Re–Os geochronology of the El Salvador porphyry Cu–Mo deposit, Chile: Tracking analytical improvements in accuracy and precision over the past decade

Abstract

Sulfide minerals from the El Salvador porphyry Cu–Mo deposit, Chile, were dated by Re–Os geochronology to clarify the timing and duration of mineralization. As these data are collected over the past 10years, they chronicle the evolution of Re–Os analytical procedures and improvements in spike-sample equilibration, mass spectrometry and data reduction. Included in the data is the first tennantite–pyrite Re–Os isochron along with tennantite–enargite–pyrite Os tracing of ore metals based on initial osmium systematics (187Os/188Osi). Porphyry-associated mineralization follows the traditional early (A), transitional (B), and late (D) stage classification of relative timing of vein formation. Most Cu- and Mo-bearing sulfides were deposited during stages A and B. Field relationships clearly show the B-stage veins are older than the D-stage veins. Samples analyzed with Carius tube digestion and mixed-double spike indicate B-stage molybdenite mineralization between 41.8 and 41.2Ma. These ages best represent the timing of main-stage mineralization at El Salvador. A five-point 187Re–187Os isochron for pyrite–tennantite from the latest D-stage veins yields an age of 42.37±0.45Ma. The isochron age, while older, is within error of early analyses of molybdenite by alkaline fusion digestion using single Re and Os spikes (42.4–42.0Ma). A separate pyrite and tennantite–enargite pair containing significant common Os yields an initial 187Os/188Os ratio of 0.134±0.042, compatible with a predominantly mantle origin. We appreciate the early analyses for their historical significance while emphasize the geological implications of the 41.8–41.2Ma molybdenite ages. Temperature estimates by sulfur thermometry and fluid inclusions are 390–510°C for the B-stage molybdenites and 230–240°C for the latest D-stage pyrite and tennantite.Analyses based on Carius tube digestion and updated spike calibrations suggest a ∼0.6m.y. duration (∼41.8 to 41.2Ma) in-line with published deposit geochronology. The timing and duration of mineralization from Re–Os dating of ore minerals is more precise than estimates from previously reported 40Ar/39Ar and K–Ar ages on alteration minerals. The Re–Os results suggest that the mineralization is temporally distinct from pre-mineral rhyolite porphyry (42.63±0.28Ma) and is immediately prior to or overlapping with post-mineral latite dike emplacement (41.16±0.48Ma).Based on the Re–Os and other geochronologic data, the Middle Eocene intrusive activity in the El Salvador district is divided into three pulses: (1) 44–42.5Ma for weakly mineralized porphyry intrusions, (2) 41.8–41.2Ma for intensely mineralized porphyry intrusions, and (3) ∼41Ma for small latite dike intrusions without major porphyry stocks. The orientation of igneous dikes and porphyry stocks changed from NNE-SSW during the first pulse to WNW-ESE for the second and third pulses. This implies that the WNW-ESE striking stress changed from σ3 (minimum principal compressive stress) during the first pulse to σHmax (maximum principal compressional stress in a horizontal plane) during the second and third pulses. Therefore, the focus of intense porphyry Cu–Mo mineralization occurred during a transient geodynamic reconfiguration just before extinction of major intrusive activity in the region.