pacific d1-2 seismic properties · these data will allow us to anticipate the seismic properties of...

PACIFIC

Passive seismic techniques for environmentally friendly and cost efficient mineral exploration

D1.2 – Report on the physical properties and

seismic characteristics of ores and host rocks

Grant agreement number 776622 Due date of Deliverable 31/10/2018

Start date of the project 01/06/2018 Actual submission date 28/11/2018

Duration 36 months Lead Beneficiary UGA

Lead Beneficiary UGA Contributors SISP, DIAS

Description

Report on the physical properties and seismic characteristics of ores and the geological structures and elastic properties of host rocks.

Dissemination Level

PU Public X

CO Confidential, only for members of the consortium (including the Commission Services)

PACIFIC_D1.2_seismic properties

D1.2 – THE PHYSICAL PROPERTIES AND SEISMIC CHARACTERISTICS OF ORES AND HOST ROCKS

Public © PACIFIC consortium / 13

Table of contents

Table of contents ..................................................................................................................................... 2

1 Executive Summary ......................................................................................................................... 3

2 Introduction ..................................................................................................................................... 4

3 Literature data on the physical properties and seismic characteristics of ores and host rocks ..... 5

4 Compilation of data on lithologies related to mineral deposits ..................................................... 8

5 Conclusion ..................................................................................................................................... 11

Bibliography ........................................................................................................................................... 13




1 Executive Summary

The physical properties of rocks and minerals, particularly their density and elasticity, control the velocity with which they transmit seismic waves. The acoustic impedance, which is the product of density and seismic velocity, is a useful property to characterize different lithologies. Available data indicates that there are strong contrasts in acoustic impedance between common types of rock and, most importantly, between common rocks and ore minerals. These differences provide a basis for relating passive seismic tomographic models with models based on geological and previously acquired geophysical data.




2 Introduction

In this deliverable, we have compiled information from the literature on the physical properties and seismic characteristics of ores and host rocks together with new data acquired during on-going projects. These data will allow us to anticipate the seismic properties of rocks and minerals that will be encountered during the projects at Marathon (WP3) and Kallak (WP4). Together, they will provide a firm basis to evaluate the reliability of interpretations made using the new seismic methods – passive reflection seismics and the multi-array method.




3 Literature data on the physical properties and seismic characteristics of ores and host rocks

Malehmir et al. (2012) provided an overview of the physical properties and seismic characteristics of ores and host rocks in their important paper on the application of seismic methods in mineral exploration and mine planning. The data are summarized in Figure 1, which builds on earlier information compiled by Salisbury et al. (2003). This figure illustrates several important features:

1) most ore minerals have higher densities that the rocks in which they are found 2) the seismic velocities (Vp) of ore minerals overlap those the host rocks 3) different types of rocks have strongly contracting Vp ranging from low in unconsolidated or

hydrated rocks such as sediment or serpentinite, to far higher values in igneous rocks with abundant mafic minerals (peridotite, eclogite).

Figure 1. P-wave velocities and densities of common rock types and ore-forming minerals (from Malehmir et al. 2012)

A better way to demonstrate the way in which rocks and minerals transmit seismic waves is to calculate the acoustic impedance, the product of density and P-wave velocity. Figure 2 shows that some ore mineral – pyrite, magnetite and hematite – have far higher acoustic impedances than common host rocks. The minerals of common base metals such as sphalerite, chalcopyrite and galena have lower acoustic impedances that are comparable to those of mafic-ultramafic rocks like gabbro and eclogite. However, ore deposits of these minerals are commonly found in more felsic rocks such as granitoids or sedimentary rocks and here again there is a strong contrast in acoustic impedance. On this basis it can be predicted that ore deposits that contain a large proportion of these minerals,




examples being massive sulfides as found in VMS and magmatic Ni or Cr deposits, have readily distinguishable seismic signatures.

A problem, however, is that in many ore deposits, the proportion of ore minerals in low, commonly no more than a few percent. Typical examples are porphyry copper, gold and almost all deposits of rare elements such as the rare earths or platinum group metals. Such ores do not have seismic signatures that distinguish them from normal rocks. Note that an impedance difference of 2.5 x 105

g/cm2 gives a reflection coefficient of 0.06 which, when the geometry is appropriate, produces sufficiently strong reflections for seismic exploration (Salisbury, 2007).

An alternative manner to identify the presence of ores is to image other lithologies that occur in association with the ores. Almost all hydrothermal deposits (which constitute a large majority of ores of most industrially important metals) are associated with alteration – the transformation of fresh rock into assemblages of secondary minerals caused by the circulation of hydrothermal fluids. These minerals are commonly hydrated or consist of carbonates, sulfates and other such minerals and they usually are far less dense and less rigid that the minerals that make up fresh rocks (certainly those of igneous or high-grade metamorphic character). For these reasons, they have relatively low seismic velocities, as shown in Figure 2. In most cases the volume of rock that is affected by alteration is far greater than the ore deposit itself. The "alteration haloes" surrounding the ores therefore constitute a larger target than the deposit itself. A problem with this approach is that reliable data on the densities and seismic velocities of the rocks in alteration haloes has not been compiled.

In addition to the impedance contrast, the geometry of the deposit (mainly its size and depth) is the second most important factor that can be used to detect the presence of ores. The minimum thickness resolvable as a tabular deposit (i.e vertical resolution) is equal to one quarter of the wavelength of seismic waves travelling within the deposit. This is known variously as the Rayleigh limit, the quarter-wavelength criterion or the tuning thickness (Salisbury, 2007). Horizontal resolution is determined by the width of the first Fresnel zone and under ideal conditions, it is possible to detect deposits as small as one wavelength (Berryhill, 1977). Therefore, the detectability of different sized deposits is highly dependent on the frequency of the seismic source being used.

Finally, since some types of ores only have small impedance contrasts with many common host rocks, it is often advisable to conduct laboratory measurements of the velocities and densities of the ores and host rocks in a potential survey area to determine whether reflections are even possible and the survey worth conducting (Salisbury and Snyder 2007).




Table 1: Example of data from Li et al. (2003) compilation of seismic properties of rocks and minerals.




4 Compilation of data on lithologies related to mineral deposits

To help obtain the data on the physical properties of rocks and minerals that are lacking, we have compiled information available in the literature and from unpublished sources. A very valuable source in the large compilation made by Ji et al. (2002). This 630-page volume contains seismic velocities (P and S waves) at pressures from 10 to 600 MPa, for a large selection of rock types and ores. Table 1, a copy of a typical page from the volume, shows data for basalt, felsic gneiss and bornite, a Cu-sulfide from the Kidd Creek deposit in Canada. Also included is an extensive reference list, to which we have added more recent publications. Table 2 is a list of representative ore deposits from which data will be collected or measured during the course of the PACIFIC project.

More information comes from the first test site of the PACIFIC program (Marathon) and information provided by colleagues and mineral exploration companies. This exercise benefitted greatly from contacts established or reinforced during clustering activities. At a later stage, data from the second site, at Kallak in Sweden, will be added.

Table 2: List of typical deposits for which data will be compiled

Name of Deposit

Marathon Kallak Tara Mine Taylor Cobalt Silver deposit

Location Ontario, Canada

Sweden Ireland Arizona Ontario, Canada

Mexico

Type of deposit

orthomagmatic hydrothermal or magmatic

sediment hosted polymetallic

porphyry copper

polymetallic hydrothermal

manto-type, hydrothermal

Type of ores sulfides containing PGE

Fe oxides Pb-Zn sulfide

disseminated Cu-Mo sulfide

sulfide and sulfosalts

massive silver-Zn-Pb sulfide

Rocks hosting ores

gabbros, ultramafic cumulates

metasediment, altered and fractured

shales, in places brecciated and altered

granitoids, metasediment; variable altered

dolerite fracture and breccia zones

metasediment, variably altered

Surrounding rocks

syenite, metavolcanic granitoid

metavolcanic shale, sandstone, carbonate

granitoids, metasediment

dolerite sills, metavolcanics

metasediment, metavolcanic




Figure 2. Density and Vp data. Dotted lines indicate acoustic impedances. Characteristics of rocks and ores from Malehmir et al (2012). Also shown are unpublished data for alteration zones surrounding a porphyry

copper deposits in Arizona.

An example of the new data is shown in Figure 2 where we plot the densities and seismic velocities of alteration zones associated with a porphyry copper deposit in Arizona. This information was acquired during a commercial project of Sisprobe and with permission of the mine operator, we reproduce only generalized information. This example illustrates clearly the low acoustic impedance of these rocks and demonstrates the potential of using these characteristics to image the alteration zones surrounding the copper mineralization.

Table 1 contains a list of data obtained by direct measurements of acoustic properties of rock and ore samples from the Marathon deposit, and Figure 3, shows selected examples of these data, plotted in the velocity vs. density diagram.




Figure 3. Density and Vp data from core measurements at Marathon. The footwall breccia and the felsic syenite have low acoustic impedance whereas the oxide gabbro has higher acoustic impedance. These

differences are sufficient to allow the geological structure to be determined using passive seismic methods.




5 Conclusion

A list of physical properties of rocks and minerals has been compiled from literature information and

data acquired during ongoing projects.




Bibliography

Berryhill, J.R., 1977, Diffraction response for non-zero separation of source and receiver: Geophysics, v. 42, p.

1158-1176.

Ji, S., Wang, Q., Xia, B. 2006. Handbook of Seismic Properties of Minerala, Rocks and Ores. University of Nanjing,

xxp.

Malehmir, A., G Bellefleur. 2010. Reflection seismic imaging and physical properties of base-metal and

associated iron deposits in the Bathurst Mining Camp, New Brunswick, Canada. Ore Geology Reviews, No. 38

(4), 319-333.

Salisbury, M., D Snyder. 2007. Application of seismic methods to mineral exploration, in Goodfellow, W.D., ed.,

Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of

Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division.

Special Publication No. 5, 971-982.

pacific d1-2 seismic properties · these data will allow us to anticipate the seismic properties of...

Documents