Large Volcanogenic Massive Sulfide Research Articles

Distribution of critical metals in evolving pyrite from massive sulfide ores of the Iberian Pyrite Belt

With >90 known deposits containing original reserves of >2400 Mt of sulfide ore, the Iberian Pyrite Belt (IPB) is the largest volcanogenic massive sulfide (VMS) province on Earth. In these evolving mineral systems, texturally different pyrite exhibits characteristic mineralogy and trace element fingerprints. Pyrite (Py-1), which is well preserved in the polymetallic ores that crystallized at the earliest stage of VMS deposit formation, consists of kernels of pyrite framboids surrounded by concentric colloform bands and ended by faceted outlines. It is rich in some metals like Pb, Zn, Sb and As (mostly hosted as nano-to-micron-sized particles, including galena, tetrahedrite and arsenopyrite) but depleted in Cu, Co and Bi. In contrast, pyrite from the pyritic and Cu-rich ore overprinted by late fluids exhibits spongy-looking (Py-2) or homogenous (Py-3) cores surrounded by external facets with crystallographic continuity across the whole single grains due to re-crystallization. Py-2 is depleted in most trace elements with the exception of Au and Bi, which occur both in solid solutions and as nano-to-micron-sized inclusions. Py-3 has the highest Cu, Ag, Co and Ni (mainly associated to nano-to-micron-sized particles of tennantite, chalcopyrite and gersdorffite) and the lowest Au contents in the form of native gold. The progressive increase in metal contents from inner to outer parts of Py-1 matches with the onset of the economic metal endowment of VMS deposits in the IPB, whereas Py-2 and Py-3 are associated with metal shoot processes that led to both leached and high-grade ores, very likely when mafic rocks were emplaced into the footwall of the deposits.

Large Fe isotope fractionations in sulfide ores and ferruginous sedimentary rocks from the Kuroko volcanogenic massive sulfide deposits in the Hokuroku district, northeast Japan

Anoxic seawater may have played an important role in the preservation of volcanogenic massive sulfide (VMS) deposits in the Hokuroku district, northeast Japan, which is the type locality for Kuroko-type VMS deposits. In this study, we investigated the Fe isotopic compositions of sulfide ores and overlying ferruginous sedimentary rocks in these deposits. These data, coupled with petrographic and geochemical data, enable us to investigate the key formation processes and conditions during the formation of large Kuroko-type VMS deposits. Large Fe isotope variations of ca. 4‰ (δ56Fe) associated with negative or positive Ce anomalies characterize the ferruginous sedimentary rocks formed in the Kuroko VMS deposits and post-Kuroko hydrothermal activity. This suggests that iron (hydr)oxides were precipitated by the partial oxidation of dissolved Fe2+ derived from hydrothermal fluids in anoxic or suboxic pools in the Hokuroku Basin. Positive Eu anomalies in the ferruginous cherts closely associated with the Kuroko VMS deposits indicate formation from high-temperature hydrothermal fluids. Zinc-rich black ores in both the Matsumine and Fukazawa deposits have lower δ56Fe values, due to rapid precipitation of pyrite triggered by the mixing of hydrothermal fluids with seawater. Positive shifts in δ56Fe values in the ferruginous cherts from the Ezuri and Fukazawa deposits may be explained by simultaneous precipitation of ferruginous sedimentary rocks with black ores, which modified the δ56Fe values of the hydrothermal fluids to positive values. However, Cu-rich yellow ores show no significant Fe isotope fractionation as compared with the dissolved Fe in the hydrothermal fluids, and were likely formed by slow growth of pyrite that replaced the black ores. The difference in the abundance of sulfide ores between the Matsumine and Fukazawa deposits may reflect the duration of hydrothermal circulation.

A combined fluid inclusion and S–H–O–He–Ar isotope study of the Devonian Ashele VMS-type copper–zinc deposit in the Altay orogenic belt, northwest China

The Altay orogenic belt of Kazakhstan and China incorporates one of the world-class volcanogenic massive sulfide (VMS) polymetallic metallogenic belt. More than 12 large and superlarge VMS deposits have been found in this belt. The Ashele Basin is located at the western end of the Chinese Altay bordering Kazakhstan and hosts the large Ashele copper–zinc deposit, which is a typical VMS deposit and the largest one in this basin. The principal orebodies are stratabound and located between basalt and tuff units in the Ashele Formation. The mineralization has a two-layered structure that consists of an upper stratiform, concordant, massive Cu–Zn (or barite) orebody, and a lower veinlet–disseminated and stockwork Cu (or Cu–Pb–Zn–Ag) orebody associated with silicic alteration. Several exhalative rocks, such as barite, hematitic jasper, pyrite and silicified units also occur.Here we investigate the ore-forming fluids of the exhalative–sedimentary units including jaspilte, stratiform barite and Cu–Zn orebodies which carry fluid inclusions that show homogenization temperatures of 100–410 °C, with two peaks around 230 and 150 °C, and low salinity (clustering between 2 and 8 wt% NaCl equiv), corresponding to NaCl–H2O fluids. The ore-forming fluids of the vein orebody and silicified zones are characterized by low to medium temperature (peak homogenization temperatures between 120 and 280 °C), low to medium salinity (0.7–12.3 wt% NaCl equiv), and H2O–CO2 (±CH4/N2)–NaCl fluids. The δ34S values of barite associated with the stratiform barite orebody range from 20.4‰ to 21.4‰, indicating that the sulfur was derived from the seawater. The sulfur isotope values of sulfide (−3.7‰ to 7.7‰ with a mean of 3.1‰) are similar to those of VMS deposits from other parts of the world. The host volcanic rocks are inferred to be the major source from which the mineralizing fluid leached the ore-forming elements. The 3He/4He ratios of fluid inclusions in pyrite are in the range of 0.136–0.260 (R/Ra), broadly similar to the helium ratios of the crust. The 40Ar/36Ar ratios range from 394 to 9515, and are higher than those of atmospheric argon. The 40Ar/4He ratios of the ore fluids range from 0.002 to 0.064 with a mean of 0.034. The helium and argon isotope compositions of fluid inclusions suggest that the ore fluids of the Ashele deposit were mainly derived from the crust and were mixed with a minor amount of mantle component. The δ18OSMOW values of quartz from the silicified zones and vein orebody range from 8.3‰ to 11.1‰, with corresponding δ18Ofluid values of −4.45‰ to 2.24‰, whereas the δD values of fluid inclusions are between −140‰ and −90‰. The δ18OSMOW values of barite from the stratiform barite orebody range from 7.7‰ to 9.1‰, with corresponding δ18Ofluid values of –3.03‰ to 1.79‰, with δD values of fluid inclusions showing a range of –162‰ and –135‰. The combined isotopic data (H–O and He–Ar) suggest that the ore-forming fluids of the Ashele deposit were mainly derived from deep circulation of seawater and mixed with magmatic fluids. The decrease in temperature and pressure, water–rock exchange, and changes in the composition of ore fluids played important roles in the ore-forming processes of the Ashele Cu–Zn deposit.

Evidence for Remobilization of Barite Affecting Radiometric Dating Using 228Ra, 228Th, and 226Ra/Ba Values: Implications for the Evolution of Sea-Floor Volcanogenic Massive Sulfides

Samples of chimneys (black smokers) and other volcanogenic massive sulfide (VMS) mineralization were collected from hydrothermal vent fields associated with submarine arc and back-arc volcanoes in the western Pacific. Each specimen was dated using radiometric techniques to establish the frequency and duration of hydrothermal activity at that site, information that is vital when establishing the time elapsed to amass an ore deposit. In each case, the radiometric dating relied on the radium isotopes 228 Ra and 226 Ra with half-lives of 5.75 and 1,600 years, respectively, which coprecipitate with barite and then decay. We began dating VMS mineralization 15 years ago, knowing that the 226 Ra/Ba decreases with time, with the Ba and Ra assumed to be from a common source such that the initial 226 Ra/Ba value, i.e., the value at the onset of mineralization, was constant. Thus, the age of an “old” chimney (up to 15,000 yr) was calculated by comparing its 226 Ra/Ba value with the initial value established from nearby “young” chimneys ( 228 Ra. Since then, we have discovered that chimneys and other forms of sea-floor VMS mineralization can contain barite from two or more hydrothermal events. That is, older barite can be dissolved and reprecipitated (remobilized) from earlier mineralization deposited subseafloor, or earlier sea-floor mineralization overprinted (crosscut) by new. In many cases, the 226 Ra/Ba dating did not provide an absolute age for the older material but, rather, a difference in age between two samples that had incorporated varying proportions of older barite that was low or “dead” (i.e., >15,000 yr) with respect to 226 Ra, due to radioactive decay. Remobilizing of older barite is consistent with zone refining of metals in large VMS deposits, but we have only considered this recently. However, our investigations show that this can affect the initial 226 Ra/Ba values for VMS mineralization from arc and back-arc volcanoes, and also from mid-ocean ridges. Where sufficient samples contained 228 Ra, we estimated an age for the remobilized barite by plotting a 226 Ra/Ba versus 228 Ra/Ba mixing line for the VMS at the time of mineralization and found that hydrothermal activity has occurred at some arc volcanoes for >15,000 years. Such long histories of hydrothermal activity indicate the propensity for producing large metal-rich deposits on the seafloor.

Gold in the Brunswick No. 12 volcanogenic massive sulfide deposit, Bathurst Mining Camp, Canada: Evidence from bulk ore analysis and laser ablation ICP─MS data on sulfide phases

The 329-Mt Brunswick No. 12 volcanogenic massive sulfide deposit (total resource of 163 Mt at 10.4% Zn, 4.2% Pb, 0.34% Cu, and 115 g/t Ag) is hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence. Massive sulfides are for the most part syngenetic, and the bulk of the sulfide ore occurs as a Zn–Pb-rich banded sulfide facies that forms an intimate relationship with a laterally extensive Algoma-type iron formation and defines the Brunswick Horizon. Zone refining of stratiform sulfides is considered to have resulted in the development of a large replacement-style Cu-rich basal sulfide facies, which is generally confined between the banded sulfide facies and an underlying stringer sulfide zone. Complex polyphase deformation and associated lower- to upper-greenschist facies regional metamorphism is responsible for the present geometry of the deposit. Textural modification has resulted in a general increase in grain size through the development of pyrite and arsenopyrite porphyroblasts, which tend to overprint primary mineral assemblages. Despite the heterogeneous ductile deformation, primary features have locally been preserved, such as fine-grained colloform pyrite and base and precious metal zonation within the Main Zone. Base metal and trace element abundances in massive sulfides from the Brunswick No. 12 deposit indicate two distinct geochemical associations. The basal sulfide facies, characterized by a proximal high-temperature hydrothermal signature (Cu–Co–Bi–Se), contains generally low Au contents averaging 0.39 ppm (n = 34). Conversely, Au is enriched in the banded sulfide facies, averaging 1.1 ppm Au (n = 21), and is associated with an exhalative suite of elements (Zn–Pb–As–Sb–Ag–Sn). Finely laminated sulfide lenses hosted by iron formation at the north end of the Main Zone are further enriched in Au, averaging 1.7 ppm (n = 41) and ranging up to 8.2 ppm. Laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) analyses of pyrite (n = 97) from the north end of the Main Zone average 2.6 ppm Au and range from the detection limit (0.015 ppm) to 21 ppm. Overall, these analyses reveal a distinct Au–Sb–As–Ag–Hg–Mn association within pyrite grains. Gold is strongly enriched in large pseudo-primary masses of pyrite that exhibit relict banding and fine-grained cores; smaller euhedral pyrite porphyroblasts, and euhedral rims of metamorphic origin surrounding the pyrite masses, contain much less Au, Sb, Ag, As, and Sn. Arsenopyrite, occurring chiefly as late porphyroblasts, contains less Au, averaging 1.0 ppm and ranging from the detection limit (0.027 ppm) to 6.9 ppm. Depth profiles for single-spot laser ablation ICP-MS analyses of pyrite and arsenopyrite display uniform values of Au and an absence of discrete microscopic inclusions of Au-bearing minerals, which is consistent with chemically bonded Au in the sulfide structure. The pervasive correlation of Au with Sn in the Zn–Pb-rich banded sulfide facies suggests similar hydrothermal behavior during the waxing stages of deposition on the seafloor. Under high temperature (>350oC) and moderate- to low-pH conditions, Au and Sn in hydrothermal fluids would be transported as chlorocomplexes. An abrupt decrease in temperature and aH2S, accompanied by an increase in fO2 and pH during mixing with seawater, would lead to the simultaneous destabilization of both Au and Sn chlorocomplexes. The enrichment of Au in fine-grained laminated sulfides on the periphery of the deposit, accompanied by sporadic occurrences of barite and Fe-poor sphalerite, supports lower hydrothermal fluid temperatures analogous to white smoker activity on the flanks of a large volcanogenic massive sulfide system. In lower temperature (<350oC) and mildly acidic hydrothermal fluids, Au would be transported by thiocomplexes, which exhibit multifunctional (retrograde–prograde) solubility and a capacity to mobilize Au to the outer parts of the sulfide mound. The sluggish nature of this low-temperature venting together with larger variations in ambient fO2 could lead to a sharp enrichment of Au towards the stratigraphic hanging wall of massive sulfide deposits.

A Special Issue Devoted to Continental Margin Massive Sulfide Deposits and Their Geodynamic Environments

The papers in this issue arise from a special session held at the May, 2003 Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC)-Society of Economic Geologists (SEG) meeting in Vancouver, British Columbia, Canada, entitled “Massive Sulfides on the Edge: The Formation of VMS and SEDEX Deposits Within Evolving Continental Margins.” Continental margin environments (e.g., continental arcs, arc rifts, and back-arc basins) host some of the world’s largest volcanogenic massive sulfide (VMS) and sedimentary-exhalative (SEDEX) deposits (e.g., Bathurst, Iberian Pyrite Belt), yet there remains an incomplete understanding of the regional- to local-scale controls on their setting and genesis. The purpose of this issue is to explore and better understand the setting, styles, and continent- to deposit-scale controls on the localization of these deposits within and along continental margins. The papers here build on recent multidisciplinary projects such as those on the Bathurst camp (Goodfellow et al., 2003), on Australian VMS environments (Large et al., 2001), and the ongoing International Geological Correlation Program (IGCP) Project 502 on “Global Comparison of Volcanic-Hosted Massive Sulfide Districts: The controls on the distribution and timing of VMS deposits” (Allen et al., 2002). The results presented here contribute new data and observations from both well-known camps (e.g., Iberian Pyrite Belt) and new camps and deposits for which little published information is available (e.g., Finlayson Lake in the Yukon-Tanana terrane, Greens Creek in the Alexander terrane, Baiyinchang in Gansu Province, China, La Plata in western Ecuador, and Cuale in the Gurrerro terrane, Mexico). The papers in the volume range from those that are regional in scope (e.g., Bissig et al., 2008; Mortensen et al., 2008; Taylor et al., 2008), to new deposit descriptions (e.g., Bradshaw et al., 2008; Chiaradia et al., 2008), to new information on known deposits (e.g., Barrett et al., 2008; …

Constraints on the energy and chemical balances of the modern TAG and ancient Cyprus seafloor sulfide deposits

The size, chemical composition, energy flux, and fluid composition of the TAG hydrothermal sulfide deposit at the Mid‐Atlantic Ridge and the size, chemical composition and reaction zone characteristics of the Skouriotissa volcanogenic massive sulfide deposit of the Troodos ophiolite in Cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit. We conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years, with episodes of activity as short as a few tens of years separated by thousands of years of inactivity. About 2×1019 J of energy supplied at high temperature is required to form a deposit the size of the TAG mound. Metals (up to 4 times the mass of any element present in the sulfide deposit) are leached out of relatively small reaction zones most likely at the base of the sheeted dikes. Chemical balance can be struck for all elements except sulfur with a reaction zone 1–2 km3 in volume from which a small proportion of iron and large proportions of copper, zinc, and manganese are removed. A sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate, as suggested by stable isotope analyses. We argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section. Furthermore, we speculate that activity of the TAG hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in Hawaii and Iceland.

High 18 O ore-forming fluids in volcanic-hosted base metal massive sulfide deposits; geologic, 18 O/ 16 O, and D/H evidence from the Iberian pyrite belt; Crandon, Wisconsin; and Blue Hill, Maine

High 18 O ore-forming fluids in volcanic-hosted base metal massive sulfide deposits; geologic, 18 O/ 16 O, and D/H evidence from the Iberian pyrite belt; Crandon, Wisconsin; and Blue Hill, Maine