Vent Zone Research Articles

Hydrothermal eruption mechanisms and hot spring gold deposits

Episodes of gold mineralization in the shallow hot spring environment are related in time to hydrothermal eruption events and in space to the resulting vent breccias and peripheral stockwork zones. It is proposed that large but short-lived overpressures in a geothermal reservoir, probably triggered by sudden magmatic heat fluxes, induce hydraulic fracturing which then evolves into hydrothermal eruptions if driven through to the surface. The maximum available energy in hot, shallow reservoirs appears easily sufficient to drive such eruptions, particularly if CO 2 -rich fluids are involved. In mineralized systems, gold-bearing fluids are subsequently channeled into the outflow conduit where they flood the permeable vent breccia and peripheral stockwork. Gold is lifted into this hot spring environment above a boiling level that is elevated by high flow rates to within several hundred meters of the surface and is precipitated with abundant quartz, pyrite, and adularia, along with a distinctive and steeply zoned trace element suite. These sequential events probably occur as a continuum, which if repeated cyclically in a single vent zone, can result in ore-grade mineralization.

Considerações sobre a origem dos grãos "olhos-de-quartzo" nas rochas porfiroidais do distrito de Western Mines, Ilha de Vancouver, Canadá

Quartz-eye bearing porphyroidal occur in straight association with the vent ore, in the vent zone, of the base metal massive sulphide deposits of the Western Mines District. Microscopic observations on these indicated two stages of development, which are part of a continuous process of rock alternation. In the earlier stage (first generation quartz) there was a total or quasi-total replacement of chloritecalcite spherulites or a filling of vesicles by quartz. It was accompanied by development of strong vertical foliation which superimposed to the primary bedding. In the final stage (second generation quartz) the quartz overprinted the vertical schistosity, and deformation was minimum. Quartz-eye typical of the earlier stage, termed first generation quartz, is surrounded by a bowed micaceous matrix; this is not the case when it is examined in a section perpendicular to the foliation. Composite grains and recrystallized single round grains may be in association with deformed or broken grains of original quartz and with latter or second generation grains. A preserved nucleus of calcite is occasionally observed. Hypidiomorphic and idiomorphic coarse grains of quartz are typical of the final fase of the vent alteration process, they are termed second generation quartz. Overgrowth of second generation quartz without optical character of continuity upon first generation quartz is commonly observed. In addition most of the round rims of quartz represent the outer zone of the crystallized second generation quartz. It becomes distinguishable from the rest of the quartz grain because of the abundance of microinclusions. Since both stages of quartz generation affected either mafic-intermediate or intermediate-acidic rocks, it is suggested that the term quartz-eye bearing porphyroidal rocks might be better regarded as a textural terminology than a compositional one.

Peralkaline Ash-Flow Tuffs in Santa Clara Canyon, North of Chihuahua City, Mexico, Possible Source Rocks for Uranium: ABSTRACT

A thick sequence of upper Oligocene peralkaline ash-flow tuffs and an associated vent complex are located in Santa Clara Canyon, 91 km north of Chihuahua City, Mexico. The principal unit, the Cryptic Tuff, is a comendite and contains about 1% phenocrysts that include sanidine (Or 45-55) quartz, hedenbergite, and magnetite. A complete cooling unit consists of: (1) a basal vitrophyre with associated spherulitic pods; (2) greenish, densely welded tuff that contains stretched pumice lapilli; (3) a banded red and light pink zone that contains flowage features and shear folds; (4) a highly porous zone with abundant quartz and sanidine in cavities; and (5) a thick zone of micropoikilitic sanidine and quartz interspersed with riebeckite and aegirine. Flow breccias that contain fragments of the different Cryptic tuff varieties usually separate micropoikilitic riebeckite tuff units. A north-south-trending vent zone (at least 3 km long) is present in lower Santa Clara Canyon where over 400 m of Cryptic tuff is exposed. Flow foliation is vertical in the vent area. Overlying the Cryptic sequence is the bluish, comenditic Campana tuff. It is densely welded and contains about 10% sanidine and quartz phenocrysts set in a groundmass that includes riebeckite and matted aegirine needles. A minimum loss of 8 mg U3O8 per gram of Cryptic tuff has been calculated using data for glassy and massive riebeckitic phases. These paralkaline tuffs could have been source rocks from which large amounts of uranium were mobilized. End_of_Article - Last_Page 758------------