Paleoproterozoic gold deposits in the Bald Hill and Coyote areas, Western Tanami, Western Australia

Significant gold deposits in the western Tanami region of Western Australia include deposits in the Bald Hill and Coyote areas. The ca. 1,864 Ma Bald Hill sequence of turbiditic and mafic volcanic rocks hosts the Kookaburra and Sandpiper deposits and a number of smaller prospects. The ca. 1,835 Ma turbiditic Killi Killi Formation hosts the Coyote deposit and several nearby prospects. The Kookaburra deposit forms as a saddle reef within a syncline, and the Sandpiper deposit is localized within graphitic metasedimentary rocks along a limb of an anticline. Gold in these deposits is hosted by anastomosing quartz–(–pyrite–arsenopyrite) veins within quartz–sericite schist with disseminated arsenopyrite, pyrite, and marcasite (after pyrrhotite). Based on relative timing relationships with structural elements, the auriferous veins are interpreted to have been emplaced before or during the ca. 1,835–1,825 Ma Tanami Orogeny (regional D₁). Gold deposition is thought to have been caused by pressure drops associated with saddle reef formation (Kookaburra) and chemical reactions with graphitic rocks (Sandpiper). The Coyote deposit, the largest in the western Tanami region, consists of a number of ore lenses localized along the limbs of the Coyote Anticline, which formed during the Tanami Orogeny. The largest lenses are associated with the Gonzalez Fault, which is located along the steeply dipping southern limb of this fold. Gold was introduced at ca. 1,790 Ma into dilatant zones that formed in local perturbations along this fault during later reactivation (regional D₅) towards the end of a period of granite emplacement. Gold is associated with quartz–chlorite–pyrite–(arsenopyrite–galena–sphalerite) veins with narrow (<?5 mm) chloritic selvages. A quartz–muscovite–biotite–K–feldspar–(tourmaline–actinolite–arsenopyrite) assemblage, which is interpreted to relate to granite emplacement, overprints the regional greenschist facies metamorphic assemblage. The mineralogical similarity between this overprinting assemblage and the vein assemblage suggests that the auriferous veins at the Coyote deposit are associated with the granite-related metamorphic–metasomatic assemblage. Gold deposition is thought to have been caused by pressure drops within dilatant zones.     

Regional exploration for channel and playa uranium deposits in Western Australia using groundwater

Shallow calcrete aquifers in the central north of the Yilgarn Craton in Western Australia are the host to numerous secondary carnotite U deposits. Sampling and analysis of approximately 1400 shallow aquifer groundwaters were conducted to test if U mineralisation of this type may be found using a >5 km sample spacing. Results show this can be achieved. All the economic deposits and most of the minor deposits and occurrences are associated with groundwater that has carnotite (KUO₂VO₄) approaching or exceeding saturated conditions. Soluble U concentrations alone identified the largest deposit (Yeelirrie) and several smaller deposits, but this parameter was not as successful as the mineral saturation indices. Palaeodrainage distribution and thickness of cover combined with surface drainage and catchment boundaries provided background information of U primary sources and for areas with the highest exploration potential for channel and playa U deposits. Granites in the SE of the study area are less prospective with regard to secondary U deposits. Groundwater geochemistry in conjunction with palaeodrainage mapping may greatly improve exploration through cover where radiometric geophysics is not effective. The study of regional, shallow groundwater for U shows multiple benefits for mineral exploration, the economy and potable water quality.     

Stratigraphic-structural-paleoclimatic controls of the newly discovered iron ore deposits of Western Australia

Until a few years ago the Australian iron-ore reserves were estimated to about 300 million t. The recent discoveries of extensive stratified iron deposits in the Hamersley Range, West Australia, raised this estimate to 15–20,000 million t. — The mineralization was controlled in a threefold manner, i.e., stratigraphical (the productive horizon forms the base of the Brockman formation), structural (the iron enrichment took place in synclinal structures) and paleoclimatic (the iron deposits are related to an old land surface of probable Miocene age). — The absence of any metamorphism, granitisation or plutonic action supports the hypothesis of a residual origin of the deposit.P.S. Together with the proofs of this paper the article on the Iron ores of the Ophthalmia Region, Western Australia by J. M. Neilson was received (Trans. Soc. Min. Eng., AIME, New York, 232, Dec. 1965, p. 327 to 338). Neilson reaches in part similar conclusions as here proposed. The differences will be discussed in a future issue of Mineralium Deposita.     

Palynostratigraphy of some Pleistocene deposits in the Western Alps: A review

In the Alps, interglacial and interstadial deposits are rarely preserved due to the intense erosive effect of glaciers in the valleys. Fortunately, some outcrops and cored sequences located in the field area ranging from Lyon to Evian provided sedimentary profiles datable by palynostratigraphy in a highly documented geomorphological context. An overview of several palynological sequences studied in this large area is proposed, and their position in a general chronostratigraphical pattern is discussed. Particular attention is paid to palynostratigraphical evidence whose relevance is tested with systematic comparisons with long reference European pollen sequences spanning several glacial cycles. Minimum ages are suggested for non-glacial episodes corresponding to the deposits studied.     

Metamorphic olivine in ultramafic rocks from Western Australia

Textural relationships plus associated mineralogy are compatible with a metamorphic origin for some olivines in altered ultramafics. Upper amphibolite facies PT conditions are indicated.     

Sulfur isotopes and origin of some sulfide deposits,New England,Australia

The S-isotopic compositions of sulfide deposits from Steinmann, granitoid and felsic volcanic associations have been examined. Ores of Steinmann association have ³⁴S values close to zero per mil (³⁴S=+0.3±3.1) it appears they are of mantle origin. Isotopically, ores of granitoid association regularly show a variable enrichment in ³²S relative to meteoritic (³⁴S=–2.7±3.3). The composition is in accord with an upper mantle/lower crustal source. Two stratiform accumulations of felsic volcanic association show a narrow spread of ³⁴S values (+0.2 to 2.4); a mantle origin for the sulfur in these deposits is favored. In contrast, vein, stockwork and cement ores are moderately enriched in ³²S relative to meteoritic (³⁴S=–4.0±6.4). These ores are polygenetic; sulfur and metals appear to have been leached from local country rocks where volcanogenic and biogenic sulfur predominate.     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造（BIF）有关,可分为2种类型：①深成—表生矿床;②表生—富集矿床,主要分布在尤恩米（Youanmi）地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造(BIF)有关,可分为2种类型:1深成—表生矿床;2表生—富集矿床,主要分布在尤恩米(Youanmi)地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

Tenor variation within komatiite-associated nickel sulphide deposits: insights from the Wannaway Deposit, Widgiemooltha Dome, Western Australia

Summary Detailed geological evaluation yields insights into the relative influence of post-volcanic structural and metamorphic processes on Ni tenor (Ni concentration in 100wt% sulphides) variation within the Wannaway N02 ore body, Western Australia. The ore body is characterised by highly variable geometrical configuration and ore zone morphology, and predominance of massive sulphides. Polyphase deformation and metamorphism to mid-amphibolite facies have modified the ore continuity, mineralogy, textures and fabrics. The up-dip and down-dip parts of the ore body are distinct from the central part, with significant structural relocation of massive ores, abundance of pyrrhotite-rich massive ores that lack mineralogical layering, and breccia ores. Massive sulphides plot within the field of Fe–Ni–S monosulphide solid solution (MSS) at 600°C and form separate low, and medium- to high-tenor populations. Nickel tenor in massive ores varies from 6.1% to 18.8% within the ore body. Moreover, massive ore tenor is highly variable (>8wt%) over distances of 20m, and the lowest tenor massive ores occur at the down-dip and up-dip parts of the ore body.Four models are evaluated as possible explanations for the wide Ni tenor variation within the Wannaway N02 ore body (i) R-factor, (ii) oxygen fugacity f(O₂), (iii) metamorphic sulphidation, and (iv) structural relocation. The highly variable massive ore tenor over such short distances and the occurrence of lowest tenor massive ores in structurally complex areas all contradict strict application of the R-factor model. The abundance of magnetite within massive ores is relatively consistent throughout the ore body, which undermines the f(O₂) model. Lack of correlation of pyrite and Ni tenor is inconsistent with the metamorphic sulphidation model. The bulk of the structural, mineralogical and geochemical evidence indicate a strong role for structural and metamorphic processes, in addition to primary volcanic processes, on Ni tenor variation. Sulphides reverted to Ni-poor MSS and Ni-rich MSS at peak metamorphism. As a result of ductility contrasts, syn- and post-peak metamorphic deformation, Ni-poor MSS was preferentially relocated along faults, producing massive ore tenor variations and destroying primary ore fabric and textures.Present address: Geoinformatics Explorations Ltd, Suite 1280, 625 Howe Street, Vancouver BC, V6C 2T6 Canada     

Groundwater processes and sedimentary uranium deposits

 Hydrologic processes are fundamental in the emplacement of all three major categories of sedimentary uranium deposits: syngenetic, syndiagenetic, and epigenetic. In each case, the basic sedimentary uranium-enrichment cycle involves: (1) leaching or erosion of uranium from a low-grade provenance; (2) transport of uranium by surface or groundwater flow; and (3) concentration of uranium by mechanical, geochemical, or physiochemical processes. Although surface flow was responsible for lower Precambrian uranium deposits, groundwater was the primary agent in upper Precambrian and Phanerozoic sedimentary uranium emplacement. Meteoric or more deeply derived groundwater flow transported uranium in solution through transmissive facies, generally sands and gravels, until it was precipitated under reducing conditions. Syndiagenetic uranium deposits are typically concentrated in reducing lacustrine and swamp environments, whereas epigenetic deposits accumulated along mineralization fronts or tabular boundaries. The role of groundwater is particularly well illustrated in the bedload fluvial systems of the South Texas uranium province. Upward migration of deep, reducing brines conditioned the host rock before oxidizing meteoric flow concentrated uranium and other secondary minerals. Interactions between uranium-transporting groundwater and the transmissive aquifer facies are also reflected in the uranium mineralization fronts in the lower Tertiary basins of Wyoming. Similar relationships are observed in the tabular uranium deposits of the Colorado Plateau. Received, May 1998 · Revised, July 1998 · Accepted, September 1998     

The central Australian groundwater discharge zone: Evolution of associated calcrete and gypcrete deposits

The groundwater discharge zone of central Australia is marked by a chain of playas extending 500 km from Lake Hopkins in Western Australia through Lake Neale, Lake Amadeus and numerous smaller playas to the Finke River in the Northern Territory. This great valley is devoid of surface drainage, and the playas and their associated landforms and chemical sediments are a result of groundwater transmission, and discharge from a large regional flow system. The valley contains extensive groundwater calcrete deposits, which are commonly silicified. The playas contain gypsum and glauberite deposits resulting from the evaporative concentration of discharging groundwater to 250 g/1total dissolved solids. Thin gypcrete crusts have developed on the playa margins and islands as a result of induration of bordering gypsum dunes. Diagenetic changes in the calcrete and gypcrete are caused by dissolution and reprecipitation through groundwater movement.

Calcrete and gypcrete samples from playas near Curtin Springs, NT, dated by electron spin resonance (ESR) — calibrated by ¹⁴C dates on calcrete and the uranium content of gypcrete — are mostly in the range 8000–16 000 years BP, and are evidence of groundwater discharge conditions similar to those of the present day. Calcrete in the vadose zone, above the present water‐table, gives ESR dates in the range 22 000–27 000 years BP suggesting episodes of high intensity rainfall at that time. Phreatic calcrete, below the water‐table, with ages in the range 34 000–75 000 years BP, provides evidence of older episodes of calcretization.

The ESR and ¹⁴C dates for the chemical sediments suggest that this groundwater flow system has been an important feature in the region for much of the Late Quaternary. Although there are problems with the interpretation of ESR dating for chemical sediments with diagenetic alteration, the technique shows promise as a dating tool.     

Management Of Saline Groundwater Discharge By Long-term Windmill Pumping In The Wheatbelt,Western Australia

Land and stream salinization in the Western Australian wheatbelt developed as a result of clearing of native vegetation. This caused a shift in the hydrological balance and a 10- to 30-fold increase in recharge. Rising water levels rejuvenated groundwater flow through paleao channels and increased baseflow to drainage lines. An increase in saline groundwater ascension (TDS 5000–30 000 mgl⁻¹) to the near-surface, induced by rising pressure heads in the deeper semi-confined aquifer system, advanced land and stream salinization. Areas in a catchment where groundwater flow is channelled upwards by subsurface geological strctures are especially susceptible to salinization and water logging. Enhanced discharge by windmill pumping is an engineering method to reduce excess pressure heads of deep semi-confined aquifers in discharge areas and, consequently, lower water levels at shallower depths. It is an interim solution to halt and prevent salinization in the wheatbelt of Western Australia. Long-term field experiments in three catchments and modelling have shown that windmill pumping (15–30 m³d⁻¹) in the wheatbelt can reduce water levels by 1 to 2 m at radial distance of more than 1 km after several years of pumping. The pumped groundwater can be reused for irrigation (TDS < 3000 mgl⁻¹) or stock water (TDS < 10 000 mgl⁻¹). In some instances, highly saline effluent can be discharged into saline streams or salt lakes. Often saline water disposal requires on-farm evaporation ponds or sacrificial basins. The enhanced discharge method should be part of an integrated catchment management program for the restoration of saline land and streams. the technology is applicable to other irrigated and non-irrigated salt-contaminated regions.

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−1 en minéralisation) à proximité de la surface, due à l'augmentation de la charge dans l'aquifère semi-captif plus profond, a provoqué la salinisation des soles et des cours d'eau. Dans un bassin versant où les écoulements souterrains s'organisent en remontant vers la surface, les structures géologiques favorisent particulièrement la salinisation et l'accumultion d'eau. Les prélèvements par pompage au moyen d'éoliennes est une technique destinée à réduire la charge dans les aquifères profonds semi-captifs dans les zones de décharge et donc à abaisser les niveaux aux plus faibles profondeurs. C'est une solution intermédiaire pour arrêter et éviter la salinisation dans les terres à blé de l'ouest australien. Des expériences de longue durée sur trois bassins versants et une modélisation ont montré que le pompage par éolienne (15–30 m₃.j₋₁) dans les terres à blé peut abaisser les niveaux piézométriques de 1 à 2 m dans un rayon de plus d'un km après plusieurs années de pompage. Les eaux pompées peuvent être utilisées pour l'irrigation (minéralisation inférieure à 3000 mg.1₋₁) ou pour des réservoirs (minéralisation inférieure à 10000 mg.1₋₁). Dans quelques cas, les eaux très salées peuvent être écoulées dans des cours d'eau ou dans des lacs salés. En général, l'épandage d'eaux salées impose l'évaporation sur place dans des bassins. La méthode de prélèvement d'eau doit être un élément du programme de gestion intégrée des bassins versants en vue de la restauration des sols et des cours d'eau salés. La technologie est transposable et appliable à d'autres régions contaminées par le sel, irriguées ou non.

     

The late orogenic timing of mineralisation in some slate belt gold deposits,Victoria, Australia

Gold mineralisation in classic Australian slate belt gold deposits at Ballarat, Bendigo, St. Arnaud and Inglewood occurred very late in the orogenic history of these rocks rather than during formation of the main slaty cleavage. This has been revealed through the examination of microstructural relationships in gold-bearing quartz veins and their host rocks from these deposits, which has established a D₁ to D₄ deformation-stage history and consistent timing for gold mineralisation over a wide area. The gold was deposited synorogenically but during the fourth deformation stage (D₄) of the orogeny, a relatively weak event occurring two deformations after the main slaty cleavage producing event, D₂. Previously, D₂ had been regarded as both the source and control of gold mineralisation as most of the quartz veins that occur in these deposits formed before or during this deformation event. However, most gold is hosted in breccia veins that formed during D₄. The wallrock clasts within these breccia veins contain a young rotated foliation and the breccia veins are spatially associated with a paragenetically consistent alteration of the host rocks in the deposits. This alteration both crosscuts and preferentially mineralises wallrock S₄ allowing the timing of the breccia veins, alteration and gold deposition to be defined as syn-D₄ in age.     

The Paroo Station Mine supergene lead deposits,Western Australia: Geological and geochemical constraints

The Paroo Station Mine lead project is situated near Wiluna in the central part of Western Australia. It is located in an outlier of the Paleoproterozoic Earaheedy Group overlying the southeast Yerrida Basin. The total Measured and Indicated Mineral Resource is currently (as at 31st of December 2014) estimated at 31.8 Mt of ore at 4.4% Pb. The deposits are hosted by deeply weathered carbonate and clastic sediments of the Paleoproterozoic Yelma Formation. The lead-only mineralisation contains only traces of sulfides and is composed of few supergene minerals, predominantly cerussite and anglesite with minor pyromorphite and plumbogummite. The Paroo Station Mine demonstrates a high degree of supergene Pb mobility which has resulted in almost complete destruction of primary Pb distribution pattern and development of characteristic for the oxidation zones coupled depletion zone and subhorizontal enrichment blankets below.The deposits are interpreted to have formed from Mississippi Valley-type (MVT), stratabound relatively low-grade sulfide PbZn mineralisation located within dolomites of the Sweetwaters Well Member of the Yelma Formation. The primary mineralisation based on modelling the Pb distribution was controlled by a network of northeast, northwest and meridional oriented fractures and small faults. Lead isotope study yielded from 1.80 to1.88 Ga ages of the mineralisation based on the Stacey and Kramers (1975) two-stage model; the mineralising event is related to the Capricorn Orogeny. This data is well in line with previous UPb dating of monazites interpreted to be related to the mineralisation (1.82 Ga; Muhling et al., 2012). High μ (²³⁸U/²⁰⁴Pb) values (10.26–10.58) are indicative of the Archean basement contribution and/or more likely with sediments originally derived from that basement. The Paroo Station Mine mineralisation is thought to be similar in style and formed synchronously with the MVT prospects in the Teague area in the Earaheedy Basin approximately 120 km to the northeast.The mineralisation at the Paroo Station Mine was significantly upgraded through a multi-stage supergene remobilisation and reprecipitation of lead in the course of long weathering history of the region. Lead was almost completely depleted from the upper zone and accumulated below as subhorizontal enrichment blankets at three levels (from top to bottom): (1) the base of the collapse quartz-clay breccia unit marking the dolomite weathering front; (2) the contact of siltstone and sandstone packages coincident with the feldspar/kaolinite weathering front and; (3) the mid- to lower parts of the sandstone unit close to the base of weathering. The lowest enrichment zone extends as a continuous blanket at a significant, up to 500 m distance (at a 1% Pb cut-off) beyond the upper mineralisation projection. This enrichment is interpreted to be formed by lateral Pb transport in waters close to the groundwater table through the most permeable layers of weathered sandstones. This active lateral lead dispersion is of high exploration significance for the region providing a much larger exploration target and vector to the primary mineralisation.     

The crustal continuum model for late-Archaean lode-gold deposits of the Yilgarn Block,Western Australia

Most Archaean gold ores belong to a coherent genetic group of structurally controlled lode-deposits that are characteristically enriched in Au with variable enrichments in Ag, As, W, Sb, Bi, Te, B and Pb, but rarely Cu or Zn, and are surrounded by wallrock alteration haloes enriched in K, LILE and CO₂, with variable Na and/or Ca addition. Evidence from the Yilgarn Block of Western Australia, combined with similar evidence from Canada and elsewhere, indicates that such deposits represent a crustal continuum that formed under a variety of crustal régimes over at least a 15 km crustal profile at PT conditions ranging from 180°C at <1 kb to 700°C at 5 kb. Individual deposits, separated by tens to hundreds of kilometres, collectively show transitional variations in structural style of mineralisation, vein textures, and mineralogy of wallrock alteration that relate to the PT conditions of their formation at varying crustal depths. Specific transitions within the total spectrum may be shown also by deposits within gold camps, although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt. Recognition of the crustal continuum of deposits implicates the existence of giant late-Archaean hydrothermal systems with a deep source for the primary ore fluid. A number of deep fluid and solute reservoirs are possible, including the basal segments of greenstone belts, deep-level intrusive granitoids, mid-to lower-crustal granitoidgneisses, mantle lithosphere, or even subducted oceanic lithosphere, given the probable convergent-margin setting of the host greenstone terranes. Individual stable and radiogenic isotope ratios of fluid and solute components implicate fluid evolution from, or equilibrium with, a number of these reservoirs, stressing the potential complexity of pathways for fluid advection to depositional sites. Lead and strontium isotope ratios of ore-associated minerals provide the most persuasive evidence for fluid advection through deep-level intrusive granitoids or granitoid-gneiss crust, whereas preliminary oxygen isotope data show that mixing of deeply sourced fluid and surface waters only occurred at the highest crustal levels recorded by the lode gold deposits.     

C and ESR dating of calcrete and gypcrete cores from the Amadeus Basin, Northern Territory, Australia

More than forty calcrete and gypcrete samples from bore holes in the Curtin Springs area of the Amadeus Basin, Central Australia, have been dated by means of the ¹⁴C and ESR techniques. The results indicate that the application of multiple dating techniques to samples from calcrete and gypcrete deposits enables age limits to be expanded and the number of datable material types to be increased with respect to Quaternary studies. The ability of the ¹⁴C and ESR techniques to determine the approximate age of calcrete and gypcrete deposits has been demonstrated.     

Secondary geochemical dispersion patterns associated with the nickel sulphide deposits at Kambalda,Western Australia

Geochemical investigations have shown that the nickel sulphide mineralisation at Kambalda is detectable by sampling and analysing the minus 80-mesh fraction of near-surface soils. Over unmineralised mafic and ultramafic rocks the distribution of Ni in soils accurately reflects bedrock Ni distribution. In the environment of sulphide mineralisation Ni is preferentially concentrated in the coarse fractions of the soils, with a corresponding depletion in the minus 80-mesh fraction. Both Ni and Cu nevertheless display strong anomalies in minus 80-mesh fraction soils in the immediate vicinity of sub-outcropping mineralisation.