Two metallogenic maps for North America

Part of the interior of the North American continent is covered by relatively undisturbed sedimentary rocks not available to the entrance of ore-bearing fluids from depths, but a wide belt along the Pacific Coast, a narrower belt along the Atlantic Coast, and most of the north-east quadrant of the continent have been mineralized. If we plot all the known ore deposits in these latter areas, we find considerable differences in the densities of occurrences but no well defined patterns except a crowding along most of the area near the Pacific Coast. If, on the other hand, we plot only the major occurrences, we can see some linear patterns transverse to the edges of the continent.Transverse ore deposit patterns in an ENE direction are conspicuous in western United States and northern Mexico and in Ontario and Quebec of Canada but are almost unknown in British Columbia and Alaska. East-west patterns (within 5 degrees of a 90degree azimuth) are much more rare: one in southern British Columbia, one in Quebec and Ontario, one in Missouri, and one in Tennessee. No north-south patterns of any great length can be recognized.In addition to these rich ridges there are many inhomogeneities in the distribution of the major ore deposits, and these I think are due to subcrustal inhomogeneities in the distribution of metals, presumably in the upper mantle and probably of a late stage in the formation of the earth. I will attempt to show that the large ore deposits of North America form belts bounded approximately by parallels of latitude, separated by gaps in which no large deposits occur. Without undue distortions I have been able to draw outlines to include all the approximately 250 major ore deposits, except in the area north of latitude 60 where recorded occurrences are at present too sparse and too widely distributed.The localization of ore deposits in North America is thus the result of linear zones, probably deep crustal or sub-crustal fracture zones, crossing latitudinal belts of anomalous concentrations of metals probably of pre-crustal age.I have no explanations for the processes that formed either the transverse patterns or the latitudinal belts. They are not prominent and will be judged by many to be nonexistent. The lack of an explanation must not, however, be used to deny their existence— an attitude that surely limits the progress of metallogenic theory. Correlations with similar studies on other continents will be most useful.

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Zusammenfassung Ein Teil des Inneren des Nordamerikanischen Kontinents ist mit relativ ungestörten Sedimentgesteinen bedeckt, die ein Eindringen von Erz-bringenden Lösungen aus der Tiefe verhinderten. Dagegen sind ein breiter Streifen entlang der Pazifikküste, ein schmalerer Streifen entlang der Atlantikküste und der größte Teil des Nordostquadranten des Kontinentes mineralisiert. Wenn wir alle bekannten Erzlagerstätten in diese Gebiete eintragen, so finden wir auffällige Unterschiede in der Verteilung der Punkte, aber kein regelmäßiges Muster, ausgenommen das Gebiet nahe der Pazifikküste. Wenn wir dagegen nur die größeren Vorkommen auftragen, so ergeben sich lineare Anordnungen, die quer auf die Ecken des Kontinentes zulaufen.Quer verlaufende Muster in ENE-Richtung sind auffallend in den westlichen Vereinigten Staaten, in Nord-Mexico, in Ontario und Quebec; diese Richtung ist jedoch in British Columbia und in Alaska unbekannt.E-W-Muster mit±5° Schwankungen sind sehr viel seltener. Eines finden wir im südlichen British Columbia, eines in Quebec und Ontario, eines in Missouri und Tennessee, N-S-streichende Lineationen sind unbekannt.Über die Anordnung in reichen Zonen hinaus finden wir viele Inhomogenitäten in der Verteilung der wichtigsten Erzlagerstätten; diese sind meiner Meinung nach von subkrustalen Inhomogenitäten in der Verteilung der Metalle bestimmt. Vielleicht spielt hier eine frühe Verteilung im oberen Mantel zur Zeit der Entstehung der Erde eine Rolle. Ich will versuchen zu zeigen, daß die großen Erzlagerstätten an Zonen gebunden sind, die annähernd parallel zu den Breitengraden verlaufen und durch erzfreie Zonen getrennt sind. Ohne Schwierigkeiten lassen sich Umgrenzungen mit dieser Orientierung aufzeigen, die alle 250 größeren Erzlagerstätten umfassen, ausgenommen im Gebiet nördlich des 60. Breitengrades, wo die Dichte der Lagerstätten zu gering ist.Die Anordnung der Erzlagerstätten Nordamerikas ist auf lineare Zonen zurückzuführen, die wahrscheinlich auf Bruchstrukturen der Unterkruste oder des oberen Mantels begründet sind. Die Konzentration der Metalle in Querstrukturen zu den Breitenkreisen ist vermutlich eine Auflage, die vor der Bildung der Kruste entstanden ist. Ich habe keine Erklärung für die Prozesse, die zu dieser Metallkonzentration geführt haben. Viele mögen eine derartig vorgegebene Anordnung ablehnen; man sollte jedoch bedenken, daß das Fehlen einer Erklärung noch nicht zur Ablehnung auffälliger Phänomene berechtigt. Es sollten auch andere Kontinente daraufhin untersucht werden.

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Résumé Une partie de l'intérieur du continent Nord Américain est couverte par des roches sédimentaires peu dérangées et défavorables à l'accès de fluides minéralisants venus de la profondeur. Par contre une large ceinture le long de la côte pacifique, un ceinture plus étroit de long de la côte atlantique, et la majeure partie du quadrant Nord-Est du continent ont été minéralisées. Si l'on reporte sur carte tous les dépôts connus dans ces domaines, on trouve des différences considérables dans les »densités d'occurrences« mais aucun réseau bien défini, à l'exception d'un fort peuplement dans presque tout le domaine proche du Pacifique. Si, d'autre part, on ne reporte que les occurrences majeures, on voit apparaître des lignes directrices transversales sur les bordures du continent.De telles transversales dirigées ENE apparaissent dans l'Ouest des Etats-Unis, le Nord du Mexique, l'Ontario et le Québec (Canada) mais sont presque inconnues en Colombie britannique et en Alaska.Des directions Est-Ouest (avec une variation de 5) sont beaucoup plus rares: une dans le Sud de la Colombie britannique, une dans le Québec et l'Ontario, une dans le Missouri et une dans le Tennessee. Aucune direction Nord-Sud de grande longueur ne peut être reconnue.En plus de ces »arêtes riches« existent beaucoup d'inhomogénéités dans la distribution des gisements importants, et celles-ci sont dues, à mon avis, à des hétérogénéités souscrustales de la distribution des métaux, probablement au niveau du manteau supérieur et remontant à une étape tardive de la formation de la Terre. Je désire essayer de montrer que les grands gisements métallifères de l'Amérique du Nord s'intègrent dans des ceintures approximativement limitées par des parallèles de latitude et séparées par des intervalles dépourvus de grands dépôts. Ainsi, sans distorsions abusives, j'ai pu tracer des enveloppes englobant environ 250 gisements majeurs, sauf dans une aire située au Nord du parallèle 60 Nord, où les occurrences jusqu'ici enregistrées sont encore trop rares et disséminées.La localisation des gisements métallifères de l'Amérique du Nord est donc due à des zones linéaires, probablement des zones de fractures crustales profondes ou sous-crustales, croisant des ceintures latitudinales de concentrations anormales de métaux, elles-mêmes probablement précrustales.Je n'offre pas d'explication pour les processus qui ont formé les ceintures latitudinales et les transversales. Les unes et les autres ne sont pas fortement apparentes, et beaucoup estimeront qu'elles n'existent pas. Cette absence d'explication ne doit cependant pas être utilisée pour nier leur existence, attitude qui limiterait certainement les progrès des théories métallogéniques. Des comparaisons avec des ètudes similaires sur d'autres continents seraient certainement très utiles.

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Holocene displacement along branch and secondary faults in the San Andreas fault zone, southern California

Detailed geologic mapping of the San Andreas fault zone in Los Angeles County since 1972 has revealed evidence for diverse histories of displacement on branch and secondary faults near Palmdale. The main trace of the San Andreas fault is well defined by a variety of physiographic features. The geologic record supports the concept of many kilometers of lateral displacement on the main trace and on some secondary faults, especially when dealing with pre-Quaternary rocks. However, the distribution of upper Pleistocene rocks along branch and secondary faults suggests a strong vertical component of displacement and, in many locations, Holocene displacement appears to be primarily vertical. The most recent movement on many secondary and some branch faults has been either high-angle (reverse and normal) or thrust. This is in contrast to the abundant evidence for lateral movement seen along the main San Andreas fault. We suggest that this change in the sense of displacement is more common than has been previously recognized.The branch and secondary faults described here have geomorphic features along them that are as fresh as similar features visible along the most recent trace of the San Andreas fault. From this we infer that surface rupture occurred on these faults in 1857, as it did on the main San Andreas fault. Branch faults commonly form “Riedel” and “thrust” shear configurations adjacent to the main San Andreas fault and affect a zone less than a few hundred meters wide. Holocene and upper Pleistocene deposits have been repeatedly offset along faults that also separate contrasting older rocks. Secondary faults are located up to 1500 m on either side of the San Andreas fault and trend subparallel to it. Moreover, our mapping indicates that some portions of these secondary faults appear to have been “inactive” throughout much of Quaternary time, even though Holocene and upper Pleistocene deposits have been repeatedly offset along other parts of these same faults. For example, near 37th Street E. and Barrel Springs Road, a limited stretch of the Nadeau fault has a very fresh normal scarp, in one place as much as 3 m high, which breaks upper Pleistocene or Holocene deposits. This scarp has two bevelled surfaces, the upper surface sloping significantly less than the lower, suggesting at least two periods of recent movement. Other exposures along this fault show undisturbed Quaternary deposits overlying the fault. The Cemetery and Little Rock faults also exhibit selected reactivation of isolated segments separated by “inactive” stretches.Activity on branch and secondary faults, as outlined above, is presumed to be the result of sympathetic movement on limited segments of older faults in response to major movement on the San Andreas fault. The recognition that Holocene activity is possible on faults where much of the evidence suggests prolonged inactivity emphasizes the need for regional, as well as detailed site studies to evaluate adequately the hazard of any fault trace in a major fault zone. Similar problems may be encountered when geodetic or other studies, Which depend on stable sites, are conducted in the vicinity of major faults.     

Radiocarbon Ages and Environments of Deposition of the Wono and Trego Hot Springs Tephra Layers in the Pyramid Lake Subbasin, Nevada

Uncalibrated radiocarbon data from core PLC92B taken from Wizards Cove in the Pyramid Lake subbasin indicate that the Trego Hot Springs and Wono tephra layers were deposited 23,200 ± 300 and 27,300 ± 300¹⁴C yr B.P. (uncorrected for reservoir effect). Sedimentological data from sites in the Pyramid Lake and Smoke Creek–Black Rock Desert subbasins indicate that the Trego Hot Springs tephra layer was deposited during a relatively dry period when Pyramid Lake was at or below its spill point (1177 m) to the Winnemucca Lake subbasin. The Wono tephra layer was deposited when lake depth was controlled by spill across Emerson Pass sill (1207 m) to the Smoke Creek–Black Rock Desert subbasin.¹⁸O data from core PLC92B also support the concept that the Trego Hot Springs tephra fell into a relatively shallow Pyramid Lake and that the Wono tephra fell into a deeper spilling lake.     

Calculations of horizontal mixing rates using222Rn and the controls on hypoxia in western Long Island Sound, 1991

Late summer hypoxia (<3 ppm oxygen) in western Long Island Sound (WLIS) is a persistent environmental and management issue whose controlling processes are poorly understood. Measured rates of sediment and water-column oxygen consumption in the bottom water suggest that a condition of no oxygen should be attained on the time scale of 13–30 d. Observations, however, indicate the onset of hypoxia is of the order 150 d. Therefore, horizontal and/or vertical transport of oxygen into the area of hypoxia must play an important role. Hypoxia decreases benthic activity and the sediment flux of²²²Rn. The resulting horizontal gradient in bottom water²²²Rn was measured and used to estimate the effective horizontal transport rate (>5–50 m² s^?1), which is considerably slower than previous estimates. Scale analysis of the hypoxia process indicates that horizontal transport rates alone can explain the slow progression of hypoxia in XLIS but that vertical processes may also be capable of delaying the onset of hypoxia especially under conditions of weak stratification or weak intermediate layer oxygen consumption. This scale analysis indicates a delicately balanced process that is sensitive to both climatologically-driven variability in the rates of horizontal and vertical transport as well as the biologically-driven rates of oxygen consumption. An improved ability to predict and/or control hypoxia must be based on a better understanding of temporal and spacial variations in circulation, mixing, and stratification as well as the biological processes in the water column and the sediments.     

Determination of the REE in Geological Reference Materials DTS-1 (Dunite) and PCC-1 (Peridotite) by Ultrasonic and Microconcentric Desolvating Nebulisation ICP-MS

Inductively coupled plasma-mass spectrometry is well suited for the precise, accurate and rapid determination of rare earth elements in most geological samples. However, determination of rare earth elements in certain mantle-derived materials, without applying preconcentration techniques, remains problematical due to low natural concentrations (generally < 1 ng g⁻¹). Consequently, USGS reference materials DTS-1 (a dunite) and PCC-1 (a partially serpentinized harzburgite) have only suggested rather than recommended values for the rare earth elements in reference material compilations. We compared results obtained using two ICP-MS instruments: a U-5000AT ultrasonic nebuliser coupled to a PQ2+ quadrupole ICP-MS and an ELEMENT sector field ICP-MS equipped with a MCN-6000 microconcentric desolvating nebuliser, with the suggested literature values for these two reference materials. Precision and accuracy of analytical methods employed by both instruments were demonstrated by excellent relative standard deviations (< 2%) and inter-laboratory agreement (< 5%) for numerous analyses of BHVO-1 and BIR-1, which are well established with rare earth elements contents at the μg g⁻¹ level. Repeat analyses of DTS-1 and PCC-1 at each laboratory indicate that each method is generally precise to better than 5% at sub-g g⁻¹ levels. Furthermore, values from both instruments generally agree to within 10%. Our DTS-1 and PCC-1 values agree reasonably well with selected data reported in the literature (except for Ce and Sm in DTS-1) but exhibit poorer agreement with reported compilation values. With the demonstrated level of precision and accuracy, we contend that these new values for DTS-1 and PCC-1, generated by two different instruments, are the best estimates of the true whole-rock composition of these samples reported to date.     

Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones,Central North Sea: an example of cement growth during bioturbation?

Unusual textural and chemical characteristics of disseminated dolomite in Upper Jurassic shelf sediments of the North Sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones. The dolomite is present throughout the Franklin Sandstone Formation of the Franklin and Elgin Fields as discrete, non‐ferroan, generally unzoned, subhedral to highly anhedral ‘jigsaw piece’ crystals. These are of a similar size to the detrital silicate grains and typically account for ≈5% of the rock volume. The dolomite crystals are never seen to form polycrystalline aggregates or concretions, or ever to envelop the adjacent silicate grains. They are uniformly dispersed throughout the sandstones, irrespective of detrital grain size or clay content. Dolomite authigenesis predated all the other significant diagenetic events visible in thin section. The dolomite is overgrown by late diagenetic ankerite, and bulk samples display stable isotope compositions that lie on a mixing trend between these components. Extrapolation of this trend suggests that the dolomite has near‐marine δ¹⁸O values and low, positive δ¹³C values. The unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near‐surface zone of active bioturbation. Sea water provided a plentiful reservoir of Mg and a pore fluid of regionally consistent δ¹⁸O. Labile bioclastic debris (e.g. aragonite, Mg‐calcite) supplied isotopically positive carbon to the pore fluids during shallow‐burial dissolution. Such dissolution took place in response to the ambient ‘calcite sea’ conditions, but may have been catalysed by organic matter oxidation reactions. Bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones, but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework. Continued exchange of Mg²⁺ and Ca²⁺ with the sea‐water reservoir maintained a sufficient Mg/Ca ratio for dolomite (rather than calcite) to form. Irregular crystal shapes resulted from dissolution, of both the dolomite and the enclosed fine calcitic shell debris, before ankerite precipitation during deep‐burial diagenesis.     

Silver and copper accumulation in two estuarine bivalves,the eastern oyster (Crassostrea virginica) and the hooked mussel (Ischadium recurvum) in the Patuxent River estuary,Maryland

In order to assess the source of trace elements and to compare uptake between different bivalve species, oysters and mussels were transplanted to five sites in the upper Patuxent River estuary during 1988. Transplant sites were located above and below the discharge of Chalk Point Steam Electric Station (CPSES)—a historic point source for copper. Organisms were sampled approximately monthly for 1 yr and analyzed for copper and silver. During spring and summer, concentrations of silver and copper increased in oysters at the upstream stations. There was very little change in silver and copper concentrations in the mussels with either time or among stations. Copper concentrations accumulated by oysters approached those found during previous periods when CPSES used Cu?Ni alloy condenser tubes (1966–1987), suggesting that corrosion from the Cu?Ni condenser tubes was only a minor contributor to the copper burdens of oysters in the river nearby. The lack of accumulation by mussels at the same sites suggests that part of the reason for the accumulation by oysters may be a taxonomically specific physiological effect caused by the salinity regime in the upper Patuxent.     

Genetic structure of bay anchovy (Anchoa mitchilli) populations in Chesapeake Bay

To determine the genetic structure of the bay anchovy (Anchoa mitchilli) within Chesapeake Bay, 16 isozyme systems encoding 21 loci for 20 population were examined using horizontal starch gel electrophoresis. Contingency Chisquare analysis revealed significant allelic frequency differences at nine loci (AAT-1, AAT-2, ALD-1, CPK-2, GAP-1, GLY-1, LDH-1, MDH-1, and MDH-2). Two loci, ALD-1 and MDH-1, were responsible for nine of 14 tests not conforming to Hardy-Weinberg expectations, with some of these deviations attributed to possible scoring and/or sampling error. Estimates for mean average heterozygosity were relatively high, ranging from 0.40 to 0.096, with 33–57% of the loci polymorphic. A low F_st value (0.041) along with high genetic identity estimates (I=0.997) indicated little substructuring of bay anchovy populations within Chesapeake Bay.     

Advances in modelling of snowpack processes utilizing remote sensing technology

Application of models over large areas requires enormous amounts of point data which are seldom available. As a result, the use of remote sensing to acquire data for these models becomes a practical alternative because from space areal measurements can be made over entire regions. Remote sensing data sets of snow are being studied and refined for input into energy balance, hydrological, and general circulation models. Snow influences the amount of solar energy retained and returned to space, the utilization of radiant energy in the hydrological cycle, and the atmospheric circulation by interacting with and modifying air masses. The continually evolving remote sensing technology will be used to provide a better understanding of how snow cover influences global climate and a better fundamental understanding of snow accumulation and ablation processes.