Structural control and hydrothermal alteration pattern of Chaucha Porphyry Copper,Ecuador

The intrusives and associated volcanic activity, the hydrothermal alteration pattern, and mineralization of the Chaucha porphyry copper deposit appear to be strongly controlled by the intersection of two major structures: the Chaucha fault and the Cordillera fault. Stockwork development and dynamic foliation of the intrusives and surrounding rocks respond to the same control. The fault movements affect the diorite and granodiorite intrusions and largely predate the mineralization and the quartz monzonite intrusion. The lithologies and peculiar geological setting favor a volcanic origin of the Chaucha porphyry copper. Using structural relations and other geological criteria, the Chaucha porphyry copper is tentatively placed in the plate tectonic model.

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Résumé L'activité ignée, la zonalité de l'altération hydrothermale et la minéralisation du type Porphyry Copper à Chaucha sont controllées d'une façon évidente par l'intersection de deux failles importantes: la «faille de Chaucha» et la «faille Cordillera». Le developpement des fractures et de la foliation dans les roches intrusives ainsi que dans les roches intrudées obéissent au même control tectonique. Le jeu des failles affecte la diorite et la granodiorite mais pas la minéralisation ni l'intrusion de monzonite quartzifère. La texture et la nature des roches aussi bien que l'environnement géologique suggérent une origine volcanique pour ce gisement. Se basant sur les relations tectoniques et d'autres arguments géologiques, l'origine du gisement (roches et minéralisation) est interpretée, d'une façon préliminaire, à la lumiere de la tectonique des plaques.

     

Overview of GIS Applications in Estuarine Monitoring and Assessment Research

Geographic information systems (GIS) tools are now considered integral in estuarine monitoring and assessment research. A synopsis is presented of our estuarine applications of GIS in the Northeast region of the U.S. The applications discussed cover sample site selection, support for field sampling activities, quality assurance of data, spatial display of geographic referenced information, quantitative spatial analysis of data, and communication of results.     

Role of melt during deformation in the deep crust

Deformation in the deep crust is strongly influenced by the presence of melt. Injected melt (or magma) weakens the crust because strain will tend to localize where melt is present. The amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate. For plutons that intrude into rocks which are near the solidus temperature of the melt, crystallization times can be quite long (> 1Myr).
Partial melting of deep crustal rocks can lead to melt-enhanced embrittlement. This occurs because the volume change for most melting reactions is positive. Therefore, when the rate of melt production outpaces the rate at which melt can leave the system, the melt pressure increases. Eventually, the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture.
Conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the Central Gneiss belt (Canada); this suggests that melt-enhanced embrittlement occurred in these rocks. An expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived. Assuming that migmatite has a small tensile strength, differential stresses are ≤ 20 MPa in migmatitic rocks at the time melt-enhanced embrittlement occurs. The occurrence of melt-enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting. Furthermore, repeated episodes of melt-enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains.     

The North Fork porphyry copper deposit of the Washington Cascades

The North Fork porphyry copper deposit in the Cascade volcanic arc of Washington displays copper mineralization confined to the potassic (biotite) and phyllic (sericite) alteration zones. No secondary potassium feldspars have been found in either alteration zone; moreover, chemical analyses indicate the potassic alteration zone contains a low K₂O content. Argillic and propylitic zones are also recognized, but these are barren of hypogene copper mineralization. Biotite-chalcopyrite intergrowths in the deposit have been given a 9.9 K-Ar age. Thus it is not only one of the youngest deposits dated in the western Cordillera, but it is associated with a volcanic arc which has no current Benioff zone seismic array or accompanying trench. The deposit appears to have developed during the period when coupling of the North American and Juan de Fuca plates probably inhibited subduction under the Cascade volcanic arc.     

The Calipuy Formation of Northern Peru, and its relation to volcanism in the Northern Andes

The Calipuy Formation is a primarily volcanic sequence deposited during the period 33 to 10 m.y. ago when basaltic and andesitic volcanoes developed concomitant with a N60°E horizontal foreshortening of the Peruvian Andes. The axis of compression is inferred from both fault tectonics in and near the Calipuy sequence and from N30°W-striking fold axes within it. Dacitic domes younger than 10 m.y. unconformably overlie the Calipuy sequence.Basaltic and andesitic effusive rocks concurrently filled a basin which developed during volcanism. During subsidence 80% of the fill was provided by primary extrusive igneous material, whereas sedimentary rocks associated with the igneous assemblage account for only 20%.Limited chemical data indicate that Calipuy andesitic rocks are slightly richer in alkalies than the average Cenozoic andesite, but petrographic data show that they are similar to other andesites of this age found in similar environments in the Andes. However, the analyses are too few to make any real generalization concerning petrogenesis.     

Geology of epithermal silver-gold bulk-mining targets,bodie district,Mono County,California

The Bodie mining district in Mono County, California, is zoned with a core polymetallic-quartz vein system and silver- and gold-bearing quartz-adularia veins north and south of the core. The veins formed as a result of repeated normal faulting during doming shortly after extrusion of felsic flows and tuffs, and the magmatic-hydrothermal event seems to span at least 2 Ma.Epithermal mineralization accompanied repeated movement of the normal faults, resulting in vein development in the planes of the faults. The veins occur in a very large area of argillic alteration. Individual mineralized structures commonly formed new fracture planes during separate fault movements, with resulting broad zones of veinlets growing in the walls of the major vein-faults. The veinlet swarms have been found to constitute a target estimated at 75,000,000 tons, averaging 0.037 ounce gold per ton. The target is amenable to bulkmining exploitation.The epithermal mineralogy is simple, with electrum being the most important precious metal mineral. The host veins are typical low-sulfide banded epithermal quartz and adularia structures that filled voids created by the faulting. Historical data show that beneficiation of the simple vein mineralogy is very efficient. On the cover: Southeast view of Bodie mining district in Mono County, California, one of the more famous pioneer epithermal gold producers in the western U.S. Recent exploration drilling suggests an additional resource of 2 million ounces of gold. The townsite is at the intersection of roads near the center of the photo. The foreground is mainly tuff breccia of the Bodie Hills volcanic field. Arcuate lines in the valley beyond the hills are old Mono Lake shorelines. The White Mountains form the ridge on the skyline. Photo by Frank Kleinhampl, about 1970.     

Khatyrka,a new CV3 find from the Koryak Mountains,Eastern Russia

A new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay‐rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel‐rich iron‐nickel metal, nepheline, and calcic pyroxene [diopside‐hedenbergite solid solution]) of oxidized‐subgroup CV3 chondrites. A few grains are fine‐grained spinel‐rich calcium‐aluminum‐rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper‐aluminum‐iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu‐Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3‐like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu‐Al‐Fe metal alloys and the alloy‐bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper‐aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia.     

An empirical solvus for CO2-H2O-2.6 wt% salt

The solvus in the system CO₂-H₂O-2.6 wt% NaCl-equivalent was determined by measuring temperature of homogenization in fluid inclusions which contained variable $\text{CO}{}_2\text{H}{}_2\text{O}$ but the same amount of salt dissolved in the aqueous phase at room temperature. The critical point of the solvus is at 340 ± 5°C, at pressures between 1 and 2 kbar; this is about 65°C higher than for the pure CO₂-H₂O system. The solvus is assymetrical, with a steeper H₂O-rich limb and with the critical point at mole fraction of water between 0.65 and 0.8.