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Abstract
This paper applies the magma-metal series classification developed by Keith and others (1991) and Keith and Swan (1996) to mineralization of southwestern Nevada, including the Nevada Test Site (NTS), Yucca Mountain, and surrounding areas. A new Ultra-Deep Hydrocarbon (UDH) hydrothermal oil model (Keith and others, 2008) also applies to the region. Mineralization was emplaced from mid-Cretaceous to late Mio- cene time. Mineralization related to the Miocene volcanism at Yucca Mountain proper did not contain sufficient hydrous minerals to suggest there was potential for economic mineralization, in contrast to well mineralized districts around Beatty to the SW.
Abstract
This paper applies the magma-metal series classification developed by Keith and others (1991) and Keith and Swan (1996) to mineralization of southwestern Nevada, including the Nevada Test Site (NTS), Yucca Mountain, and surrounding areas. A new Ultra-Deep Hydrocarbon (UDH) hydrothermal oil model (Keith and others, 2008) also applies to the region. Mineralization was emplaced from mid-Cretaceous to late Mio- cene time. Mineralization related to the Miocene volcanism at Yucca Mountain proper did not contain sufficient hydrous minerals to suggest there was potential for economic mineralization, in contrast to well mineralized districts around Beatty to the SW.
Cretaceous magmatism and related mineralization in the NTS region includes:
1) the Climax stock (metaluminous alkali-calcic [MAC]) at 102-99 Ma; 2) the Gold Meadows stock (metaluminous calc-alkalic [MCA]) at 93-96 Ma, which can produce copper-molybdenum-silver porphyry systems; 3) peraluminous calcic (PC) tungsten associated with pegmatite dikes (PC2 of Late Cretaceous age); and 4) peraluminous calc-alkalic (PCA3A) gold-quartz veins at 85-72 Ma.
Cretaceous MAC and MCA magmatism reflects flattening subduction between 100 and 90 Ma. The main structures accompanying metaluminous magmatism were east-directed thrust faults (e.g., the Belted Range thrust) that were broadly related to the Sevier orogeny in eastern Nevada and western Utah.
Peraluminous magmatism probably resulted from flat subduction beneath the region between 90 and 72 Ma. Structures related to this flat subduction include west- directed mylonite fabrics in low-angle thrust faults related to peraluminous sills in northern Bare Mountain and south of Beatty, and possibly to the WNW-directed CP thrust east of Yucca Mountain.
Tertiary mineralized systems in the region are metaluminous and include: 1) calc-alkalic [MCA] gold mineralization at 13.8-14.9 Ma; 2) alkali-calcic [MAC] base- metal mineralization of the central arc at 12.8-11.2 Ma; 3) quartz-alkalic [MQA] gold mineralization of the late arc at 10 Ma; and 4) nepheline alkalic [MNA] gold-telluride mineralization of the terminal arc at 8 Ma. Increasing alkalinity with decreasing age reflects a rapidly steepening subducting slab beneath the Yucca Mountain area be- tween 15 and 7 Ma.
MCA gold mineralization of the early Miocene arc includes mines on the east flank of Bare Mountain and Paleozoic-hosted mineralization presently deeper than 4800 feet at Yucca Mountain. Examples include Sterling and Mother Lode on the east side of Bare Mountain.
Most of the Miocene igneous rocks in and near the Nevada Test Site are associ- ated with the Southwest Nevada Volcanic Field erupted from 17 to 7 Ma. Most of the volume of volcanic rocks at Yucca Mountain is MAC magma-metal series, commonly associated with lead-zinc-silver-tin mineralization throughout the world. MAC vol-canism and associated base-metal mineralization are related to the central part of the magmatic arc. These MAC districts include: 1) silver-base metal mineralization at 12.6-12.8 Ma of the Wahmonie district; 2) hot spring and probable epithermal tin mineralization at 13.5-12.7 Ma in the Beatty Mountain sinter area, Thirsty Can- yon-Sleeping Butte area, and West Transvaal district; 3) mercury/fluorite/alunite mineralization at 12.9–11.2 Ma in the northern Bare Mountain area, Mary/Diamond Queen mine, Telluride mine, Southern Calico Hills, western Calico Hills, Claim Can- yon mercury anomaly areas, Transvaal East district; and widespread pyritic mineral- ization in the Tram Ridge Tuff, southwestern Mine Mountain, and 4) northern Yucca Mountain area.
The last major Miocene mineralization in the Yucca Mountain area is associated with quartz alkalic (MQA) magmatism in the trailing portion of the southward-mi- grating magmatic arc. MQA gold districts include gold-only/adularia mineralization at 10 Ma in the West Bare Mountain, and Bullfrog district (Bullfrog mine, Montgom- ery-Shoshone mine, Bonanza Mountain area, Original Bullfrog mine, Gold Bar mine, Mayflower-Pioneer area/North Bullfrog), Clarkdale district, and Tolicha district.
The latest metallic mineralization event was the basanitic, nepheline alkaline (MNA) volcanism associated with minor volumes of gold/telluride mineralization. These systems are dated at about 7 Ma in the Beatty area and possibly at the Oasis Mountain gold-telluride system.
Calderas and associated mineralization were probably emplaced in a transpres- sional regime that affected right slip on the Walker Zone to the north and on the Las Vegas Shear Zone–Stateline-Pahrump fault on the south. These two systems are connected by a north-south trending, dilational jog that includes most of the calde- ras. This area also includes a N-S trending normal-fault swarm and the north-south trending, Kawich-Greenwater gravity low. Extension in the dilatant jog created most of the room into which calderas and associated hydrothermal activity were emplaced. Minor antithetic tilting on the N-S faults (especially on Bare Mountain) created a gravitational ramp where local gravitational sliding occurred in the Bullfrog Hills tilt domain. Denudational sliding reused pre-existing, low-angle thrust faults that had accompanied Cretaceous magmatism.
Between 7 and 4 Ma, hydrous, iron-poor, metal-bearing magmatism changed to anhydrous, iron-rich, non-metallic magmatism. After 4 Ma, magmatism at Crater Flat contains anhydrous ferromagnesian minerals (such as olivine and pyroxenes), has no epigenetic mineralization, and has strong iron enrichment in typically low- volume felsic differentiates. Basaltic volcanism is associated with Basin and Range, high-angle, normal faults that are in a transtensional regime driven by far-field exten- sion. In contrast, earlier arc-related tectonism was driven by far-field transpression.
The youngest, most widespread hydrothermal activity in the Yucca Mountain area may be associated with hydrothermal oil developed from ultra-deep sources as- sociated with serpentinization in the lower crust. Railroad Valley is the closest system that may be an example of this process. Evidence for this hydrothermal activity in- cludes: basaltic volcanism at Crater Flat; extensive dolomitization in the Paleozoic carbonates beneath the Tertiary rocks; widespread, magnesium-charged, light <13C isotopes; and warm (25-35°C) water discharge throughout the Yucca Mountain area. These features are consistent with a UDH, hydrothermal oil system that could under- lie Crater Flat beneath a coinciding magnetic high and gravity low.
INTRODUCTION
Applying the magma-metal series classification and strato- tectonic approach to the magmatism, geochronology, mineral-
ization, and tectonics of a region is highly effective in assessing the types of ore deposits that may be expected in a region. Be- cause these classes are genetically related to igneous geochem- istry and tectonics, they are effective in predicting the types of
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Abstract
This paper applies the magma-metal series classification developed by Keith and others (1991) and Keith and Swan (1996) to mineralization of southwestern Nevada, including the Nevada Test Site (NTS), Yucca Mountain, and surrounding areas. A new Ultra-Deep Hydrocarbon (UDH) hydrothermal oil model (Keith and others, 2008) also applies to the region. Mineralization was emplaced from mid-Cretaceous to late Mio- cene time. Mineralization related to the Miocene volcanism at Yucca Mountain proper did not contain sufficient hydrous minerals to suggest there was potential for economic mineralization, in contrast to well mineralized districts around Beatty to the SW.
Cretaceous magmatism and related mineralization in the NTS region includes:
1) the Climax stock (metaluminous alkali-calcic [MAC]) at 102-99 Ma; 2) the Gold Meadows stock (metaluminous calc-alkalic [MCA]) at 93-96 Ma, which can produce copper-molybdenum-silver porphyry systems; 3) peraluminous calcic (PC) tungsten associated with pegmatite dikes (PC2 of Late Cretaceous age); and 4) peraluminous calc-alkalic (PCA3A) gold-quartz veins at 85-72 Ma.
Cretaceous MAC and MCA magmatism reflects flattening subduction between 100 and 90 Ma. The main structures accompanying metaluminous magmatism were east-directed thrust faults (e.g., the Belted Range thrust) that were broadly related to the Sevier orogeny in eastern Nevada and western Utah.
Peraluminous magmatism probably resulted from flat subduction beneath the region between 90 and 72 Ma. Structures related to this flat subduction include west- directed mylonite fabrics in low-angle thrust faults related to peraluminous sills in northern Bare Mountain and south of Beatty, and possibly to the WNW-directed CP thrust east of Yucca Mountain.
Tertiary mineralized systems in the region are metaluminous and include: 1) calc-alkalic [MCA] gold mineralization at 13.8-14.9 Ma; 2) alkali-calcic [MAC] base- metal mineralization of the central arc at 12.8-11.2 Ma; 3) quartz-alkalic [MQA] gold mineralization of the late arc at 10 Ma; and 4) nepheline alkalic [MNA] gold-telluride mineralization of the terminal arc at 8 Ma. Increasing alkalinity with decreasing age reflects a rapidly steepening subducting slab beneath the Yucca Mountain area be- tween 15 and 7 Ma.
MCA gold mineralization of the early Miocene arc includes mines on the east flank of Bare Mountain and Paleozoic-hosted mineralization presently deeper than 4800 feet at Yucca Mountain. Examples include Sterling and Mother Lode on the east side of Bare Mountain.
Most of the Miocene igneous rocks in and near the Nevada Test Site are associ- ated with the Southwest Nevada Volcanic Field erupted from 17 to 7 Ma. Most of the volume of volcanic rocks at Yucca Mountain is MAC magma-metal series, commonly associated with lead-zinc-silver-tin mineralization throughout the world. MAC vol-canism and associated base-metal mineralization are related to the central part of the magmatic arc. These MAC districts include: 1) silver-base metal mineralization at 12.6-12.8 Ma of the Wahmonie district; 2) hot spring and probable epithermal tin mineralization at 13.5-12.7 Ma in the Beatty Mountain sinter area, Thirsty Can- yon-Sleeping Butte area, and West Transvaal district; 3) mercury/fluorite/alunite mineralization at 12.9–11.2 Ma in the northern Bare Mountain area, Mary/Diamond Queen mine, Telluride mine, Southern Calico Hills, western Calico Hills, Claim Can- yon mercury anomaly areas, Transvaal East district; and widespread pyritic mineral- ization in the Tram Ridge Tuff, southwestern Mine Mountain, and 4) northern Yucca Mountain area.
The last major Miocene mineralization in the Yucca Mountain area is associated with quartz alkalic (MQA) magmatism in the trailing portion of the southward-mi- grating magmatic arc. MQA gold districts include gold-only/adularia mineralization at 10 Ma in the West Bare Mountain, and Bullfrog district (Bullfrog mine, Montgom- ery-Shoshone mine, Bonanza Mountain area, Original Bullfrog mine, Gold Bar mine, Mayflower-Pioneer area/North Bullfrog), Clarkdale district, and Tolicha district.
The latest metallic mineralization event was the basanitic, nepheline alkaline (MNA) volcanism associated with minor volumes of gold/telluride mineralization. These systems are dated at about 7 Ma in the Beatty area and possibly at the Oasis Mountain gold-telluride system.
Calderas and associated mineralization were probably emplaced in a transpres- sional regime that affected right slip on the Walker Zone to the north and on the Las Vegas Shear Zone–Stateline-Pahrump fault on the south. These two systems are connected by a north-south trending, dilational jog that includes most of the calde- ras. This area also includes a N-S trending normal-fault swarm and the north-south trending, Kawich-Greenwater gravity low. Extension in the dilatant jog created most of the room into which calderas and associated hydrothermal activity were emplaced. Minor antithetic tilting on the N-S faults (especially on Bare Mountain) created a gravitational ramp where local gravitational sliding occurred in the Bullfrog Hills tilt domain. Denudational sliding reused pre-existing, low-angle thrust faults that had accompanied Cretaceous magmatism.
Between 7 and 4 Ma, hydrous, iron-poor, metal-bearing magmatism changed to anhydrous, iron-rich, non-metallic magmatism. After 4 Ma, magmatism at Crater Flat contains anhydrous ferromagnesian minerals (such as olivine and pyroxenes), has no epigenetic mineralization, and has strong iron enrichment in typically low- volume felsic differentiates. Basaltic volcanism is associated with Basin and Range, high-angle, normal faults that are in a transtensional regime driven by far-field exten- sion. In contrast, earlier arc-related tectonism was driven by far-field transpression.
The youngest, most widespread hydrothermal activity in the Yucca Mountain area may be associated with hydrothermal oil developed from ultra-deep sources as- sociated with serpentinization in the lower crust. Railroad Valley is the closest system that may be an example of this process. Evidence for this hydrothermal activity in- cludes: basaltic volcanism at Crater Flat; extensive dolomitization in the Paleozoic carbonates beneath the Tertiary rocks; widespread, magnesium-charged, light <13C isotopes; and warm (25-35°C) water discharge throughout the Yucca Mountain area. These features are consistent with a UDH, hydrothermal oil system that could under- lie Crater Flat beneath a coinciding magnetic high and gravity low.
INTRODUCTION
Applying the magma-metal series classification and strato- tectonic approach to the magmatism, geochronology, mineral-
ization, and tectonics of a region is highly effective in assessing the types of ore deposits that may be expected in a region. Be- cause these classes are genetically related to igneous geochem- istry and tectonics, they are effective in predicting the types of
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