Main Lode Research Articles

Alteration patterns linked to high-grade gold mineralization at the Wattle Dam deposit, Western Australia

Sharp physio-chemical gradients occur in Archean gold systems, yet they are often poorly defined. These physio-chemical gradients can provide windows into fluids that deposit high-grade gold mineralization. Recognizing different alteration styles and fluid-types in mineral systems helps define processes that led to the deposition of high-grade gold. Here we report on the architecture and chemical gradients across the komatiite-hosted Wattle Dam gold deposit in the eastern Yilgarn Craton, Western Australia, which is one of Australia’s highest grade-gold deposits. The deposit notably contains millimeter- to centimeter-thick gold-only veins with an absence of quartz. Whole rock geochemistry shows that high-grade gold veins are hosted in komatiite flow-margins facies with olivine orthocumulate komatiites in the footwall, interbedded graphitic shale close to the lode and komatiite basalt in the hangingwall. The komatiite flow-margin facies that hosts high-grade gold veins have been pervasively altered by stage 1 biotite-actinolite. Stage 2 gold veins cross-cut the biotite-actinolite altered komatiite and these veins contain thin (<100 µm) selvages of sodicgedrite, riebeckite, aegirine and ullmannite ± sulfates (barite and anhydrite) ± magnetite. Millimeter-thick gold-only veins are associated with the dissolution of zircon and magnetite, and precipitation of catapleiite in veins. These textures are consistent with the contention that fluids mobilized Zr and Fe and were highly reactive. Arsenic- and Sb-bearing alteration in the form of cobaltite, arsenopyrite, nickeline, gersdorffite, breithauptite and ullmannite occur as wide halos (up to 300 m) around the main lode. These As and Sb bearing-phases form in both the hangingwall komatiite basalt unit and footwall olivine orthocumulate komatiites. Arsenides and antinomides contain inclusions of Au and can be directly linked to mineralization. The diverse array of mineral assemblages associated with high-grade and low-grade gold mineralization indicates sharp chemical gradients at the time of gold mineralization. The mobility of Zr and Fe at the time of mineralization in the core of the deposit points towards the presence of alkaline fluids, whereas peripheral zones are associated with minerals associated with the near-neutral H2O-CO2 fluids, commonly linked with Archean gold systems. The generation of carbonate-undersaturated alkaline fluids in the high-grade core of the Wattle Dam deposit, may be explained by phase-separation or a CO2-poor alkaline fluid source. Either way these alkaline fluids played a critical role in mobilizing and concentrating very high amounts of gold into veins.

Tree-ring analyses on Bronze Age mining timber from the Mitterberg Main Lode, Austria - did the miners lack wood?

Wood was an essential raw material for mining maintenance in historic and prehistoric times. During the Bronze Age, the Mitterberg mining region in the Austrian Alps was one of the most important producers of copper and, consequently, a consumer of huge amounts of wood. Since the 1960s, archaeological investigations at the Troiboden dressing site at the Mitterberg main lode have uncovered mining timber both single finds as well as box-shaped wooden constructions like wet-tyes which were used to wash and concentrate crushed copper ore. Dendrochronological analyses on a set of mining timber yield calendar dates for these mining activities: a boom phase from the 14th to the 13th century BC, including two felling phases – one in the 1370s and a second from the 1290s to 1270s BC – are verifiable so far. As a result, the wood supply from nearby forests may have been exhausted rapidly, which is confirmed by palynological records from bogs in the vicinity of the excavation site. By utilising a tree-ring growth–elevation model developed by Dittmar et al. (2012), the elevations of growing sites were estimated to detect where the mining timber might have originated. To examine the model outcomes, we used tree-ring data with known origin, i.e., series from living trees and subfossil samples from the Troiboden and its vicinity. The results for the mining timber suggest that the prehistoric miners utilised trees from the vicinity of the mining site in the 14th century but at least partly sought wood from growing sites at lower elevations to continue mining during the 13th century BC.

The Uranium Prospects in Mika Region, Northeastern Nigeria

Uranium exploration especially in currently non-producing countries like Nigeria possesses high economic prospect. This study investigates a new uranium prospect in Mika, Northeastern Nigeria. The Mika uranium mineralization is located in Mika, Taraba State. Two lodes were identified and additional nine (9) trenches were added. The main lode in the west extends about 10 m, ore vein strikes 348°, inclination 42° and the strike of the lode 306° with average uranium content of 18%. The eastern trench is about 8 × 4 × 6 m which exposes a veinlet of pitchblende. From the petrography, one can deduce that the granitic host rock has suffered deformation resulting in crushing of quartz crystals and stretching of plagioclase. The uraninite and chalcedony in the late phase filled up the fractures along the crystal grain boundaries as veinlets. The laboratory gamma ray analysis of the samples showed that the secondary uranium content is 0.1%, while the primary uranium ore grade is 1.5%. Radiometric in situ measurements showed that Th and K ranges from 47.3 - 3654 ppm and 4.26% - 6.26% respectively. From the survey, a strong radiometric zone extends 800 × 35 m in an NW-SE direction and has highest radiation content of 1200 cpm against the background count rate of 30 cpm. Generally, the uranium concentrations in the ores in Mika area range from 0.03% - 0.12%. Since only the surface occurrences have been explored, the study area is a good prospect for future development when properly explored. The North-eastern Nigeria has been roughly explored by Nigeria Uranium Mining Company (NUMCO) in 1980, but no formal exploration had been followed after that, especially near the site of this article. The company (ACE Mines, Ltd) of the first author was among the few uranium mining companies approved in the list of Nigerian Geological Survey agency of Abuja in Nigeria. The authors determined the coordinates of the site in their first survey which formed the prospect area in the licence.

Mineral equilibria and zircon, garnet and titanite U-Pb ages constraining the PTt path of granite-related hydrothermal systems at the Big Bell gold deposit, Western Australia

The Big Bell deposit (75 t gold) is located in a narrow spur of the Meekatharra greenstone belt, Yilgarn Craton, Western Australia. Two ore bodies are located in a calcic-potassic contact alteration zone overprinting lineated granodiorite dykes and amphibolite: almandine-cummingtonite-hornblende skarn (1–3 g/t Au, 1700 g/t As, 330 g/t W) and the muscovite-microcline gneiss (3–5 g/t Au, 580 g/t Sb, 620 g/t W) of the Main Lode. Genetic models vary from pre- to post-metamorphic replacement. Hornblende-plagioclase pairs in amphibolite constrain peak metamorphic temperature to 670 ± 50 °C. In contrast, garnet-biotite thermometry provides estimates of 578 ± 50 and 608 ± 50 °C for garnet-cordierite-biotite schist bordering the skarn and enveloping the Main Lode. Garnet-cordierite and garnet-hornblende pairs extend the range of fluid temperature to 540 ± 65 °C, well below peak metamorphic temperature. At 540–600 °C, the alteration assemblage andalusite + sillimanite constrains pressure to 300–400 MPa corresponding to 11–14 km crustal depth. Published U-Pb ages indicate that metamorphism took place in the aureole of the southeast granodiorite-tonalite batholith (2740–2700 Ma), followed by gold mineralization at 2662 ± 5 Ma and by the emplacement of biotite granite and Sn–Ta–Nb granite-pegmatite dykes at 2625–2610 Ma. Amphibolite xenoliths in granite northwest of the deposit record the lowest temperature (628 ± 50 °C), suggesting it lacks a metamorphic aureole. The rare metal dykes are spatially associated with epidote-albite and andradite-diopside skarns (≤1.5 g/t Au), mined where enriched in the weathered zone. We analysed hydrothermal zircon intergrown with andradite. Concordant U-Pb ages of 2612 ± 7 and 2609 ± 10 Ma confirm the presence of a second granite-related system. The zircons display oscillatory zoning and have low Th/U ratios (0.05–0.08). Low-Th titanite from an albite granite dyke has a concordant but reset U-Pb age of 2577 ± 7 Ma.

Broadband incoherent virtual array beamforming on small scale array for passive detection

It cannot be guaranteed that array spacing corresponding to half-wavelength of each frequency during broadband signals passive detection process. Therefore, for high-frequency, side lobe will rise when array spacing is half the wavelength of the low-frequency. In addition, the main lobe of small-scale array will be wider due to limited array aperture. In order to resolve these two disadvantages for passive detection of small-scale array, virtual array beamforming based on time-delay information is studied in this paper. But this technology often requires high sampling frequency which will increase computational complexity. In order to use virtual array beamforming to processing broadband signals at lower sampling frequency, broadband incoherent virtual array technology based on phase information is introduced in this paper. The effectiveness of these two methods is shown by simulation and sea experiment.

Structure of magnetite lodes at the Estyunino iron deposit in the central Urals

The structure of magnetite lodes is determined by iron and sulfur distribution, as well as texture and structure of ore. These features have been revealed by documentation of cores from ore intervals in exploration boreholes penetrating two main lodes 21 and 22 of the Estyunino iron deposit. The documentation of cores was accompanied by sampling for microscopic examination of texture and structure of ore and selection of Fe and S contents in ore. Then these data were summarized as sections of the lodes. It was established that the structure of magnetite lodes is characterized by conformable ore layers distinguished by texture, structure, and Fe and S contents. Banded and spotty ores containing less than 50% magnetite are predominant. Layers of homogeneous massive ore are infrequent. The textural pattern indicates a volcaniclastic nature of host rocks. The spotty texture is characteristic of hyaloclastites with vitreous shards. The banded texture with oriented distribution of fiamme is inherent to volcaniclastic rocks. In both cases, magnetite selectively replaces dark-colored vitreous fragments and is also dispersed in the salic matrix and lava fragments. No indications of crosscutting superposed relationships are observed. The available data can be satisfactorily explained by an impregnation-metasomatic mode of ore deposition.

An Interdisciplinary Study on the Environmental Reflection of Prehistoric Mining Activities at the Mitterberg Main Lode (Salzburg, Austria)

A multi-proxy study by palynological, geochemical, archaeological and dendrochronological analyses discloses the mining activities at the Mitterberg Main Lode. By these means, several mining phases with varying intensity are recorded during the Bronze and Early Iron Age, whereupon a west to east shift of the mining activity at the Mitterberg Main Lode can be observed. The initial mining phase (Phase II), from the 21st to the 15th centuries bc, is characterized by an opening up of the forest vegetation and, additionally, by slightly elevated heavy metal deposition. Phase III shows a first bloom phase of the chalcopyrite mining during the 14th and 13th centuries bc. Pollen analyses disclose extensive clearings used for pasture and settlement. The increased human impact and higher heavy metal pollution suggest intensive mining activity, which is corroborated by the dendrochronological and archaeological data. Phase IV is characterized by mining activities in progress during the 12th century bc. The pollen data reflect a stabilization of the vegetation and slightly elevated As/Cu/Sb to Sc ratios. During Phase V, in the 11th century bc, new clearings indicate a re-intensification of the mining activities at the Mitterberg Main Lode. Phase VI, from the ninth century bc onwards, describes a human impact with lower intensity at the mining site. This interdisciplinary study at the Mitterberg Main Lode contributes new environmental data for an important area of past metal mining and extends our understanding of the relationship between miners and their landscape.

Structural Evolution of a World-Class Epithermal Orebody: The Martha Hill Deposit, Waihi, New Zealand

We describe the complex structure in the highest (now mined-out) levels of the world-class Martha Hill gold-silver deposit, Coromandel peninsula, which is hosted in late Miocene andesitic rocks. A northeast-striking block of rock 360 m long and up to 100 m wide was occupied by a complex vein network dominated by northeast-striking gold-silver lodes up to 30 m thick. Gold production was entirely from quartz veins. The mineralization is embedded in the following geological history: (1) deposition of the host volcanic rocks, (2) initial tectonic fracturing, (3) intrusion of clastic dikes and sills, (4) start of hydrothermal alteration, (5) main phase of faulting, (6) main phase of veining, (7) erosion and deep oxidation, (8–11) deposition of further volcanic material, including prominent rhyolites, and formation of present topography. Veins occupied fractures but also followed a network of preexisting, mostly normal faults with a small to medium amount of displacement. Faulting involved northwest-southeast, north-south, and east-west extension, implying complex 3-D strain. Clastic dikes of carbonaceous sandstone intruded during the faulting phase. Fault rocks were mostly brittle cataclasites or breccias, but some clay-rich zones displayed ductile structures. Many veins overprinted faults, with a change from shear-mode to opening-mode deformation. Stockwork veins formed a mesh composed of two orthogonal sets: one with northeast-southwest– and northwest-southeast–striking veins, including the dominant mineralized lodes, and the other with north-south– and east-west–striking veins. Although northwest-striking veins were earliest, during the main vein phase, opening oscillated between all the vein directions, with the two main lodes in the open pit area, the Martha and the Welcome, capturing most of the strain. A variety of vein and breccia textures indicate repeated structurally controlled vein opening events that were associated with changes in the physical and chemical conditions of the mineralizing fluid. A special concentration of structural features facilitated focusing of fluid flow to produce this world-class deposit, which contrasts with the less well endowed vein systems around it. The overall tectonic control was dominated by northwest-southeast extension and dip-slip deformation. The 3-D strain in this area was not only due to local interference of differently oriented structural features but also to the superposition of regional tectonic north-northwest and northeast trends associated with migration of a subduction zone past the Coromandel peninsula. This demonstrates that structural control in an epithermal mineral deposit may originate from a number of different tectonic controls at different scales.

Generation of Asymmetrical Difference Patterns from Array Antennas

It is common to produce sum patterns from array antennas. These patterns are basically symmetric with one main lode in bore sight and symmetric sidelobe structure around the main beam. The antennas associated with the marine radars are required to produce patterns with asymmetric side lobes. Asymmetrical patterns are very useful for marine radars when ships sail in turbulent water where roll and pitch exists.In particular the sidelobes in one side are lower than those of the other side. It is also essential to produce difference patterns with asymmetric structure from the boresight direction. These patterns are required to have null in the bore sight and high difference slope in the same direction. When these patterns are required from marine radars, the side lobes associated with the difference pattern on the either side of the boresight is at the different heights. In the present work, such useful patterns are generated from the newly designed arrays. The amplitude and phase distributions are designed for small and large arrays. The radiation patterns are numerically computed and they are presented in udomain. Keywords—asymmetric patterns, arrays; array antennas; marine radars; amplitude and phase distributions.

A geological investigation of the Waterberg hydrothermal platinum deposit, Mookgophong, Limpopo Province, South Africa

The geology around the unusual quartz-vein hosted Waterberg platinum deposit was mapped using aerial photographs at a scale of 1 : 1000 to establish possible controls on this unique style of mineralisation. The study area is located in the Mookgophong (Naboomspruit) District of the Limpopo Province in South Africa and is dominated by a gently to moderately, NNW-dipping irregular sequence of bimodal (rhyolite–basalt) volcanics, interbedded with many sedimentary rocks, that form part of the extensive 2˙06 Ga Rooiberg Group, that are cut by variably striking, steep to subvertical quartz veins. The regional geology of the area was mapped at various scales in the 1920s and 1940s, but much of this work remains inaccessible and mapping during this study has uncovered more detail. The major platiniferous quartz vein (known as the 'Main Lode') juxtaposes volcanic rocks of Rooiberg age against Triassic sandstones and therefore occupies a fault. There is no textural evidence that quartz veining was syntectonic and may be much younger. The well-exposed vein fabrics show many features typical of low pressure epithermal quartz veins and probably formed near the present-day surface beneath a highly eroded Karoo cover, suggesting a recent age for veining and Pt mineralisation. The Main Lode can be followed for at least 3 km along strike, but Pt mineralisation is concentrated in a short section of the vein around 500 m in length. A major regional fault (the Welgevonden Fault) occurs about 2 km to the northwest of, and parallel to, the Main Lode. The Welgevonden Fault zone is suggested to extend to the northeast and merge with the Planknek–Ysterberg Fault system, south of Mokopane (Potgietersrus). The Welgevonden Fault is presently geothermally active, but this fluid system does not carry significant concentrations of Pt and may represent a younger Pt-depleted stage of hydrothermal activity related to the event that formed the Main Lode. It is unlikely that the Rooiberg volcanics and younger rocks supplied the platinum-group elements in the deposit and the source for these metals must lie elsewhere. It is suggested that shearing along the Welgevonden–Plankneck Fault system transposed PGE-bearing mafic rocks of the northern Bushveld Complex to a position at unknown depth beneath the Waterberg deposit, or provided a network of shear zones which could be exploited by a recent hydrothermal event that transported platinum and other metals westwards along the fault system and redeposited them near the surface.

Vein deflections and thickness variations of epithermal quartz veins as indicators of fracture coalescence

Epithermal quartz veins at the Broken Hills gold deposit, New Zealand, strike N-S and dip steeply westward. Small changes in the orientations of the three main lodes and associated mesoscopic veins define deflection lines that rake steeply in the vein plane. A method of constructing the opening vector from the three-dimensional geometry of a vein deflection is presented. The resulting vectors plunge steeply and are at low angles to the main veins and the deflection lines, indicating a large component of normal dip-slip shear during the opening of these veins. Vein thickness distributions are power-law, with similar fractal dimensions to previously reported values for non-stratabound vein arrays. The vein system at Broken Hills developed by linking of isolated extension-dominated shear veins with shear-dominated veins, generating sub-vertical and sub-horizontal fluid flow pathways.

Sub-Audio Magnetic Survey Experiments for High-Resolution, Subsurface Mapping of Regolith and Mineralisation at the Songvang Gold Mine near Agnew, Western Australia

Experimentation with Sub-Audio Magnetic (SAM) survey parameters over the Songvang mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and differential weathering in the regolith at high resolution to a depth of 100 m, as long as the transmitter electrodes are placed sub-parallel to the strike direction of features to be detected. The equivalent magnetometric resistivity (EQMMR) response produced by horizontal current channelling between transmitter electrodes has a very similar pattern to gradient-array apparent resistivity results using a 50 m or less dipole spacing. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and the 2 m along-line sample density provides much greater resolution. The same area was surveyed using the same parameters, except that the transmitter electrode directions were at a right angle, and this produced radically different EQMMR results. EQMMR anomaly trends running N–S were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The Songvang main lode is an elongated, shallow plunging, rod-like zone of gold-pyrite ore that is detected by a gradient-array induced polarization survey. SAM equivalent magnetometric induced polarization (EQMMIP) was ineffective at imaging the main lode using surface transmitter electrodes and 1 Hz transmitter frequency. However, the 4 Hz EQMMIP response of the main lode was later imaged in great detail when transmitter electrodes were placed down boreholes into the orebody. The downhole SAM EQMMIP results were very effective at highlighting zones of intense sulphide-related gold mineralisation ahead of drilling. SAM surveying provided EQMMR data that identified shallow, conductive shears that contained economic gold mineralisation, and the EQMMIP data helped to plan resource definition drilling by prioritising chargeable targets related to increased pyrite alteration and gold grade along the Songvang main lode.

Shear zone versus fold geometries at the Cannington Ag–Pb–Zn deposit: implications for the genesis of BHT deposits

Existing structural analysis of drill-core and sulphide zonation were taken as evidence for a large-scale synform that dominates the geometry of the Cannington Ag–Pb–Zn deposit—a Broken Hill-type (BHT) deposit. Underground mapping in this study has found that northwesterly directed thrusting took place during peak-temperature D 1, and that S 1 gneissosity is continuous around an amphibolite mega-boudin. D 2 sillimanite–biotite shear zones contain asymmetric folds previously interpreted to be parasitic folds, but in this study they are interpreted as due to easterly-directed extension of S 1. S 1 and S 2 indicate that shear zones were continuous and not folded into the current deposit geometry. D 2 is associated with garnet–pyroxene alteration that proved to be a favourable host for low-temperature sulphide precipitation. Retrograde deformation post-D 2 controlled lode geometry. Such a concept of high-temperature shear zone formation can be applied to other BHT deposits such as the Broken Hill Main Lode and the Aggeneys–Gamsberg base metal deposits in South Africa. In the shear zone model, Cannington and other BHT deposits are situated within high-temperature thrust shear zones that formed in conjunction with regional large-scale folding.

TELLURIDE MINERALOGY OF THE GOLDEN MILE DEPOSIT, KALGOORLIE, WESTERN AUSTRALIA

The Golden Mile, Kalgoorlie, Western Australia, is a mesozonal gold deposit unusual because of its enormous size (>1,457 tonnes of gold) and because tellurides are responsible for approximately 20% of gold production. Gold mineralization is hosted primarily by Archean-aged dolerites and basalts that have been metamorphosed to the greenschist facies. This mineralization occurs in hundreds of auriferous and telluride-bearing lodes, which have been classified into three structural types on the basis of orientation: main (trending NNW), caunter (trending NW) and cross lodes (trending NE). Three distinct styles of hydrothermal mineralization are present in the deposit. The early Fimiston-style lodes are characterized by main, caunter and cross lodes. The Oroya-style lodes (V- and Te-rich) occur in later dilational jogs and veinlets within a shear zone. Both of these are overprinted by the younger Mount-Charlotte-style mineralization (quartz-vein stockwork). Nineteen tellurium-bearing minerals occur in the Golden Mile deposit. Calaverite, petzite, coloradoite, altaite and native gold are most common throughout the deposit, whereas tellurantimony, melonite, hessite, stutzite, krennerite and sylvanite are rare in the main and caunter lodes. In addition, hessite and montbrayite are found in trace amounts in the cross and main lodes, respectively. There is no systematic distribution of tellurides within a given lode. Tellurides in Fimiston-style ores formed at ca. 300° to 170°C during cooling of the deposit. Values of f (Te2) and f (S2) determined at 300°C on the basis of the stability of native elements, tellurides and sulfides, are ca. log f (Te2) in the range −11.4 to −6.8 and log f (S2) in the range −12.6 to −5.5. No spatial variations in the composition of native gold, tetrahedrite-group minerals and tellurides occur in the Golden Mile deposit, although there is a weak to moderate decrease in gold fineness with depth. Marked differences in telluride mineralogy in and between individual lodes likely reflect variations in physicochemical conditions during ore formation or mechanisms of precious-metal deposition.

Microthermometry and chemical composition of fluid inclusions from the Mt Chalmers volcanic-hosted massive sulfide deposits, central Queensland, Australia: implications for ore genesis

Mt Chalmers is a mound-style, volcanic-hosted massive sulfide (VHMS) deposit in central Queensland, Australia. The ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of Early Permian age. The two ore lenses (Main lode and West lode) consist of Cu–Zn–Pb massive sulfide underlain by Cu-rich stringer mineralization. Textural and petrographic investigations of fluid inclusions indicate that primary Type I inclusions up to 25 μm are found in quartz from the stringer mineralized zone, and microthermometric studies of these inclusions yielded homogenization temperatures of 160–285 °C and salinities of 5–10.5 NaCl equiv. wt.%. Laser Raman spectroscopic (LRS) analysis indicates the presence of CO 2 (0.1–1 mol%) in the Mt Chalmers VHMS systems. Semiquantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Mt Chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids. Cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids. High base metal content and the presence of CO 2 in the fluid inclusions imply that magmatic input of ore metals, copper in particular, accompanying seawater leaching of the footwall volcanic pile is a distinct possibility. In terms of fluid composition, the K–Ca–Fe variation of the Mt Chalmers ore fluid is comparable with those of typical epithermal deposits (e.g., Thames, New Zealand) and porphyry copper deposits (Bingham, UT). The Cu–Zn–Fe/10 plot of the Mt Chalmers ore fluids, indicates that there is significant copper in the system, comparable to copper enrichment in a porphyry copper system. The Mt Chalmers ore fluids also show similar copper content with the Cu-rich end-member ore fluid composition of the mineralized Stage 2B veins of the Hellyer VHMS deposit, whereas the Stage 2A veins of Hellyer are more enriched in lead and zinc. Overall, the ore fluids have a variable chemistry with a continuum of compositional data from VHMS to epithermal-porphyry style ore fluids. Shallow-water emplacement (<300 m) for the VHMS mineralization has been postulated for the Mt Chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies. However, fluid inclusion studies do not rule out a moderate to deeper submarine environment, as there is no fluid inclusion evidence of boiling. At Mt Chalmers, boiling of ore fluids would probably have occurred (as in most epithermal systems) if the ore fluid exhaled onto the seafloor under a shallow environment.

Orogenic gold and geologic time: a global synthesis

AbstractOrogenic gold deposits have formed over more than 3 billion years of Earth’s history, episodically during the MiddleArchean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristi-cally associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustalstructures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts andfluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which arecommonly intimately associated with this mineral deposit type, recognized production and resources from economicPhanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversialWitwatersrand ores, known Precambrian gold concentrations are about half this amount.The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most ofthe world’s major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in spaceand time. There are few well-preserved blocks of Middle Archean mid-crustal rocks with gold-favorable, high-strain shearzones in generally low-strain belts. The exception is the Kaapvaal craton where a number of orogenic gold deposits arescattered through the Barberton greenstone belt. A few )3.0 Ga crustal fragments also contain smaller gold systems in theUkrainian shield and the Pilbara craton. If the placer model is correct for the Witwatersrand goldfields, then it is possiblethat an exceptional Middle Archean orogenic-gold lode-system existed in the Kaapvaal craton at one time. The latter half ofthe Late Archean ca. 2.8–2.55 Ga was an extremely favorable period for orogenic gold-vein formation, and resulting oresŽ.preserved in mid-crustal rocks contain a high percentage of the world’s gold resource. Preserved major goldfields occur ingreenstone belts of the Yilgarn craton e.g., Kalgoorlie , Superior province e.g., Timmins , Dharwar craton e.g., Kolar ,Ž. Ž . Ž.Zimbabwe craton e.g., Kwekwe , Slave craton e.g., Yellowknife , Sao Francisco craton e.g., Quadrilatero Ferrifero , andŽ. Ž . Ž .Tanzania craton e.g., Bulyanhulu , with smaller deposits exposed in the Wyoming craton and Fennoscandian shield. SomeŽ.workers also suggest that the Witwatersrand ores were formed from hydrothermal fluids in this period.The third global episode of orogenic gold-vein formation occurred at ca. 2.1–1.8 Ga, as supracrustal sedimentary rocksequences became as significant hosts as greenstones for the gold ores. Greenstone–sedimentary rock sequences nowexposed in interior Australia, northwestern Africarnorthern South America, Svecofennia, and the Canadian shield were the

A carbonate-dominated copper-cobalt breccia-vein system at the Great Australia Deposit, Mount Isa eastern succession

Great Australia is a small Cu-Co deposit in the Eastern succession of the Mount Isa inlier, northern Queensland. It may be viewed as a carbonate-rich member of the globally distributed, oxide-dominated, Fe-Cu-Au + or - rare earth element (REE) + or - U deposit group. It is also a gold-poor variant of a group of small- to medium-sized, carbonate-dominated vein-replacement deposits in the surrounding Cloncurry copper gold field, known as Mount Freda-style deposits. It lies within the Proterozoic Toole Creek Volcanics (TCV), adjacent to and within a major splay of the Cloncurry fault, which forms a regional tectonic contact with the metasedimentary Corella Formation and is consequently a fundamental structure in the inlier. The deposit has a gangue mineral assemblage of dolomite-calcite-quartz-pyrite (ore type 1) transitional to amphibole-quartz-pyrite (ore type 2), with mineralization largely restricted to thick, tabular veins and fault zones. Mineralization that is deeper in the explored deposit, within the sheared TCV/Corella Formation contact, or in magnetite-altered wall rocks is pre-dominantly ore type 2. The deposit occurs within a dilational jog that developed during D 3 dextral fault reactivation. Plunging ore shoots occur at the intersections of this contact with a north-south-striking fault (the Main lode), forming thick veins. Strong alteration halos exist around the ore zones within the TCV, and are subdivided into a regional, pre- to syn-D 2 albite-actinolite alteration (sodic-calcic I), and a local ore-related, albite-actinolite zone (sodic-calcic II); the latter has a fringing biotite alteration envelope. Alteration within the Corella Formation is geochemically similar to but more subdued than that of TCV alteration. Chalcopyrite-bearing secondary fluid inclusions from ore type 1 quartz contain high-temperature (450 degrees + or - 60 degrees C), high-salinity (45 + or - 10 wt % NaCl equiv), high-f (sub O 2 ) fluids that are interpreted to be syn-ore. Oxygen isotope values of delta 18 O quartz (11.3-13.4ppm) indicate that delta 18 O fluid = 8.3 to 10.3 per mil, consistent with a magmatic to metamorphic source. Calcite carbon isotope values require delta 13 C fluid = -0.3 to -3.7 per mil, best explained by a mixture of carbonate sedimentary carbon and magmatic carbon.The genetic model involves the emplacement of ore fluids into the previously altered Cloncurry fault at high fluid pressures and temperatures, initially forming zoned magnetite-albite-dominant alteration grading out to biotite-dominant alteration (sodic-calcic II) that was contemporaneous with carbonate-amphibole-magnetite-cobaltian pyrite-dominant veining and replacement within the dilational jog. Late in the history of carbonate precipitation, sulfate-bearing metalliferous fluids were introduced and were rapidly reduced by earlier magnetite, biotite, and pyrite-bearing alteration. Reduction of fluid sulfate and mineral sulfidation resulted in coprecipitation of chalcopyrite, chlorite, and minor gold, commonly replacing pyrite, magnetite, biotite, amphibole, and carbonate.

Geology and mineralization of the Cannington Ag-Pb-Zn deposit; an example of Broken Hill-type mineralization in the eastern succession, Mount Isa Inlier, Australia

The Cannington Ag-Pb-Zn deposit represents an important new discovery of Ag-rich Broken Hill-type mineralization in the Eastern succession of the Mount Isa inlier. The deposit is concealed beneath 10 to 60 m of Recent and Cretaceous cover, and there is no oxidation profile preserved at the basement subcrop. Mineralization is hosted by amphibolite facies migmatitic quartzofeldspathic gneisses, and is characterized by intense deformation and metamorphism, with complex metasomatic and retrograde overprints. Lithostratigraphic correlations of the host lithologies with other units in the Eastern succession are unclear. Limited dating of probable stratigraphic equivalents has given an age of 1677 + or - 9 Ma, which is broadly coeval with host depositional ages for Pb-Zn-Ag mineralization at Mount Isa, HYC and Broken Hill.The orebody is divided on the basis of late structural displacement into Northern and Southern zones. The Southern zone is the focus of current development, and mineralization occurs as crudely strata-bound massive sulfide lenses that display complex brittle and ductile disruption. A large-scale isoclinal D 2 synform within the Southern zone appears to control broad repetition patterns between ore lenses. Grade control within individual ore zones can also be related to zones of ductile strain and metasomatism influenced by strain partitioning around the termination of the Core Amphibolite.Mineralization within the Cannington Southern zone is divided into five main economic lode horizons that incorporate 10 mineralization types. These types are defined on the basis of distinctive zonations in Pb/Zn ratios, and Fe-rich versus siliceous gangue lithologies. Fe-rich mineralization types are characterized by coarse-grained, equigranular hedenbergite, Mn-Fe pyroxenoid, magnetite, olivine, and fluorite mineralogies, zones of amphibole, almandine, ilvaite, pyrosmalite-dominant mineralogies with sulfide- and fluorite-rich ductile breccias are associated with extensive postpeak metamorphic metasomatism and retrogression. Siliceous mineralization types represent late-stage metasomatism, and are associated with further modification of mineralization and retrogression of Fe silicates. Siliceous mineralization types exhibit a distinctive low abundance of magnetite and fluorite.Dominant sulfides are galena and sphalerite, which show multiple generations and variable intergrowths. Subordinate magnetite-pyrrhotite with minor arsenopyrite-lollingite-chalcopyrite are characteristic of Fe-rich mineralization types. Pyrite is generally absent and is only locally associated with late structural and low-temperature metasomatic overprints. Extreme Ag enrichment is a consistent association of all mineralization types in the Cannington deposit, and is related to argentiferous galena with freibergite inclusions. High levels of Sb, Cd, As, Cu, and F are also a feature of specific mineralization types. When in full production, Cannington will be one of the world9s largest Ag producers.Cannington shows many similarities with the Broken Hill Main lode (New South Wales), and represents an important new example of a Broken Hill-type classification. However, the Ag enrichment that characterizes Cannington is unusual even for previously considered Ag-rich members of the classification. A genetic model is proposed that involves high-temperature metasomatic zone refining of a preexisting Fe-Ca-Mn-Pb-Zn-Ag-rich mineralized system.

The lode-porphyry model as dediced from the Camaru mesothermal granitoid-hosted gold deposit, southern Pará, Brazil

A new type of gold deposit is described for the Amazon Craton, in the Carajas greenstone belt province. Instead of mesothermal mylonite-hosted lodes or exhalative deposits already recognized in the Carajas Province, the Cumuru gold deposit is hosted by a calc-alkaline 2.82 Ga granodiorite pluton of volcanic arc affiliation. The Cumaru granodiorite is intruded in the horse-tail dilation sites of a shear zone that crosscuts the volcano-sedimentary mylonitic rocks of the Archean Gradaus greenstone belt. The Cumaru gold-bearing quartz veins lie along NE-SW trending fractures at the NW edge of the granodiorite pluton where disseminated ore in a stockwork array of veinlets is associated with the main lodes in a groundmass of hydrothermal phyllic alteration. Three kinds of fluids were identified in the Cumaru quartz veins. An aquo-carbonic fluid, immiscible at the time of ore formation, is interpreted as a metamorphic fluid related to shear zone. The second type of fluid comprises brines within the H2O-NaCl-KCl-CaCl2 system, interpreted as derived from the granodiorite residual fluids. The third fluid is a low-salinity meteoric water. The T-P conditions for the gold deposition constrained by the hydrothermal chlorite geothermometer and isochors calculated from the fluid inclusions microthermometric data. Values range from 300 to 350°C and 1.3 to 3.8kb. The low fO2 values of the ore fluids and the gold-sulfide paragenesis suggest that sulfur was in reduced state at the time of ore formation, conditions that favor gold transport as thiocomplexes. Two gold deposition events have been identified in the Cumaru deposit, both related with oxidation of the ore fluids. Initially oxidation was caused by immiscibility of the aquo-carbonic fluid and by wallrock sulfidation with decrease in fSa of the ore fluids. Subsequent mixing between aquo-carbonic fluid and aqueous brines brought about additional oxidation of the ore fluids with increase in fO2 and decrease in pH, favoring gold precipitation. Stable isotope data place important constraints on the source of the ore fluids: δ18O and δD values support the mixing between metamorphic and magmatic water and the δ 13 C data are consistent with mantle-derived carbonic fluids. The cumulative geological, structural, geochemical, fluid inclusions and isotopic data of the Cumaru gold deposit are similar both to Archean greenstone-hosted lode gold deposits that occur in shear zones and to porphyry-style mineralizations typical of Phanerozoic magmatic arcs. Such a situation suggests that both shear zone activation and granodiorite intrusion were involved in the genesis of the Cumaru mesothermal gold deposit, with is here assigned to a new classification of lode-porphyry gold deposit.

IP tomography test survey in the Barrier Main Lode, Broken Hill, Australia

In both the domestic and overseas mineral exploration projects of the Metal Mining Agency of Japan (MMAJ), many holes have been drilled in order to core for geological logging and laboratory analysis. However drill holes themselves are seldom used to obtain additional information except for occasional geophysical logging such as resistivity, self-potential and induced polarisation (IP). Therefore the MMAJ has been developing an IP tomography data acquisition system since 1992 for the purpose of effectively utilising drill holes, reducing exploration cost and improving accuracy of geophysical exploration.This system is based on time-domain measurement and adopts a pole-dipole electrode configuration to avoid coupling effects compared to a pole-pole array and to deploy the electrodes more easily than a dipole-dipole array. It consists of a transmitter, receiver, personal computer, controller, non-polarisable electrodes, ground cables and borehole cables, and has the following features:It acquires resistivity and chargeability data at 10 sites simultaneously.It acquires very accurate data because of an operational amplifier at each electrode in the borehole cable and the relay box.It acquires the data automatically through the controller, computer and relay boxes.In 1997, the MMAJ conducted an IP tomography test survey in order to examine the applicability of the system for mineral exploration in the Barrier Main Lode, located northeast of Broken Hill, NSW, Australia. The quality of the acquired data was excellent because of high sensitivity of the system and absence of culture noise such as power lines. The data was analysed with the inversion technique based on the alpha centers method and the non-linear least square method. In the reconstructed resistivity and chargeability images, the ore body appears as a low resistivity zone and as a high chargeability zone. The reconstructed images are consistent with the result of IP logging although the reconstructed resistivity is lower than that obtained by IP logging. It is confirmed that the reconstructed chargeability image can delineate a massive sulphide ore deposit such as the Barrier Main Lode more clearly than the reconstructed resistivity image.