THE SIERRA GORDA PORPHYRY CU-MO(AU) DEPOSIT, REGION II, NORTHERN CHILE, PART 2: INTRUSIVE RELATIONS AND 40AR/39AR AND RE-OS MOLYBDENITE GEOCHRONOLOGY OF THE CATALINA AND 281-ZONE MINERALIZATION CENTERS

Seven Re-Os moly ages and 26 ⁴⁰Ar/³⁹Ar plateau ages (plag, bio, sericite, Kspar) suggest that Sierra Gorda’s Catalina and 281-Zone mineralization resulted from ~3.m.y. (60.06 ±0.39 to 57.31 ± 0.19 Ma) of ongoing igneous/hydrothermal activity. Sourced by an underlying monzodiorite (MD) batholith, each center hosts up to 15-20 small intrusive bodies (many <100m wide), but intrusions & breccias (bx) differed between centers. The 281-Zone is largely a dike swarm of fine-groundmass (<0.1mm) feldspar porphyry (PF) (± granodiorite porphyry PG). By contrast, Catalina is a porphyry/bx-pipe complex of PG dikes/plugs, lesser PF, tourmaline bx (BXT), potassic bx (BXH), and stocks and cupolas texturally zoned from MD-porphyry cores (with 0.1-2mm graphic groundmass) to intensely K-feldspathized carapaces of coarse (0.1-2mm) graphic aplite (Sienogranite SG). Locally, SG cupolas grade downward into qtz-feld-tourm pegmatite and upward into vuggy tourm “flash breccia” (caused by L-to-V flash). Moly veins (Kspar±bio halos) predate chloritic cpy veins (± ser, py) in both centers. Coupled samples (moly with bio or overprinted by ser) suggest that individual Sierra Gorda hydrothermal events took <250-400 Ka to cool from moly Re-Os closure (~600°C) to bio (~300-350°C) or sericite closure (~270-325°C). Re-Os and ⁴⁰Ar/³⁹Ar samples suggest at least 6 igneous/hydrothermal events of that approximate duration: (1) 60.06 ±0.39 to 59.55 ± 0.36 Ma, Cu/Mo-destructive sericite, 281-Zone only, (2) 59.74 ± 0.37 to 59.40 ± 0.20 Ma (moly) to 58.99 ± 0.33 Ma (ser), (3) 58.73 ± 0.33 to 58.60 ± 0.26 Ma (bio+ser), (4) 58.48 ± 0.23 Ma moly to 58.23 ± 0.35 Ma (ser), Catalina only, (5) 58.03 ± 0.24 to 57.38 ± 0.23 Ma (9 bio+ser ages + 1 Kspar), and (6) 57.31 ± 0.19 Ma moly to 57.05 ± 0.29 Ma Kspar, Catalina only). However, there may have been many more than 6 hydrothermal events during this ~3 m.y. history, given that (1) each of Sierra Gorda’s >30 intrusions appears to have produced at least some potassic-to-sericitic veining, and (2) data from Potrerillos (Marsh et al., 1997) suggest that small intrusions (<500m) can cool from 900°C to 200°C in <40 Ka. Youngest Sierra Gorda ages (56.55 ± 0.28 to 56.03 ± 0.24 Ma (plag+Kspar) record district cooling to feldspar closure T (~150-250°C).