BH250-240
Title
BH250-240
Subject
Diabase
Description
Major Minerals: plagioclase, clinopyroxene, olivine
Minor Minerals: chlorite
We visit this site to see a diabase dike and its contact metamoprhic effects and an outcrop that shows stratomlites. In the Death Valley region, many diabase intrusions are related to late Precambrian through Mesozoic magmatic events, although numerous mafic dikes in southern Death Valley are commonly interpreted as Neoproterozoic to Cambrian intrusions associated with rifting during the breakup of the Rodinia supercontinent (Mike Smith, Class of 1999, personal communication).
Chemically, diabase is the medium-grained intrusive equivalent of basalt. It is mafic in composition, and its dominant minerals are calcic plagioclase, clinopyroxene, and commonly olivine. The thin sections show strong alteration, where the presence of chlorite indicates later hydrothermal alteration or low-grade metamorphism, with original pyroxene and olivine partially replaced by secondary minerals during fluid interaction.
Where diabase intrudes carbonate rocks such as limestone or dolostone, common throughout Death Valley, it can produce well-developed contact metamorphism. Heat and reactive fluids from the intrusion recrystallize the surrounding rocks into marble and calc-silicate assemblages. If the host rock is magnesium-rich dolomite and silica is available, metasomatic reactions may generate talc through reactions such as:
dolomite + quartz + water →talc + calcite + CO2
3CaMg(CO3)2 + 4SiO2 + H2O→Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
This process commonly occurs in metamorphic aureoles around mafic intrusions where silica-bearing fluids migrate through dolomitic country rock. Talc-bearing zones may appear soft, pale green, white, or soapy, and can be associated with tremolite, serpentine, calcite, or chlorite depending on temperature and fluid chemistry.
These are of Death Valley also contains numerous historic talc mines and prospects, making it one of California’s most important talc-producing districts. A major talc belt extends from the Ibex Hills to the Silurian Hills. These diabase samples come from near the Saratoga Talc Mine.
This Death Valley diabase is geologically important because it records three linked processes: regional extension and magma intrusion, mafic Fe-Mg-rich magmatism, and thermal/metasomatic alteration of surrounding carbonate rocks, including talc formation.
In the BH250-collections there are also other contact metamorphic rocks. In the Wasatch Range, the Alta Stock provides an excellent comparison for intrusive contact metamorphism. This Tertiary quartz monzonitic to granodioritic intrusion metamorphosed surrounding carbonate rocks to produce marble, skarn, and Mg-rich assemblages including talc-bearing zones. Samples BH250-101 through BH250-106 document these thermal and metasomatic effects, where intrusive heat and fluids altered adjacent limestones and dolostones into coarse marble and calc-silicate mineral assemblages. Unlike the Saratoga Mine area in Death Valley, where talc is the dominant Mg-silicate phase observed, the Alta Stock preserves a broader metamorphic progression from periclase to forsterite to tremolite to talc facies. This wider range of assemblages likely reflects the larger size, higher temperature, and more prolonged thermal influence of the Alta intrusion. Like the Death Valley diabase, the Alta Stock illustrates how igneous intrusions can drive recrystallization, fluid-rock interaction, and local ore-forming processes.
Minor Minerals: chlorite
We visit this site to see a diabase dike and its contact metamoprhic effects and an outcrop that shows stratomlites. In the Death Valley region, many diabase intrusions are related to late Precambrian through Mesozoic magmatic events, although numerous mafic dikes in southern Death Valley are commonly interpreted as Neoproterozoic to Cambrian intrusions associated with rifting during the breakup of the Rodinia supercontinent (Mike Smith, Class of 1999, personal communication).
Chemically, diabase is the medium-grained intrusive equivalent of basalt. It is mafic in composition, and its dominant minerals are calcic plagioclase, clinopyroxene, and commonly olivine. The thin sections show strong alteration, where the presence of chlorite indicates later hydrothermal alteration or low-grade metamorphism, with original pyroxene and olivine partially replaced by secondary minerals during fluid interaction.
Where diabase intrudes carbonate rocks such as limestone or dolostone, common throughout Death Valley, it can produce well-developed contact metamorphism. Heat and reactive fluids from the intrusion recrystallize the surrounding rocks into marble and calc-silicate assemblages. If the host rock is magnesium-rich dolomite and silica is available, metasomatic reactions may generate talc through reactions such as:
dolomite + quartz + water →talc + calcite + CO2
3CaMg(CO3)2 + 4SiO2 + H2O→Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
This process commonly occurs in metamorphic aureoles around mafic intrusions where silica-bearing fluids migrate through dolomitic country rock. Talc-bearing zones may appear soft, pale green, white, or soapy, and can be associated with tremolite, serpentine, calcite, or chlorite depending on temperature and fluid chemistry.
These are of Death Valley also contains numerous historic talc mines and prospects, making it one of California’s most important talc-producing districts. A major talc belt extends from the Ibex Hills to the Silurian Hills. These diabase samples come from near the Saratoga Talc Mine.
This Death Valley diabase is geologically important because it records three linked processes: regional extension and magma intrusion, mafic Fe-Mg-rich magmatism, and thermal/metasomatic alteration of surrounding carbonate rocks, including talc formation.
In the BH250-collections there are also other contact metamorphic rocks. In the Wasatch Range, the Alta Stock provides an excellent comparison for intrusive contact metamorphism. This Tertiary quartz monzonitic to granodioritic intrusion metamorphosed surrounding carbonate rocks to produce marble, skarn, and Mg-rich assemblages including talc-bearing zones. Samples BH250-101 through BH250-106 document these thermal and metasomatic effects, where intrusive heat and fluids altered adjacent limestones and dolostones into coarse marble and calc-silicate mineral assemblages. Unlike the Saratoga Mine area in Death Valley, where talc is the dominant Mg-silicate phase observed, the Alta Stock preserves a broader metamorphic progression from periclase to forsterite to tremolite to talc facies. This wider range of assemblages likely reflects the larger size, higher temperature, and more prolonged thermal influence of the Alta intrusion. Like the Death Valley diabase, the Alta Stock illustrates how igneous intrusions can drive recrystallization, fluid-rock interaction, and local ore-forming processes.
Coverage
Location: Death Valley, California, USA
Nearby Geographic Feature: Death Valley
Saratoga Talc Mine
GPS Coordinates: 35.680203, -116.419136
Nearby Geographic Feature: Death Valley
Saratoga Talc Mine
GPS Coordinates: 35.680203, -116.419136
Date
2023
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 240. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-240,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/313.