Open-File Report 2006-1259

Published 2006
Online Only

Geology of the Snow Camp-Saxapahaw Area

Approximately four-fifths of the study area is underlain by volcanic and volcaniclastic rocks, the remainder by intrusive stocks and shallow plutons (fig. 2, fig. 3, fig. 4A, fig. 4B, and fig. 5), all generally similar to those found elsewhere in the Carolina slate belt. The rocks include flows and fragmental rocks ranging from very fine tuffs to coarse conglomerates and pyroclastic breccias having compositions ranging from basalt to rhyolite. The plutons are mostly granite to quartz monzonite in composition, but include some gabbros and other rocks of mafic composition. Porphyritic rocks are common in the plutons. These porphyries have very fine matrices suggesting intrusion at relatively shallow depths. Some plutons have wide contact metamorphic haloes, and other plutons have intensely hydrothermally altered interior zones.

We think that the relative ages of the volcanic and subvolcanic rocks and their present distributions can best be explained by the interpretation that if the area consists of three major structural blocks. The northwest Cane Creek structural block is separated from the central Major Hill structural block by the Snow Camp fault, and the southeastern Chestnut Hill structural block is separated by the broad South Fork fault system. We interpret the Major Hill structural block to be a northeast-trending graben. The continuation of the bounding faults outside the area was not determined.

The stratified volcanic and volcaniclastic rocks were divided into two major mappable rock types throughout much of the area. The older unnamed diverse complex of intermediate to felsic volcanic rocks is overlain unconformably by a well-defined rhyodacitic and dacitic crystal-rich tuff, here called the Reedy Branch Tuff.

Summarized Geologic History of the Snow Camp-Saxapahaw Area

A complex of andesitic to dacitic volcanic rocks, including flow-banded rhyolite and rhyodacite, as well as many tuffaceous and fragmental rocks, was erupted onto an unknown substrate in a volcanic-arc environment. Several plutons of granite, quartz monzonite, and other granitoid rocks were intruded into the volcanic complex. The intrusion of porphyritic bodies at shallow depth was accompanied by major hydrothermal alteration that resulted in intense high-sulfidation of certain existing rocks and produced bodies of pyrophyllite-rich rocks and local gold-pyrite and minor silver mineralization.

Tight folding, perhaps without significant metamorphism, and a period of extensive erosion followed. A graben formed bounded on the northwest by the Snow Camp fault and on the southeast by the South Fork fault system. During graben formation, several plutons of gabbro were intruded along the Snow Camp fault and small granitoid plutons, including the Lindley Farms Quartz Monzonite, were emplaced along the South Fork fault system. Large volumes of crystal-rich rhyodacitic and dacitic Reedy Branch Tuff were erupted within the southwest-tilting graben, probably during the sinking of the central block.

The rocks were deformed into open folds, cleaved, and regionally metamorphosed during the Taconic orogeny. Probably additional faults formed, especially along the graben margins. Rocks within large areas of the Reedy Branch Tuff were sheared and subjected to quartz-sericitic and propylitic alteration with no known mineralization.

Post-metamorphic granitoid rocks and porphyritic diorites were intruded at about 300 Ma, accompanied by autometasomatism. The strong potassic and sericitic hydrothermal alteration included formation of molybdenum-bearing greisenlike bodies, and gold-bearing pyritic zones within the porphyritic plutons. Probably some of the earlier widely disseminated gold was remobilized in epithermal veins in favorable sites. Finally, the area was eroded to the present-day land surface, deep saprolitic weathering took place, and gossans formed on favorable pyritic rocks.

Intermediate to Felsic Volcanic Complex
The intermediate to felsic volcanic complex is a heterogeneous sequence of rocks designated Zv where undivided, and Zvab, Zvrd, Zvdf, Zvh, Zvhlf, Zvqs, or Zvqg, where separate rock types were mapped. All of it is thought to have undergone generally similar deformation, regional metamorphism, and widespread intrusion by shallow plutons, probably subvolcanic bodies, and locally intense hydrothermal alteration.

Rocks within the complex are widely diverse. The most abundant are tuffs, coarse pyroclastic rocks, and lesser flows and volcaniclastic rocks, of andesitic to dacite composition. Rocks as diverse as basalt and rhyolite are also common. All are assumed to include both flows and subvolcanic bodies. Among the porphyritic volcanic rocks, rhyolites and rhyodacites probably dominate. There are many subvolcanic intrusions and shallow plutons, but because of poor exposures, some of these may have been miscategorized as volcanic rocks. Thinly bedded volcaniclastic rocks were recognized in a few places but are sparse as compared to many parts of the Carolina slate belt outside of our map area. This complex is host to all of the extensive high-sulfidation hydrothermal alteration in the region. The relative ages of the rocks within this complex are not known, nor are the total ages represented.

Volcanic rocks have been altered to hornfels at many places adjacent to the larger plutons. Extensive areas of hornfels cut by abundant dikes or septa of granitoid rock (Zic) are described separately. Because the mapping depends on recognition of the intricate repetition of the rock types, this could be done only in areas of good outcrops.

The clasts in the tuffs are generally less than 0.8 in (2 cm) long, but coarser fragments are present locally, as in outcrops found northeast of Major Hill near Highway 2351 (fig. 2, fig. 3, fig. 4A, and fig. 5, sector F), where clasts as long as 15 in (~40 cm) were easily recognized on weathered surfaces. In a few places, there are amygdaloidal rocks here interpreted to have been flows.

Andesitic or Basaltic Bedrock, Separately Mapped
Andesitic or basaltic rocks (designated Zvab, fig. 2, fig. 3, fig. 4A, fig. 4B, and fig. 5, sectors G and H) are indicated by the presence of very dark red-brown Tirzah- and Efland-type soils (Kaster, 1960). Although only a few outcrops of basalt and fragmental andesite were observed within those soils, the linear shape of the Zvab area in sectors G and H suggests a lenticular body of volcanic rock, perhaps a flow or tuff layer.

Siliceous Flow-Banded Rhyolite and Locally Flow-Banded and Locally Porphyritic Dacite
Areas of siliceous rhyolite and dacite (Zvrd, fig. 2, fig. 3, fig. 4A, and fig. 5) are present at many places within the volcanic complex, and such rocks make up most of the volcanic rock part of the Cane Creek Mountains. Other than in the Cane Creek Mountains, these rocks are in mostly elongate isolated outcrop areas and were interpreted to have formed as flows, domes, or stubby lenses. No continuous outcrops of such rocks more than 3 mi (5 km) long could be identified with certainty.

Volcanic rocks in the Cane Creek Mountains are microcrystalline and, in many places, flow banded; their high silica content and conchoidal fracture suggest that they may have been silicified. Nearer the mountain crest, they surround three granophyric intrusive bodies (Zcg). Most of these volcanic rocks were recrystallized to a very fine grained siliceous hornfels texturally similar to aplite (Zvh). The granophyres may have resulted from a late resurgent event, in which the rising magma did not reach the surface. Chemical analyses of sample 1667 from the eastern edge of the Cane Creek Mountains and sample 1665 from Bass Mountain (table 1; fig. 2, sectors A and B) indicate a rhyolitic composition. A few late felsic porphyry dikes cut both the granophyre and volcanic rocks.

Thin sections of seven samples collected on Bass Mountain (fig. 2, sector B) are of porphyritic rhyolitic or rhyodacitic flows having fine matrix grains, 0.002 to 0.004 in (0.05–0.10 millimeter (mm)) long and phenocrysts as long as 0.12 in (3 mm). Microspherulites are present in many sections. There is flow banding in about half of the Bass Mountain outcrops examined, but some exposures appear to be massive. The rocks in many outcrops are very brittle and have broken with a conchoidal fracture.

Most flow-banded rhyolites and dacites on Major Hill (Zvrd, fig. 2, fig. 3, fig. 4A, and fig. 5, sector F) contain megacrysts 0.04 to 0.16 in (1–4 mm) in longest dimension. Some are porphyries and have devitrified or chilled matrices. Others are crystal-rich fine lapilli tuffs that contain some lithic fragments. The rhyolitic and dacitic rocks in outcrops to the south and east of Major Hill include porphyritic flows and crystal tuffs, some of which are flow banded, and have welded textures, or lapilli.

Rhyolitic rocks of many textures are exposed on Chestnut Hill (Zvrd, fig. 2, fig. 3, fig. 4A, and fig. 5, sector O) and the adjacent ridge. There is tuff that contains compacted and deformed glass shards, porphyry that has a fine-grained microlitic groundmass, lapilli tuffs, and devitrified glass that preserves patches of flow-banded rocks and pockets of intergrown spherules and euhedral quartz. It is unclear as to how many of the components of these rocks are original and how many were modified by later alteration, such as sample 792 from Chestnut Hill, which contains 76.1 percent SiO2 and 11.5 percent Al2O3, yet retains 3.68 percent Na2O and 3.88 percent K2O (table 1; fig. 2).

The large area of flow-banded rhyolitic rock (Zvrd, fig. 2, fig. 3, fig. 4A, and fig. 5, sector P) in the southeastern part of the mapped area is shown as it was mapped by Wilkinson (1978). The abundance of flow banding, the cryptocrystalline matrix, and the albitic megacrysts, as described by her, suggest that the rocks there are very similar to those on and near Chestnut Hill. Although her mapped area suggests a greater thickness of flow-banded rhyolite than at Chestnut Hill, part of the southeastern area is underlain by nonrhyolitic volcanic rocks; therefore, the real thickness of the flow-banded rocks might be less. At several localities, the flow-banded rhyolitic and dacitic rocks seem to be relatively unaltered although they are adjacent to highly altered rocks, as on Major Hill (fig. 2, fig. 3, fig. 4A, and fig. 5, sector F).

The relations of the highly siliceous volcanic rocks in the isolated areas and those in the Cane Creek Mountains is problematic because they are very similar in physical appearance although less similar chemically and mineralogically. Results of ten chemical analyses of this sequence of rocks are given in table 1. Potassium feldspar is a major constituent of some of the samples from Bass Mountain, and may be the result of hydrothermal alteration, but at five sites it is almost certainly a primary mineral. In other locations, such as Major Hill, potassium feldspar is generally sparse or entirely lacking and, where identified, is probably the result of hydrothermal alteration.

The siliceous extrusive rocks and granophyric intrusive rocks of the Cane Creek Mountains are typical of those of a volcanic center, as suggested by R.E. Koeppen (oral commun., U.S. Geological Survey, 1994). The extrusive-intrusive rock relations also are permissive of the assemblage being a small caldera complex, but field data are not adequate to confirm that interpretation.

Debris Flow Strata East of the Snow Camp Community
Several thin beds of coarse conglomerate containing diverse volcanic cobbles, perhaps lahars, are interlayered with dacitic and andesitic tuff, in an elongated area east of Snow Camp community (Zvdf, fig. 2, sector F). The cobbles are mostly rounded and are as much as 6 in (15 cm) in maximum dimension. The beds are especially well exposed along the banks of Cane Creek below the dam at Holman's Mill.

Reedy Branch Tuff
The crystal-rich rhyodacitic and dacitic tuff along Reedy Branch is herein named the Reedy Branch Tuff (undivided Zr, and subunits Zra, Zrb, and Zrc, fig. 2, fig. 3, fig. 4A, and fig. 5). Typical exposures can be seen along Highway 2352 where it crosses Reedy Branch (sample 6889, fig. 2, sector J). Within the mapped area, the Reedy Branch Tuff seems to be confined to the Major Hill structural block, especially in its southwestern part. Compared to most Carolina slate belt rocks in the general area, it is remarkably uniform in both texture and composition. Where near the intermediate to felsic volcanic complex, the younger Reedy Branch Tuff is less altered and less deformed than the underlying rock. It is everywhere metamorphosed in the greenschist facies.

The tuff consists of abundant 0.12- to 0.3-in (3- to 8-mm)-long plagioclase and lesser quartz crystals set in a very fine grained matrix of muscovite, biotite, chlorite, epidote, calcite, ilmenite, and anatase. Euhedral to subhedral plagioclase crystals are 10 to 30 times as abundant as quartz by volume. Because of the abundance of epidote or clinozoisite formed in the crystals during greenschist-facies metamorphism, the plagioclase crystals are estimated to have been originally intermediate in composition. Quartz crystals are about the same size as plagioclase crystals but almost all are rounded and many are deeply embayed. It is unlikely that any potassium feldspar in this rock is primary. Where present, it is interpreted to be the result of potassic alteration. Muscovite and biotite are mostly secondary, formed, in part, during regional metamorphism and, in part, by later hydrothermal alteration. The original mafic minerals, probably biotite and hornblende, are now chlorite, epidote, biotite, calcite, ilmenite, and anatase. Matrix grains are very small, ranging from 0.0004 to 0.002 in (0.01–0.05 mm) in longest dimension. Textures characteristic of devitrified glass are present in some samples.

Small inclusions of mostly dark, fine-grained rock are common in the Reedy Branch Tuff wherever it was studied. Their color results, in part, from fine-grained biotite or from chlorite that has replaced biotite. Most of these inclusions are smaller than 0.4 in (1 cm), but they may be as much as 6 in (15 cm) long; there are generally 1 to 10 inclusions visible in one square yard of outcrop surface. Some are angular, and some are rounded. They are quite different from the inclusions in the basal debris flow and conglomeratic layers described below.

A discontinuous subunit (Zrc), as much as several feet (meters) thick, at the base of the Reedy Branch Tuff contains abundant, rounded, volcanic rock clasts set in a crystal-rich matrix and is here interpreted to have been a debris flow. This subunit was first noted by Hughes (1987) at site 5074 (south of Mine Ridge, fig. 2, sector J). Poorly sorted, rounded cobbles, as long as 6 in (15 cm) of glassy, fine-grained, unaltered porphyritic intermediate to felsic volcanic rocks, are present at this site where they appear to unconformably overlie a variety of highly altered rocks of the older complex (Zvqs). In a small outcrop along the eastern basal edge of the Reedy Branch Tuff about 1.2 mi (2 km) southwest of Mine Ridge (site 6955, fig. 2, sector J), there is a sheared conglomerate containing abundant rounded volcanic rock pebbles and cobbles as long as 4 in (10 cm) in a crystal-rich matrix. In a large exposure south of the Hinshaw pyrophyllite prospect (site 1923, fig. 2, sector N), similar conglomerate can be seen in layers separated by beds of clast-free dacitic tuff. At site 6903, about 0.3 mi (0.5 km) west of Mine Ridge (fig. 2, sector J), another outcrop of dacitic crystal tuff contains abundant rounded to angular clasts of volcanic rocks as long as 2 in (5 cm) that have a variety of textures and compositions. The rocks at these sites are interpreted to constitute a volumetrically minor basal facies. The contact is an important horizon marker in the volcanic stratigraphy. Wherever pebbles of various compositions are present, we think that they indicate a stratigraphic position near the basal contact. No upper contact of the Reedy Branch Tuff was identified.

Sericitization is widespread throughout the Reedy Branch Tuff (Zra), and strong hydrothermal alteration was separately mapped in about half the unit (Zrb). Where the relatively strong hydrothermal alteration is evident in both hand specimen and thin section, much new muscovite has formed. There was, however, very little change in rock chemistry (table 2, samples 5005 and 6184; fig. 2, sector F).

Relative Ages of the Volcanic Rocks

The volcanic rocks of the area can be tentatively separated into three age units based on the abundance of intruded igneous rocks. Rocks of the intermediate to felsic volcanic complex (Zv, Zvab, Zvrd, Zvdf, Zvh, Zvhlf, Zvqs, and Zvqg) have been invaded by many plutons (ZCambrian symbold, Ztm, Zcg, Zlf, Zic, and Zg). In contrast, the flow-banded rhyolite (Zvrd), separately mapped but placed in the same general age unit, appears to have been intruded by only the central granophyric plutons within the Cane Creek Mountain block (Zcg, fig. 2, fig. 3, fig. 4A, and fig. 5, sector A). The presence of a few felsic porphyry dikes, and probably a few small mafic stocks, is indicated only by soil type. The Reedy Branch Tuff (Zr, Zra, Zrb, and Zrc, fig. 2, fig. 3, fig. 4A, and fig. 5), the youngest volcanic rock, has not been cut by intrusives at all. The sparse postmetamorphic granitic plutons (Cg, fig. 2, fig. 3, fig. 4A, and fig. 5), for which we assume an age of ±300 Ma are clearly the youngest intrusive rocks mapped, but they are only present within older granitoid rocks in a small part of the area providing little information on relative ages. The Reedy Branch Tuff could not be interpreted as the extrusive phase of any intrusive sequence in the mapped area. The restriction of the unit to the Major Hill structural block suggests that it may have been erupted during or shortly after graben formation.

Metamorphosed Intrusive Rocks
Metamorphosed intrusive rocks are widely distributed throughout the map area other than that underlain by the Reedy Branch Tuff. Granitic rocks (Zcg) underlie the highland area in the Cane Creek Mountain structural block, and several small gabbroic stocks (Zg) were mapped farther to the south. Within the Major Hill structural block, large and small plutons, mostly of granite and quartz monzonite (Ztm, Zcg, Zlf, Zic, and ZCambrian symbold), are common, especially toward the northeast, and several gabbroic bodies (Zg) have been intruded along the bounding Snow Camp fault and in small bodies within the block. Two bodies of the Lindley Farms Quartz Monzonite were mapped: one in sector J in the Major Hill structural block and one in sectors J and N, straddling the South Fork fault system.

Granitic Intrusions in the Cane Creek Mountains
Three areas containing similar plutonic rock (Zcg), which were apparently intruded at shallow depths, were mapped in the central part of the Cane Creek Mountains. They almost certainly are the apophyses of a large body at depth. These fine- to medium-grained granites grade into porphyries on their margins that have aplitic groundmasses. Some rocks have graphic features and others have myrmekitic textures suggesting that the magma was near crystallization, indicating eutectic conditions during formation, which is compatible with shallow emplacement. A mode of a specimen from site 1691 (fig. 2, sector A) is given in table 4. The abundance of perthites and irregular granophyric and graphic intergrowths, as well as local alteration, made the modal analyses of these granites especially difficult. The rocks are feldspar rich; sodic plagioclase and potassic feldspar probably constitute more than 65 percent of the rock if the perthites are included. Perthite partly mantles plagioclase grains and is a major component of the common graphic intergrowths. Biotite is completely altered to deep-brown and red masses and, in part, to red-brown pleochroic blades or needles, perhaps ferristilpnomelane. The rock also contains sparse small grains of an unidentified opaque mineral. The ages of these granites are not known but we think that they are older than the Reedy Branch Tuff.

Relatively wide contact aureoles of siliceous hornfels (Zvh) have formed in felsic volcanic host rocks along some of their contacts with the granite bodies of the Cane Creek Mountains (fig. 2, fig. 3, fig. 4A, and fig. 5). Some wide aureoles may reflect a gently dipping contact. The hornfels weathers to an Appling-type soil similar to that which has formed on the intrusions. Our mapping of the width of the hornfels zones was based on the soils survey by Kaster (1960).

The southernmost body of siliceous hornfels does not contain outcrops of granites, but such rocks may be in the subsurface. Any relation of any of the hornfels to the several small gabbroic bodies, as suggested by dark Iredell-type soils, as mapped by Kaster (1960), could not be determined.

Granite, Quartz Monzonite, and Related Rocks in Sectors C, D, and the Northern Halves of Sectors G and H
Metamorphosed rocks of plutons and subvolcanic apophyses of granitoid rocks (Ztm) were mapped in the Major Hill structural block in the Saxapahaw-Eli Whitney area (sectors C, D, G, and H). These rocks have been noticeably silicified, and a few, that formerly contained no more than a few percent K2O, now contain as much as 20 percent potassium feldspar. Chemical and modal analyses of several samples of granite, porphyritic granite, quartz monzonite, quartz monzogabbro, and porphyritic granodiorite of this group are given in table 3 and table 4. These rocks contain relatively small amounts of calcium, magnesium, and iron, now present mostly in epidote and chlorite. Textures range from medium-grained hypidiomorphic granular to porphyries having fine-grained groundmasses. In many rocks, the quartz grains have been enlarged by replacement of adjacent feldspar, and new quartz grains have formed within feldspar, changing the original texture. Although the regional metamorphic grade is generally too low for the formation of hornblende, it appears locally as a replacement product in a small area of quartz diorite (site 6580, fig. 2, sector D; fig. 6) included within an area of younger greisenlike rock (Cg).

Gray granite (Ztm) crops out on the island in the Haw River at Saxapahaw (site 6371, fig. 2, sector D). The medium-grained rock there contains slightly more than 20 percent quartz and 6 percent of aggregates of chlorite and epidote pseudomorphous after biotite. A few dark-gray, fine-grained inclusions, as much as 2 in (5 cm) across, may be fragments of nearby volcanic wall rocks now altered to hornfels. Rocks of this general type (Ztm) also constitute the plutonic body in the southeastern corner of the area (fig. 2, fig. 3, fig. 4A, and fig. 5, sector L) that was named the Chatham Granite by Hauck (1977) and Wilkinson (1978).

Granitoid Plutons along the Postulated South Fork Fault System
Small plutons of granite, quartz monzonite, quartz monzodiorite porphyry, and related rocks (Ztm, fig. 2, fig. 3, fig. 4A, and fig. 5) crop out at several places near the trend of the South Fork fault system. Hydrothermal alteration has modified the minerals in parts of these bodies, and modal analyses of six samples are given in table 5. Graphic and (or) myrmekitic textures are generally common. The inclusion of many wall-rock xenoliths and the assimilation of wall rock by these plutons suggest that the outcrops formed near the tops of the intrusive bodies.

The possible interrelations of these plutons to the postulated South Fork fault system are not known. Some of the shallow plutons may have been sources for the fluids of hydrothermal systems that profoundly affected large volumes of surrounding volcanic rock as well as parts of the plutons themselves.

Lindley Farms Quartz Monzonite Plutons
The Lindley Farms Quartz Monzonite (Zlf, fig. 2, fig. 3, fig. 4A, and fig. 5) is named for good exposures of two related plutons near or straddling strands of the South Fork fault system. The unit is well exposed at site 7057 (on the farm of Walter Lindley), at site 1884 (on the farm of Frank Lindley) (fig. 2, sector J,) and at site A655 on the farm of Jonathan Lindley (fig. 2, sector N). There are other outcrops near the creek 1,600 ft (500 m) north of site 1884. The distribution of the unit cannot be inferred from soils types. (Note, none of these outcrops can be examined without entering private land, and permission of the current landowners should be obtained.)

The Lindley Farms Quartz Monzonite is medium- to coarse-grained rock. The rock consists of light-gray quartz and feldspar and about 20 percent of grain-size aggregates of dark minerals that appear black in hand specimen. The unit includes granite porphyry and quartz monzogabbro, phases of rock that generally have granophyric textures. There is abundant evidence at grain boundaries of replacement of plagioclase by quartz and potassium feldspar. All plagioclase grains were saussuritized during regional metamorphism, and in specimen 1884 (fig. 2, sector J), mafic mineral aggregates are partly altered to a green amphibole, perhaps ferroactinolite.

Modes of medium-grained samples A655, 1884, 5072, and 6611 of the Lindley Farms Quartz Monzonite plutons are given in table 5, and two whole-rock chemical analyses of samples 5072 and 7057 are given in table 3. These samples appear to be very similar in hand specimen.

Color and textural variations of these rocks, especially near the margins, indicate a considerable internal inhomogeneity because of the assimilation of andesitic country rock. At the northern end of the southern pluton (fig. 2), the rock is dull greenish gray, presumably from the assimilation. The grain size of the rock coarsens southwestward. The northern pluton is also more contaminated by assimilated country rock at its northern end. Inclusions of wall-rock hornfels are common along its eastern margin. Fine-grained, dark-gray inclusions as much as a few inches (centimeters) across are present in the southernmost outcrop of the northern body.

Strongly altered granite porphyry, a minor phase of the northern pluton, crops out in several places in the beds of small streams such as at site 6611 (fig. 2, sector J). Plagioclase phenocrysts as long as 0.2 in (0.5 cm) are enclosed in a matrix of grains 0.004 to 0.012 in (0.1–0.3 mm) long. Most of the plagioclase appears to have contained less than 50 percent anorthite before saussuritization, but a few grains had more calcic cores as indicated by zones richer in epidote. Matrix mineralogy was complex and bears the imprint of both regional metamorphism and potassium-feldspar metasomatism. Small grainlets of quartz have been enlarged, mostly replacing adjacent plagioclase. Potassium feldspar has replaced small grains and larger crystals of plagioclase at grain boundaries and along microfractures. Myrmekitic aggregates of quartz and feldspar are common. A modal analysis of this rock is given in table 5. A crude fabric has resulted from the crystallization of biotite, epidote, and, finally, chlorite along abundant anastomosing microfractures.

Crystal-rich porphyritic granodiorite (sample 6610H, fig. 2, sector J), having a fine matrix, can be seen as large boulders and perhaps outcrops near the microfractured granite porphyry at site 6611 (fig. 2, sector J). The rock consists of quartz and calcic plagioclase and a variety of mafic minerals, including biotite, chlorite, epidote, and secondary hornblende, probably formed by regional metamorphism from what was probably primary hornblende. Metamorphism at the greenschist- to amphibolite-facies transition is indicated. The lack of shearing and integrity of grain boundaries suggest that this rock is at least a little younger than the nearby granite porphyry at site 6611.

Other Metamorphosed Small Plutons Mapped Near the South Fork Fault System
Five small granitoid plutons (Ztm, fig. 2, sectors K, L, and H) were mapped along the South Fork fault system northeast of the Lindley Farms Quartz Monzonite bodies, to which they may be related. The first is a body of quartz monzonite and quartz monzodiorite south of the Zachary gold mines (fig. 2, sectors K and L). There are only small exposures at either end of the pluton. Its extent, however, was inferred from the presence of Appling-type soils as mapped by Kaster (1960), a general characteristic of granitoid substrates. Dull-greenish-gray quartz monzodiorite porphyry crops out at the western end of the pluton (site 6761, fig. 2). The dark mineral component constitutes nearly 25 percent of the rock, probably resulting from the assimilation of wall-rock hornfels, inclusions of which are present in outcrops. Myrmekitic aggregates of quartz and feldspar are especially abundant. A modal analysis of site 6761 is given in table 5, and there is a chemical analysis of the same sample in table 3. Mafic-rich quartz diorite grading locally into porphyritic quartz monzonite can be seen at the eastern end of the pluton in an area about 650 ft x 1,640 ft (200 x 500 m), mainly in a small creek bed (site 7076, fig. 2, sector L). Abundant wall-rock inclusions near the eastern margin and the mafic composition of the quartz monzogabbro at site 7082 (table 3; fig. 2, sector L) suggest modification by assimilation of mafic material from the walls. Biotite and chlorite, products of the regional metamorphism of original hornblende, locally make up as much as 40 percent of the rock along the eastern margin. Sample 7082 of quartz monzogabbro (table 3; fig. 2, sector L), also was mapped near the margin of this pluton. The abundance of myrmekite and of little-altered xenoliths suggests that this igneous body was emplaced near the surface. Part of this pluton was first noted by Wilkinson (1978), who described it as a tonalite or perhaps a granodiorite.

The second pluton is a granitic body about 0.31 mi (0.5 km) in diameter (Ztm) exposed along Pine Branch north of the Braxton Mine (site 6391, fig. 2, sector H). The modal composition of this body, about 35 percent quartz, 31 percent plagioclase, and 23 percent potassium feldspar (table 5), makes it quite unlike other rocks along the South Fork fault. The rock has been silicified and has biotite, chlorite, and epidote on many small shear surfaces. These minerals give the rock a dull, dark-gray aspect that belies its granitic composition. Secondary two-phase (liquid-vapor) fluid inclusions are very abundant in the quartz grains.

Three small plutons (Ztm) were mapped in sector H northeast of the body just described. Outcrops of the largest pluton are granite and quartz monzonite that have been locally sheared. The mapping of these bodies was based largely on the soils as mapped by Kaster (1960).

A sixth, a strongly altered very small pluton of porphyritic rhyolite or trachyte, was mapped southwest of the Lindley Farms Quartz Monzonite plutons near South Fork and adjoining a suspected fault strand southeast of Sheeprock (site 6136, fig. 2, sector N). In it, plagioclase phenocrysts as long as 0.2 in (5 mm) set in a fine-grained matrix are common. Secondary mineral grainlets resulting from the albitization of the plagioclase phenocrysts indicate that most were more sodic than calcic. A few of the largest grains, however, have calcic cores. Only a few phenocrysts appear to have been originally potassium feldspar, but the mineral is abundant as marginal replacements and overgrowths on plagioclase, especially adjacent to quartz grains. Biotite is interpreted to be a secondary alteration product.

Small Intrusive Bodies of Dacite, Dacite Porphyry, and Rhyodacite Porphyry, with Intense Quartz-Sericitic and Locally Potassic Alteration
Several small plutons of metamorphosed and hydrothermally altered dacite porphyry and rhyodacite porphyry were found at scattered locations in the area, all in or near strongly altered zones. All are very small and could have been overlooked easily; therefore, other such bodies may have gone undetected.

Low outcrops of a small pluton at site 6548 (Ztm, fig. 2, sector G) consist of gray rhyodacite porphyry containing hydrothermal biotite and potassium feldspar. Nearby float is siliceous dacite porphyry. The two rock types are interpreted to be phases of one hydrothermally altered intrusive body on the fringe of the Central Highland alteration zone.

An area underlain by strongly sheared and altered dacite porphyry (Ztm, sites 1599 and 1603, fig. 2, sector J) can be seen 1,150 to 1,300 ft (350–400 m) east of the Snow Camp pyrophyllite mine. Plagioclase phenocrysts, ranging in length up to 0.3 in (8 mm), are much more abundant than those of quartz. The quartz phenocrysts are generally rounded and embayed. The rock at site 1603 has undergone strong quartz-sericite alteration as have most other exposures of the unit. These rocks are pale buff, light gray, or almost white, and contain abundant flakes of muscovite that formed by alteration of the plagioclase, as well as grains of pyrite less than 0.04 in (1 mm). In contrast, hydrothermal biotite, indicating a more potassic alteration, is common at site 1599, where the rock is dark gray.

Metamorphosed Porphyritic Dacite Pluton
We mapped what we interpret to be a non-outcropping small intrusive body of porphyritic dacite (ZCambrian symbold at site 1882, fig. 2, sector J) on the Frank Lindley farm, north of the narrow outcrop band of Reedy Branch Tuff. The pluton was inferred from the presence of float cobbles and a local area of pale-buff, quartz-rich soil. The cobbles were collected from a field that has anomalous reflectance spectra in Landsat Thematic Mapper data, perhaps related to the higher quartz content. The dacite contains common plagioclase phenocrysts, 0.1 to 0.2 in (3–5 mm) long, set in a matrix of plagioclase and lesser quartz, 0.004 to 0.012 in (0.1–0.3 mm) long. No potassium feldspar was recognized. Epidote is locally abundant in altered plagioclase phenocrysts as well as in matrix grains. Quartz has also extensively replaced plagioclase, and chlorite has replaced all the original biotite. The epidote formed by albitization varies widely in the dacite cobbles, but its local abundance suggests the introduction of additional CaO beyond that supplied by plagioclase alteration. Pieces of highly altered epidosites are common within the same area and were considered to be part of the altered porphyritic dacite. They typically consist of about 70 percent epidote and 30 percent quartz. The epidosite contains as much as an estimated 5 percent ilmenite plus anatase in small grains. Probably associated epidote-rich veins have cut the nearby Reedy Branch Tuff indicating that they are younger than that unit.

Quartz Diorite-Hornfels-Volcanic Rock Injection Complexes
In parts of the area, injection complexes (designated Zic, fig. 2, fig. 3, fig. 4A, fig. 4B, and fig. 5) formed by the intrusion of quartz diorite into wall rocks in parallel layers, yielding large volumes of hornfels that have been cut by multiple apophyses of fine-grained plutonic rock and variously textured porphyry. Some of these layers are only a few feet thick. Injection complexes were mapped southeast of Eli Whitney (sectors D and H), and Wilkinson (1978) mapped the injection complex in the southeastern corner of the area (fig. 2, fig. 3, fig. 4A, and fig. 5, sectors L and P). Similar injection complexes may be present in even larger areas where outcrops are too few to allow a good interpretation of the mixed-rock float material. Excellent exposures of fine-grained quartz diorite in small bodies that have injected roof rock hornfels and in masses separated by hornfels screens can be seen along the banks of Cane Creek west of the North Carolina Highway 87 bridge (near site 6606G, fig. 2, sector H).

Metamorphosed Mafic-Rich Intrusive Rocks
Mafic-rich small plutons (designated Zg) were mapped at four sites along the Snow Camp fault and are probably related to that structure. Such rocks are present in the dump of the Foust Mine (sites 454 and 1665A, fig. 2, sector B); in small outcrops and scattered boulders about 1.4 mi (2.2 km) to the southwest (site 6716, fig. 2, sector B); in good outcrops along a tributary of Cane Creek, 3.1 mi (5.6 km) southwest of the Foust Mine (site 7017, fig. 2, sector F); and in a small streambed outcrop (site 7174, fig. 2, sector E) as well as in abundant coarse float blocks in fields and fencerows over an area 0.9 mi (1.3 km) north of Snow Camp crossroads. All four intrusive bodies are assumed to be small.

The mottled, dull-gray rock in the Foust Mine dump is, in part, quartz monzogabbro and also includes fine-grained quartz-feldspar porphyry, diorite, fine gabbro, and hornfels. The matrix texture of some of the porphyry may have been derived by devitrification of volcanic glass. The quartz monzogabbro ranges from porphyritic to medium grained equigranular, and some blocks contain parallel layering. There is much variation in texture and dark-mineral content, probably because of marginal chilling and the assimilation of mafic volcanic country rock. Quartz is common (10–15 volume percent) in angular myrmekitic and cuneiform grains that fill spaces between original plagioclase crystals of intermediate to calcic composition. The rock has been strongly altered, probably deuterically, and has been enriched in potassium feldspar, as well as common, newly formed biotite. Grain boundaries have been much modified. These compositional and mineralogic changes have been overprinted and supplemented by regional metamorphism, which yielded ragged patches of new hornblende as well as a few well-formed hornblende grains. The original abundant plagioclase is now mostly altered to muscovite, epidote, potassium feldspar, and probably some calcite.

There are no outcrops near the mine. Other mafic intrusive rocks are inferred to be present about 1,000 and 1,600 ft (300 and 500 m) east of the mine (fig. 2, sector B) on the basis of soil types (Kaster, 1960), but these locations were not checked in the field.

Greenish-gray hornblende quartz monzogabbro at site 6716 (fig. 2, sector B), southwest of the Foust Mine, has a medium-grained, hypidiomorphic granular texture. Much of the quartz, estimated to be about 10 percent of the rock, is in cuneiform intergrowths. The plagioclase has been albitized and contains resultant muscovite and epidote. The hornblende is of metamorphic origin, chlorite is common, and there are traces of biotite and apatite.

The streambed outcrop at site 7017 (fig. 2, sector F) is of a small stock or irregular dike of medium-grained, gray and greenish-gray gabbro. The intrusion has cut hornfels of gray crystal tuff and rhyodacitic volcanic rocks. The gabbro has grains as long as 0.4 in (1 cm) and consists of primary calcic plagioclase and hornblende and, as a result of metamorphism, white mica and much epidote. Some of the hornblende grains are clean and well crystallized, whereas others seem ragged and much altered by metamorphism. Quartz and potassium feldspar each constitute less than 5 percent of the rock; the potassium feldspar crystallized late or is a hydrothermal replacement of plagioclase. The abundance of epidote formed in the plagioclase grains during metamorphism indicates that the plagioclase was intermediate to calcic in original composition. The adjacent hornfels contains hornblende megacrysts as long as 0.4 in (1 cm).

Five small patches of gabbroic rock near the margin of mapping in sectors A and E (fig. 2) were inferred from dark-red Davidson soil (Kaster, 1960). Another probable patch of mafic-rich intrusive rock, inferred from the dark-red Davidson soil, was found in the southeastern corner of sector E surrounded by Reedy Branch Tuff. It may have been derived from an intrusion into the tuff, but we interpret it to be a bit of older mafic rock protruding through the overlying Reedy Branch Tuff. We have seen no outcrops in most areas of the distinctive dark-red soil.

Rocks Intruded After Regional Metamorphism
Granitoid rocks of the younger age group (±300 Ma, designated Cg) were distinguished from older rocks of similar composition because of their much lower metamorphic grade. These rocks contain both partly altered and thoroughly sericitized plagioclase. The partly altered grains have retained sharply bounded areas of pristine, complex oscillatory zoning that affirm the absence of pervasive greenschist-facies regional metamorphism (fig. 7). In contrast, all older intrusive rocks have undergone thorough greenschist-facies metamorphism, and all plagioclase crystals have been saussuritized to albite or oligoclase; relic oscillatory zonation was preserved only as sericite-rich and epidote-rich zones.

These rocks (Cg), assumed to be ±300 Ma (Fullagar and Butler, 1979), occur in low outcrops in sectors D, G, and H, north and west of Eli Whitney, as well as in small, scattered outcrops within the metamorphosed granitic rocks in the area designated Ztm, as at sites 6258, 6571, 6682, and 6707 (fig. 2, sectors D and H), and more commonly as patches of float at sites 6266B, 6268A, and 6578, within the areas where greisenlike rock is abundant (Cg, fig. 4B, sectors D and H). Deuteric alteration was strong at all localities examined. The younger granitoid rocks include low potassium granites and dacite porphyries.

The quartz-bearing monzonites are light-colored, in part equigranular and have a medium texture. Some of the smaller bodies are porphyritic and some are characterized by common 0.16 to 0.31 in (4–8 mm), rounded and partly embayed quartz phenocrysts. These rocks are quartz-rich, mostly low-potassium rocks and have a wide range (6.0–16.5 percent) of biotite, chlorite, epidote, and amphibole. In many of these rocks (table 6 and table 7), the mafic mineral was originally biotite, now mostly converted to chlorite and epidote by deuteric alteration, which in some cases penetrates along fractures and selected favorable compositional zones. In almost all of the thin sections studied, quartz grains were enlarged by the replacement of adjacent feldspar, mostly plagioclase. Up to 8 percent potassium-feldspar has formed in a few of these rocks, mostly along microfractures and against quartz grains. Some of the quartz grains have replaced adjacent feldspar grains, and also grown onto the exteriors of some of the plagioclase grains.

Dacite porphyry crops out in a small area at site 6282 (Cg), east of the Central Highland (fig. 2, sector G). The rock is characterized by abundant feldspar and deeply embayed quartz phenocrysts, commonly as much as 0.23 in (6 mm), rarely 0.39 in (10 mm), enclosed in a fine matrix. The plagioclase is intermediate to calcic in composition. Traces of potassium feldspar can be seen as replacements around microfractures. The propylitic alteration was not complete, and large parts of the feldspar phenocrysts completely retain their delicate oscillatory zonation. A group of large gossan boulders weathered from a coarse sulfide-rich stockwork is present nearby.

 
 

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