Page 53 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

52 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
In spite of all the study given to ore deposits, 23 the geologist is as yet unable to do more than speculate as to what is beyond the last visible point in a vein. He does not know the conditions that determine why a certain vein should contain a valuable ore shoot while another is barren. No one knows with certainty the exact source of the metals, or what delicate balance of complex conditions determines their deposition in a certain place. It is certain that there is a genetic connection between most ore bodies and igneous activity, and it is believed on the grounds of field evidence that the solutions which deposited the metals came in large part from the solidifying igneous mass that was intruded into the upper layers of the earth's crust, or from the exhalations from a magmatic reservoir at greater depth, but this opinion has not as yet been completely demonstrated.
In order to have a localization of the ore minerals, it is a prime requisite that the openings must exist in which solutions may migrate and accumulate. Such openings are in general created by faulting and fracturing in the sediments adjacent to igneous intrusions or in the upper parts of the intrusive rocks themselves, due to the forces exerted by later intrusions, set- tling and shrinking accompanying cooling, or to other causes. It may be accepted, therefore, that favorable areas for pros- pecting are near centers of igneous activity where considerable faulting and fracturing of either sediments or igneous rocks have occurred. Folding of the beds within these areas into low anti- clinal arches creates traps in which the solutions deposit their mineral load, and the presence of such folds is especially signifi- cant if the beds favorable for ore deposition are overlain by others that are plastic and impervious, such as shales and shaly limestones. Such favorable areas are indicated at the surface by escarpments, folding, tilting and fracturing of the rocks, abun- dant vein quartz or calcite, pronounced staining of the rocks, by iron, manganese or copper, the presence of relict sulfide minerals or of oxidized minerals in these leached areas, or an abundance
of free gold in the leached outcrops or in the stream gravels
below such outcrops. Quartz veins may outcrop boldly, whereas veins with predominating calcite gangue may be depressed at the surface and obscured by rock detritus, so that trenching is neces- sary to locate and sample them ; likewise, the outcrops of the best portions of a vein may be depressed due to oxidation of the sul- fides and breaking down of the vein matter, while the poorer por- tions may remain projecting conspicuously above the surface. There are extensive areas in the county within the exposures of Paleozoic sediments and Tertiary lavas, the two principal ore bearing horizons, that are without faulting, mineralization, or other evidence of unusual disturbance, and hence of little promise for ore deposits. On the other hand, near the known mining
23Ransome, F. L., Geology of the Oatman gold district, Arizona : U. S. Geol. Survey Bull. 743, p. 51, 1923.