Petrogenesis and Metal Content of Hornblende-Rich Xenoliths from Two Laramide-age Magma Systems in Southwestern USA: Insights into the Metal Budget of Arc Magmas

Abstract

The petrogenesis and metal content of hornblende-rich xenoliths hosted in Laramide-age magmas at Santa Rita and Cerrillos, both in New Mexico (USA), were reconstructed by detailed petrographic studies and laser ablation inductively coupled plasma mass spectrometry analysis of minerals, melt inclusions and sulfide inclusions. The xenoliths from both locations record an evolution from clinopyroxene + olivine + phlogopite cumulates (1000–1100°C), through nearly pure hornblendites (920–1000°C), to hornblende + plagioclase cumulates (830–950°C) and hornblende gabbros (850–910°C) that crystallized at a depth of 10–20 km. In contrast to cumulates formed by gravitational settling of phenocrysts from magmas, the abundant hornblendite lithologies seem to have formed by reaction of residual melts with previously crystallized clinopyroxene + olivine + phlogopite assemblages in a crystal mush environment. The mafic melts (≤47–53 wt % SiO2) that produced the clinopyroxene + olivine + phlogopite cumulates were sulfide-undersaturated, whereas more evolved melts (>53 to 65 wt % SiO2) were generally saturated in sulfides ± anhydrite. The reaction-replacement hornblendites contained up to 0·8 wt % magmatic sulfides, which occur mostly in the form of monosulfide solid solution containing 1–8 wt % Cu, 0·1–2·2 wt % Ni, 1–15 ppm Ag and 0·1–1·3 ppm Au, but rarely also in the form of intermediate solid solution containing up to 37 wt % Cu, 0·5 wt % Ni, 1100 ppm Ag and 190 ppm Au. Quantitative modeling reproduces well the Cu evolution recorded in sulfide inclusions and melt inclusions in xenoliths and porphyry dikes at Santa Rita, with the Cu content of magmatic sulfides and silicate melts decreasing dramatically within a narrow melt SiO2 interval of 55–58 wt % once sulfide saturation was reached, and then declining more gradually to ∼1·5 log units lower values. These results suggest that the formation of hornblende-rich lithologies during the ascent of primitive arc magmas through the crust should have a negative influence on the mineralization potential of the residual liquids (≥55–60 wt % SiO2), even if the magmas are relatively oxidized (∼2 log units above the fayalite–magnetite–quartz fO2 buffer). However, the same process also seems to be responsible for the characteristically high Sr/Y ratios of fertile magmas. The apparent contradiction may be solved if either significant amounts of mafic magma are involved in the formation of the upper crustal magma chambers beneath porphyry Cu ± Mo ± Au deposits or magma fertility does not depend primarily on metal content but rather on other factors.