occur within or close to volcanic piles.Three Australian examples (the Starra, Osborne, and Big Cadia deposits) form the basis for a more general volcanogenic copper-bearing oxide model. The shapes of the deposits vary from massive mounds and pods (Cadia) with no laterally developed sedimentary iron-formation, through to extensive well-banded stratiform examples (Osborne). The orebody style of the deposits correspondingly varies from small high-grade Cu-Au lenses such as Area 251 at Starra (2.6 Mt at 6.7 g/t Au, 3.4% Cu), to large lower grade deposits such as Osborne (13.6 Mt at 1.9% Cu and 1.0 g/t Au, including a higher grade pod). In all cases intense footwall alteration is present (a hallmark of vent-proximal exhalative sediments), varying from albite-iron oxide-chlorite, to massive quartz-iron oxide, to muscovite-epidote-chlorite, depending on the salinity and pH of the ore fluid, and the presence or absence of boiling solutions. At Osborne, three stratigraphic ore positions record three episodes of hydrothermal activity in a stacked system, with evidence of overprinting in the earlier exhalites. At Starra and Cadia, there is evidence that the most proximal exhalites are dominated by magnetite, quartz, and chalcopyrite, whereas the distal portions are hematite dominated and variably pyritic.Ore formation is attributed to precipitation from the oxidized and H 2 S-deficient version of a volcanic-hosted massive sulfide fluid, in an oxidized water column. Such fluids must be buffered by hematite and/or magnetite during deep footwall fluid-rock reaction below vent complexes, to provide a sink for H 2 S produced by the reduction of seawater sulfate. Thus the ore type is favored in intracontinental basins which are common hosts for the requisite oxidized fluvial and shallow-marine sediments. A second possible source of oxidized metal-bearing fluids is highly fractionated magnetite series granites; in the Australian Proterozoic these are particularly prevalent in rift settings. To achieve Cu-Au solubility as chloride complexes, and to account for the ore mineral assemblages, the ore fluids must have had a T = 260 degrees to 380 degrees C, a log f (sub O 2 ) = -27.5 to -30, a pH = 3.9 to 6, and an Sigma S approximately 10 (super -3) M. High salinities are not an essential condition but serve to greatly enhance the efficiency of metal transport by increased chloride ion pair metal solubility. Higher salinities favor the transport of the elements Au, W, Sn, Mo, and Co, which constitute a distinct secondary element association. Salinity is also the strongest control on ore deposit shape by determining the behavior of the fluid after it exits from the vent; for instance, modeling suggests that a high-salinity fluid cooling at constant pH can explain the Si/Fe ratio and thickness of some Starra ore lenses.An important geochemical feature of volcanic copper-bearing oxide mineralization is a distinct depletion of Pb and Zn (usually less than 10 ppm of each occurs in ore zones). This is attributed to the very low H 2 S content of the ore fluid and water column, which permits precipitation of Cu and Fe as chalcopyrite and pyrite at temperatures greater than approximately 280 degrees C, removing all H 2 S prior to galena and sphalerite saturation. A volcanic copper-bearing oxide brine, from which Cu was removed at the vent, could migrate across a sea floor and precipitate Pb and Zn distally in localized biogenic H 2 S-bearing deeps. By this means, or by slightly increasing the exhaled H 2 S concentration, Pb-Zn-rich iron oxide ores with greater Pb-Zn contents could be produced, mineralogically transitional between volcanic copper-bearing oxide Cu-Au banded iron-formation and Broken Hill-Pegmont-style Pb-Zn banded iron-formation ores.