Beck Iron Mine

The Beck Spring iron deposit is the largest of several bodies of iron mineralization which outcrop along the north flank of the Kingston Range. All deposits lie within the same east-west trending belt which hosts the much more areally extensive talc deposits. Only the Beck Spring deposit has been exploited, however recent airborne magnetic surveys (Calzia, 1987) have outlined a much larger iron anomaly in the alluvium that floors the valley to the south of the mine site.

General Geology

The geology and iron/talc mineralization of the Kingston Range has been described by Hewett (1948) and Wright (1968). More recently Burchfiel and Davis (1988) and McMakin (1988) have reinterpreted much of the earlier mapping. The oldest rocks in the Beck Spring Mine area are 1.7 billion year old metamorphic rocks which lie about 300 meters south of the mine. The basement rocks lie in fault contact with Late Precambrian Pahrump Group. The Pahrump Group consists of the basal Crystal Spring Formation, the ore host, overlain by Beck Spring Dolomite and Kingston Peak Formation. Cambrian units outcrop along the east flank of the Kingston Range, and to the west in the Nopah Range, but not in the immediate mine area. About two kilometers south of the mine the main mass of 13 m.y. Kingston Peak Granite forms the core of the Kingston Range. Numerous small dikes of rhyolite cut the iron deposits. Their age is thought to be slightly younger (12 m.y.) that that of the Kingston Peak Granite.

Structure

The structure of the Kingston Range is complex and continues to undergo study and debate. Fortunately, in Beck Canyon the geology is more straightforward. The major structural feature is west-northwest trending fault that separates crystalline basement from Pahrump Group. Hewett (1948) suggested this fault was a thrust with west to east transport of the upper plate (north to south in the mine area). Wright (1963) mapped the fault as such. However, McMakin (1988) questioned this interpretation. McMakin interpreted the fault as a low angle normal fault, but stopped short of labeling it a detachment since the northeast to southwest movement of the recognized portions of the Kingston Range Detachment Fault are hard to reconcile with the fault geometry in Beck Canyon. Age of the faulting is also problematic, but it would appear that the main mass of the Kingston Peak Granite has cut the fault about two kilometers south of the mine suggesting a minimum age of 13 m.y.

Numerous northeast trending faults have been mapped at or near the mine site. Most have left-lateral displacements of only a few hundred meters, but the most prominent of this group, near the Crystal Spring Talc Mine, has an offset in excess of 1000 meters.

Ore Deposits

The iron-bearing horizon outcrops along the north wall of Beck Canyon over a distance of about two kilometers. It can be subdivided into two separate ore bodies. The west ore body strikes north-northwest and dips nearly vertically. Drill data suggest a strike length of 700 meters and thickness of 10-20 meters. The east ore body is more discontinuous, splitting into two separate ore bodies near its east end. Strike is similar to the west ore body, but dip is moderate to steep to the northeast. Strike length is about 500 meters and thickness 5-15 meters.

The hanging wall of the ore horizon is comprised of altered and recrystallized carbonate of the Crystal Spring Formation. Hewett (1948) reported unaltered rock is nearly pure limestone, in contrast to the typical dolomite of the Crystal Spring. The footwall is either heavily altered and silicated carbonate or diabase. Mapping by Wright (1968) suggests the iron horizon may lie within a septa of carbonate enclosed by a thick sill of diabase. Alteration consists of tremolite, wollastonite and serpentine. Siderite (iron-carbonate) is locally a conspicuous alteration product of hanging wall limestone.

Magnetite "black iron" is the dominant iron species, but considerable hematite "red iron" is also present. Pyrite and pyrrhotite (?) occur as disseminated crystals in the altered carbonate and as thin veins cutting the iron oxides and diabase. Limonite is present as supergene coatings and boxworks in near surface mine workings.

Genesis

Hewett (1948) suggested that the iron deposits of the Kingston Range were of contact metamorphic origin. He felt the mineralizing fluids were derived from the nearby Kingston Range Granite. The fact that some iron deposits were several kilometers from the pluton was not explained. More recent mapping indicates that the Kingston pluton may cut the iron belt and hence be younger than mineralization. McMakin (1988) attributed the iron deposits to contact metamorphism due to intrusion of the footwall diabase sill during late Precambrian. This model effectively draws upon that put forth by Wright (1968) to explain the origin of the talc deposits in the Kingston Range. Unanswered is why the belt of talc deposits is nearly 100 kilometers long while that of iron is only about 12 kilometers. It would be attractive to suggest that the Crystal Spring Formation was locally pyritic in the Kingston Range, the pyrite supplying the iron during contact metamorphism. Unfortunately, Hewett (1948) indicates the pyrite is later than both the iron oxides and diabase. Another possible explanation might be the unusually thick diabase sill in proximity to the iron deposits.

REFERENCES CITED

Burchfiel, B.C., and Davis, G.A., 1988, Mesozoic thrust faults and Cenozoic low­angle normal faults, eastern Spring Mountains Nevada, and Clark Mountains thrust complex, California in This Extended Land: Geological journeys in the southern Basin and Range, Field Trip Guidebook, Geological Society of America, Western Cordilleran Section, Las Vegas, Nevada, p. 87­10.

Calzia, J., et.al., 1988, Mineral resources of the Kingston Range Wilderness Study Area, San Bernardino County California: U.S. Geological Survey Bulletin 1709-C, 21 p.

Hewett, D.F., 1948, Iron deposits of the Kingston Range, San Bernardino County, California; in The Iron Resources of California: Calif. Division of Mines and Geology Bulletin 129, p. 193-206.

McMakin, Mathew R., 1988, Cenozoic sedimentation and tectonics of the Kingston Range in This Extended Land: Geological journeys in the southern Basin and Range, Field Trip Guidebook, Geological Society of America, Western Cordilleran Section, Las Vegas, Nevada, p. 221­223.

Wright, Lauren A., 1968, Talc deposits of the south Death Valley - Kingston Range region, California: Calif. Division of Mines and Geology Special Report 95, 79 p.