Geologic Setting of the Hamme Tungsten District, North Carolina and Virginia

The Hamme tungsten district is in the eastern part of the Piedmont province, mainly in Vance County, North Carolina, but it extends a few miles into
Virginia. The district is underlain by a central lenticular pluton of albite
granodiorite that trends north-northeastward and is flanked on both sides by
metamorphic rocks of low and medium grade that dip steeply westward. The
relative ages of the metamorphic rocks are uncertain. The oldest rocks are
likely to be the biotite gneisses in the eastern part of the district; successively younger units expose westward across the district are sericite-chlorite phyllites, greenstone, metafelsites, and metabasalts.

The biotite gneisses and minor intercalated hornblende gneiss, which have
a total thickness of many thousand feet, were derived from sediments. Some
of the gneiss grades into phyllites and as probably formed by metasomatic alteration of the phyllites. Sericite-chlorite phyllite, epidote-quartz meta siltstone, quartzite, and conglomeratic phyllite occur principally in a wide belt on the west side of the central albite granodiorite. This unit is some 10,000 feet thick and originally consisted mainly of sediments of the graywacke suite. Greenstone totaling about 500 feet in thickness lies west of the phyllite and was derived from maflc lava flows and andesitic tuff. Metamorphosed massive aphanitic and porphyritic flows and dikes that range in composition from dacite to rhyolite, and phyllitic metatuffs and tuffaceous breccia are exposed west of the greenstone. These total at least 3,000 feet in thickness. Massive metabasalt that resembles greenstone but is less altered is common in the area between the Hamme district and the Virgilina district to the west. The thickness of the metabasalt is about 600 to 6,000 feet.

The metamorphic rocks of the Hamme and Virgilina districts are parts of the Carolina slate belt, but map units cannot be directly correlated. Rocks in
the Hamme district are thought by the writer to have been derived mainly
from graywackes and volcanic flows, and subordinately from pyroclastic materials, whereas the rocks of the Virgilina district were interpreted by earlier workers as being mainly volcanic with much pyroclastic material but little sediment.

Igneous, and perhaps pseudo igneous, rocks in the district include hornblende
gabbro, albite granodiorite, aplite, and pegmatite--all of which are probably
middle Paleozoic in age--and diabase and hypersthene tonalite of Late Triassic age. The gabbro forms three lenticular to subcircular bodies up to 2% miles in width in the western part of the area. Albite granodiorite forms a pluton with a maximum width of 7 miles which occupies the center of the area. At its northeastern end the pluton narrows abruptly to a point. Phyllite forms the wall rocks on all sides of the albite granodiorite. The contact is gradational and conformable in most places, but on the northwest side it cuts across wall structure for about 3 miles. Near its western edge the albite granodiorite includes a northeast-trending zone of schistose wall rock in and near which are localized the tungsten deposits. The origin of the albite granodiorite is uncertain, but it may have formed by the metasomatic replacement of the wallrocks, during which albite porphyroblasts developed first and were followed by microcline and quartz. Diabase and hypersthene tonalite occur as dikes and sills along four northward-trending belts. The dikes are a few feet to more than 300 feet thick, and several extend along strike for more than 10 miles.

The Hamme district Is in the eastern part of the Carolina slate belt, and
the Virg1l1na district lies along the western side of the belt. Rocks in the
Hamme district dip mostly westward and in the Vifg1lina district dip mainly eastward into a syncline. This syncline, here named the Spewmarrow syncline, may be a structure of regional significance.

Tungsten in the Hamme district occurs mainly