Active ore formation at a brine interface in metamorphosed deltaic lacustrine sediments; the Salton Sea geothermal system, California

Abstract

The Salton Sea geothermal system is an area of active hydrothermal metamorphism of Pliocene to Pleistocene deltaic, lacustrine, and evaporitic sediments deposited in a modern continental rift zone. Temperatures up to 365 degrees C, hypersaline metalliferous brines with up to 26 wt percent total dissolved solids and active base metal ore formation accompany low-pressure greenschist and amphibolite facies metamorphism at depths less than 3.2 km. Drill cores recovered by the Salton Sea Scientific Drilling Project shed new light on the nature and origin of ore formation in the eastern portion of the Salton Sea geothermal system.Base metal ore mineralization occurs in vertical fractures that comprise the major form of fluid permeability at 1 to 3 km in the modern geothermal system. Type 1 veins are dominated by carbonate, contain pyrrhotite and other sulfides, are completely sealed, and occur in wall rock showing minimal alteration. Fluid inclusion data indicate that they were deposited under a thermal gradient much higher than that now found in the eastern part of the system. Type 2 veins are dominated by silicates and hematite, are permeable, and occur in wall rocks showing pervasive chloritization and epidotization. Fluid inclusion data indicate that they are forming now from the geothermal brines under the modern thermal gradient.Fluid inclusions record an apparent steep, progressive salinity increase in the vein systems in the upper 2 km of the system. However, fluid production data from Salton Sea geothermal wells indicate that reduced, metalliferous hypersaline brine averaging 23 wt percent total dissolved solids is overlain by more oxidized, metal-poor fluid averaging 5 wt percent total dissolved solids. A sharp interface exists between the fluids at 1 to 2 km, paralleling the 250 degrees C isothermal surface.A model is developed for type 2 vein ore formation involving mixing at the interface between the two fluids. Magmatic intrusion induces fracturing, heating, and the expulsion of reduced connate fluids to cause early type 1 vein mineralization. Following the rising thermal front, deep hypersaline brine rises diapirically and mixes with overlying fluids in vertical fractures transecting the ascending interface, forming type 2 veins. The hypersaline brine rises to a level of density-temperature stabilization, causing pervasive greenschist metamorphism of the host sediments. The diapir then begins to cool and descend. Shallow, oxidized brine is drawn in as the hypersaline brine descends, promoting late-stage hematite in type 2 veins. The eastern Salton Sea geothermal system presently appears to be in this latter mode of cooling and retrograde brine descent.Ore formation in the system is caused by the coincidence of transaxial entry of a major river into an active rift zone, deposition of metal-bearing deltaic sediments to form a closed-basin sedimentary environment, episodic lacustrine evaporite formation, and injection of heat and elements by rift-related magmatic intrusions at depth. The presence of an upwelling brine diapir in the system may be analogous to conditions immediately prior to the exhalation of metalliferous fluids that formed stratiform Fe-Zn-Pb-Cu sulfide and oxide deposits in paleorift zone environments.