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USGS1982

Strata-bound sulfide deposits, wall-rock alteration, and associated tin-bearing minerals in the Carolina slate belt, South Carolina and Georgia

Massive sulfide deposits, gold deposits, and tin-bearing minerals occur in rocks of the Carolina slate belt in South Carolina and adjacent Georgia. The belt of greenschist metamorphic rocks in which the ore deposits occur is mostly Cambrian in age. It is divided into an upper metasedimentary unit and a lower unit of predominantly felsic metavolcanic and metavolcaniclastic rocks. The ore deposits occur in the lower unit chiefly near the contact between the two units. The rocks have been deformed into two maior sets of coaxial folds and at least two minor deformations. Coarse-grained unmetamorphosed Carboniferous, I-type granites, and abundant Triassic-Jurassic diabase dikes cut the slate belt and appear largely unrelated to the ore deposits. Remnants of Cretaceous and younger sedimentary rocks overlie the crystalline rocks near several of the larger mineralized areas.Massive sulfides, largely pyrite, and some polymetallic sulfides occur in mines opened for gold in the nineteenth century. The largest of these in South Carolina are the Haile, Brewer, and Dorn mines. These mines and the Little Mountain and Cedar Creek-Blythewood areas have many similar lithologic characteristics. These are: hydrothermally altered wall-rock consisting of quartz-sericite-kaolinite schist and quartz-sericite schist; abundant aluminous silicates such as kyanite, andalusite, pyrophyllite, and topaz; zones of iron-enriched rocks; and a suite of resistant heavy minerals that includes tin-bearing minerals. Heavy mineral concentrates from alluvium of small streams showed 20,000 ppm tin. Cassiterite and nigerite have been identified.At the Brewer mine, gold has been produced from altered felsic volcanic rocks that contain silicified breccia, massive topaz, abundant pyrite, minor enargite, and probably cassiterite. Massive pyrite and gold have been mined at the Haile mine. In the McCormick-Lincolnton area, a near-surface granitoid pluton is thought to be the source for volcaniclastic rocks that contain polymetallic massive sulfide deposits, gold, and associated deposits of kyanite, barite, and manganese. Tin was found there in heavy mineral concentrates, and rutile occurring with kyanite contains 1,000 ppm tin. At Little Mountain, cassiterite and hematite are present in rocks considered to be metamorphosed hot spring deposits. In the Cedar Creek-Blythewood area, nigerite is present, together with cassiterite, chrysoberyl, and seventeen other heavy minerals in concentrates panned from alluvium in streams draining an area of quartz-sericite-kaolinite schist.The deposition of massive sulfides, gold, and tin minerals is considered to be part of a continuum of volcanic activity that included alteration, deformation, and metamorphism. Altered rocks, which host the ore deposits, result from superimposed processes beginning with alteration syngenetic with the massive sulfides and ending with fracture-controlled alteration.

South Carolina↗
USGS1982

Evaluation of 0.46- to 2.36-mu m multispectral scanner images of the east Tintic mining district, Utah, for mapping hydrothermally altered rocks

Airborne multispectral scanner images recorded in the 0.46- to 2.36-mu m region for the East Tintic mining district, Utah, were evaluated to determine their usefulness for distinguishing six types of hydrothermally altered rocks from a wide range of sedimentary and igneous rock types. Limestone, dolomite, and argillaceous rocks were of particular interest because these have spectral reflectance features that might make them difficult to separate from some of the altered rocks when these images are used.The laboratory and field evaluation of a color ratio composite image, supported by in situ spectral reflectance measurements and an alteration map compiled from a published map, shows that silicified, argillized, and pyritized rocks can be mapped in detail utilizing an intense OH absorption band centered near 2.2 mu m. This absorption band is absent or weak in most of the unaltered rocks. However, the 2.2-mu m band is also weak in the reflectance spectra representing the other three types of altered rocks--hydrothermal dolomite, calcitic volcanic rocks, and chloritized rocks. Consequently, these altered rocks are not consistently distinguished from unaltered rocks in these images. Furthermore, very pure dolomite spectra have an intense 2.33- mu m CO (super -3) absorption band that is not distinguishable from the 2.2-mu m band in these images; therefore, the pure dolomite exposures were distinguished from the silicified, argillized, and pyritized rocks through field checking. The 2.33-mu m absorption feature is weak in spectra for less pure carbonate rocks. Separation of these altered rocks and local exposures of argillaceous rocks also posed problems because of their mineralogic and hence spectral reflectance similarities.These results are especially important for estimating the potential of the 1.6- and 2.2-mu m bands in the Landsat D thematic mapper for mapping altered rocks. Here, as we found in an earlier study in south-central Nevada, limonite-poor hydrothermally altered rocks that were not consistently distinguished in Landsat multispectral scanner images should be detectable using these two thematic mapper bands in combination with other bands at shorter wave-lengths. However, distinguishing among some hydrothermally altered and unaltered rocks will require other approaches, including analysis of high-spectral resolution measurements in the 2.0- and 2.5-mu m region and multispectral thermal infrared data.

Economic Geology↗
USGS1982

Biogenic and nonbiogenic ore-forming processes in the south Texas uranium district; evidence from the Panna Maria deposit

Geochemical and petrographic studies of core samples from the Panna Maria uranium deposit, a roll-type orebody in the Eocene Jackson Group in Karnes County, Texas, yield important information on the origin of the deposit. Organic carbon content averages about 0.42 weight percent in reduced rock and correlates postively with sulfur content. Pyrite is the dominant iron disulfide (FeS 2 ) mineral in most of the ore zone and throughout a surrounding zone of reduced barren ground, and it is commonly associated with organic debris. Marcasite is sparse except in ore adjacent to the altered tongue in one core and locally in mineralized lignite. Sulfur isotopic compositions (delta 34 S) of FeS 2 minerals range broadly from -1 to -34 per mil; the lightest delta 34 S values (less than -20 per mil) were measured in samples from mineralized lignite and from the nose of the ore roll. Petrographic and geochemical characteristics of the Panna Maria deposit contrast greatly with those of three other south Texas roll-type uranium deposits (the Benavides, Felder, and Lamprecht deposits), which are devoid of organic carbon and which contain more sulfide than does the Panna Maria. These three deposits are characterized by abundant isotopically light ore-stage marcasite and by isotopically heavy preore (in the Benavides) or postore (in the Felder and Lamprecht) pyrite. We have concluded previously that sulfide-bearing fault-leaked solutions from underlying hydrocarbon accumulations were important in the formation of the Benavides, Felder, and Lamprecht deposits. Although the Panna Maria deposit shows an apparent alignment along a fault zone, and although underlying formations in the Karnes County area contain sour gas (delta 34 S [asymp] + 14 per mil) and produce oil, the deposit lacks characteristics indicating that its formation or preservation involved extrinsically derived reductants such as fault-leaked aqueous sulfide. Mineralization of the Panna Maria, rather, appears to have been controlled by intrinsically derived reductants related either directly or indirectly to the presence of organic matter.

Texas↗
USGS1982

Uranium-lead isochron age and preliminary sulfur isotope systematics of the Felder uranium deposit, south Texas

Uranium-lead isotope systematics of roll-front ores in Miocene sandstone at the Felder and McLean uranium deposits (south Texas coastal plain) give a well-defined 207 Pb/ 204 Pb- 235 U/ 204 Pb isochron age of 5.07 + or - 0.15 m.y. The relatively slight degree of scatter of the points defining the isochron is probably due to initial Pb isotope inhomogeneity, and the resulting inferred persistence of closed system behavior for U and Pb is probably the result of the long-term presence of U- and Pb-immobilizing H 2 S. 206 Pb/ 238 U systematics are badly scattered owing to long-term migration of radioactive daughters of 238 U. Beta- and gamma-activity systematics of the ores consistently identify those with grossly anomalous Pb isotope systematics, however, and proved highly useful in identifying ores that have gained gross amounts of uranium daughters.FeS 2 minerals in the altered tongue of the host sandstone are characterized by abundant postore pyrite and heavy delta 34 S values, whereas FeS 2 minerals in mineralized and unaltered, barren rock are characterized by abundant ore-stage marcasite and by light delta 34 S values. The delta 34 S values of FeS 2 minerals in the altered tongue are similar to those defined for sour gas from the underlying Edwards Limestone of Cretaceous age, indicating the probable source for the sulfur of an inferred resulfidization event. The 5.07-m.y. isochron age probably reflects the end of roll-front development; we believe that the end was due to the resulfidization and does not preclude significant earlier periods of mineralization. Modern ground water in the area is also sulfide bearing (thus helping preserve the deposit), but with delta 34 S values distinct from those of sulfides in the resulfidized zone of the host rock.

Texas↗
USGS1982

Mineralogy and geochemistry of Fe-Ti oxide and apatite (nelsonite) deposits and evaluation of the liquid immiscibility hypothesis.

Modal mineralogy determined for 32 Fe-Ti oxide and apatite rocks from localities in Virginia, New York, Quebec, Norway, and Sweden largely supports the 2:1 oxide:apatite ratio suggested as a eutectic mixture by Philpotts (1967). A fairly consistent suite of accessory minerals, including biotite, clinoamphibole, spinel, zircon, and sulfides, is present. Some silicate accessory minerals tend to form glomeroporphyritic intergrowths in an equigranular matrix of nelsonite, suggesting that the accessories are xenocrysts that have been trapped in an oxide-apatite liquid. This liquid may represent an extreme case of partitioning of high charge-density cations into a low silica, phosphorus-enriched immiscible melt. Temperature and oxygen fugacity estimates for oxide pairs in nelsonites range from 600 degrees to 1,000 degrees C and 10 (super -20) to 10 (super -11) atm f (sub o 2 ) and bracket an equilibration path that is close to the FMQ buffer curve. Several stages of exsolution are evident in Fe-Ti oxides of some deposits. Nelsonites contain fluorapatite that is enriched in light lanthanides. The occurrence of nelsonite "dikes" in Roseland anorthosite and associated rocks of the Roseland-Piney River district of Virginia is explained by the infilling of fractures with nelsonite liquid that has settled out of an overlying ferrodiorite pluton.

Economic Geology↗
USGS1982

Comparison of platinum, palladium, and rhodium distributions in some layered intrusions with special reference to the late differentiates (upper zone) of the Bushveld Complex, South Africa

The Stillwater, Fiskenaesset, and Bushveld Complexes are all composed of layered ultramafic and mafic rocks in which the cumulus phases generally appear in a regular order, which have similar petrologic and chemical characteristics, which are all Precambrian in age, and which contain platinum-group elements that vary widely in their abundances and relative proportions. Two of the complexes, the Stillwater and Bushveld, contain platinum-group element enriched horizons not yet found in the Fiskenaesset. The ratio of Pt/(Pt + Pd) in rocks from the Stillwater tends to increase from the basal zone to the middle of the ultramafic zone, decreases to the beginning of the banded zone, and then increases again through the banded zone; many fluctuations, however, are superimposed upon this generalized trend. The trends of Pt/(Pt + Pd) ratios appear to show a positive correlation with the Mg/(Mg + Fe (super +2) ) ratio in olivine. In the Fiskenaesset Complex, the Pt/(Pt + Pd) ratio increases rapidly from 0.3 to 0.7 from the lower gabbro and ultramafic units through the leucogabbro units, then increases slowly through the anorthosite and upper gabbro unit. Superimposed on this generalized trend are increasing and decreasing patterns within each unit.Analyses of platinum, palladium, and rhodium for 69 rocks from the main and upper zones of the eastern Bushveld from the Roossenekal area show generally low concentrations of less than 1 ppb Pt; 0.1 ppb Pd; and 0.1 ppb Rh, but larger amounts occur in some magnetite layers and sulfide-bearing zones. Within the main zone up to the pyroxenite marker, from the pyroxenite marker to below lower magnetite layer 2, and in the upper zone, the Pt/(Pt + Pd) ratio decreases with stratigraphic position, although with minor fluctuations in these intervals. The concentration levels of the platinum-group metals appear to be directly related to the amount of sulfide material present and the proportion of intercumulus material. Variations in Pt/(Pt + Pd) ratios in the Bushveld and Fiskenaesset Complexes are presumably related to changes in major mineral compositions as they are in the Stillwater Complex.

Economic Geology↗
USGS1982

Palladium, platinum, rhodium, ruthenium, and iridium in chromitites from the Massif du Sud and Tiebaghi Massif, New Caledonia

Massive and disseminated podiform chromitite from 43 mines and other occurrences in the Massif du Sud and Tiebaghi Massif, both ophiolites, in New Caledonia contain up to 9 ppb Pd, up to 45 ppb Pt, up to 31 ppb Rh, up to 410 ppb Ir, and up to 1,130 ppb Ru. The chromitites are strongly depleted in Pt and Pd and mildly depleted in Ir and Ru with respect to an average chondrite composition, as are podiform chromitites from Oman, Turkey, Greece, and northern California. Stratiform chromitites from the Stillwater Complex, Montana, and the Bushveld Complex, South Africa, show inverse patterns of enrichment and depletion. These patterns suggest that either chromitite from ophiolites represents mantle rocks depleted in Pt and Pd or that the magmas that formed the ophiolites were depleted in Pt and Pd, or that the process of forming the chromitite involved the concentration of Os and Ir from a magma. The relatively high contents of Ir and Ru in the New Caledonian chromitite suggest the potential for the occurrence of placers that contain concentrations of Os-Ir alloys.

Economic Geology↗
USGS1983

Iron disulfide minerals and the genesis of roll-type uranium deposits

FeS 2 minerals in host rocks for deposits that contain fossil vegetal (organic) matter differ in abundance, distribution, texture, and sulfur isotopic ratios from FeS 2 minerals in host rocks for deposits that do not contain fossil vegetal matter. In three South Texas deposits lacking such organic matter, preore FeS 2 is dominantly euhedral pyrite which formed in response to solutions emanating from these faults. Ore-stage FeS 2 is dominantly marcasite that occurs as overgrowths on preore pyrite. In three deposits (two in Wyoming and one in Texas) that contain organic matter, preore FeS 2 is also dominantly pyrite, but it occurs commonly as framboids and as replacements of plant fragments and is formed by bacterial sulfate reduction during early diagenesis and may be isotopically distinct from pyrite formed from fault-related fluids. Ore-stage FeS 2 in these deposits is primarily pyrite. Bacterial sulfate reduction provided sulfide for ore-stage pyrite in deposits which contain organic matter. Abiologic sulfur transformations tend to produce ore-stage marcasite in deposits that do not contain organic matter.

Economic Geology↗
USGS1983

Remote detection of metal anomalies on Pilot Mountain, Randolph County, North Carolina

Pilot Mountain, a hydrothermally altered monadnock within the Carolina slate belt, contains areas of anomalously high amounts of Cu, Mo, and Sn in the soils. Leaves of canopy trees in the mineralized zone also contain more copper than trees in a nearby control area. Spectral data were processed using a wave-form analysis technique to minimize background noise caused by canopy variations and slope effects. Areas of spectral changes in the chlorophyll absorption region seem to correlate well with areas containing anomalous metals.

Economic Geology↗
USGS1983

Mineral discrimination using a portable ratio-determining radiometer

The instrument has ten bands in the visible and near-infrared portion of the spectrum (0.5-2.4 mu m). Measurements and statistical analyses were performed on 66 samples, which were characterized by microscopic and X-ray diffraction analyses. On the ability to discriminate between 16 mineralogical groups, 91 percent classification accuracy was achieved.

Economic Geology↗
USGS1983

Sequence of mineral assemblages in differentiated granitic pegmatites

Revised sequence is (1) plagioclase, quartz, muscovite, (2) plagioclase, quartz, (3) quartz, perthite, plagioclase, (4) perthite, quartz, (5) quartz, plagioclase, perthite, spodumene (or petalite), amblygonite, (6) quartz, spodumene (or petalite), (7) quartz, microcline or perthite, (8) quartz, and (9) lepidolite or lithian mica, plagioclase, quartz, microcline.

Economic Geology↗