MAGMATIC ANHYDRITE IN THE HANOVER-FIERRO STOCK AND ITS IMPLICATIONS FOR FS2-FO2 OF INTRUSIONS ASSOCIATED WITH PORPHYRY COPPER DEPOSITS

Although intrusive rocks associated with copper deposits are commonly regarded as oxidizing, few measurements to confirm this have been made and even fewer minerals characteristic of an oxidized environment have been found in these rocks. Here we report magmatic anhydrite from the Hanover-Fierro stock, which is associated with mineralization at the Continental Cu-skarn deposit and coeval with the Chino porphyry copper deposit immediately to the south. Anhydrite at Hanover-Fierro occurs in interstices and as inclusions in K-feldspar and quartz in two samples from a deep drill hole in the Hanover-Fierro stock. Four surface samples of the stock contain anhydrite as inclusions in K-feldspar and quartz. Electron microprobe analyses of the anhydrite show that it is CaSO₄, with Sr, Ba, and REE below detection limits. The coexistence of magmatic anhydrite, hemo-ilmenite, quartz, and sphene and the absence of clinopyroxene allow the use of limiting reactions to characterize fO₂ and fS₂ conditions of the crystallizing magma. Important reactions include 4CaTiSiO₅ + 2Fe₂O₃ + 2S₂ + 5O₂=4CaSO₄ + 4FeTiO₃ + 4SiO₂ and 4CaFeSi₂O₆ + 7O₂ + 2S₂=4CaSO₄ + 2Fe₂O₃ + 8SiO₂. Use of these reactions requires that corrections be made to end-member curves to account for decreased activity of Fe₂O₃ and FeTiO₃ due to extensive hemo-ilmenite solid solution. The average composition of hemo-ilmenite from anhydrite-bearing samples in the Hanover-Fierro stock is hem₅₂ilm₄₅geik_0.7pyr_2.3. This restricts log fO₂ to values greater than 0 to 1 log unit below the magnetite-hematite oxygen buffer and log fS₂ above –6.7 to -8.8. These high fO₂, low fS₂ conditions favor sulfate as the dominant form of sulfur in the Hanover-Fierro melt. The lack of sulfide inhibited formation of magmatic sulfides. Such sulfides would probably remove ore metals from the intrusion and prevent formation of a late-stage, copper-rich hydrothermal phase.