Volcanogenic Deposits Research Articles

Geophysical Signature of the Mons Cupri VMS Deposit, Western Australia

Mons Cupri is an Archaean proximal, volcanogenic massive-sulphide deposit in the west Pilbara of Western Australia. Mineralisation occurs in the Mons Cupri Volcanics of the Whim Creek Belt. Stockwork copper-sulphides in altered Mount Brown Rhyolite are overlain by shallowly dipping, massive, copper-lead-zinc sulphides in volcaniclastic sedimentary rocks. Two airborne magnetic surveys detected a 70 nT anomaly in broad correlation with the outline of the mineralisation. The likely source of this anomaly is magnetite within a chlorite alteration pipe. One trial and two surveys with airborne electromagnetics have been conducted. None of the systems, including GEOTEM II, yielded anomalies over Mons Cupri itself. Seven trials and one survey with ground electromagnetics were also completed. Again, none produced anomalous results attributable to the mineralisation. Two induced polarisation surveys detected slightly lower resistivities over the stockwork mineralisation; a magnetic induced polarisation survey obtained the opposite result. However, higher percentage frequency effects, chargeabilities, and relative phase shifts clearly coincided with the stockwork mineralisation. None of the systems unequivocally detected the massive sulphides at depth. Mons Cupri is a difficult geophysical target. Low total-sulphide content, deep massive sulphides and a lack of electrical continuity combine to produce an enigmatic response and to defeat detection by electromagnetic systems.

Geophysical Case History of the Discovery of the Aguas Tenidas East Massive Sulphide Deposit, SW Spain

The Aguas Tenidas East volcanogenic deposit was discovered after several years of geophysical surveying of the Iberian pyrite belt by Billiton Espanola in joint venture with Promotoras de Recursos Naturales. The discovery followed others made in the belt in recent years, in particular that of the large Neves Corvo deposit in 1977. The Aguas Tenidas East deposit is typical of the Pyrite Belt, comprising predominantly massive pyrite with a few percent sphalerite, chalcopyrite and galena, in a host sequence of sediments and felsic to intermediate volcanics. As of 1987 it contained 27 million tonnes at 1.1% Cu, 2.8% Zn, 0.8% Pb. The discovery in 1985 followed from TEM surveying along a prospective mineralised horizon extending from a smaller, exhausted surface mine. A strong conductor with a long decay was detected along strike and subsequently drilled. However the drill-hole was stopped after it passed the interpreted depth of the source without intersecting any mineralisation. Subsequent downhole TEM surveying lead to the drilling of another hole which intersected massive sulphides at a vertical depth of 330 metres. Other geophysical methods, such as INPUT, magnetics, gravity, IP and Max-Min were tried either before or after the discovery, but generally were either ambiguous or not cost-effective. Downhole techniques using TEM and mise-a-la-masse were very useful in delineating the deposit. Initial modelling of the TEM data assumed only an inductive response and failed to explain the observed anomalies. Subsequent modelling of the known mineralisation clearly indicates that the galvanic response (due to current gathering) is of considerable importance. Preliminary 2.5D numerical modelling allowing for current gathering appears to account for most the large Aguas Tenidas East EM response. Also of major significance in this discovery was the early, misleading extrapolation of surface geology to depth. Not only did the steep surface dips incorrectly constrain the geophysical model (since the deposit is sub-horizontal), but also the mineralised horizon at the nearby small open-cut mine, which was considered the prospective one, appears to be unrelated stratigraphically to the deeper Aguas Tenidas East host horizon. Without down-hole EM this situation may have resulted in the deposit not being discovered.

Lead mobilization during tectonic reactivation of the western Baltic Shield

Lead isotope data from sulfide deposits of the western part of the Baltic Shield define mixing lines in the 206Pb 204Pb - 207Pb 204Pb diagram. Lead from two types of sulfide deposits have been investigated: 1. (1) Exhalative and volcanogenic deposits that are syngenetic with their host rocks 2. (2) vein deposits. The syngenetic deposits locally show a very wide range of lead isotopic compositions that reflect a variable addition of highly radiogenic lead, while the vein deposits, although they have radiogenic lead isotopic compositions, exhibit only limited isotopic variations. In different provinces of the shield, both types of deposits fall on the same lead mixing array. The slope of the lead mixing lines varies as a function of the age of basement rocks and the age of the tectonic event which produced the lead mobilization and therefore relates the source rock age with the age of lead mobilization. Calculated mixing ages fall into several short time periods that correspond either to orogenic events or to major phases of continental rifting. The orogenic events are the ca 360–430 Ma Caledonian, ca 900–1100 Ma Sveconorwegian, and the ca 1800–1900 Ma Svecofennian orogenic cycles. The rifting events correspond to the formation of the ca 280 Ma Oslo rift and the Ordovician (ca 450 Ma) graben system in the area of the present Gulf of Bothnia. Each mixing age indicates that lead was mobilized, probably as a consequence of mild thermal disturbances, and that the crust was permeable to lead migration. The data show that the geographic distribution of sulfide deposits with highly radiogenic lead isotopic compositions coincides with old graben systems, orogenic belts, and orogenic forelands on the Baltic Shield. The ages of vein deposits and their geographic distribution demonstrate multiple tectonic reactivation of the interior of the Baltic Shield in response to orogenic events at its margin.

Petroleum Basins of Sakhalin and Adjacent Shelf

Sixty-seven oil and gas fields have been discovered on Sakhalin and the adjacent shelf but the distribution of fields is uneven in north Sakhalin, south Sakhalin, and the Tatar basins. The sedimentary cover is composed of sandy, clayey, and siliceous rocks, with volcanogenic and coal-bearing deposits of Upper Cretaceous, Paleogene, and Neogene 8-12 km thick. Marine clayey and siliceous oil source rocks are regionally developed in the section at different stratigraphic levels; the organic matter is of mixed type and the content varies from 0.5 to 1.5%. The upper Oligocene and middle-upper Miocene source rocks in the north Sakhalin basin are typical, and the organic carbon content ranges from 1 to 5%. The level of organic matter catagenesis and conversion into hydrocarbons is high because of the high differential geothermal gradient in the basins, 30-50[degrees]C per km. Porous sandstones in the Miocene form the reservoirs in all fields with the exception of Okruzhnoye, where the pay zone is a siliceous claystone. Growth-fault rollovers and anticlines form the main traps ranging in area from 5 to 300 km[sup 2], with amplitudes between 100 and 600 m. both stratigraphic and structural traps have been identified. Considerable volumes of reserves are associated withmore » the Miocene deposits of north Sakhalin, which are characterized by an optimum combination of oil source rocks, focused migration paths, and thick sequences of reservoirs and cap rocks. Six large fields have been discovered in the past 15 yr. Oil and condensate reserves stand at over 300 million MT, and gas reserves are about 900 billion m[sup 3].« less

Volcanic rock-hosted gold and base-metal mineralization, associated with Neoproterozoic-Early Paleozoic, back-arc extension in the Carolina terrane, southern Appalachian Piedmont

Volcanogenic mineral deposits in the Carolina terrane, southern Appalachian Piedmont,include Kuroko-type polymetallic massive sulfide deposits and disseminated gold-pyrite deposits associated with propylitic, silicic, argillic, and advanced argillic alteration. Host rocks are metavolcaniclastic and metaepiclastic rocks of a Neoproterozoic-Early Cambrian magmatic arc. The favorable gold horizon is the transition from a lower succession of andesitic and rhyolitic pyroclastic rocks with basal mafic lavas to an upper sequence of epiclastic sedimentary units and minor lava and ash flows. Kuroko-type deposits are associated with mafic to bimodal volcanic rocks in the upper sequence. Whole-rock oxygen isotope analyses indicate that gold mineralization is associated with a transition from hydrothermal systems dominated by isotopically relatively light (δ18O = -6‰ to -10‰) waters, typical of high-latitude subaerial systems, to seawater (δ18O = 0‰). Plots of δ18O vs. SiO2 of the host rocks show a compositional gap associated with mineralization at the subaerial to submarine transition. Values of δ18O for the hydrothermal waters, lithostratigraphic analyses, and tectonic models of the Carolina terrane demonstrate that mineralization coincided with extension in a rifted arc.

Geochemistry of Ordovician Keli Group basalts associated with Besshi-type Cu-Zn deposits from the southern Trondheim and Sulitjelma mining districts of Norway

Besshi-type volcanogenic Cu-Zn deposits in the Scandinavian Caledonides are hosted by Ordovician metabasalts and clastic sediments of the Storen, Fundsjo and Sulitjelma groups. The basalts are transitional between T-MORB and marginal basin tholeiites in composition and are characterised by Nd and Pb isotopic compositions which overlap the more radiogenic values of Lower Palaeozoic MORB. These features, along with the intercalation of the basalts with tuffs and continentally derived sediments, indicate an epicontinental rift or marginal basin origin, possibly analogous to the present Red Sea and Gulf of Aden rifts. This implies the development of a restricted ocean basin in the north of Iapetus between the Laurentian and Baltoscandian microcontinents during the Cambrian and Early Ordovician.

The Brioverian (Upper Proterozoic) and the Cadomian orogeny in the Armorican Massif

Abstract The Brioverian in Brittany has generally been divided into the volcanogenic Lower Brioverian and the detrital Upper Brioverian, representing respectively the first and second episodes of the Cadomian orogeny, Cadomian I and Cadomian II. This scheme was subsequently incorporated into an active margin model of evolution and related to subduction during closure of the Celtic Ocean, in a geochronological framework defined by ages of 2000 Ma for the basement, 670 Ma for magmatic rocks of the first episode (Cadomian I) and 600 Ma for those of the second (Cadomian II). Recent work has led to substantial modification of this model. The two parts of the Brioverian are now considered not to have been superposed one upon the other but to be juxtaposed along a suture, and the superimposition of Cadomian structures not to have been due to successive events but to southward migration of deformation from the ocean towards the continent. In the northern, orogenic domain the volcanogenic Brioverian represents a volcanic arc-back-arc basin system located on a rifted continental margin. At about 600 Ma the closure of this system resulted in the overthrusting of the orogenic belt onto the continental margin, with the development of anatectic magmatism up to 540 Ma. In the southern, continental domain the detrital Brioverian includes black chert in the lower part (probably contemporaneous with the volcanogenic deposits of the orogenic zone) and reworked black chert in the upper part. In Normandy, the deformation of these rocks was related to the rise of the Mancellian granites about 540 Ma. In central Brittany, the main deformation is Variscan in age, and the age of the Brioverian there is questionable as the lower part of the Palaeozoic cover is of Ordovician age.

Lithofacies and depositional pattern of sea floor sediments in the North Banda Sea, Indonesia

A systematic coring and seismic survey across the northern Banda Arc reveals various Quaternary lithofacies deposited in particular morphotectonic environments. Terrigenous and volcanogenic deposits exist in volcanic arc areas, in step-faulted basins and large local structural escarpments with steep slopes which generate debris and mass-gravity flows. Sedimentologic data suggest that volcanogenic sediments are derived from the Bandaneira volcanic complex. The eastern part is marked by benches and notches that exhibit peculiar flat-lying layers mostly of sandy turbidites. Silt turbidites are found in the deepest part in the Weber Deep. Furthermore, spatial distribution of the gravels and their lithology suggest source areas in the west and northeast. Whilst the structural highs represented by isolated sharp ridges are characterized by the presence of gravel deposits, slumping and mass-gravity flows play a major role in sediment deposition. Seram Trench is indicated by distinct flat-lying seismic reflections of trench-fill deposits of pelagic and sand turbidite deposits. The continental slopes consist of blocks of slides and slumps, and are prone to slumping. They are covered by ooze and pelagic deposits. On the other hand a flat terrace-like morphology represents the continental shelf with uniform, consistent lithology of biogenous sand and mud.

Lead- and strontium-isotope geochemistry of Paleozoic Sicker Group and Jurassic Bonanza Group volcanic rocks and Island Intrusions, Vancouver Island, British Columbia

Whole-rock and galena lead-isotope analyses have been obtained from the Sicker Group Paleozoic island-arc volcanic package and from a Jurassic island-arc represented by the Bonanza Group volcanics and Island Intrusions. Galena lead-isotope analyses from the volcanogenic ore deposits at the Buttle Lake mining camp in the Sicker Group provide estimates of the initial lead ratios for the Sicker Group. Lead-isotope signatures are uniform within each of the major orebodies, but the Myra orebody is less radiogenic than the older H–W orebody. This has major significance in terms of ore genesis for these important deposits.There are significant differences in isotopic composition between the Sicker Group and Devonian island-arc type rocks in the Shasta district, California, which rules out direct correlations between the rock units of these two areas. Relatively high initial values of 207Pb/204Pb (> 15.56) and 208Pb/204Pb (> 38.00) suggest that large quantities of crustal lead must have been involved in the formation of the Sicker Group volcanic rocks. Thus it is proposed that the trench related to the Paleozoic island arc had a substantial input of continental detritus and may have lain near a continent.The Jurassic island arc is characterized by low 207Pb/204Pb ratios (< 15.59), suggesting a more primitive arc environment than for the Paleozoic arc. Bonanza Group volcanic rocks contain lead that is less radiogenic than lead in the Island Intrusions. Present and initial lead-isotope ratios of both the Bonanza Group volcanics and Island intrusions follow the same trend, supporting the hypothesis that they are comagmatic. Lead isotopes from a galena vein within the Island Copper porphyry deposit plot with the initial ratios for Bonanza Group volcanics and Island Intrusions. This confirms the hypothesis that this mineralization is related to the Jurassic island-arc volcanic event.Initial lead-isotope ratios for the Jurassic rock suite form a linear array on both 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb plots. If interpreted as due to isotopic mixing, the more radiogenic end member has a composition that is lower in 207Pb/204Pb and higher in 206Pb/204Pb than typical upper continental crust. Assimilation of Sicker Group material during the emplacement of the Jurassic arc can explain the mixing trend.

Application of the zoning of ore-associated elements to exploration for volcanogenic massive sulphide deposits: A case study of the Pukkiharju Zn-Cu prospect, central Finland

Abstract The zoning of ore-associated elements Volcanogenic massive sulphide deposits are characterized by mineralogical alteration zones that are associated with distinct geochemical zones. The geochemical zones commonly form larger exploration targets than do the observable alteration zones. Use has been made of the depletion of Na and Ca and enrichment of Mg, Fe, and sometimes of K as a standard procedure in lithogeochemical exploration. The enrichment of ore-associated minor or trace elements that are often found in a restricted zone in the area of most intense alteration are also used in locating orebodies, even though their halos are sometimes less distinct and well defined than those of the major elements. The concept of zoning of ore-associated elements in hydrothermal deposits has been extensively applied in the Soviet Union to interpret the level of erosion and type of deposit and also to detect blind mineralization. The direction of movement of the ore-bearing solutions is expressed as axial zoning, and the standard sequence for elements considered there is (from top to bottom): Ba-Ag-Pb-Zn-Au-Cu-Ni-Co. In this study, zoning is compared to features of volcanogenic massive sulpide deposits.

The Application of Geophysics on the West Coast of Tasmania

The Tasmanian Mines Department recently initiated a large-scale project to assist exploration in Western Tasmania, one of the world's most mineralised regions. The geophysical component of the project was designed to determine the geophysical signatures of the type deposits and the effectiveness of the techniques being used in Tasmania, particularly in areas of thick cover. The study has shown that the replacement tin and polymetallic volcanogenic deposits are good targets for TEM surveys, although small occurrences of non-conducting zinc-rich deposits have been found. A comparison of FEM, TEM and CSAMT surveys over a series of moderate conductors emphasised the limitations of fixed transmitter surveys and demonstrated the potential of CSAMT for discriminating deep targets.

Geology and lithogeochemistry of the Huntingdon Deposit, cyprus-type mineralization in the ophiolite belt of the southeastern Quebec Appalachians

The Huntingdon mine, situated in the ophiolite belt of the Quebec Applachians, is a 1.9-million-metric-ton copper deposit mined from 1954 to 1957. The deposit is hosted by a pillowed mafic volcanic unit at or near a faulted contact with a serpentinite body located in a highly tectonized zone composed of discontinuous slices of graywackes and slates together with ophiolite-derived rocks. The mineralization consists of disseminated, stockwork, and massive ore types composed of chalcopyrite and pyrrhotite, and minor amounts of pyrite and sphalerite. The gangue minerals are quartz and minor chlorite and carbonate.Mineralogical and textural observations indicate that the mineralization and associated alteration took place prior to orogenesis. Major element geochemistry exhibits hydrothermal alteration patterns typical of volcanogenic deposits in mafic rocks. The host rocks show an almost total loss of alkali elements (mainly Ca) and a simultaneous gain of Fe and Mg toward the mineralized zone. Potassic metasomatism also occurs near massive mineralization. Trace element geochemistry indicates that the volcanic host rocks are similar to the volcanics of the Thetford Mines ophiolite situated in the same belt and that they are boninitic in nature.The Huntingdon deposit is interpreted as a typical ophiolitic, Cyprus-type volcanogenic deposit that was subsequently deformed, recrystallized, and partly remobilized during later tectonic events.

Base metal exploration of the Mount Read Volcanics, western Tasmania; Pt. II, Lead isotope signatures and genetic implications

Lead isotope measurements of 13 deposits and prospects from western Tasmania have been undertaken to establish a framework for the use of the lead isotope technique in exploration and to gain an understanding of the processes of ore formation. Lead isotope signatures for the Cambrian volcanogenic deposits (Mount Lyell, Rosebery, Hercules, Que River, Hellyer) differ by more than 1 percent from those characterizing the post-Cambrian mineralization, thought to be related to Devonian-Carboniferous granitoids and/or Tabberabberan metamorphism (e.g., Renison Bell, Queen Hill, Spray mine, Farrell lodes). The differences in lead isotope signatures allow discrimination of the two contrasting styles of mineralization in both surface material and drill core; the isotopic signatures have been utilized in an exploration program at Elliott Bay which is described in companion papers.The following important genetic conclusions can be drawn from the results of this exploration-oriented investigation: (1) A whole-rock Pb-Pb age of 580 + or - 170 m.y. (2 Sigma ) was measured for polymictic volcaniclastic rocks from the Que River area, which, although not tightly constrained because of the large error limits, is consistent with paleontological evidence. (2) The intersection of this whole-rock line with the average Que River ore lead isotope value is consistent with a common source for the lead. (3) The lead isotope data are consistent with an origin of the post-Cambrian mineralization by remobilization from the host volcanic pile in response to Devonian-Carboniferous metamorphism and/or plutonism. (4) All isotopic data for the volcanic rocks, massive sulfides, and post-Cambrian mineralization lie on linear arrays, and on the uranogenic lead plot ( 207 Pb/ 204 Pb vs 206 Pb/ 204 Pb) they intersect the lead evolution (growth) curve for other massive sulfide deposits at approximately 1,000 m.y. This is interpreted as evidence of a Precambrian source for the metals and also of the cogenesis of the volcanics, massive sulfide, and later mineralization. (5) The Proterozoic source age preserved in the lead isotope data for the volcanics (and mineralization) is consistent with their formation in a rifting environment or a continental margin. (6) The 206 Pb/ 204 Pb values of 18.27 to 18.35 are unusually high for Cambrian massive sulfides and the calculated model ages of approximately 270 m.y. are about 50 percent too young and possibly result from an increase in the U/Pb ratio of the source in the last billion years. A calculated 238 U/ 204 Pb (mu ) value for the stage from 1,000 (or 1,500) to 540 m.y. ago is high at approximately 13, similar to that measured in the volcanic rocks today, and may account for the unusual 206 Pb/ 204 Pb ratios. (7) The Th/U value of 4 for volcanics and mineralization is normal for continental crustal rocks but is markedly different from the low Th/U of 1 to 2 in Devonian granitoids. (8) The unusual lead isotope signatures for the deposits cast doubt on the proposed conjunction of Tasmania with the Australian mainland in Cambrian times; the relationships with Antarctica await future investigations.

Stratigraphy and sedimentation in the western Strabo Trench, Eastern Mediterranean

Three piston cores ranging in length from 501 to 921 cm, raised from the adjacent Mediterranean Tyro and Kretheus basins and their intervening sill, confirmed previous findings of persistent anoxia and extended our knowledge of the stratigraphic record. The two cores from the basinal settings contain a Bronze-age homogenite. This tsunami-induced turbidite is only 28 cm thick in the shallower Kretheus Basin whereas it has a thickness of 235 cm in the deeper Tyro Basin. The recovery of the homogenite in these subbasins of the western Strabo Trench demonstrates that the tsunami wave turned west of Crete and arrived here with a velocity sufficient to remobilize unconsolidated sediments draping the basin slopes. The Holocene and uppermost Pleistocene sedimentary record is characterized by numerous millimetric to centimetric sand layers containing reworked Pliocene and Miocene foraminifers. The repetitive, siliciclastic turbidites interfered with the normal Eastern Mediterranean succession of sapropels and tephras and document a strong neotectonic activity, related to the presence of an active trench. On the floor of ponded, rimmed basins like the Tyro and Kretheus basins the sand layers are associated with anoxic sediments, whereas they are associated with normal, well-oxygenated marls on the sill which lies above the anoxic/oxic interface. Basin anoxia is interpreted as the result of submarine dissolution of Messinian evaporites, giving rise to deep-seated brines. Extraformational reworking documents the occurrence of pre-Messinian fossiliferous sediments exposed on the trench wall and subject to submarine erosion. The stratigraphic time framework is given by nannofossil biostratigraphy, corroborated by lithostratigraphic correlation of isochronous lithologies (sapropel and tephra-chronology). A drastic change in sedimentary regime is recorded at approximately 70 ka B.P. ( Emiliania huxleyi Acme- E. huxleyi zonal boundary). Older sediments which were only recovered in Core 37 from the sill separating the Tyro and Kretheus basins are pelagic, with an almost typical Eastern Mediterranean succession of sapropels and tephras. No siliciclastic turbidites and no extraformational reworking are noticed in sediments older than 70,000 years. We conclude that the trench system became active just at that time. A thick layer of plurimillimetric pumice bubbles was discovered within sapropel S-6, indicating a strong explosion of a nearby volcano at 180 ka B.P., but we have no analytical data yet to substantiate the provenance of the volcanogenic deposits.

A Proterozoic island-arc-related volcanogenic sulfide deposit near Bahrah, Saudi Arabia

A Proterozoic island-arc-related volcanogenic sulfide deposit near Bahrah, Saudi Arabia

Electrical geophysical investigations of massive sulfide deposits and their host rocks, West Shasta copper-zinc district

The West Shasta copper-zinc district, Shasta County, California, contains many volcanogenic sulfide deposits within Middle Devonian rhyolites that have not been highly metamorphosed. The district was selected by the U.S. Geological Survey for intensive geological, geochemical, and geophysical study under the Development of Assessment Techniques (DAT) project because accessible exposures have been created by erosion and mining. This report describes the geophysical methods applied to characterize the electrical properties of selected West Shasta massive sulfide deposits and their host rocks, at both small (less than 25 ft) and large (greater than 25 ft) scales. The electrical techniques used galvanic (spectral induced polarization--SIP) and induction (very low frequency--VLF, slingram, and time domain electro-magnetics--TDEM) methods.In situ spectral induced polarization measurements were carried out to determine whether or not conductive anomalies in the district could be differentiated by their polarization signatures. The sulfide, in situ, induced polarization-phase spectral signatures (the induced polarization effect as a function of frequency) have much less character and lack the distinctive shape reported for other massive sulfide deposits; however, they do have some identifiable massive sulfide traits, such as low resistivity and variable polarizability. The nondescript sulfide spectral signature is attributed to the poor development of polarization processes due to a high percentage of resistive, nonpolarizable gangue minerals, lack of pore space, and limited electrolytic fluids. Large-scale spectral induced polarization measurements over the Hornet orebody have a greater polarization than the in situ measurements. This observation, in addition to the fact that much of the Hornet sulfide body has been removed by previous mining activity, suggests that the dominant polarization processes occur at the ground-water-sulfide interface.Combined use of induction techniques, which have different depths of penetration, were used to locate conductive anomalies and determine their shape and depth. All the induction surveys over the Hornet orebody detected the conductive tabular-shaped massive pyritic sulfide deposit hosted in resistive rhyolite. Shallow penetrating induction methods near the Keystone mine detected a conductive fault zone where a block of shale has been downfaulted into volcanic rock. Integrated interpretation of deeper penetrating induction data over this conductive fault zone indicates that parts of the shale are also conductive, demonstrating that the integrated use of several induction methods provides better conductor definition than a single method.

Geological setting of the Tverrfjell copper/zinc deposit, Central Norway

The volcanogenic exhalative Tverrfjell deposit occurs in a sequence of predominantely mafic submarine meta-volcanics, interlayered with geosynclinal pelitic sediments, turbidites and volcanic breccias, belonging to the Early Cambrian to Early Arenigian Storen Group. Two major deformational phases and low to medium grade metamorphic conditions are recognized in the study area. Basalts are mainly tholeiitic but alkaline types occur as well. Extensive fractionation produced highly evolved basalts and even andesites. Basalt compositions are comparable to Type II-ocean floor basalt. The copper/zinc ores of the Tverrfjell deposit are strictly confined to an andesitic extrusive body. An extensive magma chamber is postulated to explain magma fractionation, and as a heat source that generated the exhalative Tverrfjell ore body. It is suggested that the deposit was formed at an intraplate volcanic center or back-arc spreading center.

The geophysical characteristics of the Thalanga volcanogenic sulphide deposit, Queensland

The Thalanga Zn?Pb?Cu?Ag deposit is situated approximately 65 km southwest of Charters Towers in Queensland (Fig. 1). It was discovered in 1975 by Penarroya (Australia) geologist C. Douglas-Brown, who found the gossan during a reconnaissance programme. Since that time numerous geophysical surveys have been conducted over the deposit to assist in delineating ore reserves, which enable comparison of the cost-effectiveness of different techniques in exploration for similar deposits in the region.

Conodont Thermal Maturation Patterns in Paleozoic and Triassic Rocks, Northern Alaska--Geologic and Exploration Implications: ABSTRACT

The Paleozoic through Jurassic stratigraphic sequence in the Brooks Range consists of platform to intraplatform basin deposits 1-5 km thick. This comparatively thin sequence of heterogeneous lithologies was tectonically disrupted and shortened at least 600 km to form a stack of allochthons that were transported relatively northward and emplaced during the earliest Cretaceous. Thermal patterns in Paleozoic and Triassic rocks, based on conodont color alteration indices (CAI) from about 600 localities, show: (1) a gradual increase in thermal level from the northern margin to about 3/4 of the distance southward across the range (from CAI 1 to 5.5 and higher); (2) a belt of mixed high values (CAI 4.5 to 7) along the south border of the range; (3) thermal levels in surface and ubsurface samples in the range related to tectonic burial and not to pre-thrust burial metamorphism; (4) the same CAI values in rocks above and below the Ellesmerian unconformity in the northeast Brooks Range; (5) an association of anomalously high CAI values with mineralized areas and plutonic rocks; (6) a few anomalously high CAI areas of unknown origin that deserve further study; (7) thermal potential of hydrocarbons (CAI = 4.5) only in the westernmost and northern margins of the Brooks Range; and (9) mineralization potential related to anomalously high CAI values in the southern Brooks Range. Triassic through Mississippian rocks in wells on the north flank of the Colville basin show conventional burial metamorphism patterns within each well and from well to well. All rocks indexed have thermal potential for hydrocarbons (CAI = 1-4.5). The geology of the Seaward Peninsula appears to be a southwestern continuation of that in the Brooks Range. Most of the western one-fifth of the Seaward Peninsula contains 4-5 km of unmetamorphosed platformal, dominantly shallow-water carbonate rocks of Early Ordovician through Devonian age having CAI values of 3-4. Eastward, these rocks are thrust onto a blueschist terrane of mafic volcanogenic and clastic deposits and mixed carbonate and clastic rocks of probable Ordovician through Silurian age. Adjacent to, and possibly infolded with, these rocks are metamorphosed shallow-water carbonates of Early Ordovician through Devonian age similar to the rocks of the westernmost Seward Peninsula. Conodonts from the metacarbonate rocks have CAI values of 5.5-7, indicating temperatures of 350-4 0°C that are consistent with blueschist metamorphism. End_of_Article - Last_Page 666------------

Geologic setting of selected copper deposits of China

Seven selected major copper deposits of China are described: Baiyin of Gansu province, Dexing of Jiangxi province, Yulong of Xizang (Tibet) autonomous region, Zhongtiaoshan of Shanxi province, Daye of Hubei province, Tongling of Anhui province, and Dongchuan of Yunnan province. These copper deposits can be classified into four types according to their geologic settings: (1) volcanogenic deposits--Baiyin, (2) porphyry copper deposits--Dexing, Yulong and Zongtiaoshan, (3) skarn deposits--Daye and Tongling, and (4) strata-bound deposits--Dongchuan. The Dexing mines have by far the largest proven reserves--1.5 billion tons of copper ores.