THE FORMATION OF QUARTZ VEINS 



By JOHN A. BURGESS 



How are quartz veins formed ? This is a question that is frequently 

 asked of geologists, and the answer is not altogether an easy one. Geolo- 

 gists ask it of themselves and in trying to explain it come out with 

 answers that are different in some respects. The differences, however, are 

 of a kind that are of more interest to the geologist than to those not 

 engaged in that profession. The subject is a broad one ; books have been 

 written on it ; and in order not to cover too much ground this short dis- 

 cussion will refer especially to veins of the Mother Lode of California. 

 There are other types of veins to which it does not wholly apply. It should 

 be made clear that in this brief essay only the high spots of the subject 

 can be touched upon. 



The things we know about quartz veins are the features that can be 

 seen at the surface and in mines of which only a few are over a mile deep. 

 We know the appearance of the veins, their general form, the materials 

 and minerals of which they are composed, the arrangement of the 

 minerals within them, and to a considerable extent the reactions between 

 the substances that formed the vein and the rocks on its borders. The 

 parts of the vein that we now see were formed deep below a pre-existing 

 surface from which thousands of feet have been stripped by erosion, and 

 they doubtless extend several miles below the present surface. Veins at 

 the Argonaut and the Kennedy mines at Jackson were worked to more 

 than a mile in depth without showing any change in their character. 

 It is within this range that we have definite knowledge of the veins. 



Among the things of which we have no positive knowledge are the 

 depth to which the fissures extend, and the depth to which quartz con- 

 tinues in the fissures. Nor do we know the character of the rocks where 

 the fissures terminate. These are features of the form and structure of 

 the veins. And among the processes of vein formation, we do not know 

 how the vein fissures were forced open to accommodate veins of the width 

 that are found on the Mother Lode, nor do we know the source of the 

 water that arose in the fissures to form the veins. There is not sufficient 

 evidence from which to deduce with certainty how these things came 

 about ; but, short of certainty, there are reasonable explanations of how 

 they could have happened. 



It is estimated that the veins were formed over a hundred million 

 years ago, and at great depths below what was then the surface of the 

 earth. Obviously no one has seen a vein in the process of formation. 

 Nevertheless geologists must face the problem and try to explain the 

 origin of veins. This is done by reasoning from known facts to the unknown 

 factors. The conclusions reached in this way give us all that can be 

 expected under the circumstances ; that is, reasonable explanations of 

 the origin of the veins. 



The deductions on which the leading geologists agree are : The veins 

 are formed in deep fissures that extended to a zone of hot igneous rocks 

 newly solidified, or partly solidified, from a molten condition. The gold 



in the veins is derived from these hot or molten rocks. It is known that 

 molten rock, called magma, has a considerable quantity of water in its 

 composition ; in fact the magma is fluid not only because of its heat but 

 also because of the presence of water. As the magma cools and hardens 

 this water is expelled and carries with it silica and certain other mobile 

 materials such as chlorine, fluorine, boron, sulphur, tellurium, etc., which 

 are called mineralizers. The water also carries with it gold and other 

 metals that are present in some magmas, dissolved with the help of the 

 mineralizers. It is by means of the water from the magma that the metals 

 are carried into the vein fissures. 



Quartz is pure crystallized silica ; it is not called quartz until it is 

 deposited as we see it in the veins. The silica is transported upward in 

 the vein fissure by the water in which it is dissolved, the solution of the 

 quartz being aided by other substances in the water, especially by carbonic 

 acid gas, which is the most abundant gas in magmas. The waters are at 

 extremely high temperatures and pressures in the depths where they 

 originate, so hot that they would explode into steam at atmospheric 

 pressure. As the water flows upward into cooler rocks, both its temperature 

 and its pressure decrease, and it is this decrease in temperature and 

 pressure that causes the deposition of the quartz and metals and leaves 

 the vein as we see it. 



FIG. 1. Quartz vein outcropping near Indian Gulch, Mariposa County. Xote 

 the large crystal faces showing in the quartz to the left of the man. Some are more 

 than four feet in length. 



(87) 



