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
mining-industry standards.
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.