ANTITHETIC BEHAVIOR OF GOLD IN THE VOLCANOGENIC MASSIVE SULFIDE DEPOSITS OF THE IBERIAN PYRITE BELT

Detailed mineralogical and geochemical studios of the volcanogenic sul phide mineralization in the Spanish part of the Iberian Pyrite Belt (I PB) define two geochemical, mineralogical and spatial gold association s: (1) the Tharsis-Sotiel-Migollas type in which the gold is enriched with (Co +/- Bi) in the stockworks and interaction zones at the base o f the massive sulphide mound: and (2) the Rio Tinto-Aznalcollar-La Zar za type in which the gold is enriched in facies with a polymetallic (Z n + Ag +/- As +/- Tl +/- Hg) signature in distal position or blocked b eneath the massive sulphides. The first type is localized within a dom ain covering the southern half of the belt which is characterized by a n abundance of sedimentary facies. The paragenesis shows that the gold association formed at high temperature (> 300 degrees C) during the i nitial phases of massive sulphide genesis; the gold, which occurs in p atches of very auriferous electrum (Au > 75 wt.%), was transported by chloride complexes. The second type is found in the northern domain of the belt where volcanic facies are predominant. The paragenesis shows that the gold association formed at lower temperature (< 280 degrees C) late in the massive sulphide genesis. This gold was transported by bisulphide complexes [Au(HS)(2)(-)] and is contained in Ag-and Hg-rich electrum (up to 61.0 and 30.5 wt.% respectively) and/or auriferous ar senopyrite (mean of 280 ppm Au), two mineral expressions that are able to coexist. It would appear that sulphur activity and oxygen fugacity were important factors in controlling the distribution of gold betwee n the two host minerals and also in determining the Ag content of the electrum, This antithetic behaviour of the gold in the IPB reflects di fferences in the gold mineralizing fluids that may be due to the geolo gic environment; i.e. either dominantly sedimentary and acting as a me chanical barrier for gold bearing fluids, or dominantly volcanic and m ore open to seawater circulation. The fact that possible complications can occur during massive sulphide genesis, in response to the source and evolution of the fluids, raises the question of whether one or two gold influxes are involved. For example, the two gold associations co uld derive from a single gold influx, with remobilization and redistri bution of the gold from the early (Co +/- Bi) facies giving rise to th e later Sold paragenesis of the (Zn + Ag +/- As +/- Tl +/- Hg) facies; this would not have occurred or would have been limited at the Tharsi s-Sotiel-Migollas type orebodies. Alternatively, the two gold associat ions could reflect two separate evolutionary processes distinguished b y the gold appearing either early or late in the hydrothermal fluids. Knowing the gold association of a massive sulphide deposit is an advan tage when exploring for potential host facies.