TEMPORAL OVERLAP OF SUPERGENE ALTERATION AND HIGH-SULFIDATION MINERALIZATION IN THE SPENCE PORPHYRY COPPER DEPOSIT, II REGION, CHILE

Hypogene mineralization in the Spence porphyry Cu (-Mo) deposit (22°47'S; 69°15'W; 400 Mt @ 1.0% Cu), a link in the Central Andean lower Eocene porphyry belt, largely accompanied intrusion of a NE/SW array of dacite porphyry stocks, articulated by NW/SE faults within an evolving strike-slip duplex. A-type Cu and subordinate B-type Cu-Mo veins were emplaced at 57.00 ± 0.69 (2 s) - to - 56.61 ± 0.63 Ma (laser ⁴⁰Ar-³⁹Ar plateaus for magmatic biotite in pre- and late-mineralization stocks); subsequent D-veins introduced mainly pyrite. 13 inverse isochron ages for unambiguously  supergene alunites and natroalunites from leached and chalcocite enrichment zones range  from 44.44 ± 0.54 Ma, the oldest yet recorded for a N Chilean porphyry system, to 27.74 ± 5.42 Ma, a protracted history extended by a  K-Ar date of 20.9 ± 2.2 Ma.  Supergene activity therefore persisted from at least the initiation of the late Eocene – early Oligocene Incaic orogeny, when the deposit was gently folded, to the major uplift and erosional events of the Aymará tectonic phase.  The intermediate stages of this long history coincided with the emplacement in the central part of the deposit of a series of high-sulfidation veins bearing varying proportions of enargite, tennantite, bornite, covellite, pyrite and alunite.  Three hypogene alunites, from sites 46 to 113m  below the base of the enrichment blanket, yield ⁴⁰Ar-³⁹Ar dates of 33.22 ± 1.86, 33.44 ± 1.09 and 35.57 ± 1.43 Ma. Spence lies 20-25 km W of the Argomedo-Calama strand of the Incaic Precordilleran (Domeyko) Fault System, but experienced minor Cu mineralization simultaneously with the initiation of  major hypogene activity on that structure at this latitude. This represents the first clearly-defined example of “crossover” between the two Paleogene porphyry Cu belts.