U.S. Geological
Survey Professional Paper 1578
Online version 1.0
Hydrothermal Mineralogy of Core from Geothermal Drill Holes at Newberry Volcano, Oregon
By
Keith E. Bargar and Terry E. C. Keith
ABSTRACT
Twenty geothermal exploration drill holes, funded by both industry and various government agencies, were completed between 1976 and 1986 on the flanks and within the caldera of Newberry volcano, central Oregon. These holes were drilled in order to evaluate the geothermal energy potential of this young Cascade volcano. Part of the evaluation process involved studies of the hydrothermal alteration mineralogy in nine of the drill holes. Geothermal drill holes GEO-N1, GEO-N4, and GEO-N3 were located on the south, east, and north flanks of the volcano, respectively. Four holes (GEO-N5, GEO-N2, SF NC-01, and SF NC72-03) were drilled on the west flank of Newberry volcano, and two holes (USGS-N2 and RDO-1) were completed within the volcano's caldera. Maximum temperatures measured at the bottoms of the flank drill holes GEO-N1, GEO-N3, GEO-N4, and GEO-N5 were below 100°C. Fractures, vesicles, spaces between breccia fragments, and glassy drill-core samples usually exhibit little or no hydrothermal alteration in the upper parts of these drill holes. Similar open spaces in the lower parts of the drill holes contain low-temperature hydrothermal minerals (carbonates, silica minerals, zeolites, clay minerals, sulfates, iron oxides, and sulfides) that could have formed at the temperatures measured during drilling. Temperatures in the three remaining flank drill holes (GEO-N2, SF NC-01, and SF NC 72-03) exceeded 150°C at the hole bottoms. The extent of hydrothermal alteration in these drill holes is low to moderate, and the hydrothermal minerals include laumontite, quartz, and mixed-layer chlorite-smectite that are consistent with temperatures above 150°C.
The extent of hydrothermal alteration is greater in some intracaldera drill-hole specimens from depths where the measured temperatures were significantly higher than temperatures encountered at equivalent depths within the flank drill holes. Measured temperatures above about 300-m depth in the USGS-N2 drill hole ranged between about 18 and 39ÁC, and hydrothermal alteration minerals are nearly absent in the drill core. The temperatures increased from 31°C at 300-m depth to 265°C at 930-m depth, and hydrothermal alteration is slight to extensive. The more pervasive alteration occurs along fractures and fracture margins and in the more permeable brecciated and volcaniclastic zones. Hydrothermal zeolites, apophyllite, gyrolite, carbonates, apatite, hydrogrossular, clay minerals, silica minerals, sulfides, sulfur, anhydrite, epidote, and iron oxides and hydroxides were identified. Temperatures in the RDO-1 drill hole were greater than 158°C at about 350.5-m depth. Many of the hydrothermal alteration minerals (analcime, aragonite, siderite, rhodochrosite, calcite, smectite, chlorite, quartz, mordenite, pyrite, pyrrhotite, and hematite) occur at shallower depths than in the nearby USGS-N2 drill core.
The most important controls of hydrothermal alteration in the Newberry volcano drill holes are permeability, temperature, and fluid composition. Hydrothermal fluids flow more readily through the interflow breccias and have altered these breccias and other volcaniclastic layers where initial permeability was highest; massive lava flows having low permeabilities are generally unaltered or only slightly altered except where fractured or vesicular.
Fluid-inclusion studies of quartz and calcite crystals from the lower parts of the three west-flank drill holes indicate that past temperatures were hotter. Temperatures were significantly hotter—commonly by 20 to 40°C and by as much as 160ÁC-in the SF NC72-03 drill hole, which is near the ring fracture on the west side of the volcano. The ring fractures may provide conduits for movement of deep geothermal water to shallower levels. Hydrothermal alteration minerals identified from the two intracaldera drill holes suggest that past temperatures could have been similar to the measured temperatures. However, fluid-inclusion heating studies indicate that temperatures within both drill holes have been signifi-cantly hotter (by as much as 80°C in RDO-1 and by ~100°C at the bottom of USGS-N2) than the measured temperatures. Lacustrine deposits in the upper parts of these drill holes suggest that transient higher past temperatures may have resulted from higher pressure caused by the additional weight of an intracaldera lake about 100 to 150 m deep.
The few published chemical analyses of waters from the Newberry volcano drill holes and the low measured temperatures in the upper parts of all the drill holes indicate that descending cold meteoric waters exert a substantial control over any alteration in the shallower levels of the drill holes. Limited fluid-inclusion melting-point temperature measurements suggest that the geothermal waters become more saline with depth, increasing from 0 at about 801-m depth to 1.9 weight percent NaCl equivalent near the bottom of the USGS-N2 drill hole. Salinity also appears to be higher near the caldera ring fractures where it ranges from 0.2 weight percent NaCl equivalent in GEO-N2 to as much as 0.7 weight percent NaCl equivalent in SF NC72-03.
Comparison of major- and trace-element contents of chemically analyzed rocks from the GEO-N1, GEO-N5, and SF NC72-03 drill holes with those of published analyses of unaltered surface rocks shows little or no evidence of element mobility resulting from hydrothermal alteration of core from these flank drill holes. Conversely, comparison of the published analyses with chemical data from many USGS-N2 drill-core specimens suggests that hydrothermal alteration caused insignificant to moderate decreases in SiO2, Na2O, and K2O as well as possible slight increases in total Fe and CaO. Considerable scatter of the data for Al2O3 may indicate both loss and gain of this element. Gains or losses involving MgO, TiO2, and total Fe are somewhat difficult to discern because analyses of these elements show strong trends that result from dif-ferences in the composition of precaldera and postcaldera lavas and volcaniclastic rocks. The concentration of several trace elements, as well as P2O5 and MnO, appears to vary according to whether the rock is basaltic or rhyolitic; trace-element abundance in several USGS-N2 drill-core samples having intermediate compositions appear to complete the differentiation trend.
Possibly the Newberry hydrothermal system is very young and has not evolved to a stage where trace elements and some major elements have been mobilized and reconcentrated in significant amounts. The Newberry hydrothermal system appears to be very youthful because unaltered glassy rocks are present at elevated temperatures. The lack of indications of self-sealing, together with the absence of multiple or cross-cutting fracture systems, also provides evidence that the hydrothermal system is young.
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Maintained by: Michael Diggles
Created: May 30, 2000
Last modified: June 28, 2005 (mfd)