Unaltered Volcanic Rocks Research Articles

Geophysical exploration for epithermal gold deposits: Case studies from the Hishikari Gold Mine, Kagoshima, Japan

Various geophysical techniques were applied experimentally to exploration for epithermal gold deposits. The Hishikari gold?silver deposit, located in northeastern Kagoshima, Kyushu, Japan, was selected for this study. This paper presents the results from these case studies and discusses the applicability of each method in several stages of the exploration; the reconnaissance stage, the detailed follow-up stage and the final stage where the surveys are directed at detecting veins. The results are summarized as follows; 1. A high-gravity anomaly was interpreted as the swollen basement. This subsurface structure was considered to be local doming of basement associated with volcanic activity. Local gravity highs are worthy of notice because the doming of basement might cause fractures; 2. A resistivity cross-section was characterized by three layers; an uppermost layer with high resistivity (100?1000 Wm) corresponding to unaltered volcanic rocks, an intermediate layer with low resistivity (3?8 Wm) corresponding to altered volcanic rocks and a bottom layer with relatively high resistivity (80?200 Wm) corresponding to the basement. Resistivity mapping and sounding are useful to identify lithologies and structures that may control gold mineralization; 3. IP anomalies (high chargeability) within the area of low resistivity are the most important indicators of vein zones. For this reason, in the detailed follow-up stage and the final stage of exploration, IP surveys play the most important role.

Perspective. Certified Ontario reference materials programme at the Geoscience Laboratories, Sudbury, Ontario, Canada

The Ontario Geoscience Laboratories (hereafter referred to as the Geoscience Laboratories) have produced 30 in-house materials since 1970. This experience, the limited availability of international rock standards in volume, and the recent acquisition of specialized equipment (capable of homogenizing up to 1 t of sample), has led to the development of the Certified Ontario Reference Materials programme for the production of certified reference materials (CRMs). Given the geological diversity of Ontario, the decision was made to create two streams of reference materials. One is a suite of unaltered and altered volcanic rock reference materials that reflected the wide compositional range of Archean rocks of Ontario. Sample sites are clustered in the Abitibi Greenstone Belt which includes the Timmins and Kirkland Lake mining camps. This region also hosts some of the world's largest and most prolific mines of Au and Cu–Zn, smaller Ni deposits and one of the two international type-localities of komatiite. Advantages of such a choice are extensive. The rocks in this area have been studied by the Ontario Geological Survey (OGS) for 50 years and this expertise is readily available both in terms of appropriate chemistry and in sampling sites. Geologically, Archean volcanic rocks were typically extruded into a subaqueous environment and cooled quickly. This facilitates a high degree of homogeneity. There are a wide variety of compositions available, clear association with mineral deposits and thus the reference materials may also provide excellent baseline material for environmental studies. In the 1970s, the OGS devised internationally used major element classification criteria which divided Archean volcanic rocks into three major clans that have several subtypes (komatiitic clan has ultramafic and basaltic members; tholeiitic clan has Fe-, Mg-, basaltic, dacitic, andesitic, and rhyolitic members; and calc-alkalic clan has basaltic, andesitic, dacitic and rhyolitic members). These rock types typically show different styles of alteration near ore deposits (i.e., chloritization, sericitization, carbonatization). The Archean volcanic rock suite will be comprised of these unaltered and altered rock types. The other group will focus on materials that fill a specific geoanalytical need. Based on the information above and the capacity of the Geoscience Laboratories geostandards programme (5 CRMs per year), it is anticipated that several rock types will be prepared yearly. A calc-alkaline rhyolite and an ultramafic komatiite have now been collected and other sites have been identified. Collection has commenced. After the sample is bottled, the homogeneity will be tested in our laboratories using validated methods. This will provide a tentative composition and will be followed by round-robin determinations. The reference materials will be released in 100 and 500 g plastic bottles to both the geoanalytical community and the exploration industry to monitor precision and accuracy in commercial surveys.

Mass, volume and chemical changes in the alteration zone at the Norbec mine, Noranda, Quebec

A funnel-shaped alteration pipe in Archean rhyolite and andesite below massive sulphide Zn-Cu ores at the Norbec mine in northwestern Quebec was outlined from drill core samples, geochemical parameters, and normative alteration mineralogy. The pipe has a mass of 37.9 million tonnes giving a volume of 13 × 106 m3, which represents volume increase of 10% relative to the unaltered host volcanic rocks. A bulk chemical composition was calculated using “weighting” procedures for “volumes of influence” for the samples. Net mobile mass change, or chemical flux, for the alteration pipe was + 5.8 × 106 tonnes; inclusion of the massive ore lens yields a flux of + 9.6 × 106 tonnes for the whole hydrothermal system. The largest additions to the system in millions of tonnes were: FeO(+4.2), SiO2(+3.8), S(+1.8), K2O(+0.55), and MgO(+0.5); the only depletions were Na2O (−1.4), and CaO(−0.44). Base and precious metals accounted for 3.4% of the total element flux.

Hydrothermal alteration of basic island‐arc volcanic rocks north and south of Mytilini Town, Lesvos Island, Greece

ABSTRACTThe mobilities of major and trace elements have been investigated in hydrothermally altered volcanic rocks of Pliocene age on the island of Lesvos. One type of hydrothermal assemblage, montmorillonite‐K‐feldspar‐goethite, was recognized in outcrops north of Mytilini town.The alteration occurs a few metres under the surface of unaltered volcanic rocks of high‐K andesite and high‐K basaltic andesite composition. The unaltered latite‐andesite consists of plagioclase, clinopyroxene, sanidine and magnetite with quartz, calcite, pumpellyite and montmorillonite as subordinate secondary minerals.The main control over major oxide distribution appears to be the extent of conversion of plagioclase to K‐feldspar.Three distinct groups of elements were revealed to be positively correlated: Si, Mg, Ca, Na, Mn, Sr and Ba, which are leached during the alteration process. This group of elements reflects their association with plagioclase feldspar and clinopyroxene. Fe, K, Ti, P, Nb, Zr, Y, Rb, Ni, Cr, V. This group includes elements which are enriched relative to the unaltered volcanic rock, reflecting their association with K‐feldspar, iron hydroxides and Ti and P‐bearing minerals. Ti, P, Nd, La, Ce. From this group of elements it is indicated that Nd, La and Ce are associated with Ti‐ and P‐bearing minerals. There is no evidence of Cu or Zn addition to the system by hydrothermal fluids since there are no correlations between these base metals and elements enriched in the K‐feldspar, montmorillonite‐rich zone. This potassic rich zone may be connected with hidden epithermal sulphide mineralization with gold.

Electrical structure of Newberry Volcano, Oregon

From the interpretation of magnetotelluric, transient electromagnetic, and Schlumberger resistivity soundings, the electrical structure of Newberry Volcano in central Oregon is found to consist of four units. From the surface downward, the geoelectrical units are (1) very resistive, young, unaltered volcanic rock, (2) a conductive layer of older volcanic material composed of altered tuffs, (3) a thick resistive layer thought to be in part intrusive rocks, and (4) a lower‐crustal conductor. This model is similar to the regional geoelectrical structure found throughout the Cascade Range. Inside the caldera, the conductive second layer corresponds to the steep temperature gradient and alteration minerals observed in the USGS Newberry 2 test hole. Drill hole information on the south and north flanks of the volcano (test holes GEO N‐1 and GEO N‐3, respectively) indicates that outside the caldera the conductor is due to alteration minerals (primarily smectite) and not high‐temperature pore fluids. On the flanks of Newberry the conductor is generally deeper than inside the caldera, and it deepens with distance from the summit. A notable exception to this pattern is seen just west of the caldera rim, where the conductive zone is shallower than at other flank locations. The volcano sits atop a rise in the resistive layer, interpreted to be due to intrusive rocks. The intrusive material has served as a heat source to produce enhanced hydrothermal alteration and, perhaps in the case of the west‐flank anomaly, elevated fluid temperatures. While no public drill hole information is available to confirm this hypothesis, the west‐flank anomaly appears to be a good geothermal target. In addition to the possibility that a region on the west side of the volcano could be favorable for prospecting, part of the resistive structure under the center of the volcano could be due to a vapor‐dominated environment with temperatures above 300°C. In other parts of the Cascades, pervasive alteration has produced mixed layer clays and zeolites, resulting in low‐resistivity anomalies. Low resistivities cannot be assumed to indicate high‐temperature pore fluids. The use of electrical methods that measure resistivity as a function of excitation frequency, such as spectral induced polarization, may provide a way of obtaining information about the type and extent of alteration.

Major- and trace-element mobility in altered volcanic rocks near Stypsi, Lesbos, Greece and genesis of a kaolin deposit

Major- and trace-element mobilities have been investigated in hydrothermally altered zones of Miocene volcanic rocks on the island Lesbos. Three types of alteration have been recognized, and are characterized by the mineralogical composition: (a) zone of illite and smectite; (a-b) transitional zone of kaolinite, illite, smectite; and (b) zone of kaolinite. The alteration zones are encountered in the Lower Lava Unit (latite—andesite) and are related to a channel (fault) which allowed the circulation of hydrothermal fluids. The zonation of hydrothermal alteration is attributed to the step-wise character of the kaolinitization process. The following relationships between mineral paragenesis of the different alteration zones and element behaviour are apparent. 1. (1) K and Rb enrichment relative to fresh volcanic rock occurs in zones a and a-b, while K and Rb are leached during destruction of illite in the kaolinite zone (b). 2. (2) Zones a and a-b are characterized by depletion of Fe, Mg, Ca, Na, P, Sr, Zn, Cu, Ni and Ba when compared with the unaltered volcanic rock. 3. (3) Relative to the volcanic rock, the kaolinitic zone (b) is characterized by depletion of Fe, Mg, Ca, Na, K, P, and Ti and enrichment of Al. Of the trace elements Nb, Zr, Ce, Nd and La are enriched while Sr, Rb, Zn, Cu, Ni, Cr, V, Ba and Sc are depleted. 4. (4) There is no evidence of Cu, Ni, Cr, Zn or Mo addition to the system by the hydrothermal fluids. Recognition of the hydrothermal origin of Stypsi Kaolin is based on the following criteria: (a) mineralogical zonation; (b) association of pyrite with hydrothermally altered zones; (c) Hg-geochemical anomalies of the alteration zones, which are indicative of postvolcanic geothermal activity; (d) comparison of the analytical data derived from the present study with relevant data on kaolins of known hydrothermal origin.

Effects of Soil pH, Competition, and Seed Predation on the Distributions of Two Tree Species

Field experiments were conducted to evaluate the effects of soil pH, competition, and seed predation on seedling establishment of two tree species in the Sierra Madre of northwestern Mexico. Quercus albocincta and Lysiloma divaricata occur on the same mountain slopes but have almost no overlap in distribution on a more local scale: Quercus dominates in distinct patches of open woodland found only on acid, nutrient—poor soil derived from hydrothermally altered volcanic rock while Lysiloma dominates in adjacent patches of drought—deciduous forest found only on less acid and more fertile soil derived from unaltered volcanic rock. The experiments were designed to test the hypotheses that Lysiloma is excluded from the oak woodland by the low soil pH and that Quercus is excluded from the deciduous forest by competition and/or seed predation. In the oak woodland, Lysiloma seedlings survived on the acid soil only if soil pH was increased, while Quercus seedlings showed no significant response to increase in soil pH. In the deciduous forest, Quercus seedlings required the exclusion of seed predators as well as removal of surrounding vegetation to survive. Lysiloma seedlings also required removal of some of the surrounding vegetation to establish in the deciduous forest, but less reduction in competition intensity was required than for Quercus seedlings.

Mapping of hydrothermally altered rocks using airborne multispectral scanner data, Marysvale, Utah, mining district

Multispectral data covering an area near Marysvale, Utah, collected with the airborne National Aeronautics and Space Administration (NASA) 24-channel Bendix multispectral scanner, were analyzed to detect areas of hydrothermally altered, potentially mineralized rocks. Spectral bands were selected for analysis that approximate those of the Landsat 4 Thematic Mapper and which are diagnostic of the presence of hydrothermally derived products. Hydrothermally altered rocks, particularly volcanic rocks affected by solutions rich in sulfuric acid, are commonly characterized by concentrations of argillic minerals such as alunite and kaolinite. These minerals are important for identifying hydrothermally altered rocks in multispectral images because they have intense absorption bands centered near a wavelength of 2.2 μm. Unaltered volcanic rocks commonly do not contain these minerals and hence do not have the absorption bands. A color-composite image was constructed using the following spectral band ratios: 1.6μm/2.2μm, 1.6μm/0.48μm, and 0.67μm/1.0μm. The particular bands were chosen to emphasize the spectral contrasts that exist for argillic versus non-argillic rocks, limonitic versus nonlimonitic rocks, and rocks versus vegetation, respectively. The color-ratio composite successfully distinguished most types of altered rocks from unaltered rocks. Some previously unrecognized areas of hydrothermal alteration were mapped. The altered rocks included those having high alunite and/or kaolinite content, siliceous rocks containing some kaolinite, and ash-fall tuffs containing zeolitic minerals. The color-ratio-composite image allowed further division of these rocks into limonitic and nonlimonitic phases. The image did not allow separation of highly siliceous or hematitically altered rocks containing no clays or alunite from unaltered rocks. A color-coded density slice image of the 1.6μm/2.2μm band ratio allowed further discrimination among the altered units. Areas containing zeolites and some ash-fall tuffs containing montmorillonite were readily recognized on the color-coded density slice as having less intense 2.2-μm absorption than areas of highly altered rocks. The areas of most intense absorption, as depicted in the color-coded density slice, are dominated by highly altered rocks containing large amounts of alunite and kaolinite. These areas form an annulus, approximately 10 km in diameter, which surrounds a quartz monzonite intrusive body of Miocene age. The patterns of most intense alteration are interpreted as the remnants of paleohydrothermal convective cells set into motion during the emplacement of the central intrusive body.

The Distribution of Evergreen and Deciduous Trees Relative to Soil Type: An Example from the Sierra Madre, Mexico, and a General Model

An example of the association of evergreen plants with nutrient—poor soils is described from the Sierra Occidental of northwestern Mexico. At elevations from 600 to 1100 m in this mountain range, distinct patches of evergreen—oak woodland are surrounded by subtropical deciduous forest in a mosaic pattern. To quantify the vegetation pattern and examine its environmental basis, vegetation and soil samples were taken throughout the elevational range of the mosaic. Analysis of the samples indicated that the patches of evergreen vegetation occur on extremely acid, infertile soils (derived from hydrothermally altered rock), while deciduous vegetation dominates on less acid, more fertile soil (derived from unaltered volcanic rock). The two community types have very little floristic overlap, and the deciduous forest has much higher density of woody plants and greater species richness. The actual location of boundaries between patches of the two community types can be more abrupt than can be accounted for by soil change. At higher elevations (above 1100 m) evergreen woodland is not restricted to acid soils and occurs on all soil types. These data are discussed in terms of a general model to account for the association of evergreen plants with nutrient—poor soils in the Sierra Madre as well as in other areas with similar growth form soil fertility patterns. I propose that evergreen plants would grow and survive at least as well on fertile soils as on infertile soils in isolation, but are unable to establish in dense, productive vegetation because of low growth rates associated with high carbon costs and low photosynthetic rates of evergreen leaves.

18O-Enrichment of silicic magmas caused by crystal fractionation at the Galapagos Spreading Center

An extremely differentiated suite of unaltered volcanic rocks dredged from the Galapagos Spreading Center ranges in δ18O from 5.7 to 7.1‰ At 95°W, low K-tholeiites, FeTi-basalts, andesites and rhyodacites were recovered. Their lithologic and major element geochemical variation can be accounted for by crystal fractionation of plagioclase, pyroxenes, olivine and titanomagnetite in the same proportions and amounts needed to model the δ18O variation by simple Rayleigh fractionation. More complicated behaviour was observed in a FeTi-basalt suite from 85°W. This study shows that 90% fractionation only enriches the residual melt by about 1.2‰ in 18O. It also implies that the magma chambers along parts of the Galapagos Spreading Center were static and isolated such that extreme differentiation could occur.

Mineral Reactions in Zeolitic Triassic Tuff, Hokonui Hills, New Zealand

Textural evidence and other considerations indicate the following paragenetic sequence of reactions in marine Triassic tuff beds of rhyolitic to andesitic parentage that are scattered through a 4.8 to 8.5 km thickness of the Murihiku Supergroup, Hokonui Hills, Southland, New Zealand: (1) Glass → montmorillonite ± illite; (2) glass →heulandite + chlorite and celadonite; and (3) heulandite → laumontite, or prehnite, or calcite or analcime, or albite. Chemical analyses of the altered tuff indicate that Ca and Na ions have been relatively mobile. Heulandite and laumontite altered tuffs are Ca-enriched, whereas analcime tuff is Na-enriched relative to unaltered volcanic rocks. Heulandite, chlorite, and celadonite have been analyzed by electron microprobe. Heulandite with high Si/Al ratio, sometimes in the clinoptilolite range, is associated with calcium-poor pyroclastic feldspar, whereas heulandite with low Si/Al ratio is associated with calcium-rich pyroclastic plagioclase. Such data indicate that the Si/Al ratio in the heulandite was controlled by the Si/Al ratio of the glass precursor. Chlorite and celadonite have high Fe/Mg ratios and variable Al contents. Some celadonite appears to form interlayered structures with chlorite. Distribution patterns and stability relations of analcime with quartz and of laumontite show that average temperature gradients did not exceed about 25°C/km. The breakdown of heulandite to Na-aluminosilicates (analcime or albite) or to Ca-aluminosilicates (laumontite or prehnite) over a wide stratigraphic interval suggests that such factors as P H 2 o and activity of various ions in stratal waters played a more significant role than depth of burial in controlling distribution of the diagenetic and very low grade metamorphic phases in the Hokonui Hills.

Unmetamorphosed Volcanic and Sedimentary Xenoliths in the Kimberlites of Sierra Leone

THE dominant xenolith types in the kimberlites of the Sefadu area of eastern Sierra Leone are granite, amphibolite and amphibole-schist derived from the wall rocks. Eclogite and dunite xenoliths are only rarely encountered. Small inclusions of a suite of virtually unaltered volcanic rocks and derived sediments for which no counterpart is found in the mapped country rocks of the area are a feature of the kimberlite now exposed by mining at Koidu. I recently collected a series which included undeformed amygdaloidal basalt, clastic basaltic sandstone, fine-grained basic tuff, fragmental andesitic tuff and calcareous tuff. The kimberlites occur in granite emplaced in the Pre-Cambrian Kambui Schists, a strongly metamorphosed schist series of largely volcano-sedimentary parentage1,2. The unaltered xenolithic material in the kimberlites is most readily explained as being derived from an overlying sequence of volcanic and associated sedimentary rocks, now removed by erosion.

Distribution of fluorine in unaltered silicic volcanic rocks of the western conterminous United States

An investigation of more than 170 samples of glass-rich volcanic rocks of rhyolitic or rhyodacitic composition shows marked regional variations in the fluorine content. The uniformity of the class of rocks was controlled by index of refraction determinations on fused beads and by chemical analyses of about one-seventh of the total number. The fluorine content ranges from 20 to 4,900 ppm. The frequency distribution closely approaches a log-normal one; the median value is 520 ppm, the 90 percentile 1,820 ppm. The mean for 167 samples is 820 ppm. These correspond fairly well with earlier averages for similar types of rocks. The highest values are predominantly from a belt in central Colorado and west-central New Mexico, from the Big Bend region of Texas, and from southeastern Idaho and western Utah and northeastern Nevada. The distribution of the fluorine in these rocks correlates well with the distribution of significant fluorspar deposits and somewhat less well with areas where volcanic rocks are prevalent and ground waters carry large amounts of fluorine.