Richard J. Herrington

Colloidal gold and silica in mesothermal vein systems

Some of the textural features of mesothermal gold-quartz veins may be best explained by the initial precipitation of amorphous silica gel (colloid), which subsequently crystallizes to quartz. This can occur in brittle-ductile shear zones where a significant fluid-pressure drop occurs during stick-slip failure. Such a process rapidly supersaturates the hydrothermal fluid with respect to amorphous silica, which precipitates instead of quartz, owing to favorable kinetics. Depressurization also commonly leads to fluid unmixing and destabilization of soluble gold complexes. However, the presence of colloidal silica can stabilize gold colloid, allowing further transport of particulate gold in suspension in the hydrothermal fluid. Silica gel would be highly unstable under mesothermal conditions and would undergo rapid syneresis and crystallization to form quartz; solid impurities would tend to be expelled toward grain boundaries. This model can account for the primary anhedral aggregate textures typical of mesothermal quartz veins, the concentration of gold along grain boundaries and the formation of discrete gold nuggets, and the rare occurrence of low-order silica polymorphs and relict spheroidal structures. The transport of gold in colloidal form may be one reason for the frequently consistent bulk grade distribution in gold-quartz vein systems over many hundreds of metres (in some cases kilometres) of depth. In addition, the formation of charged colloidal particles may help to explain the attraction of gold grains to specific mineral surfaces.

Early Jurassic hydrothermal vent community from the Franciscan Complex, San Rafael Mountains, California

The Figueroa massive sulfide deposit, located in Franciscan Complex rocks in the San Rafael Mountains of California, preserves the only known Jurassic hydrothermal vent fossils. The Figueroa fossil assemblage is specimen rich but of low diversity and comprises, in order of decreasing abundance, vestimentiferan worm tubes, the rhynchonellid brachiopod Anarhynchia cf. gabbi and a species of ?nododelphinulid gastropod. The Figueroa fossil organisms lived at a deep-water, high-temperature vent site located on a mid-ocean ridge or seamount at an equatorial latitude. The fossil vent site was then translated northwestward by the motion of the Farallon plate and was subsequently accreted to its present location. An iron-silica exhalite bed, the probable lateral equivalent of the Figueroa deposit, contains abundant filamentous microfossils with two distinct morphologies and probably represents a lower-temperature, diffuse-flow environment. The Figueroa fossil community was subject to the same environmental conditions as modern vent communities, but it is unique among modern and other fossil vent communities in having rhynchonellid brachiopods.

Late Cretaceous hydrothermal vent communities from the Troodos ophiolite, Cyprus

The Kinousa, Memi, Kambia, Kapedhes, and Sha massive sulfide deposits located in the Troodos ophiolite, Cyprus, contain fossils from Late Cretaceous hydrothermal vent communities that lived on a spreading ridge above a subduction zone in the Neotethys ocean. The Troodos vent fossils provide unequivocal evidence for the exhalative origin of the host massive sulfide deposits, including those that are now located deep within the lava pile. The fossil vent assemblages are of low diversity; they contain numerous vestimentiferan worm tubes, uncommon cerithioid and epitoniid gastropods, and rare (?)serpulid worm tubes. Among the reported modern and ancient vent communities the presence of epitoniid gastropods is unique to Cyprus. At least three of the Troodos vent communities were living on the sea floor around the same time and were as closely spaced as vent communities on modern fast-spreading ridges. Together with slightly older vent worm tubes from the Semail ophiolite of Oman, currently 2500 km from Cyprus, the Troodos fossils show that hydrothermal vent communities were present in the Neotethys ocean from the Cenomanian to the Turonian, a time span of ∼5 m.y.

Arc-Continent Collision: The Making of an Orogen

There is no one model, no paradigm, that uniquely defines arc–continent collision. Natural examples and modelling of arc–continent collision show that there is a large degree of, and variation in, complexity that depend on a number of key first-order parameters and the nature of the main players; the continental margin and the arc–trench complex (the arc–trench complex includes the arc and the subduction zone). Although modelling techniques can be used to gain insights into these, they cannot and do not aim at reproducing the messiness of nature. In natural examples, identifying the nature of the main players involved, such as the age, physical properties, and pre-existing structure of the margin and the arc is just a beginning. Once this is done, parameters such as time, convergence velocity and vector need to be taken into account when determining the tectonic processes that were operative in any one arc–continent collision. In active examples, such as those in the southwest Pacific, some of these first-order parameters can be readily determined, and the nature of the main players easily assessed. Fossil arc–continent collisions, however, have commonly undergone post-collision deformation, erosion, and possibly partial dispersion to be left outcropping in the middle of a forest, with many of the key ingredients missing or hidden. This leaves the geologist to resort to comparison with other natural examples and with models that are mechanically constrained and simplified reproductions of the process to reconstruct and explain what may have been there and, importantly, what processes may have been operating and when. We attempt to show that this is not an easy task that can be put into one simple model. In this chapter we do not present a model for arc–continent collision. Instead, we begin with the main players involved, highlighting the characteristics of each that likely have a major influence on an arc–continent collision. Then, we investigate a range of possible processes that could take place once an intra-oceanic volcanic arc collides with a continental margin.

EARLY JURASSIC HYDROTHERMAL VENT COMMUNITY FROM THE FRANCISCAN COMPLEX, CALIFORNIA

Abstract The Figueroa sulfide deposit located in Franciscan Complex rocks in the San Rafael Mountains, California, contains the only known Jurassic hydrothermal vent community. Based on radiolarian biostratigraphy it is Pliensbachian (early Jurassic) in age. The Figueroa fossil organisms lived at a deepwater, high temperature vent site located on a mid-ocean ridge or seamount at an equatorial latitude. The vent site was then translated northeastward by the motion of the Farallon Plate and was subsequently accreted to its present location. The vent fossils are preserved as molds of pyrite and there is no remaining shell or tube material. The fossil assemblage is specimen rich, but of low diversity, and comprises, in order of decreasing abundance, vestimentiferan worm tubes, rhynchonellide brachiopods (Anarhynchia cf. gabbi), and trochoidean gastropods (Francisciconcha maslennikovi new genus and species). These fossils represent only primary consuming organisms, some of which may have had chemosynthetic microbial endosymbionts, like many modern dominant vent animals. The Figueroa vent assemblage shares vestimentiferan tube worms and gastropods with other fossil and modern vent communities, but is unique in having rhynchonellide brachiopods. It shares this feature with contemporary Mesozoic cold seep communities. Many other taxonomic groups found at modern vent sites are missing from the Figueroa assemblage. The presence of vestimentiferan tube worm fossils in the Figueroa deposit is at odds with the supposed time of origin of the modern vestimentiferans (∼100 Ma), based on molecular data.

Mineral Deposits and Earth Evolution

Mineral deposits are not only primary sources of wealth generation, but also act as windows through which to view the evolution and interrelationships of the Earth system. Deposits formed throughout the last 3.8 billion years of the Earth’s history preserve key evidence with which to test fundamental questions about the evolution of the Earth. These include: the nature of early magmatic and tectonic processes, supercontinent reconstructions, the state of the atmosphere and hydrosphere with time, and the emergence and development of life. The interlinking processes that form mineral deposits have always sat at the heart of the Earth system and the potential for using deposits as tools to understand that evolving system over geological time is increasingly recognized. This volume contains research aimed both at understanding the origins of mineral deposits and at using mineral deposits as tools to explore different long-term Earth processes.

The fossil record of hydrothermal vent communities

Abstract There are 19 known fossiliferous volcanogenic massive sulphide (VMS) deposits which range in age from the Silurian to the Eocene. Most of these are in the Ural Mountains, Russia. The deposits contain assemblages of inarticulate and rhynchonellid brachiopods; gastropod, bivalve and monoplacophoran molluscs; and a small diversity of worm tube morphologies, some of which may be attributable to alvinellid polychaetes and vestimentiferans. The fossils are preserved mainly as external moulds of pyrite, which is consistent with biomineralization processes occurring at modern vent sites. Most of the fossil taxa are new to science, but the lack of original shells and organic tubes makes placement in existing phylogenetic schemes difficult. A comparison between modern vent communities and fossil vent assemblages shows that vestimentiferans, alvinellid polychaetes, bivalves, gastropods, monoplacophorans and perhaps brachiopods are shared at higher taxonomic levels, but that arthropods are found only in the modern communities. There are no direct ancestor-descendant relationships between the fossil and modern vent molluscs and brachiopods. This demonstrates that the modern vent environment is not a refuge for the known Palaeozoic and Mesozoic shelly vent taxa. Hence, taxonomic groups have moved in and out of the vent ecosystem through time. These findings are discussed in relation to alternative hypotheses for the origins of modern vent communities.

Age constraints and geochemistry of the Ordovician Tyrone Igneous Complex, Northern Ireland: implications for the Grampian orogeny

Abstract: The Tyrone Igneous Complex is one of the largest areas of ophiolitic and arc-related rocks exposed along the northern margin of Iapetus within the British and Irish Caledonides. New U–Pb zircon data and regional geochemistry suggest that the Tyrone Plutonic Group represents the uppermost portions of a c. 480 Ma suprasubduction-zone ophiolite accreted onto an outboard segment of Laurentia prior to 470.3 ± 1.9 Ma. The overlying Tyrone Volcanic Group formed as an island arc that collided with the Laurentian margin during the Grampian phase of the Caledonidan orogeny. Early magmatism is characterized by transitional to calc-alkaline, light REE (LREE)-enriched island-arc signatures, with an increasing component of continentally derived material up sequence. Tholeiitic rhyolites with flat to U-shaped REE profiles and LREE-depleted basalts, located stratigraphically below a c. 473 Ma rhyolite of the upper Tyrone Volcanic Group, suggest initiation of intra-arc rifting at c. 475 Ma. Metamorphic cooling ages from the Tyrone Central Inlier imply arc–continent collision before 468 ± 1.4 Ma, with the emplacement of the Tyrone Volcanic Group onto the margin. A suite of 470.3 ± 1.9 Ma to 464.3 ± 1.5 Ma calc-alkaline intrusions are associated with the continued closure of Iapetus. Supplementary material: Geochemical data and petrography are available at http://www.geolsoc.org.uk/SUP18467.

Massive Sulfide Deposits in the South Urals: Geological Setting within the Framework of the Uralide Orogen

The south Urals is host to more than 80 Paleozoic volcanic-hosted massive sulfide (VMS) deposits developed in four distinct metallogenic zones. From west to east these are: the Sakmara zone, Main Uralian fault zone, and the east and west Magnitogorsk zones. In the Sakmara zone, the chemistry of host volcanic suites is consistent with development of the zone in a Silurian oceanic arc. The Main Uralian fault marks a line of paleosubduction and contains VMS deposits similar to those formed in modern mid-ocean ridge settings. The Magnitogorsk zones contain VMS deposits formed in a Devonian fore-arc, arc and inter-arc or proto-back arc setting. The earliest volcanics of the Magnitogorsk zone, the Baimak-Buribai formation, form a boninitic fore-arc sequence, evolving later to more calc-alkalic volcanics with evidence for a contribution from subducted slab to the volcanics. Later, and farther east of the subduction suture, a rifted, more mature arc setting formed where the Karamalytash formation volcanics developed in an inter-arc or proto-back arc setting. The Karamalytash formation shows little evidence of contribution from subducted sediment to the melt. Stratigraphically overlying the Baimak-Buribai formation, and partly time equivalent to the Karamalytash formation, is the Irendyk formation. The Irendyk formation is VMS-poor, but contains abundant epiclastic volcanosediments and epithermal-like gold-barite deposits, indicative of shallower sea conditions. The Irendyk formation appears to form a long linear geographic feature, perhaps marking the line of an emerging arc sequence behind which the Karamalytash formation developed in a rift. Previous authors suggest that the west and eastern Magnitogorsk zones developed as separate arcs, but the arc-like volcanics in the east Magnitogorsk zone may simply indicate the migration of the volcanic arc eastwards as the East European craton approached the Main Uralian fault.

Self-organization of submarine hydrothermal siliceous deposits: Evidence from the TAG hydrothermal mound, 26°N Mid-Atlantic Ridge

This study documents a novel form of ferric iron oxyhydroxide-rich moss agate, taken from the flanks of the TAG (Trans-Atlantic Geotraverse) submarine hydrothermal mound at 26°N Mid-Atlantic Ridge. The genesis of the agate is related to a series of rapid irreversible changes, with cooling across a redox and pH front separating oxidizing highly viscous siliceous gels from a mixed pyrite–iron oxide sediment. This study shows a fraction of the inorganic self-organized mechanisms of mineralogical, textural, and geochemical patterning operative within these far from equilibrium settings, and how iron inclusion morphologies suggestive of a biogenic origin may be generated by inorganic processes.