Cu-Zn Mine Research Articles

A review of Archean volcanic-hosted massive sulfide and sulfate mineralization in Western Australia

Archean volcanic-hosted massive sulfide mineralization in Western Australia ranges in age from ca. 3.46 to 2.70 Ga. Barite-rich, Zn-Pb-Cu mineralization is hosted by relatively large ion lithophile element-rich calc-alkaline volcanic rocks in the ca. 3.46 Ga Warrawoona Group and ca. 3.3 Ga Gorge Creek Group in the eastern Pilbara craton. The high lead contents and 100 Zn/Zn + Pb ratios of between 60 and 80 are typical ofPhanerozoic mineralization hosted by silicic volcanic rocks, rather than of most Archean massive Cu-Zn sulfide deposits. The abundance of sulfate in these deposits, combined with evidence for evaporitic sulfates elsewhere in the Warrawoona Group, also suggests at least local oxidation of reduced suliur in the upper layer of the ocean at this time. Lead-rich, Zn-Pb-Cu mineralization also occurs in the ca. 2.99 Ga Whim Creek Group in the western Pilbara craton. This volcanic and sedimentary sequence formed in a rapidly subsiding basin within older continental crust, and the volcanic rocks are characterized by relatively high large ion lithophile element contents. In the east Yilgarn craton, Cu-Zn mineralization occurs in a bimodal assemblage of tholeiitic basalts and rhyolites at Teutonic Bore, in the Norseman-Wiluna belt. This deposit was mined during the 1980s. In the western Yilgarn block, Cu-Zn mineralization occurs in an assemblage of ca. 3.0 Ga. deep-water, calc-alkaline volcanic and sedimentary rocks in the Golden Grove belt. Mineralization in this belt is typically associated with magnetite and includes the Scuddles and Gossan Hill deposits. Alteration at these deposits is similar to that of Mattabi-type deposits in the Canadian Archcan and includes iron-rich chlorite and carbonates in poorly developed, or laterally extensive, feeder zones. The lead content of the host volcanic sequence and mineralization in this belt is typically much lower than that of the Pb-rich Pilbara mineralization. The Scuddles deposit with a mine recoverable reserve of about 10 million tons of 11 percent Zn, 0.8 percent Pb, and 1.2 percent Cu is currently being mined.

Deciphering the tectonic evolution of Archaean greenstone belts: the importance of contrasting histories to the distribution of mineralization in the Yilgarn Craton, Western Australia

Abstract The distribution of Archaean metallic mineral deposits reflects tectonic controls operating during the evolution of granitoid-greenstone terranes. Tectonic analysis of these terranes not only allows recognition of parameters that may be important in interpreting the tectonic histories of greenstone belts, but also the resultant tectonic control on mineral deposit distribution. In the Yilgarn Craton of Western Australia, the late Archaean (∼ 2.7 Ga) Norseman-Wiluna Belt is most intensely mineralized with both komatiite-associated Ni-Cu deposits and epigenetic gold deposits. This belt is dominated by ∼ 2.7 Ga supracrustal successions, and its tectonic history is that of an orogenic belt at an obliquely convergent craton or proto-craton margin. Tectonostratigraphic associations preserved resemble younger volcanic arc and back-arc basin crust, and many represent the remains of a volcanic arc and back-arc basin complex developed in response to westward-dipping subduction. These sequences were deformed, metamorphosed and intruded by granitoids relatively soon (30 Ma) after the major period of volcanism, with gold mineralization, occurring late in the orogenic event, possibly during uplift. In contrast, greenstone belts in the Murchison and Southern Cross provinces are dominated by older supra-crustal successions that contain > 2.9 Ga volcanosedimentary associations characterized by extensive oxide-facies banded iron formations (BIFs). BIFs appear to have been deposited in shelf or platform environments in intra-oceanic or passive craton margin settings. Komatiite-associated Ni-Cu mineralization and volcanogenic Cu-Zn mineralization are locally hosted by favourable tectonostratigraphic associations. The main period of deformation, metamorphism, and gold mineralization in these provinces was broadly synchronous with that in the Norseman-Wiluna Belt, as was regional granitoid emplacement. During this event, strain was probably transmitted from the eastern margin of the craton via a Yilgarn-wide system of shear zones, faults and fold belts. Gold mineralization is developed best in greenstone belts, such as the Meekathara-Mt. Magnet Belt, that contain large-scale fault systems and internal granitoids. Similar tectonic controls on the distribution of mineral deposits can also be recognized in other Archaean cratons, and the heterogeneous distribution of Archaean mineral deposits probably reflects a range of tectonic settings as well as different stages in the evolution of individual cratons.

Dispersion patterns of carbonyl sulphide above mineral deposits

Thermodynamic calculations and experiments in vitro have pointed to the potential value of carbonyl sulphide (COS) as a gaseous pathfinder for sulphide mineral deposits concealed beneath overburden. Convenient sampling and analytical techniques have, therefore, been developed for the determination of COS in overburden materials. In temperate regions, samples of soil are taken below the normal rooting depth of vegetation; in arid and semi-arid regions the surface microlayer is collected. The analytical procedure involves the selective thermal desorption of COS from the < 150 μm fraction of overburden materials and quantitative determination by a rapid gas chromatographic method. Field studies of surficial dispersion patterns of COS have been carried out in the vicinity of replacement-type Cu (-Zn) and porphyry Cu deposits in the southwestern U.S.A., meta-sedimentary Cu-Zn mineralization in Saudi Arabia, volcano-sedimentary polymetallic sulphides in South Africa and stratabound Pb-Zn mineralization in Ireland, and are described here. These deposits are covered by different types and various thicknesses of overburden material. Anomalous concentrations of COS occur in the overburden above all of these deposits. The anomalies tend to be of modest but satisfactory contrast and are in some instances discontinuous or patchy. Results indicate that COS may be used as a guide to concealed mineral deposits in a variety of geologic and physiographic settings. Significant anomalies can be recognized even where mineralization lies beneath more than 90 m of transported overburden.

Some petrologic and oxygen isotopic relationships in the Amulet Mine, Noranda, Quebec, and their bearing on the origin of Archean massive sulfide deposits

concentrically zoned alteration pipe is present at the Amulet A Cu-Zn mine in the Archean Abitibi greenstone belt. The pipe consists of a central core zone of so-called (=spotted cordierite-anthophyllite rock) surrounded by a zone of biotite-bearing grid-fracture alteration. Study of the dalmatianite indicates that these rocks experienced two episodes of recrystallization. First, the original andesite was completely reconstituted during a hydrothermal event, producing a quartz-chlorite assemblage. This transformation was associated with the formation of the massive sulfide lens and presumably took place in a submarine environment. Later contact metamorphism associated with the intrusion of the nearby Lake Dufault granodiorite resulted in partial recrystallization of the chlorite and quartz to cordierite and anthophyllite.Whole-rock delta 18 O values decrease from about 6 to 10 in most of the Abitibi belt to 5 to 7 in the country rocks surrounding the ore deposit, to values as low as 3.6 in the core of the dalmatianite zone. This oxygen isotopic zoning must have been produced by the hydrothermal activity, because the dehydration reactions associated with contact metamorphism cannot have affected delta 18 O by more than 0.5 per mil. Assuming an alteration temperature of 300 degrees + or - 50 degrees C and a water/rock ratio greater than two, the hydrothermal fluid must have had delta 18 O = 0.5 + or - 1.0. This indicates that the Amulet ore deposit formed from a hydrothermal fluid that had a delta 18 O value similar both to modern seawater and to the fluids which formed the Phanerozoic massive sulfide deposits of the Kuroko and Cyprus types. The apparent constancy of delta 18 O of seawater during the Precambrian and the continued importance of seawater hydrothermal processes are important constraints which must be considered in developing models of the history of the earth.

Origin of a complex alteration assemblage, Four Corners Cu-Zn prospect, Quebec, Canada

Three types of hydrothermal alteration spatially associated with Cu-Zn mineralization within the Archean Blake River Group of greenschist facies metavolcanic rocks, Noranda, Quebec, were studied. Alteration was mapped around a partly fragmental rhyolite dome, enclosed in mafic flows. Sodic regional alteration and calcic-silicic background alterations were overprinted by potassic and magnesian pipe alteration. Pipe alteration was mapped only in conical breccia pipes within the rhyolite dome, which terminate at a pyroclastic rhyolite-rhyolite breccia contact. Cu-Zn sulfide mineralization was found within the pipes and in stratiform exhalative tuff on overlying volcanic rock contacts. Mass balance studies suggest a general trend from base-fixing regional and background alteration to base-leaching pipe alteration.Textural and geochemical evidence and similarities with presently active geothermal systems indicate alteration and mineralization were products of an Archean convective system, closely related in time and space to volcanism. Albite-rich rocks (regional alteration) are mineralogically similar but chemically distinct from greenschist facies metavolcanic rocks in the rest of the Noranda area. They are chemically similar to spilitic rocks from Thingmuli volcano, Iceland, and are the earliest evidence of seawater-rock interaction. This interaction, in conjunction with further reactions during subsequent background alteration, converted neutral to alkaline seawater into a hot, metal-enriched acid brine. Final reactions between this brine and the rocks through which it passed immediately beneath the sea floor resulted in pipe alteration and in Cu-Zn mineralization where the brine was expelled onto the sea floor. Stratiform pillowed andesite flows showing calcic background alteration were altered during their deposition by continuing exhalative activity. These distinctive altered flows represent excellent field indicators of base metal mineralization, best identified and traced through detailed mapping.

Reply to D. Large's comment

The relationship between the original magma type and exhalative mineralization has been discussed by many workers. In deposits such as Meggen, Rammelsberg, Mr. Isa and McArthur River the presence of minor acid tuffaceous units may be a result of volcanism genetically associated with mineralization or, alternatively, could be quite fortuitous. With reference to the Iberian pyritite deposits, the relationship to ca!c-alkaline volcanism is much clearer. The pyrJtite deposits in the Volcanic Siliceous ComFlex (VS) are associated with a differentiated suite of mafic to acid volcanics with intercalated clastic and chemical sediments. At Rio Tinto, the VS sequence comprises a basal sequence of thick mafic-intermediate volcanics overlain by felsic volcanics with pyritite mineralization at the top of the felsic pile (Palomero, 1977). Similarily at La Zarza and Aljustrel, the mineralization is at the top of a thick sequence of felsic volcanics (Strauss and Madel, 1974; Carvalho et al., 1976) whereas the mineralization at Tharsis is intimately associated with the acid Tharsis I volcanic sequence. The rock type names keratophyre and spilite are unfortunately generally very loosely applied and the data of Aye (1974) shows that VS acid volcanics display a continuous variation from sodic to potassic rhyolite. Classification of rocks such as keratophyre or spilite give little indication of the calc-alkaline or tholeiitic affinity of these rocks and SiO~ vs. FeO/MgO, CaO vs. FeO/MgO and AFM plots of the VS volcanics (Fig. I) shows that they are dominantly calc-alkaline with minor tholeiitic units. In the Iberian pyrite belt and many other areas, a bimodal suite of calc-alkaline and tholeiitic volcanics is present and one magma type is intimately associated with mineralization. For example, Descarreaux (1973) has shown that the volcanics of the Abitibi Belt in Canada are eharacterised by rocks belonging to both the calc-alkaline and tholeiitic magma series and the stratiform Cu-Zn mineralization is associated with calc-alkaline rocks (predominantly rhyolite).