Geology topics

Matthew Steele-MacInnis

Publications and source records attributed to Matthew Steele-MacInnis.

4 recordsLinked to original sources

USGS2022

Vein-type gold formation during late extensional collapse of the Eastern Desert, Egypt: the Gidami deposit

Orogenic gold deposits, though construed to focused fluid flow during orogenesis, commonly post-date the main accretionary events. Several lines of evidence indicate that orogenic gold formation in the Arabian–Nubian Shield continued through the orogen collapse stage and associated rapid exhumation and thermal re-equilibration. The Gidami gold deposit in the Eastern Desert of Egypt is associated with post-foliation, brittle-ductile shear zones that deformed a weakly foliated tonalite-trondhjemite massif dated as ~ 704 Ma (U–Pb zircon age). Gold-sulfide quartz veins exhibit textural features indicative of repeated mylonitization, recrystallization, and muscovite crystallization. New 40 Ar/ 39 Ar ages of muscovite flakes from the auriferous quartz veins and from the altered wallrock overlap within analytical uncertainty at ~ 583 Ma, which corresponds to the climax of extension-related wrenching and rapid exhumation in the region (~ 596 to 582 Ma). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data coupled with microtextural characteristics suggest that early formed pyrite generations experienced substantial fluid-mediated recrystallization, and that a set of metals was remobilized by later fluids. A late generation of fibrous pyrite, ubiquitous in microfractures, deposited while the veins re-opened and deformed. The occurrence of free gold particles along with a late-paragenetic assemblage of galena-sphalerite-chalcopyrite(± hessite ± cervelleite) was related to influx of low salinity, metalliferous H 2 O-NaCl-CO 2 -CH 4 fluids as indicated by the fluid inclusion laser Raman spectroscopy and microthermometry results. Au-mobilization and redeposition at T ≤ 350 °C and P ~ 1 to 1.7 kbar, triggered by intermittent fluid pluses and thermal re-equilibration, were most likely stimulated by extensional structures and within-plate magmatism. Coincident province- and deposit-scale pressure–temperature-time data highlight the pivotal role of the orogenic collapse tectonics in gold endowment in the Central Eastern Desert’s crust.

Gidami deposit↗

USGS2022

Fluids and melts at the magmatic-hydrothermal transition, recorded by unidirectional solidification textures at Saginaw Hill, Arizona, USA

Fluid exsolution and melt evolution at the magmatic-hydrothermal transition are critical processes driving the metal enrichment of porphyry systems. Coeval fluid and melt inclusion assemblages in unidirectional solidification textures (USTs) at Saginaw Hill—a small, porphyry Cu system in southwestern Arizona—record a dynamic and repetitious process of fluid accumulation and release. The cores of quartz crystals throughout the UST bands host coeval silicate melt and brine inclusions but lack vapor-rich inclusions. This could indicate preferential expulsion of vapor and trapping of high-density brine during episodes of fracturing or the direct exsolution of single-phase high-salinity brine from the silicate melt. In contrast, the rims of UST quartz host abundant coeval brine and vapor inclusions, consistent with liquid-vapor immiscibility at lower pressures compared to the corresponding quartz cores. This transition from dominantly coeval silicate melt inclusions and brine in phenocryst cores to coeval brine and vapor in the rims suggests that the Saginaw Hill system underwent cyclic processes of fluid exsolution, accumulation, overpressure, and decompression at relatively stable temperatures (consistently ~650°C) during UST formation. Melt inclusion data indicate that the melt at this stage was highly fractionated and tended toward muscovite saturation. Metal concentrations in the brine were comparable to or higher than those in fluids reported in world-class porphyry Cu systems and were likely the result of both igneous fractionation and the high chloride content of the exsolved fluids. While limited in scale, Saginaw Hill provides evidence for processes that are predicted to occur at the magmatic-hydrothermal transition during the formation of large, well-mineralized porphyry systems.

Arizona↗

USGS2020

A fundamental role of carbonate–sulfate melts in the formation of iron oxide–apatite deposits

Genetic models for iron oxide–apatite deposits are controversial and span a spectrum from orthomagmatic to hydrothermal endmembers. This lack of consensus is rooted in uncertainties as to the nature and origin of ore-forming fluids in these systems. Here, we present a fluid-inclusion study of mineralizing fluids at two iron oxide–apatite deposits (Buena Vista, Nevada and Iron Springs, Utah). We found that the inclusions in both systems comprise both aqueous brine and ubiquitous iron-rich carbonate–sulfate melts. These melts were found throughout the paragenesis of both deposits and show a tremendous capacity to transport ferric iron. Hence, we argue that orthomagmatic fluids played a role in mineralization at both Buena Vista and Iron Springs, and that the main ore-forming fluid was an iron-rich carbonate–sulfate melt formed by the assimilation and anatexis of evaporite-bearing carbonate rocks. The geological conditions that give rise to carbonate–sulfate melts are also a common feature of other classic iron oxide–apatite systems worldwide. Hence, we argue that the process of assimilation, anatexis and immiscibility of carbonate–sulfate melts is fundamental to iron oxide–apatite formation and provides a common link between iron oxide–apatite systems in different geological settings.

Nature Geoscience↗

USGS2019

Shift in the Raman symmetric stretching band of N2, CO2, and CH4 as a function of temperature, pressure, and density

The Raman spectra of pure N 2 , CO 2 , and CH 4 were analyzed over the range 10 to 500 bars and from −160°C to 200°C (N 2 ), 22°C to 350°C (CO 2 ), and −100°C to 450°C (CH 4 ). At constant temperature, Raman peak position, including the more intense CO 2 peak ( ν +), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature ( PT ) range studied. At constant pressure, the peak position for CO 2 and CH 4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N 2 first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N 2 is located between 0°C and 50°C at pressures above ~50 bars and is pressure dependent. Below ~50 bars, the inflection temperature was observed as low as −120°C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N 2 , CO 2 , and CH 4 to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature-dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N 2 -CO 2 -CH 4 gas systems in a future study.

Journal of Raman Spectroscopy↗