EXPLORATION FOR EARTH RESOURCES:
APPLICATION OF GEOLOGICAL AND GEOCHEMICAL
MODELING AND EXPERIMENTATION
(modified from the original version written for the College of Computer , Mathematics and Physical Sciences for the alumni newletter, Continuum)
(Philip Candela and Philip Piccoli)
Metallic resources form an integral part of the raw materials that fuel our modern economic society. For example, gold is used not only in the fabrication of ingots and jewelry, but also in dentistry and in the construction of scientific instruments and other electronic equipment. Copper has a yearly per capita consumption in the USA of ~24 pounds; annual domestic consumption has risen (at about 2.5 % per annum) to 3 million tonnes per year in the U.S. in 1999 ( about a quarter of world production). Although there are many actively mined ore deposits distributed world wide, active mineral exploration is necessary to maintain a constant supply of copper and gold, as well as zinc and other metals. Unfortunately, the U.S. has gone from importing just 7% of its copper in 1995, to bringing in over 25% in 1999. 99% of our domestic copper comes from the states of Arizona, Utah, New Mexico, Nevada, and Montana.
The exploration for metallic resources must become ever more sophisticated as economic mineral (ore) deposits become harder and more expensive to find. On a seemingly disparate note, the search for alternate energy sources is a global endeavor, and geothermal resources represent one way to reduce global fossil fuel consumption. What do gold deposits have in common with a geothermal power plant? Well, in the Geology Department's Laboratory for Mineral Deposits Research (LMDR) Professor Philip Candela and Dr. Philip Piccoli lead a research team that is actively developing models for the sub-volcanic, hydrothermal (i.e., "hot water") processes that have formed fracture-hosted ore deposits of Cu,Au,Zn,Sn,Mo,Bi,W etc. in the geologic past, and that are occuring today beneath some modern-day geothermal fields. Hydrothermal ore deposits form when hot aqueous fluids (water and rock temperatures of 250 C to greater than 600 C), circulating in fractures in the Earth's brittle upper crust, deposit economically viable concentrations of minerals such as chalcopyrite (CuFeS2) or native gold in fractured rock as the fluids rise buoyantly through the colder, meteorically-derived (rain) waters that normally fill the fractures in upper crustal rocks. In geothermal fields, present day reservoirs of hot water, some of which may have zones of economic or subeconomic mineralization forming within them at depths of hundreds to thousands of meters, may be mined for their heat by pumping water from the reservoir to the surface, extracting the heat, and reinjecting the spent fluid back into the Earth's subsurface. Pockets or "chambers" of molten rock (magma) are the immediate source of the heat, a large proportion of the dissolved metals, and some of the water, in both modern day and ancient hydrothermal systems. The high level magma, now residing at shallow (near surface to ca. 10 km) depths has risen from lower crustal, or even upper mantle depths (the Earth's mantle extends from the base of the crust at ca. 40 km depth, to the core-mantle boundary at a depth of thousands of kilometers). Magma is a major agent of irreversible heat and mass transport from the Earth's interior to our near-surface environment, and the flow of magma is responsible for part of the radial inhomogeneity we can detect in the earth by variations in seismic (earthquake) sound speeds in the earth.
The chemical elements that tend to concentrate in silica (SiO2)-rich magmas (these magmas have on the order of 2/3 SiO2 by weight, and are represented well by the lavas of Mt. St. Helens of washington State, or Mt. Pinatubo of the Philippines) are those that have not been incorporated in the core or in the earth's mantle. These include elements such as K, Rb, Na, F, Pb, Zn, Sn, W, Mo, U, Th, Bi, Nb, Ta and other metals. Elements such as Au have a large portion of their inventory residing in the core; luckily, entropy, with some help from the complex geological history of the early Earth, and the effective (near?) isolation of the upper mantle and crust from the core for a long period of geological history, have left some iron core - compatible elements (Pt, Au, Cu, Ag among many others) in the Earth's outer spheres. When these silica-rich magmas crystallize in the earth's subsurface, they form granite, a macroscopically heterogeneous mixture of quartz and K,Na- and Ca,Na-feldspar, that represents the crystallized remains of subterranean magma chambers now exhumed by (tectonic) crustal uplift and subsequent erosion. Granitic rocks, broadly defined, make up the bulk of the Earth's continental crust. However, only a very small proportion of the earth's granitic rocks are associated with mineral deposits. (The study of granite genesis is one of the major research thrusts in the Geology Department here at College Park. In fact, the major international meeting for granite researchers, the Hutton Symposium, held every four years, held its 4th Hutton Symposium in the U.S. at College Park in 1995. The meeting was attended by approximately 175 participants from ca. 20 countries.)
In their research, members of the LMDR perform field, experimental and theoretical investigations of past geological activity that included the crystallization and boiling of molten rock in shallow (~1-10 kilometer deep) chambers of magma within the Earth's crust. Our group models the exsolution of water vapor (+carbon dioxide+ HCl + metal chlorides + more), and brines (e.g. 60% chloride salts + 40% water) from a crystallizing magma, and compute, given chemical thermodynamic data (equilibrium constants) collected in the laboratory on the partitioning of ore metals among melts, crystallizing phases, vapors and brines, the efficiency with which ore metals such as copper and gold can be extracted into the mobile and buoyant vapor or brine phases from the crystallizing magma. As a subterranean chamber of magma cools and crystallizes irreversibly, the exsolved vapors and brines move into the overlying meteorically dominated, fracture-hosted, hot water hydrological systems, and deposit minerals containing ore metals when hot fluids cool, mix with colder waters of different composition, or react with the surrounding rock.
The research results of the LMDR group are consistent with oxidized granite (granite with relatively high ratios of FeIII to FeII) hosting deposits rich in molybdenum and copper, whereas more reduced granites host deposits richer in W and poorer in Mo and Cu. Our data support the occurrence of Au in association with granite that are oxidized, but not quite as oxidized as those associated with Mo deposits. These effects result primarily from the selective sequestration of ore metals in the dispersed crystalline phases of the granitic rocks, a process that precludes the partitioning of ore metals into the exsolving "ore-generative" vapors and brines. Our work has also demonstrated the importance of chloride as an agent for transporting copper and some gold in the magmatic volatile phases, and we have developed techniques for the estimation of the original magmatic chloride concentration by the analysis of the mineral apatite (a major constituent of bones and teeth and a minor, but ubiquitous constituent of granitic rocks) by Electron Probe Microanalysis. These analyses also allow us to estimate the concentration of hydrochloric acid in these high temperature fluids. By virtue of our thermodynamic model for the behavior of HCl in magma-volatile systems, we can now predict the HCl concentration in a magmatically-derived vapor or brine given the composition of the magma, if the composition of recent magmatic activity is known from deep drilling into young granite or from recent proximal volcanic activity.
The LMDR has done physical modeling of how the magmatic vapor gains egress from the crystal-melt-vapor-brine mixture; modeling suggests that three dimensional percolation networks of vapor in a melt/crystal matrix form when the proportion of vapor reaches a critical threshold. The predicted geometry of finer grained, randomly oriented crystals of quartz and feldspar (grown from melt) and interconnected zones of wall-nucleated crystals (grown from the melt into pockets of vapor) have now been found in many ore-related granites (see the associated figure at http://www.geol.umd.edu/~candela/rock.gif).
The results of this
ongoing research effort can be employed to determine exploration
"vectors": which geographic regions are most prospective for a given
metallic resource, and what areas in a given region are likely targets for