M. Alexandra Skewes

Tectonic trigger for the formation of late Miocene Cu-rich breccia pipes in the Andes of central Chile

During the late Miocene, the decreasing angle of subduction below the Andes of central Chile caused a reduction in the influx of new magma and heat into the base of magmatic systems that had been active throughout the Miocene. This led to their cooling and solidification, triggering the release of large volumes of metal-rich magmatic fluids from middle- and upper-crustal magma chambers and generating a group of economically significant Cu-rich breccia pipes. The Sr- and Nd-isotopic compositions of the magmatic fluids that formed these brecccas, determined from breccia-matrix minerals, were variable, implying that these fluids were not exsolved from a specific Cu-porphyry magma, as is often invoked to explain Andean Cu deposits. Instead, the formation of the late Miocene deposits in central Chile was tectonically triggered by cooling of a variety of magma types during the last stages of existence of long-lived Andean magmatic systems as both subduction angle and, as a result, subarc magma supply decreased.

Genesis of the Giant Late Miocene to Pliocene Copper Deposits of Central Chile in the Context of Andean Magmatic and Tectonic Evolution

Multiple large mineralized breccia pipes (Cu grades up to >10%; individual pipes with >10 × 106 metric tons of Cu) are prominent, if not dominant, features in the three giant Andean Cu deposits of ...

Correction to: Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within aplite dikes

In the version of the paper originally published the current address of author Jeremy T Ross was given incorrectly as Untied Status Marine Corp, 3351 Onslow Drive, Camp Lejeune, NC 28547, USA.

Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within aplite dikes

Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, εNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N <50 with significant negative Eu anomalies, which contrast with higher (La/Yb)N >100 and absence of an Eu anomaly in both the aplites and segregations. These data are consistent with the aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher εNd1.42Ga values for the aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.