Structural and geochronological studies on the Liba goldfield of the West Qinling Orogen, Central China

The Liba goldfield, located to the northeast of the Zhongchuan Granite in the West Qinling Orogen (WQO) of mainland China, contains the largest known gold resource of 2.8 Moz in the Zhongchuan area. Devonian metasedimentary rocks host the structurally controlled gold mineralization, which is associated with silica–sericite–chlorite–carbonate alteration. Two major styles of mineralization occurred at the goldfield, which are disseminated sediment-hosted and quartz vein hosted types. Pyrite, arsenopyrite, and arsenian pyrite are major gold carriers and gold also occurs as native gold grains and electrum spatially associated with the sulfides. Numerous felsic/intermediate dykes have a similar structural control as the mineralization, and their contacts with host rocks are recognized as favorable zones for mineralization. Detailed fieldwork in conjunction with geochronological studies has helped to define the deformation history and gold metallogenesis of the goldfield. Three major phases of deformation have been recognized in the Zhongchuan area. The first deformation (D₁) event was compressional in broadly a N–S orientation, the second (D₂) event was also compressional and orientated in a NE–SW direction, and the third (D₃) event was post-mineralization and was associated with the emplacement of barren calcite and anhydrite veins. Compression related to D₂ is the key process that controlled the distribution of igneous dykes and gold mineralization in the Liba goldfield. Both igneous and hydrothermal fluids preferentially focused along dilational jogs under local trans-extension, which took place during the late stage of D₂. Precise dating with high-resolution ion microprobe (SHRIMP) U–Pb on zircon and ⁴⁰Ar/³⁹Ar on muscovite, biotite, hornblende, and plagioclase of crosscutting pre-mineralization granitic porphyry and diorite dykes have constrained the mineralization age to after ca. 227 Ma. ⁴⁰Ar/³⁹Ar analysis of minerals formed in hydrothermal alteration zones associated with gold mineralization indicates that there was a widespread ca. 216 Ma hydrothermal event that affected almost all lithologies in the area. This detailed investigation is the first study to tightly constrain the timing of gold mineralization in the WQO. The broadly overlapping timing and similar structural control of the mineralization and igneous dykes show a promising correlation, which could be potentially used to map this Late Triassic gold mineralization event in the WQO.     

Urban parks as shared spaces? The utility of alert distances as indicators of avian tolerance of humans in Stirling, Scotland

Urban avian foraging is strongly affected by approaching people. Avian alert distances, the instant of a bird's appraisal of human intrusion, have not been studied in a comparative mode for passerines, despite their prominent ecological importance in parks. Alert distances for 13 passerines were examined in parks in Stirling, Scotland, using an innovative approach emphasizing human behaviour and periods between alert and flight distances, termed alert periods. Larger species were less tolerant of humans, although attracted to human wastes. Alert distances also varied according to vegetation variation. Park design may use alert distances to plan vegetation layout as well as reduce disturbing human behaviour.     

Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: application to porphyry copper deposits of northern Chile

Major- and trace-element compositions of zircons and whole rocks from 14 barren and seven ore-bearing calc-alkaline intrusions from the Chuquicamata-El Abra porphyry copper belt of northern Chile have been measured in situ by excimer laser ablation (ELA) ICP-MS. These data permit the Ce(IV)/Ce(III) ratio within zircon to be calculated using a lattice-strain model for mineral-melt partitioning of Ce(IV) and Ce(III). Zircon Ce(IV)/Ce(III) and Eu_N/Eu_N* ratios, and by inference magmatic oxidation states, generally increase from older, mafic to younger, felsic units. Within this sequence, porphyry copper mineralization is directly associated only with intrusions with zircon Ce(IV)/Ce(III)>300 and Eu_N/Eu_N*>0.4. Such trends can be understood in terms of interdependent relations between oxygen fugacity, sulfur speciation and solubility, and chalcophile element partitioning in silicate magmas. Because zircon occurs in most calc-alkaline intrusions and is resistant to subsolidus alteration, zircon Ce(IV)/Ce(III) ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic-hydrothermal Cu-Au mineralization. The approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks.     

P—T—t—deformation—fluid characteristics of lode gold deposits: evidence from alteration systematics

Structurally hosted lode gold-bearing quartz vein systems in metamorphic terranes possess many characteristics in common, spatially and through time; they constitute a single class of epigenetic precious metal deposit, formed during accretionary tectonics or delamination. The ore and alteration paragenesis encode numerous intensive and extensive variables that constrain the pressure—temperature—time—deformation—fluid (P—T—t—d—f) evolution of the host terrane and hence the origin of the deposits. The majority of lode gold deposits formed proximal to regional translithospheric terrane—boundary structures that acted as vertically extensive hydrothermal plumbing systems; the structures record variably thrust, and transpressional—transtensional displacements. Major mining camps are sited near deflections, strike slip or thrust duplexes, or dilational jogs on the major structures. In detail most deposits are sited in second or third order splays, or fault intersections, that define domains of low mean stress and correspondingly high fluid fluxes. Accordingly, the mineralization and associated alteration is most intense in these flanking domains. The largest lode gold mining camps are in terranes at greenschist facies; they possess greenschist facies hydrothermal alteration assemblages developed in cyclic ductile to brittle deformation that reflects interseismic—coseismic cycles. Interseismic episodes involve the development of ductile S—C shear zone fabrics that lead to strain softening. Pressure solution and dislocation glide microstructures signify low differential stress, slow strain rates of ≤ 10⁻¹³ s⁻¹, relatively high confining stress, and suprahydrostatic fluid pressures. Seismic episodes are induced by buildup of fluid pressures to supralithostatic levels that induce hydraulic fracturing with enhanced hydraulic conductivity, accompanied by massive fluid flow that in turn generates mineralized quartz veins. Hydrothermal cementing of ductile fabrics creates ‘hardening’, lowers hydraulic conductivity, and hence promotes fault valve behaviour. Repeated interseismic (fault valve closed), coseismic (valve open) cycles results in banded and/or progressively deformed veins. Alteration during both interseismic and coseismic episodes typically involves the hydrolysis of metamorphic feldspars and Fe, Mg, Ca-silicates to a muscovite/paragonite—chlorite ± albite/K-feldspars assemblage; carbonization of the metamorphic minerals to Ca, Fe, Mg-carbonates; and sulphidation of Fe-silicates and oxides to sulphides. Geochemically this is expressed as additions of K, Rb, Ba, Cs, and the volatiles H₂O, CO₂, CH₄, H₂S in envelopes of meter to kilometer scale. K/Rb and K/Ba ratios are close to average crustal values, potentially ruling out late stage magmatic fluids where K/Rb and K/Ba are respectively lower and higher than crustal values. Smaller deposits are present in subgreenschist, and amphibolite to granulite facies terranes. The former are characterized by subgreenschist facies alteration assemblages, vein stockworks, brittle fracturing and cataclastic microstructures, whereas the latter feature amphibolite to granulite facies alteration assemblages, ductile shear zones, ductilely deformed veins, and microstructures indicative of dislocation climb during interseismic episodes. Hence the lode gold deposits constitute a crustal continuum of deposits from subgreenschist to granulite facies, that all formed synkinematically in broad thermal and rheological equilibrium with their host terranes. These characteristics, combined with the low variance of alteration assemblages in the higher temperature deposits, rules out those being metamorphosed counterparts of greenschist facies deposits. Deposits at all grades have a comparable metal inventory with high concentrations of Au and Ag, where Au/Ag averages 5, with enrichments of a suite of rare metals and semi-metals (As, Sb, ± Se, Te, Bi, W, Mo and B), but low enrichments of the base (Cu, Pb, Zn, Cd) and other transition (Cr, Ni, Co, V, PGE, Sc) metals relative to average crust. The hydrothermal ore-forming fluids were dilute, aqueous, carbonic fluids, with salinities generally ≤ 3 wt.% NaCl equivalent, and X_{(CO2 ± CH4)} 10–24 wt.%. They possess low Cl but relatively high S, possibly reflecting the fact that metamorphic fluids are generated in crust with ∼ 200 ppm Cl, but ∼ 1 wt.%S. Primary fluid inclusions are: (1) H₂OCO₂, (2) CO₂-rich with variable CH₄ and small amounts of H₂O, and (3) 2-phase H₂O (liquid-vapor) inclusions. Inclusion types 2 and 3 represent immiscibility of the type 1 original ore fluid. Immiscibility was triggered by fluid pressure drop during the coseismic events and possibly by shock nucleation, leading to highly variably homogenization temperatures in an isothermal system. A thermodynamic evaluation of alteration assemblages constrains the ore fluid pH to 5–6; redox controlled by the HSO₄/H₂S and CO₂/CH₄ buffers; and X_CO2 that varies. The higher temperature deposits formed under marginally more oxidizing conditions. Stable isotope systematics of the ore and gangue minerals yields temperatures of 200–420°C, consistent with the crustal spectrum of the deposits, very high fluid rock ratios, and disequilibrium of the externally derived ore fluids with wall rocks. The ore fluid δD and δ¹⁸O overlap the metamorphic and magmatic ranges, but the total dataset for all deposits is consistent only with dominantly metamorphic fluids. Carbon isotope compositions of carbonates span −11 to +2% and show provinciality: this is consistent with variable proportions of reduced C (low δ¹³C) and oxidized C (higher δ¹³C) in the source regions contributing CO₂ and CH₄ to the ore fluids. In some instances, C appears to have been derived dominantly from proximal to the deposits, as in the Meguma terrane (δ¹³C ∼ − 22%). Sulphur isotope compositions range from 0 to +9‰, and are consistent with magmatic S, dissolution or desulphidation of magmatic sulphides, or average crustal sulphides. ³⁴S-depleted sulphides occur in ore bodies such as Hemlo where fluid immiscibility led to loss of H₂S and consequent fluid oxidation. Gold is probably transported as an Au(HS)⁻₂ complex. Relatively high S but low Cl in the hydrothermal fluid may explain the high Au slow base metal characteristic of the deposits. Gold precipitated in ore bodies due to loss of S from the ore fluid by sulphidation of wall rock, or immiscibility of H₂S; and by oxidation or reduction of the fluid, or by chemisorption, or some combination of these processes. Most lode gold deposits have been brittly reactivated during uplift of host terranes, with secondary brines or meteoric water advecting through the structures. These secondary fluids may remobilize gold, generate retrograde stable isotope shifts, reset mineral geochronometers, and leave trails of secondary fluid inclusions. Data on disturbed minerals has led to invalid models for lode gold deposits. The sum of alteration data leads to a model for lode gold deposits involving a clockwise P—T—t evolution and synkinematic and synmetamorphic mineralization of the ‘deep later’ type. During terrane accretion oceanic crust and sediments are subcreted beneath the terrane boundary. Thermal equilibration generates metamorphic fluids that advect up the terrane structure, at lithostatic fluid pressure, into the seismogenic zone where the majority of deposits form. Thus many lode gold deposits are on intrinsic part of the development of subduction—accretion complexes of the high-T, low-P type.     

Amazonian magnetostratigraphy: Dating the first pulse of the Great American Faunal Interchange

The chronostratigraphy of the youngest Neogene deposits of the Amazon Basin, which comprise the Madre de Dios Formation in eastern Peru, remains unresolved. Although ⁴⁰Ar/³⁹Ar dates on two volcanic ashes from this formation in Peru provide critical baseline data points, stratigraphic correlations among scattered riverine outcrops in adjacent drainage basins remain problematic. To refine the chronostratigraphy of the Madre de Dios Formation, we report here the magnetostratigraphy of an outcrop on the Madre de Dios River in southeastern Peru. A total of 18 polarity zones was obtained in the ～65-m-thick Cerro Colorado section, which we correlate to magnetozones Chrons C4Ar to C2An (9.5–3.0 Ma) based on the prior ⁴⁰Ar/³⁹Ar dates. These results confirm the late Miocene age of a gomphothere recovered from the Ipururo Formation, which underlies the late Miocene Ucayali Unconformity at the base of the Cerro Colorado outcrop. The results also support earlier interpretations of a late Miocene age for other fossils of North American mammals recovered from basal conglomeratic deposits of the Madre de Dios Formation immediately above the Ucayali Unconformity. These mammals include other gomphotheres, peccaries, and tapirs, and their presence in South America in the late Miocene is recognized as part of the first pulse of the Great American Faunal Interchange.     

Keeping the stress off the sheep? Agricultural intensification, neoliberalism, and ‘good’ farming in New Zealand

Under neoliberal schemes like audit systems, consumer demands born of concerns about food safety, the environment and animal welfare are theoretically poised to influence agricultural production systems (Campbell and Le Heron, 2007). Whether such influences might reverse or redirect the trend toward environmentally-damaging rampant productivism of the 20th century hinges in part on the subjective positions of farmers and the ways in which they inform how farmers respond to policy and market signals.In this paper we argue the need for a genuine engagement with both the complexities of farmer subjectivity and the interactions amongst farmer subjectivity and agro-ecologies, and animal bodies in particular. This paper presents a case study of sheep farmers on the South Island that reveals contestation and transitions in traditional markers of “good farming”, particularly animal health. We observe how such transitions arise from reconfigurations of the relationships between agro-ecological, political and social histories. In this paper’s formulation, neither state subsidies nor neoliberalism in agriculture is primary cause or ultimate effect of the transformation of agricultural practice. Rather, changes in the political economy expose contradictions in farmer subjectivities, the resolution of which may block or reinforce trends suggested by the political economy. We suggest that contested ideas about animal health within the social field of pastoral farming in New Zealand makes it possible that New Zealand’s sheep growers may take the high road of best environmental practice via highly audited environmental standards of production demanded by elite consumer markets, or that they may remain in the intensifying trajectory of continuing to drive the sheep’s body to its maximum possible intensity of production. The mixed legacy of neoliberal reform is that it has simultaneously enabled both of these contradictory trajectories in New Zealand pastoralism.     

Particulate trace metal speciation in stream sediments and relationships with grain size: Implications for geochemical exploration

Sediment samples were collected from streambeds in an undisturbed watershed in eastern Quebec (Gaspé Peninsula). Two sampling sites were located on a stream draining an area of known mineralization (Cu, Pb, Zn) and two on a control stream. The sediment samples were separated into 8 distinct size classes in the 850 μm to <1 μm size range by wet sieving, gravity sedimentation or centrifugation. Each sediment subsample was then subjected to a sequential extraction procedure designed to partition the particulate heavy metals into five fractions: (1) exchangeable; (2) specifically adsorbed or bound to carbonates; (3) bound to Fe-Mn oxides; (4) bound to organic matter; (5) residual. The following metals were analyzed in each extract: Cu, Pb, Zn; Fe, Mn.Comparison of samples from the mineralized area with control samples revealed the expected increase in total concentrations for Cu, Pb and Zn. Non-detrital metals were mainly associated with Fe oxides (specifically adsorbed; occluded) and with organic matter or resistant sulfides. For a given sample, variation of trace metal levels in fractions 2 and 3 with grain size reflected changes in the available quantities of the inorganic scavenging phase (FeOx/MnOx); normalization with respect to Fe and Mn content in fraction 3 greatly reduced the apparent dependency on grain size.The results of this study suggest that a single reducing extraction (NH₂OH.HCl) could be used advantageously to detect anomalies in routine geochemical surveys. A second leaching step with acidified H₂O₂ could also be included, as the trace metal concentrations in fraction 4, normalized with respect to organic carbon content, also showed high {anomaly/background} ratios.     

Growth rate of the preserved continental crust: II. Constraints from Hf and O isotopes in detrital zircons from Greater Russian Rivers

Detrital zircons from the Ob, Yenisey, Lena, Amur, Volga, Dnieper, Don and Pechora rivers have been analyzed for U-Th-Pb, O and Lu-Hf isotopes to constrain the growth rate of the preserved continental crust in Greater Russia. Four major periods of zircon crystallization, 0.1-0.55, 0.95-1.3, 1.45-2.0 and 2.5-2.9 Ga, were resolved from a compilation of 1366 zircon U/Pb ages. The Archean zircons have δ¹⁸O values lying between 4.53‰ and 7.33‰, whereas Proterozoic and Phanerozoic zircons have a larger range of δ¹⁸O values in each of the recognized U/Pb time intervals with maximum δ¹⁸O values up to 12‰. We interpret the zircons with δ¹⁸O between 4.5‰ and 6.5‰ to have been derived from a magmatic precursor that contains little or no sedimentary component. The variable δ¹⁸O values of the zircons were used to constrain the ¹⁷⁶Lu/¹⁷⁷Hf ratios of the crustal source region of the zircons, which, in turn, were used to calculate Hf model ages (T_DM^V). The crustal incubation time, the time difference between primitive crust formation (dated by T_DM^V) and crustal melting (dated by zircon U/Pb age), varies between 300 to 1000 Myr for the majority of analyzed zircon grains, but can be up to 2500 Myr. The average T_DM^V Hf model age weighted by the fraction of zircons in the river load is 2.12 Ga, which is in reasonable agreement with the area-weighted average of 2.25 Ga. The T_DM^V Hf model age crustal growth curve for zircons with mantle-like δ¹⁸O values (4.5-6.5‰), weighted by area, shows that growth of the Great Russian continental crust started at 4.2 Ga, and that there are two principal periods of crustal growth, 3.6-3.3 Ga and 0.8-0.6 Ga, which are separated by an interval of low but more or less continuous growth. An alternative interpretation, in which the average ¹⁷⁶Lu/¹⁷⁷Hf ratio (0.0115) of the continental crust is used for the Paleoproterozoic zircons from the Lena River, lowers the average T_DM^V age of these grains by about 500 Myr and delays the onset of significant crustal growth to 3.5 Ga.The two principal growth periods recognized in Greater Russia differ from those identified from the Gondwana and the Mississippi river basin, which show peaks at 1.7-1.9 and 2.9-3.1 Ga (Hawkesworth and Kemp, 2006a) and 1.6-2.2 and 2.9-3.4 Ga (Wang et al., 2009), respectively. The older 3.6-3.3 Ga or 3.5-3.3 Ga peak for Greater Russia is slightly older than the older Gondwana-Mississippi peaks, whereas the younger 0.8-0.6 Ga peak is distinctly younger than the youngest peak in either Gondwana or the Mississippi river basin. This suggests that the two major peaks of crustal growth identified in Gondwana and the Mississippi river basin may not be global periods of enhanced continental growth and that the major periods of crustal growth may differ from continent to continent.