GEOCHEMISTRY AND GENESIS OF THE MURRAY-BROOK PRECIOUS-METAL GOSSAN DEPOSIT, BATHURST-MINING-CAMP, NEW-BRUNSWICK

The Murray Brook precious metal (Au, Ag) gossan deposit is one of nine supergene mineral deposits that have developed over polymetallic mass ive sulfide deposits in the Bathurst Camp of New Brunswick. Reserves c onsist of 1.9 Mt of gossan containing 1.53 g/t Au and 65.9 g/t Ag. The supergene zone at Murray Brook consists of six distinct units: a) alt ered massive sulfide, b) pyrite-quartz sand, c) massive sulfide gossan , d) disseminated sulfide gossan, e) ferruginized wallrock, and f) lea ched bedrock. The massive sulfide gossan constitutes the main body of economic mineralization and consists of goethite, primary quartz, seco ndary amorphous silica, K-Fe-Pb-As-Sb-Ag hydrated sulfate and oxide mi nerals (beudantite, plumbojarosite, jarosite, bindheimite, scorodite), trace cinnabar, and cassiterite of primary origin. The order of stabi lity of sulfide minerals during oxidation was pyrite > arsenopyrite > galena > Bi-Sb sulfosalts > chalcopyrite > tetrahedrite group > sphale rite. Two mass balance techniques (isovolumetric and conservative elem ent) indicate elemental depletion in gossan relative to primary ore in the order: n>Cu=Mn>Na=Ca>Fe=Al=V>Mg=K=Se=Hg>Pb>Ti>Ag>Mo=P>Ba; and enr ichment in the order Au>As>Sb>Si>Bi. Tin, as cassiterite, is conservat ive. Mineralogic and paleomagnetic data suggest that the gossan deposi t formed under a warm temperate climate throughout the Pliocene. The r ate of oxidation and the compositional nature of the ore was significa ntly influenced by: a) the position of the primary sulfide zone in a d ownward moving hydrological system, b) primary sulfide composition and zonation that promoted the development of a strong electrochemical ox idation system, and c) pronounced intercalation of carbonate and sphal erite mineralization in sulfide zones, which promoted rapid developmen t of secondary porosity, permeability and downward flow of oxygenated groundwater. During progressive oxidation and physico-chemical erosion of the gossan zone, Au was transported downward in groundwaters, prob ably as an Au-o colloid complex, to be concentrated in the lower horiz ons of the gossan profile. The precipitation of Au was greatly increas ed in the pyrite/arsenopyrite zones. Gold concentration was multi-cycl ic with continual dissolution and nucleation of the metal until final concentration in the void structures of the gossan. Leaching experimen ts and microprobe analyses indicate that Au is present in the gossan a s submicron composite sols of Au-Ag-silica.