Jean S. Cline

Can economic porphyry copper mineralization be generated by a typical calc‐alkaline melt?

Numerical simulation of chlorine and copper partitioning between a crystallizing melt and exsolving aqueous fluids indicates that “typical” calc-alkaline magmas contain sufficient copper, chlorine, and water to produce economic porphyry copper mineralization. Neither an elevated copper content in the magma nor an additional large volume of magma are required to provide metals or volatiles. The most important variables that determine the volume of melt necessary to produce sufficient copper are the degree of compatible behavior displayed by copper, the ratio of initial water in the melt to the water saturation level, and the initial chlorine/water ratio of the melt. The absolute values for initial water in the melt and water content at saturation are relatively unimportant in determining the required melt volume. The bulk salinity of the exsolved fluid may vary from < 2.0 wt.% NaCl to saturation levels (84 wt.% NaCl at 700°C) indicating that boiling is not necessary to produce high salinity brines. At appropriate P-T-XNaCl conditions the magmatic aqueous fluid separates into a saline liquid, which transports most of the copper, and a low-salinity vapor. The salinities of the two immiscible phases are governed by the P-T conditions, while the bulk fluid salinity determines the mass fractions of liquid and vapor formed. Pressure quenching causes rapid crystallization of the aplitic groundmass in porphyritic rocks associated with copper mineralization and significantly reduces the amount of chlorine and copper partitioning to the aqueous fluid. This results in abrupt and possibly large reductions in fluid salinity and causes copper to become concentrated in the melt. As copper is transported from the melt by the earliest exsolving fluids in deep (2.0 kbar) systems and by late exsolving fluids in shallow (0.5 kbar) systems, the relative timing of pressure quenching/aplite formation and fluid transport of copper from the melt can vary significantly in systems produced under different confining pressures. Model results incorporating petrologic constraints determined for Yerington, Nevada, are in good agreement with observed mineralization.

Origin, timing, and temperature of secondary calcite-silica mineral formation at Yucca Mountain, Nevada

The origin of secondary calcite-silica minerals in primary and secondary porosity of the host Miocene tuffs at Yucca Mountain has been hotly debated during the last decade. Proponents of a high-level nuclear waste repository beneath Yucca Mountain have interpreted the secondary minerals to have formed from cool, descending meteoric fluids in the vadose zone; critics, citing the presence of two-phase fluid inclusions, argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids. Understanding the origin, temperature, and timing of these minerals is critical in characterizing geologically recent fluid flux at the site, and has significant implications to whether waste should be stored at Yucca Mountain. Petrographic and paragenetic studies of 155 samples collected from the Exploratory Studies Facility (ESF) and repository block cross drift (ECRB) tunnels indicate that heterogeneously distributed calcite with lesser chalcedony, quartz, opal, and fluorite comprise the oldest secondary minerals. These are typically overgrown by intermediate-aged calcite, often exhibiting distinctive bladed habits. The youngest event recorded across the site is the deposition of Mg-enriched (up to 1 wt%) and depleted, growth-zoned calcite intergrown with U-enriched opal. The cyclical variation in Mg enrichment and depletion is probably related to climate changes that have occurred during the last few million years. The distribution of secondary minerals is consistent with precipitation in the vadose zone. Fluid inclusion petrography of sections from the 155 samples determined that 96% of the fluid inclusion assemblages (FIAs) contained liquid-only inclusions that formed at ambient temperatures (35°C). However, 50% of the samples (n 78) contained relatively rare FIA that contain both liquid-only and liquid plus vapor inclusions (herein termed two-phase FIAs) that formed at temperatures above 35°C. Virtually all of these two-phase FIAs occur in paragenetically old calcite; rare two-phase inclusion assemblages were also observed in early fluorite and quartz, and early-intermediate calcite. Homogenization temperatures ( trapping temperatures) across Yucca Mountain are generally 45 to 60°C, but higher temperatures reaching 83°C were recorded in calcite from the north portal and ramp of the ESF. Cooler temperatures of 35 to 45°C were recorded in the intensely fractured zone. Multiple populations of two-phase FIAs from lithophysal cavities in the ESF and ECRB cross drift indicate early fluid cooling with time from temperatures 45°C in early calcite, to 35 to 45°C in paragenetically younger calcite. Freezing point depressions range from 0.2 to 1.6°C, indicating trapping of a low salinity fluid. The majority of intermediate calcite and all outermost Mg-enriched calcite contains rare all-liquid inclusions and formed from ambient temperature (35°C) fluids. Carbon and oxygen isotope data reveal a consistent trend of decreasing 13 C (from 9.5 to 8.5‰) and increasing 18 O (from 5.2 to 22.1‰) values from paragenetically early calcite to Mg-enriched growth-zoned calcite. Depleted D values (131 to 90‰) of inclusion fluids from intermediate and the youngest Mg-enriched calcite indicate derivation from surface meteoric fluids. Recalculation of 18 OH2O values of 12 to 10‰ is consistent with derivation from paleometeoric fluids. Results of integrated U-Pb dating (opal and chalcedony) and fluid inclusion microthermometry indicate that two-phase FIAs that trapped fluids of 50°C are older than 6.29 0.30 Ma. Two-phase FIAs in parage- netically later calcite, which formed from fluids of 35 to 45°C, are older than 5.32 0.02 Ma. There is no evidence for trapping of fluids with elevated temperatures during the past 5.32 my. The youngest Mg-enriched calcite intergrown with opal began to precipitate between about 1.9 to 2.9 Ma and has continued to precipitate within the past half million years. The presence of liquid-only inclusions and the consistent occurrence of Mg-enriched calcite and opal as the youngest event indicate a minor, but chemically distinct, ambient temperature (35°C) fluid flux during the past 2 to 3 my. Copyright

Ore-fluid evolution at the Getchell Carlin-type gold deposit, Nevada, USA

Minerals and fluid-inclusion populations were examined using petrography, microthermometry, quadrupole mass-spectrometer gas analyses and stable-isotope studies to characterize fluids responsible for gold mineralization at the Getchell Carlin-type gold deposit. The gold-ore assemblage at Getchell is superimposed on quartz-pyrite vein mineralization associated with a Late-Cretaceous granodiorite stock that intruded Lower-Paleozoic sedimentary rocks. The ore assemblage, of mid-Tertiary age, consists of disseminated arsenian pyrite that contains submicrometer gold, jasperoid quartz, and later fluorite and orpiment that fill fractures and vugs. Late ore-stage realgar and calcite enclose ore-stage minerals. Pre-ore quartz trapped fluids with a wide range of salinities (1 to 21 wt.% NaCl equivalent), gas compositions (H2O, CO2, and CH4), and temperatures (120 to > 360°C). Oxygen- and hydrogen-isotope ratios indicate that pre-ore fluids likely had a magmatic source, and were associated with intrusion of the granodiorite stock and related dikes. Ore-stage jasperoid contains moderate salinity, aqueous fluid inclusions trapped at 180 to 220°C. Ore fluids contain minor CO2 and trace H2S that allowed the fluid to react with limestone host rocks and transport gold, respectively. Aqueous inclusions in fluorite indicate that fluid temperatures declined to ∼ 175°C by the end of ore-stage mineralization. As the hydrothermal system collapsed, fluid temperatures declined to 155 to 115°C and realgar and calcite precipitated. Inclusion fluids in ore-stage minerals have high δDH2O and δ18OH2O values that indicate that the fluid had a deep source, and had a metamorphic or magmatic origin, or both. Late ore-stage fluids extend to lower δH2O values, and have a wider range of δ18OH2O values suggesting dilution by variably exchanged meteoric waters. Results show that deeply sourced ore fluids rose along the Getchell fault system, where they dissolved carbonate wall rocks and deposited gold-enriched pyrite and jasperoid quartz. Gold and pyrite precipitated together as H2S in the ore fluids reacted with iron in the host rocks. As ore fluids mixed with local aquifer fluids, ore fluids became cooler and more dilute. Cooling caused precipitation of ore-stage fluorite and orpiment, and late ore-stage realgar. Phase separation and/or neutralization of the ore fluid during the waning stages of the hydrothermal ore system led to deposition of late ore-stage calcite.

Numerical simulation of fluid flow and silica transport and deposition in boiling hydrothermal solutions: Application to epithermal gold deposits

A numerical fluid flow model quantifying silica precipitation in hydrothermal systems as a function of temperature, permeability, and mass flux has been employed to evaluate the role of boiling in precipitating quartz in the epithermal environment. Relative permeability relationships of liquid and vapor phases incorporated into the physical model have been modified to be temperature dependent and are consistent with current understanding of fluid flow in one-component, two-phase systems. The modifications extend the height of the boiling column and eliminate discontinuities in steam content on initiation of boiling. Results indicate that the degree to which boiling contributes to quartz precipitation is dependent on three related factors: the temperature of the fluid entering the base of the boiling system, the rate of fluid temperature decrease with respect to the distance of fluid travel, and the extent of fluid vaporization, particularly in regions of gradual temperature decline. Boiling contributes significantly to quartz precipitation in systems with high-temperature fluids, and in deeper portions of systems in which extensive vaporization occurs. Temperature reduction is the dominant quartz precipitation mechanism in regions where temperature reduction is rapid, and in lower temperature systems. Owing to the smaller absolute difference in quartz solubility between the liquid and vapor phases at low temperatures as compared to higher temperatures, boiling is a less important precipitation mechanism in low temperature, nearsurface regions. Quartz precipitation is most intense in systems with short column heights, i.e., systems with high mass flux/permeability ratios, and low initial fluid temperatures. Vertical permeability variations within the flow channel produce steam jumps, or discontinuities, in mass fraction steam profiles, resulting in local zones of enhanced quartz precipitation or dissolution. Ore grade calculations indicate that high mass flux rates, low permeabilities, and low initial fluid temperatures promote precipitation of high gold grades in response to boiling. Under these conditions gold precipitates over a smaller interval of temperature decline and over a shorter ore horizon height.