SEDEX Deposits Research Articles

Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits, 1. Lady Loretta Deposit, Queensland

Stratiform sediment hosted Zn–Pb–Ag deposits, often referred to as SEDEX deposits, represent an economically important class of ore, that have received relatively little attention in terms of defining lithochemical halos and geochemical vectors useful to exploration. This study concentrates on the Lady Loretta deposit which is a typical example of the class of Proterozoic SEDEX deposits in northern Australia. We examined the major and trace element chemistry of carbonate-bearing sediments surrounding the deposit and defined a series of halos which extend for several hundred metres across strike and up to 1.5 km along strike. The stratiform ore lens is surrounded by an inner sideritic halo [Carr, G.R., 1984. Primary geochemical and mineralogical dispersion in the vicinity of the Lady Loretta Zn–Pb–Ag deposit, North Queensland. J. Geochem. Expl. 22, 217–238], followed by an outer ankerite/ferroan dolomite halo which merges with low iron dolomitic sediments representative of the regional background compositions. Carbonate within the inner siderite halo varies in composition from siderite to pistomesite (Fe 0.6Mg 0.4CO 3), whereas carbonate in the outer ankerite halo varies from ferroan dolomite to ankerite (Ca 0.5Mg 0.3Fe 0.2CO 3). Element dispersion around the stratiform ore lens is variable with Pb, Cu, Ba and Sr showing very little dispersion (<50 m across strike), Zn and Fe showing moderate dispersion (<100 m) and Mn and Tl showing broad dispersion (<200 m). Within the siderite halo Cu, Mg and Na show marked depletion compared to the surrounding sediments. The magnitude of element dispersion and change in carbonate chemistry around the Lady Loretta orebody has enabled the development of three geochemical vectors applicable to exploration. Whole rock analyses are used to calculate the three vector quantities as follows: (1) SEDEX metal index = Zn + 100Pb + 100Tl; (2) SEDEX alteration index = (FeO + 10MnO)100/(FeO + 10MnO + MgO); (3) manganese content of dolomite: MnO d = (MnO × 30.41)/CaO. All three vectors increase to ore both across strike and along strike. The manganese content of dolomite (MnO d) exhibits the most systematic pattern increasing from background values of about 0.2 wt% to a maximum of around 0.6 wt% at the boundary between the ankerite and siderite halos. Siderite within the inner halo contains considerably more Mn with MnO values of 0.4 to 4.0 wt%. It is suggested here that the basket of indices defined at Lady Loretta (Zn, Tl, metal index, alteration index, MnO d and MnO s) is applicable in the exploration for stratiform Zn–Pb–Ag deposits in dolomite-rich sedimentary basins generally. The indices defined can firstly assist in the identification of sedimentary units favourable for SEDEX mineralisation, and secondly provide vectors along these units to ore. The alteration index and MnO d, however, should only be used for exploration dolomitic sequences; they are not recommended for exploration in clastic sequences devoid of carbonates.

Abstract: Regional Association of Petroleum Productive Breccias, Mississippi Valley Type Lead-Zinc Deposits and Sedex Deposits; Strategy for Exploration of Albion-Scipio (150 + Mmbbls) Type Deposits

Abstract: Regional Association of Petroleum Productive Breccias, Mississippi Valley Type Lead-Zinc Deposits and Sedex Deposits; Strategy for Exploration of Albion-Scipio (150 + Mmbbls) Type Deposits

REE Geochemistry of VMS and SEDEX Ores in China

AbstractREE ratios and patterns for massive pyrite ore and massive cuprous pyrite ore of the Baiyinchang orefield are quite similar to those for quartz keratophyritic tuff and quartz albitophyre of the same orefield. However, massive cuprous pyrite ore of the Ashele Cu–Zn deposit is similar to basalt–diabase of the same district in REE geochemistry. Comparison of the Chinese VMS ores with those from Rio Tinto, Spain and Que River, Australia, in REE geochemistry has been made. REE ratios and patterns of bedded–massive and massive ores of the Changba–Lijiagou Zn–Pb deposit, the second largest SEDEX deposit in China are similar to those of their host rocks, the Qinling shales of Devonian age and the Changba adamellite. The three types of ore and their host rocks of the Dongshengmiao pyrite (pyrrhotite)–Zn–Pb deposit have parallel REE ratios and patterns.

The generation at hot springs of sedimentary ore deposits, microbialites and life

Notwithstanding the current fashion which favours an epigenetic origin for what used to be termed SEDEX deposits, there are several lines of evidence to indicate that Phanerozoic base-metal orebodies of this type have at least some exhalative aspects. The fossil polychaete worms, which occur in Lower Carboniferous pyrite mounds at Tynagh and Silvermines in Ireland, have affinities to Paralvinella, an organism that lives attached to hydrothermal chimneys at the Juan de Fuca hot spring site in the Northeast Pacific. In addition, fossil tube worms, and their moulds, occur both in silica masses underlying the Carboniferous giant Red Dog sulphide orebody in Alaska and in Devonian barite and base-metal deposits in North America and in Russia, respectively. The development of sulphide and carbonate fossil microbialites over exhalative centres further supports generation of some mineral deposits on sea or lake floors. Carbonate microbialite mounds are also developing today over warm springs and seepages. The existence of an environment in which sulphide mineralisation developed at the sea floor has implications also in a different sphere. Life itself may have emerged in a similar milieu at ∼ 4.2 Ga from iron monosulphide bubbles. A primitive metabolism could have been driven by the high, long-lived and constant, redox potential of ∼ 300 mV made available across an iron monosulphide membrane which would have been spontaneously generated where sulphide-bearing, submarine, alkaline springs issued into the acidic, iron-bearing, Hadean ocean. The alkaline spring provided bisulphide to the iron-rich (carbonic) acid ocean for the precipitation of iron-monosulphide bubbles (probotryoids), as well as acetate (Shock, 1992) — the feeder to the biochemical Krebs cycle, driven in reverse by the high partial pressure of CO 2. In addition to its scientific significance, an understanding of these beginnings may well benefit research into many aspects of economic geology. Even more extreme redox contrasts are revealed by the presence of sedimentary jasper or iron formation in three of the major Carboniferous sulphide orebodies in Ireland. Pyritic sulphide microbialites also grew over some of the associated fossil hot-spring sites and may be recognised by their bacteriogenic δ 34S values (−20 to −40‰). Recognition of such fossil hot-spring sites could lead to further discoveries of SEDEX deposits.

Early proterozoic tectonics and metallogeny of the lake huron region of the Canadian shield

Abstract Deposition of the Huronian Supergroup, dominantly fluvial clastic sediments with local basal volcanic accumulations, began at ∼2.49 Ga in a broad trough that extended far south of the present Lake Huron area atop a large Archean continent that was assembled at ∼2.7 Ga and broken up at ∼2.15 Ga after intrusion of widespread gabbro sheets and dykes including Nipissing diabase. The Wyoming Province, which contains sequences similar in age and stratigraphy to the distinctive Huronian Supergroup, may be a fragment of this continent; the Superior Province is a much larger fragment. Tectonic controls of deposition of the Huronian and its paleoplacer uranium deposits, and of the pre-2.2 Ga deformation of these strata, were probably unrelated to processes that controlled deposition of the Animikie and Marquette Range sequences in the Lake Superior region at ∼2.0 Ga. The Huronian, Animikie, and Marquette Range sequences were deformed during the ∼1.85 Ga Penokean orogeny which occurred when an assembly of island arc terranes collided with the Superior block. The Sudbury structure, including the Whitewater Group and its contained sedex deposits and the Sudbury igneous complex and its NiCuPGE ores, were also formed at ∼1.85 Ga. The Elliot Lake pyritic paleoplacer uranium ores, myriad lenses of radioactive pyritic gravels, were deposited by early Huronian fluvial systems. Other endogenic Huronian mineral deposits include disseminated NiCu sulphides in gabbros, Cu in red beds high in the Huronian succession, and silica in quartz arenites. Intrusions of Nipissing gabbro near Sudbury contain minor NiCu mineralization. These intrusions also caused recycling of metals from Archean and Huronian rocks to form concentrations of Cu, Au, Ag, Fe and W in veins and other epigenetic deposits. Most important of these are the unconformity-related Ag veins of the Cobalt area which had particularly complex, still undeciphered, histories that probably included pre-Huronian weathering. Metallogenic patterns in the Early Proterozoic rocks were controlled by a complex interplay of factors including the distribution of mineral deposits in the Archean basement, tectonically controlled variations in Huronian deposition and deformation, and oxyatmoversion, the transition from anoxic to oxygenic atmospheric conditions that occurred during deposition of the Huronian Supergroup.

Lead-isotope study of the sulphide ore and alteration zone, Bleikvassli zinc-lead deposit, northern Norway

The Bleikvassli Zn-Pb deposit is located in the Uppermost Allochthon of the northern Norwegian Caledonides and is enclosed in amphibolite facies, multiply deformed supracrustal rocks. The stratiform orebody occurs stratigraphically above a sequence of gneiss and amphibolite and below a thick carbonate unit. The orebody, spatially associated with a footwall microcline gneiss that contains as much as 12wt‰ K2O, occurs in the lower part of the Mine Sequence which also comprises (kyanite-) mica schist and quartzo-feldspathic to siliceous rocks. The host rock lithology and the metal content of the Bleikvassli orebody are consistent with a SEDEX origin of the deposit. Field relationships and chemistry suggest that the microcline gneiss represents a potassic alteration of pelitic sediments related to the ore-forming process. A 464 ± 22 Ma Rb-Sr isochron for the microcline gneiss is interpreted to be a metamorphic age resulting from resetting of the Rb-Sr isotopic system during the Caledonian orogeny. The U-Pb in the whole rock shows evidence of recent mobilization of uranium and a partial or total resetting of the system during peak metamorphism. As with most SEDEX deposits, the lead isotope composition of the Bleikvassli ore plots close to the orogen growth curve. The geological setting of the ore and the lead — isotope compositions of the galenas indicate a Cambrian age of mineralization. However, the slope of the lead isotope data indicate an age of about 1000 Ma, which is also a maximum age of ore deposition. The lead isotope data for the galena, in conjunction with the compositions of the microcline gneiss during peak metamorphism, support a model whereby the microcline rock was formed as an alteration product by the ore forming fluid and the initial lead isotope composition of the microcline rock was similar to that of the galenas during ore deposition.

Lithogeochemical patterns related to sedex mineralization, Sudbury Basin, Canada

Distal sediments stratigraphically equivalent to sedex Zn-Cu-Pb deposits at the base of the Proterozoic Onwatin argillite formation, Sudbury Basin, are depleted in S and enriched in Mn, occurring as kutnohorite, manganosiderite and spessartine, and in Ba in the form of the K-Ba-feldspar, hyalophane. Fluid inclusion data indicate deposition of the distal sediments at ∼ 125°C and of the sedex carbonate gangue at ∼ 170°C. The overlying Onwatin argillites, deposited at ∼ 60°C, are characterized by a distinctive pattern of S C org ≈ 0.36 indicative of diagenetic S, alternating with S C org ⪢ 0.36 , probably reflecting episodic expulsion of S-bearing fluids into the water column during the waning stages of hydrothermal activity. Similar to the S/C org pattern are patterns of rhythmically alternating low and high Zn, Pb, As, Re, Ni and Pd values in the Onwatin argillite. Coinciding with the onset of hydrothermal activity, δ 34S-values in pyrite decrease abruptly from ∼ + 7‰ at the top of the Onaping Formation to + 3‰ at the base of the overlying Onwatin argillites. δ 13C-values in the carbonate gangue of the sedex deposits, −8.9 to −2.4‰, lie in or close to the range for magmatic carbon, in contrast to values of −18.7 to −14.9‰ in carbonate derived by oxidation of organic carbon in the overlying and underlying rocks. Low δ 34S-values in the sedex deposits and basal Onwatin rocks and enrichment of the Onwatin argillite in S, Ni, Re and Pd suggest derivation of these metals, at least part of the S and possibly Cu, Zn, Pb from the magmatic Ni-Cu ores and/or associated Sudbury Igneous Complex which probably also provided the energy to drive the convective-exhalative system responsible for the sedex mineralization. The model implies potential for further sedex deposits along the 110 km of the Onwatin-Onaping contact.

Terrigenous dilution: using a method for quantifying hydrothermal input in the sedimentary environment

The formation and preservation of sedimentary exhalative ( sedex) massive sulfide deposits in oceanic basins is tightly constrained by the rate of sedimentation. To estimate the relative input of terrigenous sedimentation and hydrothermal precipitation, the use of a simple graphic method, known as the isocon method and applied previously to metasomatic alteration processes is proposed. On two sedex deposits, the Champagne deposit in the Quebec Appalachian sapropelic sediments and the Atlantis II Deep in the Red Sea, this method allows a good estimate of the sedimentation input/hydrothermal input ratio ( S H ), and also enables a mechanism for computing the duration of fossil hydrothermal exhalative phenomena to be developed.

Isotopic evidence for hydrothermal discharge into anoxic seawater, Sudbury basin, Ontario, Canada

The Errington and Vermilion sedex ZnCuPb deposits occur within hydrothermal carbonate and chert that form part of a carbonaceous slate unit, the contact unit, at the base of the Onwatin formation. Within the underlying Onaping formation sulphur and organic carbon increase upward but maintain S/C org ratios near 0.36, indicative of diagenetic sulphide formed by biogenic sulphate reduction, a process that also accounts for 13C-depleted carbonates in these rocks. δ 34S-values in pyrite are close to +7‰ throughout the Onaping but decrease abruptly to +3‰ in the contact unit; this decrease coincides with the first appearance of S/Cog⪢ 0.36. Both the decrease in σ 34S and increase in S/C org ratios result from venting of hydrothermal sulphur into the water column. Fluid-inclusion homogenization temperatures average 170°C at the proposed vent site and 125°C in the distal contact unit sediment. The isotopic compositions of water, calculated using these temperatures and the isotopic compositions of carbonates, are −1‰ for the sedex deposits at the vent site and -3‰ in the distal sediments.

Oxygen-isotope composition and temperature of fluids involved in deposition of Proterozoic sedex deposits, Sudbury Basin, Ontario

Sedex Cu–Pb–Zn deposits in the Sudbury Basin were deposited on the sea floor from fluids in which δ18O = −1‰ and at temperatures around 170 °C. Distal Mn-bearing sediments were deposited in seawater in which δ18O ≈ −3‰ and at temperatures around 125 °C. The fluids at the vent site (δ18O ≈ −1‰) probably represent a mixture of normal seawater and isotopically positive hydrothermal fluid generated in the substrate by seawater–rock reactions. The heat source responsible for convection and venting onto the sea floor and into the water column was the underlying Sudbury Irruptive, which was emplaced immediately following deposition of the Onaping Formation, directly above which the sedex deposits lie.

Origin of a giant orebody at Navan, Ireland

The Navan base-metal deposit in Ireland formed 360 Myr ago1 and is one of the world's largest orebodies, with 70 million tonnes of 12% Zn + Pb. Much of the mineralization was sedimentary, exhalative: other well known examples of this class of the so-called SEDEX deposits are Mount Isa in Australia, Sullivan in Canada, and Meggen in Germany. Several models for the formation of such deposits have been suggested. Here we report the first systematic stratigraphic Pb isotopic study of galenas in any giant exhalative Zn + Pb orebody. We document evidence that the sources of Pb in the Navan orebody evolved to produce increased dispersion and less radiogenic averages of lead isotopic compositions. This is as expected if the orebody formed from a hydrothermal convection cell which expanded to scavenge metals from progressively deeper parts of the continental crust.