Silicate Melt Inclusions in Porphyry Copper Deposits: Identification and Homogenization Behavior

Abstract

Silicate melt inclusions are ubiquitous in phenocrysts in porphyry copper deposits. Compared to melt inclusions in other igneous environments, those in porphyry copper deposits are more difficult to recognize and analyze, for a variety of reasons. The inclusions are usually partially to completely crystallized or altered by hydrothermal fluids, resulting in a dark, granular appearance. In this study, a protocol for identifying and homogenizing crystallized melt-inclusions from porphyry copper deposits is described. The protocol has been used to obtain preliminary data from melt inclusions in the Red Mountain, Arizona, and the Tyrone, New Mexico, porphyry copper deposits. Many melt inclusions in syn-and post-mineralization samples show evidence of alteration by magmatic-hydrothermal fluids that produced alteration and mineralization in the plutons, and many inclusions trapped crystals or fluid (or both) along with the melt. Elevated concentrations of metal in pre-mineralization (H 2 O-undersaturated) melt inclusions, and lower concentrations of metal in later syn-and post-mineralization (H 2 O-saturated) melt inclusions are consistent with models that invoke quantitative transfer of copper and other metals from the melt into the magmatic aqueous phase when the melt reaches H 2 O saturation. Results from Red Mountain and Tyrone are compared to melt inclusions from the White Island, New Zealand, active volcano.

Application of the Linkam TS1400XY heating stage to melt inclusion studies

“…Also, the optics improved if the crystal was free of matrix glass. As previously noted by [15], groundmass adhering to phenocrysts affects the optics because it consists of dark recrystallized glass that generally melts before the MI has completely homogenized and flows around the crystal (or through fractures in the crystal) causing the optics to deteriorate. In order to remove matrix glass from the crystals before heating experiments, some authors have suggested that the samples should be placed in concentrated hydrofluoric acid solution for several seconds [15,36], but this was not done for the phenocrysts studied here.…”

“…A few of the MI could not be heated to homogenization, either because the optics deteriorated before homogenization was achieved or, in one case, because the crystals began to fracture and break into smaller pieces owing to the high internal pressures generated in the volatile-rich MI during heating. For such samples, it is necessary to heat the MI in a pressurized vessel to eliminate or minimize decrepitation and/or leakage [14,15,24,35].…”

“…The most important advantage of using a microscopemounted heating stage (such as the Linkam TS1400XY) relative to one atmosphere furnaces and high pressure autoclaves is that the MI can be observed continuously during the heating/cooling experiments [15]. For example, the appearance/disappearance of a microscopically recognizable phase is possible during heating experiments using these types of devices, but is not possible when the MI is heated in a furnace.…”

“…Various techniques have been used to successfully homogenize MI, including heating in one-atmosphere furnaces [9][10][11][12], heating in high-pressure (cold-seal or internally-heated) vessels [13][14][15][16], heating in piston cylinder apparatus [17][18][19], and heating in a microscope-mounted high temperature stage [8,9,[20][21][22][23]. Each of these techniques has advantages and disadvantages, as described by [15,24].…”

“…This approach allows information such as the temperature of first melting and the temperatures (and order) of disappearance of various phases to be observed and measured ( Fig. 1 in [15]), and prevents overheating of the MI because heating can be stopped at the moment of homogenization. Importantly, homogenization of the MI using a microscope mounted heating stage allows workers to determine if phases present after quenching represent phases that were still present at the highest temperature to which the MI were heated, or if they represent phases that nucleated from the melt during quenching.…”