Gold distribution at the E26 porphyry copper-gold deposit, Goonumbla N.S.W.

The E26 porphyry copper-gold deposit is the largest of several copper-gold deposits discovered to date in the Goonumbla region of central west N.S.W. Cu and Au mineralisation occurs in areas of strong pervasive potassic alteration and quartz veining, associated with a small pipe-like quartz monzonite porphyry (QMP1). The mineralisation occurs both within the porphyry and the covolcanic late Ordovician trachyte-latite lavas and volcanic breccias that it has intruded. The deposit is bounded at depth and to the east by a biotite quartz monzonite, while to the immediate north, is another younger QMP (QMP2) which has stoped out the Cu-Au mineralisation. Several other QMP's occur to the northwest and at deeper levels in the E26 deposit. Copper mineralisation occurs principally as bornite with lesser chalcopyrite and chalcocitedigentite. Two bornite phases have been identified based on colour and mineral associations. Gold occurs principally as fine inclusions in bornite. The sulphides are zoned from a bornite-rich core, through a chalcopyrite zone to an outer pyrite halo associated with propylitic alteration. A weak base metal/gold association occurs in thin structurally controlled zones of quartz-sericite-pyrite alteration, away from the main E26 mineralisation. Within E26 itself, the bulk of the Au is confined to the central and deeper parts of the system and immediately around QMP 1. Cu however, continues to shallower depths and laterally further away from QMP 1 . This pattern is reflected in the Cu/Au ratios, and has led to the definition of two distinct vertical domains within the higher grade areas of E26. The upper domain is characterised by high copper grades but low gold grades; the lower domain contains similar copper grades but much higher gold grades. Sulphide composition, timing and alteration are similar within the Cu/Au domains defined. A geochemical model developed shows that the Cu/Au relationships observed at E26 can be explained by coprecipitation of Cu and Au, but at different rates due to differing changes in saturation levels between the two metals. Deposition of both metals from chloride complexes in high temperature, saline fluids was in response to a decrease in T, and changes in f02nand pH associated with hematisation of secondary magnetite associated with earlier biotite (potassic) alteration. The deposits at E22, E27 and E31N do not show the same degree of magnetite destruction as E26 and E48, are more deeply eroded, and therefore not as enriched in copper.