Showing papers in "Economic Geology in 1967"

Origin of certain iron-titanium oxide and apatite rocks

Abstract: Rocks consisting essentially of iron-titanium oxides and apatite occur as small intrusive-like bodies associated with anorthosites and some alkaline igneous complexes. They have a consistent composition of two-thirds by volume oxides and one-third apatite, and invariably have dikes rich in ferromagnesian minerals and apatite associated with them.Reconnaissance experiments in the system magnetite-fluorapatite indicate a eutectic at a composition of approximately two-thirds by volume of magnetite and one-third apatite, which provides an explanation for the common occurrence of rocks with this composition. Experiments indicate that eutectic mixtures of magnetite and apatite form immiscible liquids with silicate melts having the composition of the dioritic dike rocks commonly associated with oxide-apatite deposits. Mixtures of magnetite, diorite and apatite, containing apatite in excess of thirty percent form three immiscible liquids on melting: an apatite-rich one, a magnetite-apatite melt and a silicate melt.Analysis of coexisting magnetites and ilmenites from naturally occurring oxide-apatite rocks indicate temperatures of formation in the range of 850 to 1,000 degrees C.Oxide-apatite rocks are concluded to have formed as immiscible liquids, which separated from magmas that underwent strong differentiation. It is postulated that high sodium contents in the silicate magmas play an important role in forming these immiscible liquids.

The central portion of the Fe-Ni-S system and its bearing on pentlandite exsolution in iron-nickel sulfide ores

Abstract: A detailed study of the central portion of the Fe-Ni-S system between 600 degrees and 250 degrees C has been conducted by means of silica-tube techniques. Particular emphasis has been placed on defining the limits of the mono-sulfide solid solution (Mss) that extends across the system from Fe (sub 1-x) S to Ni (sub 1-x) S; the sulfur-poor limit is significant with respect to the formation of pentlandite in iron-nickel sulfide ores.At 600 degrees C the Mss appears on the phase diagram as a nearly straight-sided band, thinner at the nickel-rich than at the iron-rich end. The band becomes narrower as the temperature falls: the sulfur-rich limit remains nearly straight but moves towards the sulfur-poor limit; the sulfur-poor limit moves rapidly towards the sulfur-rich limit in the center but not at the ends of the solid solution, so that at lower temperatures the Mss is concave outwards on the sulfur-poor side. The temperature at which the Mss becomes incomplete and below which the pyrite-pentlandite assemblage is stabilized is not known precisely, although it is below 300 degrees C.Magnetite and chalcopyrite, which often occur with pyrrhotite and pentlandite in ores, do not change significantly the temperature at which pentlandite exsolves from Mss of any given composition. When projected from chalcopyrite onto the Fe-Ni-S face of the Cu-Fe-Ni-S system, compositions of most pyrrhotite-pentlandite-chalcopyrite ores fall within the boundaries of the Mss at 500 degrees C. If these deposits were formed as equilibrium assemblages at or above this temperature, the pentlandite must have exsolved from Mss.The shape of the pentlandite-pyrrhotite solvus is such that pentlandite exsolution is more dependent on the metal: sulfur ratio than the nickel content of the Mss. Two copper-poor deposits serve as examples: pentlandite exsolution would have started at the Alexo deposit ( approximately 6.5 percent Ni in sulfides, 38-38.2 percent S in pyrrhotite) at 350 degrees -400 degrees C, but in the massive ore at the Marbridge deposit ( approximately 6.5 percent Ni in sulfides, 39.2-39.8 percent S in pyrrhotite) it would not have begun above 300 degrees C. Since most if not all of the nickel in iron-nickel sulfide deposits is in solid solution as nickeliferous pyrrhotite at temperatures as low as 300 degrees C, the pyrrhotite-pyrite solvus is invalidated as a geothermometer in these ores.

Sulfides associated with the Salton Sea geothermal brine

Abstract: Concentrated saline brine tapped by a deep well drilled for geothermal power near the Salton Sea, California, deposited metal-rich siliceous scale at the rate of 2 to 3 tons per month. The iron-rich opaline scale contains an average of 20% Cu and up to 6% Ag present in bornite, digenite, chalcopyrite, a new dense polymorph of chalcocite, stromeyerite, and native silver.The brines are in equilibrium with an assemblage of sulfide minerals in the reservoir rocks. Sphalerite containing 16.6 mol % FeS is in equilibrium with the brine in which the activity of sulfur (a (sub s 2 ) ) is estimated at 10 (super -10.2) at 325 degrees C. This is in good agreement with the projected high-temperature data of Barton and Toulmin (2) for the system Fe-Zn-S. Other sulfides identified in the reservoir rocks are pyrite, pyrrhotite, chalcopyrite, and galena.The heavy metals in solution are apparently derived from the sediments of the brine reservoir, being released from the silicate minerals in which they occur in trace amounts, as metamorphism of the sediment proceeds. Although the total of the heavy metals in solution greatly exceeds the sulfur, on a molal basis, the brine is saturated with respect to sulfide components and addition of more sulfide ion would only cause precipitation of sulfides.

Tin belts on the continents around the Atlantic Ocean

Abstract: Tin is inhomogeneously distributed on the continents, in relatively narrow, continent-sized belts. On a reconstruction of the continents around the Atlantic Ocean before continental drift, the belts extend unbroken from one continent to another. As the ages of the tin mineralizations vary within the belts, an argument can be derived that the source of the tin and its associated elements must be in the crust. It is further speculated that concentrations of workable tin deposits occur in the intersection of orogenic belts with zones of primitive enrichment of tin. The ultimate cause of such primary geochemical culminations may well lie back in the early history of the earth.

The Fe-Pb-S system

Abstract: The phase relations in the ternary system Fe-Pb-S were determined by quenching and differential thermal analysis experiments in rigid silica-glass tubes. The solubilities of FeS in galena and in liquid Pb and the extent of solid solution of PbS in pyrite and pyrrhotite at 700 degrees C are too small to be determined by the methods employed and are probably less than 0.1 wt. percent. Less than 1 wt. percent S is soluble in liquid Pb at 700 degrees C.Ternary compounds do not occur in this system. An invariant point involving galena, pyrite, sulfide liquid, liquid sulfur, and vapor occurs at 716 degrees C. The ternary liquid at this temperature contains about 60 wt. percent Pb, 13 wt. percent Fe, and 27 wt. percent S, and is thus situated slightly on the sulfur side of the PbS-FeS 2 join. At 719 degrees C tie lines between galena and pyrite are replaced by sulfide liquid-pyrrhotite tie lines in an invariant reaction involving vapor. The sulfide liquid field expands rapidly and intersects the PbS-FeS 2 join at 723 degrees C. At 848 degrees C the ternary liquid field intersects the FeS-PbS join. Diagrams are presented to illustrate changes in phase relations from 700 degrees to 1,115 degrees C.The experimental results demonstrate that mixtures of galena (about 70 wt. percent) and pyrite (about 30 wt. percent) can crystallize directly from a liquid at 716 degrees C. Additional components may lower this temperature so that it appears possible that pyrite-galena ores may melt at temperatures below 700 degrees C when exposed to thermal metamorphism.

Ore solution chemistry; [Part] 2, Solubility of HgS in sulfide solutions

Abstract: The solubility of cinnabar (red HgS) has been measured in aqueous sulfide solutions under varying conditions: up to 2.5 molal HS (super -) , pH 3.5 to 7.8; 25 to 200 degrees C; and 4 to 140 atm. total pressure. Concentrations were determined by using neutron activation analysis. From these measurements plus those of Schwarzenbach and Widmer for metacinnabar (black HgS), the following constants were derived for ionic strengths (mu ) and temperature as indicated:HgS + 2H 2 S(aq) --> HgS(H 2 S) 2 (aq)K = 10 (super -4.25) (black HgS, mu = 1.0, 20 degrees C)K = 10 (super -4.31) (red HgS, mu = 0, 20 degrees C)K = 10 (super -3.0) (red HgS, mu = 0, 100 degrees C)delta H = + 8.3 Kcal. (red HgS, 25-100 degrees Based on an ionization constant for NaHS of 5.0 at 20 degrees C, we also find:HgS + HS (super -) + H 2 S(aq) --> Hg(HS) 3 (super -) K = 10 (super -3.50) (black HgS, mu = 1,0, 20 degrees C)K = 10 (super -3.59) (red HgS, mu = 1.0, 20 degrees C)HgS + 2HS (super -) --> HgS(HS) 2 (super =) K = 10 (super -3.51) (black HgS, mu = 1.0, 20 degrees C)K = 10 (super -3.60) (red HgS, mu = 1.0, 20 degrees C)and from the constant of Schwarzenbach and Widmer,HgS + S (super =) --> HgS 2 (super =) K = 10 (super +0.57) (black HgS, mu = 1.0, 20 degrees C),we find K = 10 (super +0.48) (red HgS, mu = 1.0, 20 degrees C)The pH boundaries between the regions of predominance of these complexes in equilibrium with metacinnabar at 20 degrees C, mu = 1.0, are:HgS(H 2 S) 2 , 6.13, Hg(HS) 3 (super -) , 6.89, HgS(HS) 2 (super =) , approximately 8.5, HgS 2 (super =) where only the last pH is dependent on sulfide concentration.The maximum measured solubilities were 950 mg./l at 25 degrees C and 4,300 mg./l at 200 degrees C. In general, the solubility between 25 and 200 degrees C in an isochemical, isochoric system increases 4-7 times in bisulfide solutions, and the increase in H 2 S solutions is about twice as great as in HS (super -) solutions.Transport of HgS or Hg to form mercury deposits, by bisulfide complexes or, in strongly alkaline solutions, by HgS 2 (super =) , is possible under geologically reasonable pressures at 100 degrees to 200 degrees C in neutral to weakly alkaline sulfide solutions. On the basis of present evidence, the relative geological importance of the bisulfide and sulfide complexes is uncertain.

Geochemical implications of chrysocolla-bearing alluvial gravels

Abstract: At Chuquicamata, Chile, the location of chrysocolla-bearing gravels with respect to the parent ore body indicates lateral migration of some four kilometers. Experiments suggest that through mechanisms of oxidation and hydrolysis, surface waters draining copper ore bodies readily acquire the copper and silica needed to form chrysocolla. The chrysocolla at Chuquicamata may have formed by precipitation of copper and silica on increase in pH of transporting solutions. Alternatively, transported copper may have combined with silica derived from alteration of silicates in the gravel.

Subsolidus studies in the ZnS-FeS-FeS 2 system

Abstract: The sluggish nature, measured in periods of up to twelve months, of solid state reactions in the Zn-Fe-S system hampered the low-temperature phase studies of earlier workers. New techniques involving the preparation of sulfide starting material by precipitation and fluxing of these sulfides with salts has made it possible to grow (Zn,Fe)S crystals in equilibrium, established in one direction only, with pyrrhotite and pyrite in less than a week at 395 degrees C. As a result it was possible to investigate the sphalerite solvus in the ZnS-FeS-FeS 2 system from 714 degrees C to a new low temperature of 303 degrees C.After reaching an FeS composition maximum at 580 degrees C, the sphalerite solvus does not reverse its slope but instead drops vertically at a constant composition of 20.8 + or - 0.5 mole percent FeS to the hexagonal-monoclinic pyrrhotite inversion temperature. At 303 degrees C hexagonal pyrrhotite, pyrite and sphalerite constitute the equilibrium assemblage on the sphalerite solvus. Monoclinic pyrrhotite was present in a run at 210 degrees C. Therefore, over the temperature range of geological interest, below 600 degrees C, the assemblage pyrite-hexagonal pyrrhotite-sphalerite is unsuitable for geothermometry.

Trace-element content of magnetites and hematites, southeast Missouri metallogenetic province, U.S.A

Abstract: Six major and numerous smaller Precambrian iron deposits of magmatic origin occur in the volcanic rocks of the Southeast Missouri Iron Metallogenic Province in an area approximately 70 miles long and 40 miles wide.The Precambrian igneous complex is composed of mainly high silica, with some intermediate, volcanic rocks intruded by granitic rocks and some diorites and gabbros. Biotite schists occur locally. Some of the volcanic and intrusive rocks are characterized by an unusual abundance of alkali metals. Local enrichments in fluorite, calcite, and iron oxides are present.The shapes of the iron deposits range from massive tabular to irregular. Emplacement characteristics vary from cross-cutting masses and breccia cementations to intensive replacements of moderately to steeply dipping volcanic host rocks.The major iron minerals are magnetite and hematite; smaller amounts of martitic hematite, chalcopyrite, and bornite occur. Even less abundant are spinels of the magnetite series: Magnesio-ferrite, Mg-jacobsite, Zn-Fe spinel. Apatite, garnet, pyroxenes, actinolite, tremolite, crocidolite, epidote, biotite, quartz, pyrite, sericite, chlorite, fluorite, calcite, dolomite and barite are more or less common gangue minerals. Relatively rare geikielite, Ca and Mg hydrosilicates and Mg-chamosite are probably present. Alteration minerals of the iron oxides and silicates also occur.Eighty-two purified magnetite, hematite and bulk ore samples were analyzed for Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Mo, Mg and Al by emission spectrography in the ranges of 1-10,000 ppm. Other details were obtained from polished sections, thin sections and X-ray analyses of the minerals.Standard deviation and variance tests showed several statistically significant differences in the trace element population between individual iron deposits. These may partly be explained by mineralogical, geological and genetic differences. A distinct pattern of higher trace element content near the center of the iron deposits decreasing toward the margins occurs in two well sampled bodies. High and low electronegativity pairs such as V/Zn, V/Cu, and Mg/Co show the same pattern and strongly suggest a temperature control of trace element concentration. The range of trace element content, the Ti/V ratio and the V content of purified magnetites resemble those of magmatic injection iron deposits of the world. The geochemical data and the geologic character of the ore bodies suggest that possible ore magma injection, contact metasomatism and hydrothermal replacement were the most important ore forming processes in this Metallogenic Province.

Geochronology of the Keweenawan rocks, White Pine, Michigan

Abstract: Age determinations by the total-rock Rb-Sr method of several suites of felsite from the White Pine area of Michigan indicate dates ranging from 978 + or - 40 m.y. to 1,100 + or - 25 m.y. A single specimen of felsite from Government Peak of the Porcupine Mountains was dated at 1,042 + or - 32 m.y. Assuming that the Porcupine Mountains are an anticline, this date sets an upper limit to the time of deposition of the overlying Nonesuch Shale. Another upper limit is provided by dates of 1,107 and 1,180 m.y. for two felsite pebbles from the lower sandstone unit of the Parting Shale member exposed in the old Nonesuch Mine.Nine samples of mineralized and unmineralized rock from the basal section of the Nonesuch Shale exposed in the mine workings of the White Pine Mine were analyzed. These samples form a good co-linear array in coordinates of Rb 87 /Sr 86 and Sr 87 /Sr 86 . The apparent age, calculated from the slope of the isochron, is 1,075 + or - 50 m.y. The initial Sr 87 /Sr 86 ratio is 0.7080 + or - 0.0008. The apparent age of the Nonesuch Shale is interpreted to be slightly greater than the time of deposition because of the probable incorporation of inherited Sr 87 into the sediment at the time of deposition.The isotope composition of three lead samples extracted from chalcocite of the ore body at White Pine was found to be anomalous. This suggests that the lead in the ore was mixed with radiogenic lead and favors an epigenetic or late diagenetic rather than a syngenetic origin for the copper sulfide in the basal portion of the Nonesuch Shale.

Grid-drilling exploration and its application to the search for petroleum

Abstract: A probabilistic model describing the grid-drilling method of exploration has been developed and tested with specific application to the exploration for petroleum in the United States. It may also have application to non petroleum resources.This method of exploration is shown to be economically feasible when applied to fifteen test areas in the United States and to the United States as a whole.The objective of the analysis was to maximize the net value of petroleum located. The optimum grid spacings for each of the test areas and for the United States as a whole were determined subject to this criterion.

A new copper-iron sulfide

Abstract: The mineral, from Norilsk, Siberia, has a composition close to Cu 18 (Fe,Ni) 18 S 32 , and a large cubic cell with a = 10.64 8 Aaa, possible space group I43m. The strongest x-ray powder diffraction lines are, in Aaa, 3.04, 2.656, 1.879, 1.598, 1.210, 1.079, and 1.0193. On heating, the mineral breaks down to 'tetragonal' cubanite and minor bornite; the original mineral is reformed on slow cooling. It is clearly a discrete mineral entity, not to be confused with the cubic high-temperature polymorph of chalcopyrite.

A study of temperature conditions at igneous contacts with certain Permian coals of India

Abstract: Contact zones with mica peridotite intrusions have been studied in ten Permian coal seams of the lower Gondwana coal fields, Bihar. Carbon and hydrogen contents and reflectance show a sharp change near the contact. Comparison of the reflectance measurements of natural cokes with those of laboratory carbonized coals suggests that the pressure effect of the intrusion was negligible and that the maximum temperature of metamorphism was about 600 degrees C.

The relationship between paleotopography and sulfur distribution in some coals of western Pennsylvania

Abstract: The effects of paleotopography on the distribution of pyritic sulfur were determined by comparing coals deposited in paleotopographic lows with superjacent coals deposited on relatively flat surfaces. The latter coals, used as experimental controls, exhibited little lateral variation and consisted mainly of bright clarain with low pyritic sulfur (< 2%). The coals deposited in topographic lows showed complex lateral and vertical changes in coal petrography which were paralleled by variations in the mineral composition of the coal ash and the percentage of pyritic sulfur. Durains and bone coals located in paleotopographic depressions, and associated with clay partings, contain relatively large amounts of detrital illite and quartz and pyritic sulfur (2-5%). Bright clarains have low sulfur contents (< 2%) and contain relatively little detrital material, the principle clay mineral being authigenic kaolinite. The clay partings are composed mainly of illite and quartz and have sulfur values less than one percent.The abundance of pyritic sulfur in coals with relatively abundant detrital components, is interpreted to mean that the availability of ferrous iron is a critical factor in controlling the distribution of pyritic sulfur. Ferric iron, insoluble in alkaline solutions, is adsorbed on clays and transported into the peat swamp by currents which follow topographic lows. Below the sediment-water interface, ferric iron is reduced to ferrous which then migrates into adjacent, reduced parts of the peat, there combining with sulfide sulfur to form pyrite.

Manganese nodules; nature and significance of internal structure

Abstract: Polished sections of manganese nodules from the western Pacific Ocean reveal variations in the composition and texture of layers, inclusions of foreign material along certain horizons, and unconformable relationships between groups of layers. These features 'have a direct bearing on problems of nodule formation such as rate, mechanism, and environment of growth, and mineralogy.'

Copper mineralization in the upper part of the Copper Harbor Conglomerate at White Pine, Michigan

Abstract: Native copper is irregularly disseminated in cross-stratified sandstones in the upper 85 feet of the Copper Harbor Conglomerate at White Pine, Michigan. The topmost 100 feet of this formation can be divided into an upper chloritic facies, 1 to 20 feet thick, and a lower hematitic facies, 80 to 100 feet thick. Most of the native copper occurs as lenses in the chloritic facies in the upper 7 feet of the formation, but small amounts are found in chloritic layers within sandstones of the hematitic facies. The native copper occurs mostly as rims around interstitial carbonaceous material, and the carbonaceous material controls the distribution of native copper. Both carbonaceous material and native copper are typically found along cross-laminae, but there are significant departures from exact correspondence between mineralization and cross-stratification. These departures reflect the irregular distribution of carbonaceous material along cross-laminae. This distribution is in turn related to variations in porosity, probably due to variations in sorting.The native copper is restricted to chloritic sandstone, and its presence must be related to conditions that existed in the chloritic layers. The chlorite is diagenetic and formed in a reducing environment. The reducing environment may have been created as water, squeezed out of the overlying Nonesuch Shale during compaction, circulated through the sandstone.Native copper is later than carbonaceous material but did not replace carbonaceous material. Copper therefore either replaced some other material or filled open space. There is no evidence of replacement of any other material. It seems most likely that copper ions were adsorbed onto the surface of carbonaceous material before the interstices were filled with other minerals during diagenesis. The native copper is therefore diagenetic.

Experimental studies of anthracite coals at high pressures and temperatures

Abstract: Three samples of anthracite, Middle Mammoth, Lykens Valley, and St. Nicholas have been subjected to temperatures in the range of 400-900 degrees C and pressures of 2,000 to 20,000 psi in hydrothermal pressure vessels. Selected runs were taken to 30 Kb in an opposed anvil device.The altered materials were studied primarily to determine differences in morphology of the grains, their reflectance values and degree of anisotropy. X-ray diffraction analyses were run to determine the mineral assemblages present before and after treatment.The reflectance values of the altered materials increase with temperature while pressure alone appears to have little effect. When temperature and pressure are held constant, reflectance increases with increasing time of the experimental runs. The rate of increase of reflectance is greatest in the temperature range where evolution of volatile matter is also the greatest. As might be expected the highest "rank" coal showed the least change in reflectance under the conditions applied.It was possible to fuse the anthracite particles with temperatures and pressures as low as 400 degrees C and 2,000 psi. Increased retention time at constant temperature and pressure causes an increase in the degree to which the grains are fused. The extreme temperature and pressure conditions to which the anthracites were subjected have caused them to behave in a fashion similar to bituminous coals at much lower pressures and somewhat lower temperatures.Observations made during this investigation correlate with existing theories on coal metamorphosis, pyrolysis and structure, extended from lower rank coals.

Aurorite, argentian todorokite, and hydrous silver-bearing lead manganese oxide

Abstract: During a study of hypogene manganese minerals, three silver-bearing manganese oxides were identified in "black calcite" associated with silver ores at the Aurora mine (Treasure Hill), Hamilton, Nevada. Specifically these are: (1) argentian chalcophanite (aurorite) (Ag 2 Ba,Ca,Pb,-K 2 ,Cu,Mn (super +2) )Mn 3 (super +4) O 7 . 3H 2 O; (2) argentian todorokite (Ag 2 ,K 2 ,Ca,Ba,-Na 2 ,Cu,Pb)Mn 4 (super +4) O 9 . 5H 2 O; and (3) hydrous silver-bearing lead manganese oxide (Pb,Cu,Ba,Sb,Ag 2 ,Ca,K 2 )Mn 5 (super +4) O 11 . 5H 2 O. Based on the large concentration of silver in argentian chalcophanite (7.50% Ag 2 O) and the low zinc content (0.25% ZnO) this mineral is recognized as a new species and named "aurorite."Physical and optical properties, and chemical analyses of the three minerals are presented together with X-ray powder diffraction data for aurorite and argentian todorokite. All chemical analyses were done with the electron microprobe analyzer. Due to extremely small particle size and textural relations, no X-ray powder data were obtained for the hydrous silver-bearing lead manganese oxide. Associated minerals include cryptomelane, pyrolusite, birnessite(?), cerargyrite, native silver, quartz, and manganoan calcite. Although the general uniform dark color of the "black calcite" and apparent uniform distribution of the inclusions suggests contemporaneous formation, examination of polished sections at high magnifications shows a definite paragenetic sequence. Manganoan calcite is replaced by fine-grained intergrowths of cryptomelane, pyrolusite, birnessite(?), and cerargyrite. Native silver formed early and the silver-bearing manganese oxides formed late in the sequence; aurorite fills microfractures that cut through all other minerals.These three silver-bearing manganese oxides were recognized only at the Aurora mine, Hamilton, Nevada, although black calcite veins were studied from numerous areas in the western United States. The geologic features of silver-bearing black calcites are described by Hewett and Radtke in the preceding paper.

Mineralogy of manganese deposits, Olympic Peninsula, Washington

Abstract: The mineralogy of specimens from seven manganese deposits in the Olympic Peninsula has been studied by microscope and x-ray diffraction methods. Primary minerals are chiefly silicates and oxides. Among the silicates bementite is predominant, and minor minerals include hibschite, rhodonite, johannsenite, tephroite, inesite, grossularite, and alleghanyite. Hausmannite is the chief oxide mineral, but in some deposits braunite and jacobsite are common. Crednerite is present in trace amounts, as is hematite. Carbonates, including rhodochrosite, manganoan calcite, and calcite, are widespread but are nowhere abundant. Alabandite, barite, quartz, and native copper, with associated cuprite, are rare.Interpretation of microscopic textural relationships leads to the recognition of three general periods in the primary paragenetic sequence. Pre-bementite minerals include hibschite, rhodonite, tephroite, johannsenite, and alleghanyite. Minerals more or less contemporaneous with bementite are braunite, hausmannite, jacobsite, crednerite, alabandite, barite, copper, manganiferous carbonate, and probably grossularite. Post-bementite minerals include manganiferous carbonate and calcite, inesite, and quartz.Supergene minerals are widespread but not abundant. They generally form thin veinlets in the primary material and consist chiefly of hydrous calcium-manganese oxides (rancieite, todorokite, and birnessite). Nsutite is the only manganese dioxide mineral recognized. Neotocite accompanies most of the supergene oxides.The primary deposits show microscopic features consistent with deposition in a sedimentary environment. These include micro-conglomerates, tuffaceous breccias, probable fossils, crystalline micro-pellets, and rare stratification. Some hausmannite specimens show nodular colloform structure that is suggestive of accretionary growth similar to that of modern marine manganese nodules.It is concluded that the mineralogy is consistent with a hypothesis of marine origin similar to that proposed by Park (38). Evidence is lacking for appreciable post-diagenetic change or for later introduction of the manganese and silica. The ultimate source of these constituents, however, is uncertain. Some connection with submarine vulcanism, however, seems probable.

Gem corundum pegmatites in NE Tanganyika

Abstract: Mafic pegmatites cut serpentinite in Precambrian gneisses Three varieties contain (a) vermiculite only; (b) vermiculite and corundum; or (c) calcic plagioclase, vermiculite, and corundum (plumasitic type) Chemical compositions suggest a trend from anorthosite through plumasite to vermiculite-corundum pegmatite The absence of primary olivine suggests emplacement temperatures below 500 degrees C Gems are rubies and sapphires

Sulfur isotopes and the origin of the Quemont ore body

Abstract: Physical age dates suggest that the sulfides in the Noranda area were deposited 3,250 my ago, the granites were intruded 2,450 my ago and diabase dikes between 1,200 to 1,700 my ago Five analyses of pre-ore extrusives and seven analyses of post-ore intrusives show that the rocks are near the Qz-Ab join in the granite system, and that the extrusive phases contain more normative quartz than the later intrusivesThe delta S 34 sulfur isotope data are presented for 21 pyrite, 9 pyrrhotite, 7 sphalerite, and 6 chalcopyrite samples Two isotopic effects are noted, one related to the paragenetic sequence, the other a depth effect which is attributed to the melting of the ores by post-ore granite

The geology of the Thanksgiving Mine, Baguio District, Mountain Province, Philippines

Abstract: Small pyrometasomatic ore bodies contain massive sulfide, mainly sphaleirte and pyrite, with minor chalcopyrite, galena, arsenopyrite, magnetite, hematite, chlorite, calcite, garnet, and sphalerite Gold, silver, zinc, copper, lead, sulfur, and cadmium are recovered. The tellurides altaite, hessite, petzite, and sylvanite are scattered through the sulfides and gangue minerals. Free gold occurs in pyrite and the telluride minerals. The ore occurs as veins and pods, mainly along contacts between diorite porphyry dikes and limestone; a few veins are entirely enclosed within the limestone. The diorite porphyry is regarded as the source of the ore.

Sulfur isotope ratios in co-existing galena and sphalerite from Broken Hill, New South Wales

Abstract: In 15 out of 16 sphalerite-galena pairs, there has been a relative concentration of the heavier sulfur isotope in the sphalerite. Comparison of results from many areas suggests that fractionation decreases with increasing metamorphic grade. Applications to geologic thermometry may be significant.

Regional environment of the Jacobina auriferous conglomerate, Brazil

Abstract: Gold occurs in conglomerate beds and in veins in a quartzite-conglomerate unit of the Jacobina series of Precambrian age in northeastern Brazil. The series is 8,000 m thick and includes pelitic schist and quartzite. The pelitic schist and the mafic and ultramafic rocks that intrude the series contain mineral assemblages corresponding to the amphibolite facies of metamorphism. Evidence of a placer origin for the gold may have been obscured by redistribution of the metal during metamorphism.

Late Cenozoic drainage reversal, east-central Idaho, and its relation to possible undiscovered placer deposits

Abstract: The Lemhi River, a northwest-trending river in east-central Idaho, apparently flowed to the southeast until late Pliocene or early Pleistocene time, and was then reversed by the rise of the Gilmore Summit. The river and its parallel--and probably also reversed--sister streams the Pahsimeroi River and Warm Spring Creek, are now tributaries to the Salmon River, but the Salmon could not have existed in anything resembling its present form until after the reversal. The Big Hole River, in adjacent southwestern Montana, may also have drained to south at one time, which suggests that fossil placer deposits of gold might be found in the Big Hole drainage area by analysis of late Cenozoic tilting and faulting and reconstruction of late Pliocene and early Pleistocene drainage systems.

The porphyry copper deposit exposed in the Liberty open-pit mine near Ely, Nevada; Part 2, The Formation of hydrothermal alteration zones

Abstract: In the southwest part of the Liberty pit a large porphyry body exhibits a zonal alteration pattern related to fissures and pyrite-bearing veins. The outermost zone contains unaltered K-feldspar, kaolinite after plagioclase, and black biotite after hornblende. An intermediate zone contains reconstituted K-feldspar, phlogopitic biotite, and muscovite or muscovite plus K-feldspar replacing only plagioclase. In the innermost zone the only silicates are quartz and muscovite. In some places an alteration band of K-feldspar after plagioclase separates kaolinite and muscovite.The widespread association of kaolinite and K-feldspar in hydrothermal alteration zones suggests that these two minerals were stable together at the time of formation. Experimental data show that, where quartz controls the activity of silica in solution, kaolinite and K-feldspar should react to form muscovite. However, calculations predict that, at 15,000 psi, solutions with silica activities compatible with amorphous or gelatinous silica could stabilize K-feldspar and kaolinite relative to muscovite at temperatures below about 250 degrees C. The rapid decomposition either of plagioclase feldspar or montmorillonite is considered a prime cause for the attainment of such high activities of silica in pore solutions of rock. The supersaturation of pore solutions with silica is a transient condition; quartz precipitates where kaolinite and K-feldspar react to give muscovite. Thus, the zonally distributed alteration at Ely is considered to have taken place after the rock had cooled to below 250 degrees C. In contrast, the diffusely distributed muscovite and montmorillonite found in another portion of the pit probably commenced at 600 degrees to 700 degrees C with only minor formation of alteration products below 250 degrees C.

Belts of major mineralization in western United States

Abstract: A study of the literature on the centers of major mineralization in western United States reveals that in the preponderant number of those of the mesothermal and hypothermal environments the mineralizing fractures strike northeasterly; that in some of these centers the mineralization and the igneous masses with which it is genetically related are confined to a relatively narrow zone that trends northeasterly in general parallel to the strike pattern of the mineralizing fractures; and that the contained dikes usually parallel the zone, and often the larger igneous masses are elongated in that direction. Furthermore, the centers of the three main metallogenic epochs of the region--Precambrian, Nevadan-Laramide, and late Tertiary--are aligned in belts that also trend northeasterly, closely paralleling the trends in the centers.It is proposed that the belts of mineralization reflect zones of deep rupture in the earth9s crust all of which formed simultaneously in response to a common regional stress, and that there is a genetic relationship between the zones of rupture and the magma reservoirs from which the mineralizing solutions were derived.

Evidence for the origin and age of some mineralized Laramide intrusives in the southwestern United States from strontium isotope and rubidium-strontium measurements

Abstract: Initial Sr 87 /Sr 86 values in the range 0.706-0.708 are reported for i) Laramide intrusive porphyries associated with major copper deposits in Arizona, New Mexico and Utah, ii) an intrusive Laramide stock associated with lead-silver mineralization in the Sawatch range, Colorado, iii) the Cripple Creek gold-silver deposit. Rb-Sr ages of micas from these rocks fall in the range 53-73 million years. The whole rock Sr 87 /Sr 86 ratios in Laramide times of five representative samples of Precambrian (ca. 1,600-1,700 million years) granite-gneiss country rock in the Sawatch Range, Colorado, fall in the range 0.76-0.80, which is probably close to the average range for the whole basement complex. It is concluded that the intrusive rocks did not originate by fusion of ancient, sialic basement, but must have developed in an environment with Rb/Sr and Sr 87 /Sr 86 ratios as low, or nearly as low, as the source regions of basalt.

Two varieties of pyrite from the Basal Reef of the Witwatersrand system

Abstract: Compared to isotropic pyrite, anisotropic pyrite contains much higher trace arsenic, nickel, and cobalt, has slightly lower reflectivity, and is not as hard. Pyrite is not a reliable standard for reflectivity and other types of measurement when trace-element content is unknown. Since the two types are intergrown, it is clear that cobalt and nickel values do not have genetic implications.

Bauxite genesis in Jamaica; new evidence from trace-element distribution

Abstract: The question of whether the bauxite deposits of Jamaica originated from the insoluble residue of the underlying White Limestone Formation or from alluvium transported onto the limestone surface from the Cretaceous inliers has been the subject of considerable discussion.Analyses of the trace element contents of samples of bauxite, limestone residues and rocks from the inliers suggest that the limestone residue is the principal source material from which the bauxite has been derived.