Precious metals in ocean-ridge basalts; implications for basalts as source rocks for gold mineralization

Abstract

The Au, Ag, and S contents of the glass rims of tholeiitic pillow basalts dredged between 25 degrees N and 30 degrees N on the Mid-Atlantic Ridges are up to 7, 5, and 2.5 times greater, respectively, than those of the crystalline interiors of the basalts. Si, Fe, and Mn are also lower in the crystalline interiors relative to the glass rims, whereas K is higher. The differences in chemical composition between the rims and the interiors are attributed to loss of Au, Ag, S, Si, Fe, and Mn from the interiors of the basaltic pillows due to interaction of the hot basalt with sea water and gain in K by the interiors during subsequent low temperature interactions.Manganese crusts from an area of hydrothermal activity at 26 degrees N have exceptionally low Cu, Ni, Co, Fe contents and low Ir/Au and Ir/Ag ratios when compared to normal hydrogenous crusts that grow away from areas of submarine hydrothermal activity. Only a small fraction of the metals that are lost from the pillow interiors may be transported to the sea floor via the hydrothermal solutions that initially interact with the basalts; the remainder may reside within the basalt pile, for example, in interpillow regions.The Au contents of the fresh crystalline interiors of the basalts correlate with their Cr, Ni, and MgO contents. This suggests that Au in the basalt interiors is hosted by an early-crystallized phase such as Cr-spinel or olivine. The difference in Au content of the fresh interior (hosted by Cr-spinel and/or olivine) and that of the glass rim of the basalt is believed to represent that Au which would have been deposited on loosely bound sites (e.g., deuteric alteration products, grain boundaries, mesostasis phases, and, most important, sulfides within the pillow interior) had it not interacted at a high temperature with sea water.The Au and Ag contents of the glass rims vary by a factor of 10, confirming the existence of regional variations in the Au contents of basalts. The lowest Au and Ag contents in the glass rims are for basalts dredged from an area with no known hydrothermal activity.The results of this study suggest that basalts with high MgO, Cr, Ni, Cu, and Ir contents and low K 2 O and TiO 2 contents may constitute the most favorable type of source basalt. The study demonstrates that the metal available for ore generation in a source rock is not a function of the total metal content of the rock but rather a function of the mineralogical siting of the metal.