Exploration For Massive Sulfide Deposits Research Articles

Application of stable isotope and minor elements analyses to the exploration of massive sulfide deposits

Application of stable isotope and minor elements analyses to the exploration of massive sulfide deposits

Alteration of Felsic Volcanics Hosting the Thalanga Massive Sulfide Deposit (Northern Queensland, Australia) and Geochemical Proximity Indicators to Ore

Thalanga is an Early Ordovician, stratiform Zn-Pb-Cu-Ag-rich massive sulfide deposit in northern Queensland (Australia) with a total resource of 6.6 Mt of ore. The host-rock succession consists of variably altered rhyolite in the footwall and a dacite-dominated volcano-sedimentary sequence in the hanging wall. The sulfide deposit and the enclosing volcanic sequence were metamorphosed under upper greenschist conditions. A laterally continuous footwall alteration zone extends beneath the entire deposit (~3,000 m) and to a stratigraphic depth of at least 300 m below the ore lenses. The bulk of this zone is occupied by feldspar-destructive, muscovite-biotite-chlorite-rich, mottled alteration facies with disseminated pyrite, representing the metamorphic equivalent of phyllosilicate-dominated hydrothermal alteration. Discordant zones of intense quartz-pyrite alteration represent the principal fluid pathways during mineralizing hydrothermal activity. Locally, quartz-K feldspar alteration facies exist on the fringes of the system, and calcareous alteration and chlorite-pyrite alteration facies exist in the upper part of footwall rhyolite, proximal to sulfide lenses. Quartz-feldspar-porphyritic rhyolite laterally surrounding the footwall alteration zone and feldspar-porphyritic dacite in the hanging wall are unaltered or weakly altered. The compositional diversity of altered footwall rhyolite implies that hydrothermal alteration at Thalanga was a complex processes. This paper presents a model for the evolution of the footwall alteration zone. Calcareous alteration probably represents the initial phase of hydrothermal activity. This was followed by diffuse upwelling of acidic, seawater-dominated fluids causing destruction of primary feldspar, precipitation of pyrite, and formation of hydrothermal sericite, chlorite, and clay minerals. Subsequent, intense quartz-pyrite alteration was directly associated with mineralization. The Thalanga footwall alteration zone has several geochemical characteristics that show systematic changes with increasing proximity to ore. These include Na depletion and elevated Mg, S, alteration index (AI), chlorite-carbonate-pyrite index (CCPI), Mo, Bi, and As. Furthermore, the X Mg of chlorite and biotite increases systematically from values of 0.45 to 0.5 in least altered rhyolite to values >0.9 immediately below ore. These geochemical features can be used in exploration for massive sulfide deposits as vectors to ore, at the prospect scale, and in discrimination of prospective hydrothermal from unprospective diagenetic alteration systems, at the regional scale.

Biogeochemical exploration for massive sulphide deposits in areas of dense vegetation: an orientation survey around the Kanköy Deposit (Trabzon, northeastern Turkey)

The eastern Pontides region is a mountainous terrain favourable for massive sulphide deposits. Besides the known deposits, it is highly likely that there are others to be discovered. This paper, therefore, describes the results of a soil and biogeochemical sampling program to assess the suitability of biogeochemical methods for the exploration of hidden deposits in temperate terrains. Soil sampling in the Pontides is shown to be a reliable follow-up method. It produced a significant geochemical response characterised by extensive Cu and Zn but localised Pb anomalies. Lead therefore is the element which can be used to pinpoint the mineralisation due to its relatively less mobile character. Hydromorphic dispersion enhanced the anomalies indicating that base of slope samples would produce reliable results. Of the plant species sampled, Corylus avellana (hazelnut tree) with deep penetrating roots shows comparable results to soil sampling and could be used to delineate the mineralisation. Rhododendron luteum may also be of help. In both species Pb shows limited dispersion and accumulates in the plants near the mineralised site whereas Cu and Zn tend to move away downslope. This shows that biogeochemical sampling could, with proper attention to species selection, be successfully used as an alternative method in the search of massive sulphide deposits in mountainous terrains with a high rainfall such as the eastern Pontides.

Pb isotopic ratios and elemental abundances for selective leachates from near-surface till: implications for mineral exploration

Pb isotope ratios obtained from fine-grained fractions ( < 63 and < 2 gmm from near-surface ( < 1 m depth) till surrounding ore deposits show isotopic overprinting from the underlying sulfide mineralization, and provide a new approach to mineral exploration for massive sulfide deposits (VMS) in glaciated terrains. In this study, Pb isotopic measurements, and selective leaching of 6 near-surface till samples down-ice from the Chisel Lake (Manitoba) and Manitouwadge (Ontario) VMS deposits were carried out in order to determine the location and nature of the Pb within till. Elemental abundances from selective leachates for all 6 samples display similar patterns and show that chalcophile elements (Cu, Ni, Pb and Zn), derived predominantly from the underlying VMS deposits, occur as (i) adsorbed/exchangeable metals; (ii) associated with oxyhydrous Fe and Mn; (iii) crystalline Fe oxides, and/or (iv) silicate. Despite the relative proximity of some of the till samples to the VMS deposits, only a very small component of the chalcophile elements is present as sulfide. This result is consistent with those from studies of weathered (oxidized) tills, which show that labile minerals such as sulfides have been completely destroyed and their chemical constituents reprecipitated or scavenged locally by clay-sized phyllosilicates and secondary oxides/hydroxides. Pb isotopic ratios for selective leachates from till samples with VMS-like (anomalous) signatures are similar to those from ore (galena) within the proximal VMS deposits. This indicates that the Pb is of a secondary nature and was probably scavenged and deposited after destruction of original sulfide minerals during till formation. The lack of a predominant sulfide-held Pb component within the selective leachates supports this interpretation. In contrast, Pb isotopic ratios for the same selective leachates from “background” samples are significantly higher and show that the Pb is not derived from proximal VMS deposits but from a more radiogenic source. Till samples were also leached using 2.5 M HCl (ldconventional” leaching). The Pb isotope ratios from the conventional leachates are similar to those obtained from the selective leachates, and show a large difference in Pb isotopic ratios between anomalous and background samples. We propose, therefore, that the conventional leaching rather than selective leaching or complete dissolution of a particular grain-size fraction be adopted for mineral exploration purposes using glacial sediments. The results from this study support the effective use of Pb isotope ratios from near-surface till as an exploration tool despite the weathered nature of the latter. We feel that this represents a more cost-effective technique over traditional geochemical prospecting methods, if used in conjunction with Pb abundance data.

Oxygen isotope alteration in the Noranda mining district, Abitibi greenstone belt, Quebec

A regional whole-rock oxygen isotope survey carried out in the Noranda district shows how oxygen isotope alteration can be used to guide exploration for massive sulfide deposits. The survey is based on 588 outcrop whole-rock oxygen isotope determinations from 478 sites in a approximately 35- by 50-km area between the Destor-Porcupine and Cadillac-Larder Lake breaks from east of the Horne mine to west of the Magusi River mine. The mean isotopic composition of all samples is 7.4 + or - 2 per mil (SMOW). Nested analysis of variance of 478 isotopic determinations from 371 of these sites shows that 74 percent of the total variance in whole-rock delta 18 O r (r = whole rock) is associated with outcrop location, 21 percent with outcrop sampling, and 5 percent with sample analysis. An analysis of 259 determinations indicates that rock type is statistically no more important than outcrop sampling. The statistical analysis indicates that isotopic variations greater than + or -1 per mil can be used to map the hydrothermal circulation responsible for massive sulfide mineralization. Specific traverses visually confirm the lack of 18 O dependence on rock type and the utility of a 2 per mil interpretive band. The large-scale pattern of isotopic alteration in the Noranda district is dominated by a nearly continuous 2- to 3-km-wide annulus at the margins of the Flavrian pluton of delta 18 O r 18 O r 18 O r depletion is surrounded up section by a approximately 10-km-wide band of patchy but intense 18 O enrichment (8.5-14ppm). The combined pattern of concentric light and heavy alteration has a diameter of approximately 30 km. The Clericy intrusion shows a similar pattern of isotope alteration on a smaller (10 km diam) scale. The Lake Dufault pluton seems to have been isotopically altered by continuing convection driven by the Flavrian hydrothermal system. Modeling suggests that the 18 O depletion is produced by water circulating toward and along the edges of an intrusion, and that the surrounding heavy alteration is produced either by seawater recharge or limited hydrothermal discharge. The large scale and coherency of the alteration pattern centered on the Flavrian pluton is remarkable and indicates that this pluton drove a major, long-lived ( approximately 10 m.y.) hydrothermal system in which convection penetrated to an >8-kin depth. The intensity and coherency of the 18 O depletion correlates with the tonnage of associated massive sulfide mineralization. Oxygen isotope depletion may in general mark the margins of plutons that have driven sufficient hydrothermal circulation to produce economic massive sulfide deposits up section. It may be possible to screen plutons for up-section massive sulfide exploration potential by collecting and isotopically analyzing approximately 50 samples from five traverses across a pluton9s margins. Additional samples may provide details (light fingers) that guide exploration to particular deposits.

Application of lead isotopes to mineral exploration in glaciated terrains

The 206Pb/204Pb ratios of fine-grained fractions from glacial tills indicate the presence of known Proterozoic volcanogenic massive sulfide deposits at Chisel Lake, near Snow Lake, Manitoba, Canada. Data from <2 μm and other size fractions define a linear array on a 207Pb/204Pb vs. 206Pb/204Pb diagram, with a slope that yields an age similar to the age of the deposits and the surrounding country rocks. Some of the least radiogenic till samples were found down ice from the deposits, suggesting that the Pb isotopic compositions from till may provide a new approach to mineral exploration for massive sulfide deposits in glaciated terrains.

Heavy-mineral concentrates from rocks in exploration for massive sulphide deposits

A number of programs have investigated the use of rock geochemistry in the search for volcanogenic massive sulphide deposits in the Canadian Shield. Regional-scale studies have been successful in differentiating productive from nonproductive volcanic cycles. Wall-rock studies have successfully delineated alteration halos related to the mineralizing event. While an alteration halo has been identified around the South Bay massive sulphide deposit, this halo does not extend far enough from the deposit to be useful for reconnaissance purposes. The authors therefore tested the possibility of enhancing detection of a primary trace-element halo by using the heavy mineral fraction of the rocks. The geochemical dispersion of trace elements in the heavy-mineral fraction of rocks was investigated around the South Bay massive sulphide deposit, in the Superior Province of the Canadian Shield. Approximately 270 samples were ground to 74–500 μm (−35 +200 mesh) and separated using the heavy liquid bromoform. Following removal of the magnetic fraction, the samples were further pulverized, and analyzed by atomic absorption spectrophotometry for Cu, Pb, Zn, Ag, Fe, Mn, Co and Ni. Corresponding whole-rock samples were analyzed to provide for a comparative study with the whole-rock geochemistry. Analysis of the heavy-mineral fraction of rocks revealed strong and extensive halos of Cu, Pb, Zn and Ag persisting in some cases up to 10 km along strike away from the South Bay Deposit. By comparison, in the whole-rock data, halos of Pb, Ag and Zn were detected no farther than 1–2 km away from the deposit. Furthermore, trace-element content in the whole rocks appeared to be dominated by rock type; either multivariate statistical techniques, or separation of the data by rock type, was necessary to distinguish the anomaly related to mineralization. Trace-element content in the heavy-mineral concentrates was dominated by the presence of the sulphide minerals pyrite, chalcopyrite, and sphalerite, thus directly reflecting mineralization. Use of the heavy-mineral fraction of the rock eliminates the dilution effects of quartz and feldspar, allowing enhancement of trace-metal concentrations in sulphide minerals, and the delineation of strong and extensive halos of Cu, Pb, Zn, Ag and Mn around the South Bay massive sulphide deposit. While the cost of preparation of heavy-mineral separates is higher than that for whole-rock samples, the anomaly clearly defined by the trace-element content of the heavy fraction avoids the need for costly major-element and subsequent statistical analysis, and increases target size by an order of magnitude. The heavy-mineral fraction obtained from rocks shows great potential as an exploration guide to volcanogenic massive sulphide deposits.

Petrographic and geochemical characteristics of iron-rich rocks and their significance in exploration for massive sulfide deposits, Bathurst, New Brunswick, Canada

ABSTRACT Saif, S.-I., 1983. Petrographic and geochemical characteristics of iron-rich rocks and their significance in exploration for massive sulfide deposits, Bathurst, New Brunswick, Canada. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 705–721. About 25 economically significant, Kuroko-type massive sulfide bodies lie in a metamorphosed volcano-sedimentary complex (probable Middle Ordovician) known as Tetagouche Group, in Bathurst area, New Brunswick. Despite unresolved structural complexities, it does appear that they were deposited during a particular phase of volcanic activity and are, therefore, contemporaneous. Most of the sulfide bodies are closely associated with iron-rich rocks representing various facies of iron formation, and together with sulfides it constitutes the “ore horizon” which, therefore, is highly magnetic. Aeromagnetic and ground-magnetic techniques are useful to locate the ore horizon but problems are created because of the occurrence of iron-rich rocks with no sulfide, along another horizon in the Tetagouche Group. Petrographic and geochemical characteristics of various types of iron-rich rocks have been studied to see if the iron-rich rocks of the ore horizon can be distinguished from the iron-rich rocks of the other horizon. The iron-rich rocks found in the Tetagouche Group can be classified into five types: (1) cherty magnetitic rocks; (2) iron-rich chloritic rocks; (3) sideritic rocks; (4) basic iron formation; and (5) maroon shale. The basic iron formation, which is quite magnetic, gives a false indication of the ore horizon wherein the presence of any of the first three types of rocks is expected. Moreover, the basic iron formation is generally similar in appearance and mineralogy to some of the cherty magnetitic and chloritic rocks. Regarding major element composition, TiO2, Na2O, Al2O3 and CaO are higher whereas Fe2O3, FeO and MnO are lower in the basic iron formation than in the other iron-rich rocks. These geochemical characteristics can help distinguish the barren rocks of the basic iron formation from those of the ore horizon during the exploration programs.

The use of the “immobile” elements Zr and Ti in lithogeochemical exploration for massive sulphide deposits in the precambrian pecos greenstone belt of northern New Mexico

Mineralogical and geochemical data of surface rocks of the Mid-Proterozoic Pecos greenstone belt in Northern New Mexico and results of drill core data of the Jones Hill deposit are presented and discussed within the geological framework derived from detailed geological mapping. Over 800 rock samples were analyzed for 25 elements (all major and several trace elements) and the lithologies were mineralogically and petrographically determined. The results show that the stratigraphic sequence of the Jones Hill deposit is composed of felsic pyroclastics, exhalites and pyroclastics of mafic and mixed felsic-mafic origin, upon which are superimposed certain alteration effects. In volcanic rocks of felsic origin Mg, Fe, Mn, Cu, Pb, Zn, As are enriched and Na, Ca, Sr are clearly depleted when compared with their unaltered source rocks (Table I). Zirconium and Ti are found to be relatively immobile during alteration and subsequent intrusive, deformational and metamorphic events. Alteration trends, best defined by the Na 2 O/MgO ratios and systematic metal zoning, confirm that the stratigraphic sequence is overturned. Lithogeochemical anomalies outline a target which is bigger (over 300 m) and more precisely defined than do other methods, mapping of alteration assemblages or soil geochemistry. New important aspects have evolved from this study, which are applicable to rock geochemical exploration and the recognition of alteration/mineralization anomalies in general. Ratios of Zr/TiO 2 can be shown to reflect the primary pre-alteration-premetamorphic composition of metavolcanic rocks (including shallow intrusives, flows, tuffs, and volcaniclastics). This ratio also enables the determination of the original composition of variably altered rocks of identical parentage (source rock). This determination is a fundamental prerequisite for the recognition and quantification of alteration trends. Direct applications are, for example, that apparently similar chlorite schists derived through hydrothermal alteration, metamorphism, and deformation Zr/TiO 2 ratio of 0.04 (and greater); however chlorite schists derived from mafic volcanic sources and metamorphosed without much hydrothermal alteration have a much lower Zr/TiO 2 ratio of 0.0075 (and less). The altered rock related to massive sulphide alteration can thus be recognized and distinguished from similar rocks of different origin. In addition, the relative concentrations of the immobile elements Zr and Ti are useful in making stratigraphic correlations in structurally complex terrain such as the Pecos greenstone belt. The assumption of a relative immobility of these elements in samples from Jones Hill is supported by the following observation: 1. (1) The TiO 2 and Zr values of Jones Hill rocks fall in narrow clusters (Fig. 1) as opposed to a random scatter plot. 2. (2) The element abundances (ratios) are very similar to: (a) granite (Type A), fresh felsic volcanics (Type A) of the same area and same age (Table I) and basalt (Type B) of the same area; and (b) fresh basalts and rhyolites reported from young volcanic island arcs. 3. (3) Highly altered chlorite schists directly underlying the main ore horizon and within the ore horizon, display Zr/TiO 2 ratios identical to less altered felsic rocks in the foot wall. These rock geochemical methods can be demonstrated to be applicable at all scales ranging from regional reconnaissance through local prospect evaluation down to detailed drill core interpretations during deposit delineation stages.

Case History Examples Comparing Dipole - Dipole Induced Polarization and Dual Horizontal Loop Transient Electromagnetic Surveys in Australia

This paper presents a series of case history examples which enable comparison of dipole-dipole induced polarization (IP) and dual small horizontal loop transient electromagnetic survey results. This work was done in the course of routine exploration for massive sulphide deposits in Australia by Aquitaine Australia Minerals Pty. Ltd.

Magnetic curve fit for a thin dike: Calculator program (TI 59)

In the exploration for massive sulfide deposits, many of the sources of magnetic anomalies are long tabular dike‐like bodies which are thin in relation to the depth of burial. That is, the width of the dike is less than the depth of burial. A program is presented to aid in the inverse solution of this interpretation problem. Demagnetization effects are ignored, and the program will not function for symmetrical anomalies. Given field‐observed amplitudes and coordinates of maximum and minimum points on a total‐ or vertical‐field magnetic profile, this program fits the corresponding theoretical profile for a thin dipping magnetic dike. Coordinate and depth of the top of dike, dip of dike, and width‐susceptibility contrast of the dike are calculated. A theoretical anomaly profile may also be calculated for comparison with the original field curve. Successive iterations may be used to refine estimates of the calculated parameters.

Case history examples comparing dipole-dipole induced polarization and dual horizontal loop transient electromagnetic surveys in Australia

"Case history examples comparing dipole-dipole induced polarization and dual horizontal loop transient electromagnetic surveys in Australia." Exploration Geophysics, 10(3), pp. 199–200

ELECTROMAGNETIC SCREENING DUE TO A DISSEMINATED SPHERICAL ZONE OVER A CONDUCTING SPHERE

The electromagnetic field response under the influence of an alternating magnetic field is investigated for an isometric ore body covered by a disseminated spherical zone. The embedding medium is homogeneous and a relatively poor electrical conductor. The formal solution of the diffraction problem is obtained when the overlying non‐uniform conducting shell has a general spherical symmetry. More complete analysis is made for three important cases where the screening shell is assumed to have: 1. a power law conductivity distribution, 2. propagation constant decreasing linearly outside, and 3. a homogeneous electrical behaviour. The computational procedure for the evaluation of the in‐phase and quadrature components of the screening factor for the last case is presented along with some relevant curves for parametric values suitable in the exploration of massive sulfide deposits. The computed results establish a paradoxical phenomenon of negative screening, meaning that under favorable conditions an ore body with a conducting cover will show up better than an uncovered one. Though these results are contrary to the present philosophy of prospecting, they are able to explain some observations recorded in the field and model experiments. Because of the generalized treatment the analysis with suitable modifications can find applications in many other diffraction problems.