Time scales of porphyry Cu deposit formation: insights from titanium diffusion in quartz

Economic Geology
By: , and 

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Abstract

Porphyry dikes and hydrothermal veins from the porphyry Cu-Mo deposit at Butte, Montana, contain multiple generations of quartz that are distinct in scanning electron microscope-cathodoluminescence (SEM-CL) images and in Ti concentrations. A comparison of microprobe trace element profiles and maps to SEM-CL images shows that the concentration of Ti in quartz correlates positively with CL brightness but Al, K, and Fe do not. After calibrating CL brightness in relation to Ti concentration, we use the brightness gradient between different quartz generations as a proxy for Ti gradients that we model to determine time scales of quartz formation and cooling. Model results indicate that time scales of porphyry magma residence are ~1,000s of years and time scales from porphyry quartz phenocryst rim formation to porphyry dike injection and cooling are ~10s of years. Time scales for the formation and cooling of various generations of hydrothermal vein quartz range from 10s to 10,000s of years. These time scales are considerably shorter than the ~0.6 m.y. overall time frame for each porphyry-style mineralization pulse determined from isotopic studies at Butte, Montana. Simple heat conduction models provide a temporal reference point to compare chemical diffusion time scales, and we find that they support short dike and vein formation time scales. We interpret these relatively short time scales to indicate that the Butte porphyry deposit formed by short-lived episodes of hydrofracturing, dike injection, and vein formation, each with discrete thermal pulses, which repeated over the ~3 m.y. generation of the deposit.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Time scales of porphyry Cu deposit formation: insights from titanium diffusion in quartz
Series title Economic Geology
DOI 10.2113/econgeo.110.3.587
Volume 110
Issue 3
Year Published 2015
Language English
Publisher Society of Economic Geologists
Contributing office(s) Central Mineral and Environmental Resources Science Center
Description 16 p.
First page 587
Last page 602
Country United States
State Montana
City Butte
Online Only (Y/N) N
Additional Online Files (Y/N) N