Geology of the Old Hickory heavy mineral sand deposit,Dinwiddie and Sussex counties,Virginia

摘要

Old Hickory is a heavy mineral sand deposit located 60 km south of Richmond, Virginia, USA. It has been an active mine since 1997, operated by Iluka Resources, Inc. The deposit is one of several known large occurrences of heavy mineral sand along the "Fall Zone" from southern Virginia to central North Carolina. These deposits were formed by marine processes, probably at the intersection of shorelines and major paleorivers, during multiple transgressive/regressive cycles throughout Tertiary time. The pre- mining resource was estimated at 75 million tons averaging 9% heavy mineral. The average heavy mineral assemblage comprises 63% ilmenite, 19% zircon and 2.5% rutile plus leucoxene. The remaining non-valuable heavy minerals are predominantly staurolite, kyanite and sillimanite. The host sediment is a dominantly a clayey sand, which contains an average of 33% material finer than 75 microns, mainly clay. The original sediment was dominantly feldspar quartz sand and quartz sand. The clay currently disseminated in the deposit is thought to have been derived by weathering of feldspar and infiltration through the pile in descending meteoric water. Much of the sediment is massive from bioturbation, but locally planar cross-bedding, trough crossbedding, planar laminations, and dewatering structures are preserved. Most fossils have been obliterated, but ophiomorpha and scolithus burrows have been identified. The bulk of the heavy mineral appears to have been deposited in foreshore and backshore (dune) facies, although some of the basal material may have been fluvial. Bouldery units serve as horizon markers and were probably deposited by debris flows related to storms or earthquake activity. Unusual for heavy mineral deposits, Old Hickory experienced widespread faulting, which spanned pre- to post- mineral deposition. Most faults appear to be highangle with reverse sense of shear. Ridges were uplifted, which commonly have highgrade heavy mineral along their flanks and in intervening swales. Paleochannels have been identified cutting across stratigraphy, which commonly have high- grade heavy mineral with elevated zircon content. The deposit can be separated into two parts: a topographically higher western, older part and a lower younger eastern part. The eastern part commonly has higher titanium values than the western part, and, in the eastern part, rutile and zircon values increase to the east and in cross-cutting paleochannels.