Geochemistry of altered basalts (continental tholeiites) and associated copper deposits

Abstract

Copper concentrations occur in the upper part of an altered sequence of continental tholeiites and continental rhyolites in the Warburton area. The copper mineralisation consists mainly of chalcocite-djurleite within faults in hematite-rich rocks. The alteration mineral assemblages in the basalts consist of varying amounts of albite-chlorite-epidote-quartz-hematiteprehnite- pumpelleyite-montmorillonite. These are the result of basalt alteration at 200° to 400°C by a nearsurface hot spring system. The copper concentrations were. formed as a result of reactions oxidising Fe++ to Fe+++ and reducing so4 to HS- or s--. The removal of Fe++ resulted in the precipitation of copper-rich sulfides. Because of the high oxidation state of the alteration system, sulfides initially contained in the basalts were leached and most of the mobilised copper and sulfur moved out of the alteration system. In most hydrothermal systems, increases in the pH cause sulfide precipitation, but because of the high oxidation state, the effects of pH increase in causing sulfide precipit~tion are minimised. Sulfides which were precipitated as a result of pH increase are quantitatively insignificant relative to those precipitated by a lowering of the oxidation state. Analytical data on modern hot springs indicates that, during alteration, Sio 2 , cao, Na 2o, K2o are the most mobile components, and that MgO, Al 2o 3 , MnO, P 2o 5 and Tio 2 are inert. Environmental features of the altered basalts, the similarity of their rare earth element, Al 2o 3 , Tio 2 , MgO, MnO, P 2o 5 , Th, Zr, V, Ni, Ga and U contents with those of modern continental tholeiites with less than 53.5% Sio 2 indicate that the altered basalts are continental tholeiites. This is confirmed by the associated rhyolite ash flow tuffs which have Na 2o;K2o ratios less than one. Modern continental tholeiites are often associated with rhyolites with Na 2o;K2o ratios less than one. Rhyolites with Na 2o;K2o ratios less than one occur in continental environments; those with Na 2o;K2o ratios greater than one occur in island arc environments. The altered basalts have gained Na and have been depleted in Sr, Cu, Cl and S; Si, Ca, K, Ba, Rb, Zn and Pb underwent redistribution during alteration. The altered basalts have lost ~40 ppm Cu and ~250 ppm s, and the total amount of mobilised copper as a result of alteration was ~10 8 tons. The copper concentrations occur within basalts that have undergone severe local depletion of Cu and s, and which contain higher Fe 2o 3 , Na 2o and Cl contents than altered basalts elsewhere in the area. The copper occurred in the original basalts mainly as a sulfide and as cu++ within Fe++ sites in the iron-titanium oxides. The sulfide copper has been removed from the Cassidy Group (lower group) of basalts as a result of oxidation, but in the vicinity of the copper concentrations in the Mission Group (upper group) , the cu++ in the iron-titanium oxides has been removed as well because of oxidation of the iron-titanium oxides to hematite. Both lead and zinc are retained in either unaltered primary minerals or within the alteration assemblages. In the wallrocks of the copper concentrations, zinc mobilised as a result of oxidation of the iron-titanium oxides to hematite has been retained in chlorite. Many of the large continental copper concentrations such as those at White Pine, Boleo, Mt. Isa, and many others overlie altered continental tholeiites. They have resulted from precipitation of the copper mobilised during basalt alteration because of reduction of the contained sulfate in the aqueous phase within an appropriate euxinic environment. An outside source of sulfur is not necessary; there is enough mobilised "basalt" sulfur to precipitate all of the mobilised copper. If an efficient sulfide precipitating mechanism does not exist, copper-rich sediments may be formed over a large area. This alt.erat;ion model i.llust:rat:.es the inadequacy of using ore deposit metal ratios to identify the source rock type.