Fluid evolution in zoned Cordilleran polymetallic veins — Insights from microthermometry and LA-ICP-MS of fluid inclusions

Abstract

Fluid inclusion analysis through the paragenetic sequence of one symmetrically zoned vein sample is used to reconstruct the P–T–X fluid evolution of a porphyry intrusion-related Cordilleran polymetallic vein from Morococha, central Peru. Results record an evolution from initial deep-seated precipitation of quartz–pyrite and base metal sulphides to final near-surface deposition of carbonates, demonstrating progressive mineralisation during uplift and erosion. This is the first detailed study addressing meso- to epithermal Zn–Pb–Ag–Cu-rich ore in a magmatic–hydrothermal system by combination of fluid inclusion microthermometry with laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) quantifying metal, as well as sulphur concentrations in the evolving hydrothermal fluid. Scanning electron microscopy cathodoluminescence (SEM-CL) imaging of quartz and detailed transmitted- and reflected-light petrography provide textural evidence that early, moderately saline (4–5 wt.% NaCl eq.) and CO 2-bearing fluids with homogenisation temperatures of 340°–380 °C precipitate Cu-bearing minerals. In this open hydrothermal system the fluids record decreasing salinities, CO 2-contents and temperatures, while Zn-, Pb-, and Ag-sulphides precipitate. Fluids related to early precipitation in the vein have metal contents of several 1000 μg/g S and Fe, over 1000 μg/g Cu, 100 μg/g Pb, 10 μg/g Ag, and several 100 μg/g Zn. Sulphur concentrations in the fluid are sufficiently high to precipitate all metals in solution as sulphides. The latest generation of fluid inclusions associated with abundant carbonate precipitation in the centre of the vein has homogenisation temperatures ranging from 260° to 220 °C, low metal concentrations, and no measurable CO 2. During vein formation, cooling and several kilometres of erosion resulted in “telescoping” of consecutively precipitated mineral assemblages. The deep input fluid dominating in the early vein stage is interpreted to be of magmatic origin, most likely a single phase magmatic fluid of intermediate salinity and density. It cooled to an aqueous liquid, separated minor CO 2-rich vapour, and was eventually diluted by meteoric water in the late stages of vein formation when the progressively eroded land surface was only several hundred meters above the vein location.