Gold mobilization during cleavage development in sedimentary rocks from the auriferous slate belt of central Victoria, Australia; some important boundary conditions

Abstract

The development of differentiated cleavage in metasedimentary rocks from the auriferous slate belt of central Victoria has made it possible to quantify the amounts of Au, Ag, As, Sb (determined by neutron activation analysis) Cu, and Zn released during metamorphism of the sedimentary rocks. This has been done by assuming that Ti, Zr, Y, and Ir, which are approximately twice as high in the cleavage layers as in the adjacent uncleaved layers, were little affected relative to other species during cleavage development and hence can be used to ascertain the gains and losses of the more mobile species. The quantities of the ore-forming metals released from the sedimentary rocks during cleavage development were controlled by their pyrite contents, the main host for these metals. Although 70 weight percent of the pyrite in the cleavage layers was dissolved, these layers constitute only 15 percent of the rocks and therefore the net loss of sulfur from the metasediments was 10.5 percent of their original sulfur contents. Similarily, although their average precleavage gold content was 5.0 ppb Au, only 0.53 ppb Au was released during metamorphism. It is improbable that this gold was deposited in the saddle reefs and other reef structures which are typical of the slate belt. For example, a column of sediments 40 km deep would have been required to have supplied the 42,466 kg of gold produced from the Clunes gold field. It is probable that the bulk of the gold released during cleavage development left the system well before the formation of the quartz reefs. This (along with the other constituents released) may have formed lower temperature epigenetic deposits in the tectonically younger rocks which overlaid the deformed sediments. Examples of precious metal deposits which formed in this way may be the Carlin-type deposits which are characterized by a low-temperature suite of elements. A Victorian example may be the gold-antimony ores at Costerfield.This investigation emphasizes the importance of both the availability of metals in rocks and the timing of their release from the rocks, if they are to be source rocks for economic concentrations of metals.