T36 Role of Mafic Magmas in the Generation of Porphyry Copper Deposits

Sunday, 27 October 2002
1:00 PM - 3:45 PM

MOBILITY OF COPPER, CHLORINE, AND SULFUR DURING QUENCHING OF HAWAIIAN BASALTIC MAGMAS

Paper number: T36-7
Program: Technical Sessions
Time: 2:40 PM – 2:55 PM

Authors

Abstract

We have examined a suite of young Hawaiian lava samples (<50 ybp), quenched Kilauea lava samples, and young dikes with chilled margins (Koolau dike swarm). Magmatic sulfides were not found in any samples. However, heterogeneous particles of Cu-Sn-Co alloys and coatings of Fe-Cu oxides were found to occur in some vesicles of Kilauea Iki lava lake samples and quenched lava samples from Pu’u ‘O’o. Element maps of the alloys reveal that chlorides are occasionally present in and around the metals. Sulfur content of the metal alloys rarely exceeds about 0.4 wt%. Our current model for the origin of the alloys suggests that the metals are transported to vesicles as chlorides and then deposited as sulfides and/or native metals. The origin of the Fe-Cu oxide coatings needs additional study. Other workers have noted the presence of Ni-bearing magmatic sulfides and oxidized sulfides in some quenched samples from Kilauea Iki; however, none of these were found in our samples (possibly due to resorption of sulfides and mobilization of metals during degassing of the magma). Chlorides (sylvite and others), sulfates, and other sulfur-bearing particles are present on the outer surface of the quenched lava samples, but none contain readily detectable Cu.

Fresh basaltic samples of the Koolau dike swarm (dike widths < 1 m) were obtained from an active quarry on Oahu. Again, we found no magmatic sulfides, but dike samples exhibited chilled margins and often contained conspicuous hydrothermal pyrite and rare chalcopyrite. We infer that the sulfur and copper incorporated in these sulfides originated from quenched dikes alone.

These data suggest that in some circumstances Cu is readily partitioned into escaping magmatic volatiles during quenching of mafic magma. If broadly similar processes occur at the base of porphyry Cu magmatic systems during episodes of magma mixing, then significant amounts of Cu, S, Cl, and H2O may be transferred to the intermediate magma or ore-related fluids.