Structure and Ore DepositsStructure and Ore Deposits

presented by:presented by:Stephen BrownStephen Brown

Agricola (1556) provided a solid basis for modern theories

Attempted to classify ores

Based on genesis:

Alluvial

In-situ: fissure veins, bedded or horizontal deposits, impregnations, stringers, seams, stockworks

Based on form: straight, curved, inclined, vertical

Mechanisms

Ore channels are secondary features

Ores have been deposited from solutions circulating in channels

Modern thought: although no two deposits are exactly alike, they share enough unifying characteristics that they can be grouped into exploration-genetic sets, which have distinct lithotectonic-geologic characteristics, and can therefore be hunted and found

Our understanding of Ore Genesis

Genetic Models only exist as a general understanding of ore genesis processes and the controlling variables.

… as a practical substitute ...

Mineral Deposit Models represent an attempt to make broad classification to using known relevant variables to restrict variability in description

Resource Assessment Process

Basis of USGS established methods for:● Mineral resources● Petroleum resources

Summary

Collect map data: geologic maps, geophysical maps, field samples, known deposits

Compare to mineral deposit models: delineate “permissible tracts” each with a classifier label identifying its archetype.

Assign mineral potential: permissible tract boundaries filled with probability using by grade and tonnage models from a world database

Principal processes

Migration of Magma

Magma is in general buoyant - it is liquid, and hence less dense than a solid phase of the same composition would be at the same pressure.

It normally contains dissolved gases, especially water, and especially in the upper portions of magma chambers.

Magma may be injected into overlying rocks or force its way in between rock layers by breaking rocks apart. Magmas may make their own paths by assimilation – engulfing, melting, and homogenation of country rocks.

We know that magmas are guided by "major structures," by which is meant regional scale deep reaching faults or district scale faults at least 10's of kilometers long.

Some magmatic fluids become "ore" by simply freezing in place, but more commonly generate water-rich fluids which dissolve, carry, concentrate, and deposit ores.

Movement of Ore-Bearing Fluids

Any level of understanding of the processes that form ore deposits must include an understanding of how various fluids actually move through rocks..

Movement of Ore-Bearing Fluids

The movement of fluids underground is as significant in ore genesis as it is in the concentration of oil and gas.

It should be appreciated that all subsurface fluid movement occurs according to physical or physiochemical laws and reasonable explanations of how a particular ore body formed must be framed in those terms.

The days of mere fantasy in explaining ore bodies as due to "underground rivers" and "subterranean lakes" are long past.

Movement of Ore-Bearing Fluids

In general aqueous fluids obey the precepts of hydrology in that their migration is controlled by a combination of the avenues of net permeability, the abundance of interconnected pores, presence of interconnected fractures and fault planes, the fluid pressure gradients, and time.

Time must be appreciated: fluids can move in rocks of exceedingly low permeability if sufficient time is allowed.

Porosity and Permeability

Porosity refers to the ratio of the pore volume to the total volume.

Permeability describes the capacity of a rock, sediment, or soil to transmit a fluid.

Another concept is that of diffusion ... motion due to thermal or chemical gradients without influence of pressure gradients or bulk motion of the fluid.

A porous rock may not be permeable, yet a permeable rock must have connected porosity.

Matrix porosity and permeability

Flow through one tube

Permeability of n tubes

Equivalent channel model

porosity

d

Fracture porosity and permeability

Flow through one fracture

Permeability of n fractures

Parallel plate model

porosity

t

05/12/15

Physics to Consider

Fluid

Solute

Heat

ElectricCurrent

HydraulicHead

ChemicalConcentration

Temperature VoltageFlowsFlows

Gradients

Darcy's

Ohm's

Fourier's

Fick's

StreamingCurrent

Electro-osmosis

Relate Flow Physics to Pore Geometry

Porosity and Permeability

Geologists consider two categories:

Primary or intrinsic porosity and permeability - part of what the rock is

Secondary or induced permeability - a result of the formation's geologic history

Intrinsic and Induced

Migration of Hydrothermal Fluids

Deep

Ordinarily permeability and porosity both decrease with depth in the crust because the pressure of overlying rocks tends to close any openings.

Nevertheless, large amounts ore-bearing solutions do move through tight rocks at depth. The answer is time.

Permeabilities of most near-surface rocks - those within 2-3 km of the surface - can transport large amounts of fluids, enough to figure in significantly into ore deposit formation.

Migration of Hydrothermal Fluids

Shallow

Geologists have suggested that hydraulic pressures in ore-bearing fluids can keep fissures open, allowing the fluids to circulate and permitting time for reaction and deposition.

The apparent impermeability of many rocks such as fresh granitic plutons indicates that superimposed permeability due to faults and other secondary structures is more important than intrinsic permeability.

Range of permeability

intrinsic superimposed

Principal processes

Structural Control

Introduction

Detailed studies of structures are essential in exploration and they unquestionably have led to more discoveries of ore than any other approach.

This is true because the movement of fluids underground is controlled by the permeability, which in turn is a function of both the original character of the rock and of the superimposed structure.

Structural Control

Primary or intrinsic permeability

Since primary structures and textures represent spatial variations in permeability and porosity, they can control the distribution of fluids and thus the localization of ores.

Examples of such controls are:

clastic or autobrecciated limestone or dolomite

reef structures.

well-sorted conglomerates that permit easy circulation of fluids.

broken and scoriaceous tops of lava flows.

permeable sandstones such as channel sands and beach deposits.

Structural Control

Secondary or superimposed permeability

Most important in most epigenetic ore deposits (those formed later than enclosing rocks).

Faults and folds are probably the most common secondary structures, although breccia zones, pipes, and others are locally of great significance.

Because fault surfaces are typically not smooth, movement along a fault can produce breccia and gouge where the two blocks grind together.

Fine-grained gouge can plug the fluid paths, inhibiting flow, but clean, coarse breccia with minimum of fines may increase permeability ... and reactive surface area.

Structure and Permeability

Fault Types

Fault Controls

Minor movement along a curved fault causes pinching and swelling of veins.

Ore shoots are known as richer portions within veins.

Ore bearing fluids are channeled through swells and are deflected around the pinches.

Rolls, or changes in attitude of strike or dip of a vein commonly mark the beginning or end of an ore shoot.

Pipes or chimneys are normally steeply plunging sub-cylindrical bodies.

Pipes are commonly formed at the intersection of tabular features, such as faults, dikes, bedding, lava flows, or joints.

When the controlling tabular features are faults, brecciation is likely to be most extensive if the fractures intersect at small angles.

Flow Paths Along Faults

Faults and Voids

Faults and Voids

Breccia

Breccia Pipes

Breccia Pipes

Breccia Pipes

Breccia Pipes

Diatremes

Diatremes, or high-velocity volcanic explosion vents, form where gases have expended at explosive rates, causing a violent upward movement.

Since diatremes form in areas of igneous activity and represent highly permeable avenues for escape of hydrothermal fluids, they stand a reasonable chance of becoming the locus of pipe-shaped ore deposits.

Veins – direct evidence of flow

Veins are classified as:

simple - if they represent mineralization of a single simple fissure

complex - if they are made up of multiple laminae or layerings along the same fracture or they are multiple fillings of the same reopened fracture.

irregular - if they are of variable thickness

anastomosing or branching - if they are braided or in mutually interlacing patterns.

A leed or fissure zone is a tabular zone with distributed veinlets.

A stockwork is a 3D zone so laced with closely spaced irregular veinlets as to be pervasively fractured and commonly mineralized.

Important points

A broad class of ore deposits arise from circulating fluids driven by heat and pressure

Important flow paths are secondary features … joints and faults

Therefore we want not only to map and describe rock lithology, mineralogy, geochemistry, but also flow paths

Important points

Joints, faults, and veins:

Record history of stress and stain

Important keys to understanding tectonics

ARE the flow paths and barriers for mineralizing fluids - therefore are sites of ore deposition

These secondary features have notable characteristics:

Fabric – lines, planes

Sense of motion

Timing

Upcoming classes

Structural Geology - (1) Fractures and Joints

Practical skills for the field Hands-on experiments and introduction to fractography

Structural Geology - (2) Faults Practical skills for the field

Hands-on experiments and fault slip indicators

Geophysics and Mapping Introduction to and report on the tiny magnetometer