Area Of Hydrothermal Activity Research Articles

Archean abiogenic and probable biogenic structures associated with mineralized hydrothermal vent systems and regional metasomatism, with implications for greenstone belt studies

Abiogenic structures, formed by fluid emission in subaerial mud pools and nearby stromatolites, from stratigraphically related carbonate sediments, occur in the southern part of the approximately 3.3 to 3.5-b.y.-old Barberton greenstone belt. The mud pool structures are associated with ferruginous shales, banded iron-formations, and in places barite deposits; they relate stratigraphically to transgressive pods of ironstone. The pods probably represent buried, mineralized hydrothermal channels and chimneys. Soft sediment deformation structures adjacent to these pods suggest fossil fluid circulation patterns in the sediments with horizontal flow over distances greater than 300 m and vertical motions an order of magnitude less. Associated metasomatism has significantly altered the original chemistry of the host rocks on a regional scale, prior to 13.8 b.y. This is described with specific reference to silicification. Silicification patterns in the igneous rocks underlying the sediments also indicate fluid circulation patterns an order of magnitude greater than those in the sediments. Thin but regionally extensive and originally horizontal alteration planes were probably zones of high fluid pressure along which large-scale, gravity-induced mass transportations occurred. Such transportations greatly influenced the subsurface mineral precipitation and the surface geologic processes within the extensive area of hydrothermal activity, in which the inferred high geothermal gradient was probably related to shallow-level igneous activity. At least minor mineralization, including local gold concentration several orders of magnitude greater than that found in average crustal rocks (e.g., ppb to ppm), occurred simultaneously with these processes of metasomatism during sedimentation, igneous activity, and tectonism within the belt, and prior to the intrusion of the greenstone belt by surrounding granitoids.

Mid-Cretaceous Black Shales: A Result of Excessive Fertility or Global Anoxia?: ABSTRACT

In Holocene marine environments, organic carbon accumulation rates are high in areas of high productivity, rapid deposition, and in anoxic basins. Heavy metal accumulation rates are high in areas of high bulk sedimentation (resulting from the large detrital component), in areas of high fertility (heavy metal enrichment in organic tissue), and in areas of hydrothermal activity. The various environments can be characterized by the relations between their bulk sedimentation rates, amounts and types of organic carbon in the sediment, organic carbon accumulation rates, primary productivity, preservation factors (fraction of organic carbon fixed in the photic zone that becomes buried in the sediment), and accumulation rates of heavy metals (such as Cu, Ni, Zn). Differences in t ese relations exist, in particular, between areas where sapropel deposition is due to high supply (i.e., high fertility in localized upwelling areas) and where it is due to anoxic deep waters (such as in the Black Sea). Application of an improved calcareous nannofossil biochronology to compute accumulation rates in various DSDP sections show that the mid-Cretaceous black shales are characterized by substantially higher accumulation rates of heavy metals relative to organic carbon than is observed in Holocene upwelling settings. The implied widespread deep-water anoxia suggests high organic carbon preservation factors and correspondingly low primary productivity for large parts of the mid-Cretaceous oceans. End_of_Article - Last_Page 552------------

Precious metals in ocean-ridge basalts; implications for basalts as source rocks for gold mineralization

The Au, Ag, and S contents of the glass rims of tholeiitic pillow basalts dredged between 25 degrees N and 30 degrees N on the Mid-Atlantic Ridges are up to 7, 5, and 2.5 times greater, respectively, than those of the crystalline interiors of the basalts. Si, Fe, and Mn are also lower in the crystalline interiors relative to the glass rims, whereas K is higher. The differences in chemical composition between the rims and the interiors are attributed to loss of Au, Ag, S, Si, Fe, and Mn from the interiors of the basaltic pillows due to interaction of the hot basalt with sea water and gain in K by the interiors during subsequent low temperature interactions.Manganese crusts from an area of hydrothermal activity at 26 degrees N have exceptionally low Cu, Ni, Co, Fe contents and low Ir/Au and Ir/Ag ratios when compared to normal hydrogenous crusts that grow away from areas of submarine hydrothermal activity. Only a small fraction of the metals that are lost from the pillow interiors may be transported to the sea floor via the hydrothermal solutions that initially interact with the basalts; the remainder may reside within the basalt pile, for example, in interpillow regions.The Au contents of the fresh crystalline interiors of the basalts correlate with their Cr, Ni, and MgO contents. This suggests that Au in the basalt interiors is hosted by an early-crystallized phase such as Cr-spinel or olivine. The difference in Au content of the fresh interior (hosted by Cr-spinel and/or olivine) and that of the glass rim of the basalt is believed to represent that Au which would have been deposited on loosely bound sites (e.g., deuteric alteration products, grain boundaries, mesostasis phases, and, most important, sulfides within the pillow interior) had it not interacted at a high temperature with sea water.The Au and Ag contents of the glass rims vary by a factor of 10, confirming the existence of regional variations in the Au contents of basalts. The lowest Au and Ag contents in the glass rims are for basalts dredged from an area with no known hydrothermal activity.The results of this study suggest that basalts with high MgO, Cr, Ni, Cu, and Ir contents and low K 2 O and TiO 2 contents may constitute the most favorable type of source basalt. The study demonstrates that the metal available for ore generation in a source rock is not a function of the total metal content of the rock but rather a function of the mineralogical siting of the metal.

Underground temperatures in hydrothermal areas in Iceland as deduced from the silica content of the thermal water

Experimental work found in the literature on the solubility of various silica minerals at different temperatures is briefly reviewed, and previous geological applications mentioned, of the use of the amount of dissolved silica in thermal springs to estimate underground temperatures. The common silica minerals (quartz, chalcedony, opal) are very widely distributed in vesicles and cracks of the flood basalt formations of Iceland and hydrothermal quartz and chalcedony have been found in drillholes in active hydrothermal areas. Quartz is formed if water temperatures are above about 110 °C but chalcedony at temperatures below about 100 °C. Measured temperatures in thirty 28 to 2199 metres deep drillholes in the low temperature areas of Iceland are often very similar to temperatures estimated from the silica content of the water assuming equilibrium with quartz or chalcedony. There is a close comparison between temperatures measured in wet steam drillholes and temperatures derived from the silica content of the water assuming equilibrium with quartz. Mixing of cold ground water with the thermal water leads invariably to some silica supersaturation at the final temperature of the mixture. Temperatures of the last equilibrium with quartz reach a maximum of about 130 °C to about 180 °C in the most prominent low temperature areas and a maximum of about 200 °C to about 270 °C in the five high temperature areas where alkaline springs are known. The conditions for the precipitation of silica in wet steam drillholes are predicted.

The Tuscan soffioni: A review

In geological, geophysical, and chemical aspects the soffioni are quite similar to hydrothermal phenomena in the western United states, particularly in Sonoma County, California. Though described as recently as the last decade as “an entirely distinct class of pseudovolcanic manifestations,” the soffioni and lagoni are really fumaroles and spouting hot‐springs distinguished by degree of activity rather than by difference in kind. Their association in Tuscany with fissures, with older intrusive rocks, with veins of copper, lead, and antimony, as well as the high temperature of the steam and other characteristic gases, proclaims their magmatic origin. These phenomena have a peculiar claim to the interest of volcanologists and geophysiclsts, (1) because Tuscany is the only place in the world where volcanic steam has been successfully utilized on a commercial scale (the credit for this belongs to Prince Piero Ginori Conti); (2) because this is probably the only active hydrothermal area where the volcanic products, boric acid, carbon dioxide, and ammonia, have been systematically and continuously recovered; (3) because of the extensive cooperation between science and industry in the exploitation of the soffioni. Besides an immense amount of work involving engineering, physics, and chemistry, drilling for steam has added much to geological information. Systematic meteorological observations are now in progress, an exact series of topographic levels has been determined, and magnetic and gravimetric surveys have been made, but important deductions from them remain for the future.