COMPARATIVE GEOCHEMICAL STUDY OF SCHEELITE FROM SKARN SYSTEMS: IMPLICATIONS FOR MINERAL EXPLORATION

Scheelite occurs commonly in magmatic-hydrothermal systems (skarn, greisen, stockworks and porphyry), and its chemistry has been recently used to constrain ore-forming process in W and Au deposits. In skarns systems, scheelite precipitation is often spatially and temporally related to that of native gold, molybdenite, chalcopy­rite and cassiterite, which makes it a good indicator mineral. In this study, the chemistry of scheelite from reduced and oxidized skarns from 20 localities with different metal associations was investigated using EPMA and LA-ICP-MS. Gold, W-Au and W-Cu-Au mineralization are associated with reduced skarns, whereas Cu, W-Sn and W-Cu-Mo are related to oxidized skarns. The W and W-Cu mineralization occur in both skarn types. Cathodoluminescence images reveal oscillatory and patchy zoning, and dissolution textures are common in scheelite from skarn systems. Multielement maps reveal that zoning are mainly caused by the differences in the concentration of Mo and Nb. Scheelite associated with oxidized skarns is strongly depleted in HREE (La/Lu >400), whereas scheelite from reduced skarns is characterized by an enrichment in LREE (La/Lu <100). Principal component analyses (PCA) shows that scheelite from oxidized skarns contains higher Mo, As and Ti, and lower HREE and Y relative to scheelite from reduced skarns. Scheelite from Cu mineralization differs from other scheelites due to higher V contents. Scheelite from oxidized W-Cu and W-Sn is richer in Mg, which differs from that of W skarns. Scheelite from Au mineralization differs from that of W-Au and W-Cu-Au having lower Mn contents. Our results support that scheelite can be used to track the redox conditions of a given system and that the aforementioned metallogenetic associations are mainly controlled by source and redox conditions of the magma, fractionation degree and specially fluid-rock interaction with wall rocks.