Elemental partitioning into Mn- and Fe-oxides derived from dolomitic shale-hosted PbZn deposits, northwest Queensland, Australia

Abstract

Exploration for dolomitic shale-hosted base-metal mineralization in northwest Queensland is largely dependent upon geochemical evaluation of ironstones. Fe-oxides, because of their abundance in the weathered zone, often quantitatively account for most of the base-metal content of an ironstone. Mn-oxides are less well developed but provide particularly good hosts for Ba, Zn and Pb and hence assume importance where present. The MnFe coatings found in stream coatings vigorously scavenge and concentrate solution-transported base metals and hence have to be very carefully evaluated when used during exploration. However, the present survey of base-metal contents in gossans and ironstones in northwest Queensland has shown that the Ba, Cu, P̄, Pb, Ni, S and Zn contents of the Mn- and Fe-oxides can be used to accurately indicate their parent material. Only Ce and Co are possibly preferentially incorporated into Mn-oxides but neither element is a pathfinder for PbZn mineralization. Development of the Mn-oxides, chalcophanite, todorokite, lithiophorite and the cryptomelane group, incorporating K, Ca, Mg, Ba, Co, Zn and Pb, reflects acid conditions during sulfide oxidation. When an ironstone consistently contains Mn-oxides with greater than 1% Pb or 10% Zn it can be regarded as derived from Pb- or Zn-rich mineralization. As Ba is often associated with sulfides and facilitates the incorporation of Pb into cryptomelane, Ba without accompanying base metals in a Mn-oxide implies derivation from base-metal-barren pyrite and/or pyrrhotite. Cu, Pb, Zn, Al, phosphate, sulfate (and arsenate) are all incorporated into Fe-oxides. However, only Pb which is more strongly partitioned into hematite and Al and phosphate which are preferentially incorporated into goethite show clear partitioning between the Fe-oxide phases. The preferential incorporation of phosphate rather than sulfate into an Fe-oxide makes areas showing higher sulfate contents particularly good targets. Knowledge of the nature of the distributions of pathfinder elements helps understanding of the associations seen in whole-rock analyses and leads to rational application of geochemical data during exploration.