Five geothermal wells in the southern Salton Sea geothermal
field (SSGF) penetrate at least 30 m of vein-controlled
and stratabound, epithermal lead-zinc mineralization with
at least 2.5 wt.% sphalerite plus galena. The richest of these
intervals is hosted by a steeply-dipping fracture zone, and assays
an average 3.5% Zn, 1.4% Pb, and 0.8 oz/T Ag over the
depth range 305-369 m. The 17.4-m true thickness of this
shallow mineral deposit is impressive at these grades even by
The base-metal sulfides of this deposit formed during
a complex paragenesis marked by alternating precipitation
and dissolution of ore and gangue minerals, and by mildly
fluctuating temperatures and salinities of the mineralizing
hydrothermal fluids. Sphalerite and galena were preceded by
anhydrite, then barite, and were followed in sequence by quartz,
fluorite, and mixed-layer chlorite/smectite. Fluid-inclusion
data demonstrate: (1) that sphalerite crystallized primarily
from 192-218oC brines having apparent salinities in the range
13-19 equivalent wt.% NaCl; (2) that slightly hotter (up to
223oC) and much cooler (173-178oC) fluids intermittently
accessed the sphalerite as it was crystallizing; and (3) that following
sphalerite precipitation, hydrothermal brines in the ore
zone maintained similar solute concentrations but experienced
a gradual warming trend culminating in the modern reservoir
temperature range (228-237oC) and salinity (14.4 wt.%).
Ore-zone mineralogy, paragenesis, and fluid-inclusion
systematics for this Pb-Zn deposit point to sulfide precipitation
through fluid mixing and fluid-rock chemical interaction
leading to cooling, pH change, and H2S enrichment in the
upper levels of (for this geothermal field) an unusually shallow thermal-fluid upflow zone beneath an impermeable mudstone
caprock. Based on the calculated maximum age of its host
siliciclastic strata, the mineralization is no older than 68-96 ka.
The deposit shares a compelling number of physical-chemical
attributes with the fossil hydrothermal systems that formed
“higher-salinity” epithermal silver-base metal orebodies like
those at Creede, Colorado, and Fresnillo, Mexico.
Prolific geothermal well Magmamax-6B (M-6B), in the southwestern Salton Sea geothermal field, yields initially 290-298oC hypersaline brine from an interval of hydrothermally altered mudstone and sandstone disrupted by veinlets and breccias mineralized with specular hematite and anhydrite. About half the well’s total production is from one such breccia zone that coincides with an abrupt, 8°C reversal in the static-temperature profile. Converging textural, petrologic, chemical, and thermodynamic evidence suggests that: (1) the M-6B breccias were formed in a strike-slip fault zone as a result of hydraulic implosion abetted by explosion of overpressured, high-temperature pore fluids; (2) the hematite and anhydrite cementing these breccias were precipitated from a downflowing brine originally cooler than the 300°C reservoir rock which the fluid eventually invaded; (3) the downflowing brine may have slightly cooled and mixed with a counterflow of hot brine already ascending the fault zone; (4)
the M-6B temperature reversal may be a further consequence of the above process; (5) the reversal nonetheless is only a minor local perturbation in a system that is still thermally prograde; (6) the observed brecciation and mineralization as well as the inferred brine downflow are geologically recent phenomena; (7) sandstones in the reservoir interval are productive only where comparatively shallow and just incipiently altered; and (8) a late-stage assemblage of anhydrite and hematite in this geothermal system may signal favorably rejuvenated porosity and permeability.
A working conceptul model has been developed for the southwestern portion of the Salton Sea geothermal system, the region encompsing CalEnergy Operating Company’s imminent “Unit 6” field expansion (185 megawatts). The model is based on examination and analysis of several thousand borehole rock samples combined with a wealth of subsurface information made available for the first time from the databases of present and prior field operators.
The Unit 6 sector of the system is hosted by fluvial and lacustrine, silciclastisc sediments and sedimentary rocks of Quaternary age. These strata are gently folded and cut by high-angle fault zones with a component of strike-slip displacement. The thicker of these zones (1) are mineralized and enriched in gouge and crush breccia (both commonly slickensided) as well as dilational microbreccia with a “jigsaw-puzzle” texture; and (2) are hosts for the most productive thermal-fluid conduits yet encountered in this part of the field. Much of this production is derived from major faults apparently forming the upper portion of a “negative flower structure”, a common feature of transtensional wrench-fault regimes like the one in which the field is situated.
A unique, ~100-200 m-thick, evaporitic anhydrite-rich layer in the mudstone capping the sedimentary sequence is continuous except above the faults most productive at depth. We believe that only these faults penetrate significantly upward into the cap, providing ingress for cooler, sulfate-dissolving waters from above.
Unit 6 as drilled to date shares numerous attributes with the broader Salton Sea geothermal resource. The production fluids are hypersaline brines (total dissolved solids content 20-25%) circulating at temperatures generally in excess of 290°C. Porosity and Permeability for fluid flow and storage are provided primarily by fractures, breccias, and veinlets, but also, in the upper part of the reservoir (and in a supra-reservoir outflow plume), by porous sandstones in which calcite has been hydrothermally dissolved. Overlying strata have not only retained their calcite, but have been mineralized locally with intergranular anhydrite, therefore providing an effective reservoir cap.
In addition to a paucity or absence of calcite, the following hydrothermal features are closely correlated with the Unit 6 geothermal reservoir: (1) pervasive veinlets of various compositions; and (2) widespread and commonly abundant epidote, accomponied locally at deeper levels by actinolite and clinopyroxene. The most prolific thermal-fluid channels coincide with fault-controlled concentrations of veinlets and dilational breccias mineralized with post-calc-silicate specular hematite +/- anhydrite.
The foregoing observations and deductions are consistent with a conceptual geothermal reservoir centered above a still-cooling granitic pluton at least 2 km in diameter and -3.5 km below the modern ground level. Major zones of buoyantly upwelling hot brine above the intrusion are focused along faults. More diffuse upflow occurs in a stockwork of interconnected, mineralized fractures (veinlets). This stockwork probably formed by hydraulic rock rupture induced by explosion of isolated, fluid-filled pores heated and consequently overpressured at an expanding (prograde) thermal front emanating from the magmatic heat source. Subhorizontal stratigraphic permeability in this model is concentrated in the upper portion of the reservoir, where the balance between carbonate dissolution and calc-silicate mineralization has favored formation of sandstone aquifers. Local downflow and warming of initially cooler brine from below the cap along major faults leads to slight (5-10°C) cooling of the upflow concomitant with open-space hematite +/- anhydrite mineralization.
The Unit 6 geothermal reservoir is clearly open at depth and for at least a kilometer to the northwest. It extends to the southwest and (especially) to the northeast for considerably greater distances. The reservoir plunges abruptly to the southeast, but even here, by analogy with the northern part of the geothermal field, there is a high probability for encountering productive reservoir rock at depths below 2 km. Our conceptual model, coupled with documented reservoir behavior here and elsewhere in the field, strongly suggests that the immediate resource is more than sufficient to support the planned expansion.
An empirical graphical method, based on type curve matching is presented. Using only flowing pressure-temperature surveys, the flowing enthalpy vs depth in two-phase wellbores can be determined. Also fluid entries are identified in the two-phase as well as the single-phase region. In some cases the relative flow rates of each entry and the enthalpy of the entry itself can be determined. Six example surveys from the Coso field are given and interpreted.
The Hot Springs Bay Valley (HSBV) geothermal resource area on Akutan Island, Alaska, has been exploredsince 2009. Geological, geochemical, geophysical surveys and the drilling of two thermal gradient wellssuggest a mature neutral-chloride reservoir between 240 and 300◦C, with outflow temperatures ∼180◦C. A network of regional and local structures control near-surface permeability. Alteration, mineralizationand geophysical patterns, including high-temperature hydrothermal minerals at unexpectedly shallowdepths of formation, indicate a poorly to moderately developed clay cap, likely the result of erosion ofthe upper portion of an older, well developed system.
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