Class 1 Overview Of Physical Geology

Class 1 Overview of Physical Geology - Notes • • • •

Introduction, Overview of Exploration Geology Cosmic Origin of the Earth, Overview of Plate Tectonics Geochemical Characteristics of the Earth Geophysical Characteristics of the Earth

INTRODUCTION:-Exploration geology is the process and science of locating valuable mineral or petroleum deposits, ie, those which have commercial value. The term “prospecting” is almost synonymous with the term “exploration”. Mineral deposits of commercial value are called “ore bodies” (compared to commercially viable deposits of oil which are called “oil fields”). This course will be focused largely on mineral exploration, although many of the same techniques are used in petroleum exploration. Each class discusses a combination of physical geology and exploration geology principles. Each class contains a lab activity which refers to these principles in the context of a real world application to exploration geology. Each class includes a quiz which evaluates the level of understanding of these principles and applications.The initial signs of potentially significant mineralization are called “prospects”. Through the exploration process, the prospect is investigated to acquire more and more detailed information. The goal is to prove the existence of an ore body (or oil field in the case of petroleum exploration) which can be mined (or “developed”). The exploration process typically occurs in stages, with early stages focusing on gathering surface data (which is easier to acquire), and later stages focusing on gathering subsurface data, including drilling data and detailed geophysical survey data.Determining the value of an ore body (or “deposit”) requires determining two main features: 1) “tonnage” (or volume), and 2) “grade” (or concentration). The volume is determined by using drill data to outline the deposit in the subsurface, and by using a geometric models to calculate the volume. If the ore body is exposed at the surface, then the dimensions of length and width can be gathered at the surface, possibly with the aid of some trenching or blasting methods. However, most of the volume which must be defined is typically located at depth and requires the use of extensive drilling or underground excavation methods to define. The volume is difficult to delineate because ore deposits often have irregular shapes. The “grade” is the average concentration determined from numerous assays of drill samples. The grade can vary considerably within different parts of the same ore body. Development usually consists of extensive, close-spaced drilling which outlines the geometry of the deposit in great detail. The development stage will also conduct extensive testing, with some preliminary metallurgical testing, to precisely determine grade of the deposit and the “recovery” (the amount of metal possible to extract, compared to the total amount of metal present in the ore body). The final stage before actual mining or extraction is called “feasibility”. During this stage, the actual mining or extraction method is proposed, taking into consideration all of the economic variables which effect the bottom-line profit (commodity price, milling cost, transportation cost, labor cost, etc...). At this stage, a decision is made whether to mine the deposit from the surface (called “open-pit mining”), or to mine the deposit by tunneling (called “underground mining”).

Overview of Exploration Geology:- Mineral seldom occur at the surface and are seldom obvious. Most often they are buried, sometimes at considerable depth. Since they are not visible we must detect their presence indirectly and extrapolate between points where data is known. Many different techniques can be used to detect an ore body. This class will discuss the more important techniques in some detail; others are only briefly mentioned. The most important techniques used in exploration geology include geological field methods, geochemical sampling methods, and geophysical methods. Exploration conducted from the surface is far less expensive than drilling or underground excavation, so thorough surface exploration usually precedes either of these activities. The Exploration Process:-Exploration for a mineral deposit is usually conducted in a step-wise fashion which progresses through stages, each of which moves closer to making a valuation of the ore body. Geological reconnaissance and surface geochemical sampling prevail in the earliest stage. Simultaneously or afterwards, geophysical surveys are typically conducted. Following surface exploration, the project moves into the drilling stage. Drilling may begin with a small number of exploratory drill holes on select targets. After this drilling stage, extensive, closespaced drilling (called “development drilling”) is conducted. Finally, pending good results, “reserve drilling” is conducted, which is the type of drilling which makes the final assessment of the deposit before actual mining begins. Generally, some amount of drilling will continue throughout the life of the mine, as further definition is required and new information is obtained and used to refine the deposit model. Exploration Methods :-If bedrock is exposed anywhere at or around a prospect, then surface bedrock mapping is an essential beginning step for an exploration program. This would include mapping and sampling (field geologic methods). This work focuses on identifying and mapping outcrops, describing mineralization and alteration, measuring structural features (geometry), and making geologic cross sections.Geochemical methods involve the collection and geochemical analysis of geological materials, including rocks, soils and stream sediments. The results mapping and sampling may suggest patterns indicating the direction where an ore deposit could be present underground or at the surface. Geophysical methods focus on measuring physical characteristics (such as magnetism, density or conductivity) of rocks at or near the earth’s surface. The measured values are then used to compare with the values and models of known ore deposits. COSMIC ORIGIN OF THE EARTH;-Theories suggest the universe consists of an unknown number of localized concentrations of helium and hydrogen gas and other elements (galaxies). Galaxies change over time. Clouds of gas within them collapse due to gravitational attraction between matter of different types. Some of the local concentrations of gases collapse and form stars. This is thought to result from hydrogen atoms fusing to form helium atoms by thermonuclear reaction, which produces enormous heat. Stars vary in size and composition, depending on their starting size and composition and on the progress through an evolutionary process of further collapse or expansion. Our sun started out with some leftover gases surrounding it or perhaps forming an orbital disk. Condensation occurred in local areas in the disk, and gravitational attraction of smaller masses towards larger ones (ie, accretion) began to occur. Eventually the masses of solids and gases ended up either in the larger ones (the planets), or are still floating around in space (asteroids and meteorites). We estimate the composition of the original concoction of materials by observing the chemistry of meteorites, which are thought to represent it: Type

% of Total Seen to Fall Characteristics mostly silicate minerals; also known as 1) Stones 92.8 “chondrites” 2) Stony Irons 1.5 mixture of nickel-iron masses and FeMg silicates 3) Irons 5.7 nearly solid iron nickel; most frequently “found”

The planets further collapsed and each underwent its own separate, unique history because of the different starting composition and the different distances from the sun (causing a different thermal history). Heat energy was generated by the accretion process, and also probably by the decay of radioactive elements.Eventually heat was sufficient to cause total melting of the collection of materials. Then as the molten mass cooled it underwent differentiation, that is, it separated into different layers. At the center is a core of solid iron, which is surrounded by an outer core layer of molten liquid iron. Even though the inner core is hotter than the outer core, the inner core is solid because of the incredibly high pressures at the center of the earth. Further out is an intermediate layer called mantle, which consists largely of iron and magnesium silicate minerals. At the surface is a very thin, rigid, crustal layer which is not homogenous. It has some areas which were thicker than others and differ in their chemical composition. Continental plates are thicker and consist mostly of light elements, like silicon and aluminum and oxygen. Other large areas consist of thinner plates which consist mostly of the elements iron and magnesium and much lower silicon. OVERVIEW OF PLATE TECTONICS WEB SITE: “SEA FLOOR SPREADING” http://www.scienceprof.com/plate.html Current geological theories consider the outer core and mantle of the earth to consist of liquid and plastic (respectively) materials with slow-moving, convective currents. The rigid crust and solid portion of the mantle essentially “float” on the liquid mantle and outer core. Convective motions in the mantle and outer core has caused the thin, crustal layer to break apart into a series of large plates, which continue to move slowly or “drift”. Each plate has had its own unique drift direction and speed. Collisions between plates occur, causing a great amount of fracturing, crumpling, overthrusting and underthrusting, particulary aroung the margins of the continents. The deep convective currents also cause molten material to upwell to the surface along linear fracture zones called “rifts”. Here the mafic material also forms a rigid shell called oceanic crust. Most of the earth’s rifts occur within the major ocean basins, although a few occur on the continents. Along the oceanic rift zones, new oceanic crustal material is being generated as the mafic (basaltic) molten material crystallizes and moves away from the axis of the rift. One of the most obvious is the mid-Atlantic Rift, which exists midway between the continents of Africa and South America. It’s discovery was brought about by the observation that the coastline geometry of these two continents are so similar. Boundaries between the crustal plates, continental or oceanic, are one of three types. The first, called a “rift” has already been mentioned. This type of boundary is where the plates move away from each other. The second type of plate boundary is called a convergent boundary where the plates move towards each other. This process typically results in one plate being forced downward below the other along a linear zone, called a “subduction zone”. The downward moving plate becomes hotter with depth, and can eventually get deep enough to become partially or wholly melted, thus creating new molten material. he third possible type of plate boundary is where two plates move sideways past each other, which is called a transform fault. As the continental and oceanic plates are shifting they are also fluctuating in elevation. Some portions of plates have become elevated while others sink and become buried by new rocks. The more deeply buried portions can become so deeply buried that local remelting occurs. New magma (molten rock) is formed in large masses which creep upward toward the surface. Many reach the surface by flowing up through zones of weakness where fractures formed, or other areas where crystallization eventually occurs. Later uplift movements have pushed the crystallized masses, called batholiths or plutons (larger and smaller). Erosion at the surface destroys the intrusions, and water transport carries the grains to their site of deposition (while decreasing the size by abrasion). Here the grains may contribute to the formation of layered sedimentary rocks. Theoretically the cycle can be repeated, if these new layers are eventually buried again.

Geochemical Characteristics of the Earth:-As mentioned previously, rather than being uniform in composition, the earth’s crust is highly varied in composition. There are broad areas of continental crust containing abundant light elements, and broad areas of oceanic crust which contain abundant iron and magnesium. Over 99 % of the earth consists of only about eight different elements out of the 103 in the “periodic table” (below). These are iron, magnesium, oxygen, silicon, aluminum, calcium, potassium and sodium. Periodic Table of Elements WEB SITE: “WEB PERIODIC TABLE” http://www.webelements.com/webelements/scholar/indes.html At present, there are 103 known elements. Each element is unique, and is defined by its atomic number, or number of protons in the nucleus. Gold, with an atomic number of 79, is heavier than copper, which has an atomic number of 29. Elements can occur as solids, liquids or gases.Solids and liquids in nature are never completely pure. They always have some minor amount of at least one other substance. For example, native gold is almost pure, but there are always minor amounts of silver, and sometimes other elements. In nature, there are many more “compounds” than there are native elements. Compounds consist of two or more elements. A “mineral” is a naturally occurring compound in the earths crust. An aggregate of one or more minerals is a “rock”. GEOPHYSICAL FEATURES OF THE EARTH Earth’s Gravity Field WEB SITE: “EARTH’S GRAVITY FIELD” http://www.mines.edu/fs_home/tboyd/GP311/MODULES/GRAV/NOTES/gtogeo. html The gravity field of the earth is caused by the earth’s mass. The strength of the field is a function of the composition of the mass (iron-rich center with silicate minerals near the surface) and the distance away from it’s center. Where we stand on the surface, the gravity field has a field strength, which we can measure (as weight). The earth is not completely round. Instead, the radius in the polar areas is approximately 21 kilometers shorter than at the equator. Since you are closer to the earth’s center at the poles, you body weight will be greater there than at the equator. The gravity field is also not perfectly spherical: there are many irregularities. In spite of the complexities in the shape and strength of the field, geophysicists have created a reliable model of the field (with numerous instrumental readings all over the globe). Using an instrument called a gravimeter, a geophysicist can measure the strength of the gravity field over a buried ore deposit to test for its presence. If the gravity readings show a departure from the model’s prediction of the normal field strength, it may indicate an abnormally dense rock mass at depth. The gravity data can be plotted as a vertical profile, or in a map view, to highlight the anomalous readings.The gravity field also effects our perspective of the way to measure density. Gravity causes objects with a dense mass (or “high density”) to feel like they weigh more. A good way to compare different mineral or rock substances is to compare their density, which we refer to as the “specific gravity”. The specific gravity for a substance is an expression stating the number of times heavier it is than an equal volume of water. For example, the specific gravity of gold is approximately 19, which means nineteen times heavier than water. Earth’s Magnetic Field WEB SITE: “EARTH’S MAGNETIC FIELD”http://wwwspof.gsfc.nasa.gov/Education/wmfield.html

The Earth has a magnetic field which resembles the magnetic field of a simple bar magnet with axis of the magnet closely aligned with the spin axis of the earth. From a point on the surface where we are standing, we call the angle between the magnetic north pole and true north is called the declination. Since a compass needle points to magnetic north, the compass declination is typically adjusted to compensate and cause north on the compass to read as true north. In detail, the earth’s magnetic field, like the earth’s gravity field, is much more complex than the simple bar magnet model would suggest. For one thing, the location of the north pole slowly migrates, and has been slowly migrating throughout geologic time. For example, in England over the last two centuries, the direction towards the north pole has migrated from 15 degrees east of north, to 25 degrees west of north, and then back to 5 degrees west of north. Certain minerals containing iron or nickel can be magnetic and so can have small magnetic fields of their own which are readily measurable. Many minerals have a trace of magnetism at least which can be measured. The strength of the magnetism associated with the mineral is called the “susceptibility”. The minerals magnetite and pyrrhotite have strong magnetic susceptibility compared to the mineral zircon, which has a low susceptibility. Where magnetite and other magnetic minerals are present in abundance in a rock, the rock can be measured and traced with sophisticated magnetometers. Geophysicists measure the field strength and orientation on the surface and compare it with predicted values using a model. The magnetic field has been measured in numerous locations to model its shape and strength. When the measured data on the surface shows anomalous values, there may be rocks below the surface causing the anomalies.