GEOLOGIC SETTING

 

Rock units exposed the district range in age from Precambrian to late Miocene and include sedimentary, metamorphic, granitic and gabbroic intrusive and volcanic rocks. Locally, rhyolitic and lamprophyre dikes intrude most older rock formations.

 

Precambrian and Cambrian Age Units

 

The oldest unit is the Precambrian Wyman Formation, which is in unconformable contact with the underlying Cretaceous age Lone Mountain plutonic rocks. The exposed area of the unit is approximately 3 miles long by 1000 ft to 2000 ft wide. The exposed unit underlies approximately half of the SIOLFOR claim block.

 

The Wyman Formation consists of approximately 45% marble, interbedded in 2- to 5-cm layers with phyllite, quartzite and calc-silicate hornfels. Regional metamorphism is widespread and the unit has locally been affected by contact metamorphism with the plutonic rocks. Contact metamorphism usually only extends a few meters outward from the pluton and has changed limestone and shale to calc-silicate hornfels and garnet-tremolite- diposide skarn.

 

Conformably overlying the Wyman Formation is the Precambrian Reed Dolomite. In the SIOLFOR claim block area the contact has been defined by Bonham and Garside (1979) as the point where marble and calc-silicate hornfels change to massive, tan dolomite. Exposures of the Reed Dolomite parallel those of the Wyman Formation and cover an area 3 miles long and up to 4,000 ft wide. The exposed unit also underlies approximately 45% of the SIOLFOR claim block. Nearly the entire Reed Dolomite consists of light tan weathering, white sugary, massive recrystallized dolomite.

 

Overlying the Reed Dolomite, and in conformable contact, is the Precambrian age Deep Spring Formation. Exposed southeast of the SIOLFOR claim block, the unit covers an area 2.5 miles long by 2000 ft wide. Near the base, the unit consists of tan-colored dolomite, similar to the Reed Dolomite, thus distinguishing the contact can be difficult. The formation is composed of alternating gray, thin-bedded marble, mica schist, and lesser amounts of light-gray quartzite.

 

Throughout its outcrop area, the unit is metamorphosed, marble consisting of calcite, dolomite and varying tremolite. Near intrusive contacts, impure limestone is changed to calc-silicate hornfels, consisting of calcite, tremolite, zoisite, chlorite, sericite and potassium feldspar. Quartzite consist of a mosaic of xenomorphic quartz, potassium feldspar, sericite, calcite and actinolite.

 

The only Cambrian age unit exposed in the area is what is believed to be the Harkless Formation, which is in thrust fault contact with the underlying Deep Spring Formation and Reed Dolomite. Exposure of the unit is in excess of 8 miles long by 2.5 miles wide. The unit has been extensively intruded by Cretaceous and Tertiary age granitic units north and east of the SIOLFOR claim block. The unit is exposed in the eastern most portion of the claim block. The unit is predominantly a detrital unit that includes dar-brown, black, and olive hornfels with minor amount of thin, gray to black quartzite, impure sandstone and recrystallized, gray-blue limestone. The unit is metamorphosed throughout the area.

 

Cretaceous Age Units

 

Cretaceous units exposed in the area are broadly grouped by Bonham and Garside (1979) as Lone Mountain plutonic rocks. These include gabbro, quartz monzodiorite and Lone Mountain Pluton in descending age.

Tertiary age units are the silicic porphyry dikes which, in some locations, have been cut by lamprophyre dikes.

 

Four Cretaceous age plutonic units occur in the SIOLFOR area. The oldest is a hornblende gabbro (113.0 my, Bonham and Garside, 1979). The gabbro intrudes the Harkless(?) Formation and is intruded by quartz monzodiorite and granite and rhyolite porphyry dikes.

 

The gabbro appears to be intruded approximately parallel to the regional grain of host rocks. The unit is a medium- to coarse-grained rock. In hand specimen it appears to consist mostly of greenish-black hornblende and very light-gray plagioclase.

 

Next oldest is quartz monzodiorite which is equigranular and medium-grained. The unit intrudes the Harkless(?) Formation and gabbro and is intruded by Miocene age (22.1 my, Bonham and Garside, 1979) dikes. The quartz monzodiorite and gabbro occur only in the upper thrust plate, which has displaced the Harkless(?) Formation.

Since neither the gabbro or quartz monzodiorite intrude the thrust fault, this relationship suggests the thrust fault developed after emplacement of these two units.

 

The youngest Cretaceous age unit exposed in the area is the Lone Mountain pluton (63.0 to 71.1 my, Bonham and Garside, 1979). The unit intrudes the Wyman Formation, Reed Dolomite, Harkless Formation and Deep Spring Formation. It is intruded by lamprophyre and rhyolite porphyry dikes. The intrusive contact with the

 

encompassing rocks is sharp and discordant in detail. Much of the pluton is very light-gray, coarse- to medium- grained, hypautomorphic-granular biotite granite.

 

Tertiary Age Units

 

Two units are exposed in the SIOLFOR area, the older is silicic porphyry dikes of early Micoene age, dated 22.1 my (Bonham and Garside, 1979) and younger lamprophyre. Silicic porphyry dikes consist of a swarm of quartz monzonite, granodiorite, granite, and rhyolite units. These dikes intrude all older rock units in the area.

 

Quartz monzonite and granodiorite dikes are light gray to yellowish gray and contain prominent phenocrysts of plagioclase plus smaller phenocrysts of quartz, orthoclase and biotite in a felsitic matrix of quartz, orthoclase and plagioclase. The granite porphyry is pale red, pinkish gray, or grayish orange pink and has phenocrysts of orthoclase, quartz, plagioclase and biotite in a micrographic or aplitic matrix of quartz and alkali feldspar. The rhyolite porphyry is light gray, pinkish gray, grayish orange pink, pale red or very pale orange and contains sparse phenocrysts of quartz, orthoclase, plagioclase and biotite in a spherulitic matrix of quartz and alkali feldspar.

 

All dikes are hydrothermally altered. The alteration ranges from propylitic to argillic and phyllic. Most of the dikes contain sparse disseminated pyrite. Hydrothermal alkali feldspar occurs in quartz veinlets in rocks displaying phyllic alteration.

 

Bonham and Garside (1979) suggest that a subjacent pluton was the magmatic source of the dike swarm and that the emplacement of this pluton caused doming and extensional fractures of its roof. The dike swarm was emplaced in fractures developed as a result of the extension of the roof.

 

Lamprophyre dikes are the youngest exposed rock unit and intrudes all units older. The dikes are dark greenish gray and contain prominent hornblende phenocrysts in a fine-grained pilotaxitic matrix of plagioclase, augite, and alkali feldspar. Magnetite and apatite occur as accessory minerals. The dikes are deuterically altered and contain abundant calcite, chlorite, and epidote replacing the primary minerals.

 

Structure

 

Two types of faults have been mapped in the area 1) a thrust fault and 2) high angle normal faults. Thrust faulting at Lone Mountain emplaced the Harkless(?) Formation over the Reed Dolomite and Deep Spring Formation, cutting out 3,000 ft to 6,000 ft of section. The thrust fault plane seems nearly parallel to beddin in the upper and lower plates. In some locations, this would result in a near vertical fault plane. Major offset is indicated by younger-over-older relations and progressive crosscutting of units. The originally low-angle thrust has been tilted by high-angle faults of Tertiary ages. Previously mentioned observations suggest the thrust is younger that Mesozoic intrusive units.

 

MINERALIZATION

 

As noted by Bonham and Garside (1982), most of the ore bodies in the Lone Mountain district are predominantly replacement bodies in Precambrian and Cambrian rocks. The ore in the replacement bodies is largely oxidized and is composed of hemimorphite, smithsonite, cerussite, chrysocolla and tenorite. The large Cretaceous stock og granite intruding the Precambrian and Cambrian rocks is essentially unaltered and does not appear to have any direct relationship to the metallization in the district.

 

In the SIOLFOR claim block area, the ore bodies are spatially and perhaps genetically related to the northwest- trending granite porphyry dikes. The dikes are hydrothermally altered and now consist essentially of K-mica, K- feldspar and quartz. Unoxidized ore at the Alpine Mine has been described by Phariss (1974) as galena, sphalerite, molybdentie, enargite and pyrargyrite in a maganosiderite gangue.

 

Ore grade lead-zinc-silver mineralization is present as replacement bodies along fracture zones in carbonate rocks of the Reed Dolomite and Deep Spring Formation in the area. Geochemical anomalies of several metals are shown in Table 1. These anomalies are associated with the porphyry dike complex. These dikes exhibit pervasive hydrothermal alteration and the dike swarm almost certainly is the upper portion of an intrusive complex which might contain porphyry molybdenum mineralization.

 

Also mentioned by Bonham and Garside (1982), the area north of the SIOLFOR claim block contains anomalous amounts of arsenic, bismuth, silver, barium, mercury, and manganese indicating possible silver-gold mineralization.