The geochemistry and alteration of the White Devil porphyry: implications to intrusion timing

Abstract

Quartz-feldspar porphyry dikes in and around the White Devil ore deposit are weakly to strongly altered, with weakly altered cores grading outward toward intensely chloritized marginal zones in contact with massive ironstone or chlorite altered sediments. Petrographic studies indicate the following sequence of alteration: sericitization of the groundmass, sericitization and chloritization of feldspar phenocrysts, chloritization of the groundmass, and alteration of igneous biotite to hydrothermal biotite and then to chlorite, intense chloritization of the groundmass as to remove quartz, and, finally, destruction of quartz phenocrysts. Isocon analyses of whole rock geochemical data from altered porphyry indicate the following relative changes during alteration: (1) consistent, near total losses of Na and Pb, (2) increasing gains of Mg, Fe, Mn and Zn, and increasing losses of Si and Sr with increasing alteration intensity, (3) gains of Ti, Cr and U, and losses of K and Rb upon alteration of feldspar phenocrysts, and (4) losses of Zr, Nb, Y and Th upon the loss of quartz in the groundmass. Of the elements studied only Ca, Al and P did not undergo significant relative changes. “Immobile” elements such as Ti, Zr, Nb, Y and Cr are highly mobile during the most intense alteration of the White Devil porphyry. Based on evidence from field relationships, alteration studies and petrographic observations, the quartz-feldspar porphyry dikes intruded after the formation of ironstones, but prior to Au-Cu-Bi mineralization. This conclusion is consistent with previous interpretations by Nguyen et al. (1989) and Edwards et al. (1990), but inconsistent with the interpretation by McPhie (1993) that quartz-feldspar porphyry sills at the Peko smelter site intruded into wet sediments. Two or more periods of quartz-feldspar porphyry intrusion occurred in the Tennant Creek district.