The Geology and Ore Deposits of the Manzana Quadrangle, Los Angeles County

Abstract

This paper describes the geology and ore deposits of the Manzana quadrangle, an area of about 40 square miles in north-central Los Angeles County, California. Topographically the quadrangle is divisible into two separate parts: a northern half covering parts of alluvium-filled Antelope Valley, the westernmost arm of the Mojave Desert; and a southern half covering a mountainous region that extends eastward into the San Gabriel Mountains. Nearly all of the mountainous portion of the Manzana quadrangle is made up of intrusive igneous rocks. They range in composition from granite to quartz diorite, but on the average are thought to approximate a quartz monzonite or granodiorite. These rocks can be traced almost continuously into the Sierran granites farther north and hence are believed to be of early Cretaceous age. Genetically related pegmatite and aplite dikes are exceedingly abundant over the entire area of crystalline rocks and are especially abundant along the northern edge of the mountains. The pegmatites range in composition from granitic to syenitic; perthitic and graphic textures indicate that replacement has been an important process in the genesis of the pegmatites. Of especial interest is the presence of gold in many pegmatite dikes of widely differing composition. This gold is shown to be of hydrothermal origin in one case, but in other dikes appears to have been a primary constituent of the pegmatite solutions. Sedimentary rocks in the quadrangle are confined to a number of small isolated roof pendants making up a discontinuous belt along the northern edge of the mountains bordering Antelope Valley. These rocks have been highly metamorphosed by the intrusive rocks. The age of the sediments cannot be stated more definitely than pre-Cretaceous, but they are thought to be the correlatives of the Bean Canyon Series (Triassic?) which has been described from an area some 20 miles to the northeast. Volcanic rocks, presumably of Miocene age, occur in an extensive series of flows and tuffs in the western part of the quadrangle. These rocks lap up on the intrusive igneous rocks in depositional contact and dip westward at moderate angles. The principal structural feature of the Manzana quadrangle is the San Andreas rift, which trends thru the southern part of the area with a direction of about N75W. Its position is marked by a series of straight, narrow canyons which cut across the normal drainage direction of the region almost at right angles. Other physiographic and structural features characteristic of tear faults are numerous within the fault zone. Steep vertical and normal faults have played an important part in shaping the topography of the rest of the area. The mineral deposits of the quadrangle include contact metasomatic deposits, and fissure veins, each genetically related to the intrusive igneous rocks. The contact deposits are restricted to areas of sedimentary rocks where there has been a widespread, though lean, dissemination of pyrrhotite, Pyrite, sphalerite and gold. In only a few localities have these minerals been concentrated enough to induce mining operations. Nearly the entire production of precious metals from the quadrangle has come from fissure veins which, so far as known, are confined, to a relatively small area near the center of the quadrangle. These deposits include slightly mineralized gouge-filled faults and quartz veins with walls of gouge. The latter type shows the following metallic minerals distributed sparingly thru white or grey crystalline quartz: arsenopyrite, chalcopyrite, pyrite, galena, sphalerite, gold telluride and gold. The structural and mineralogical features of the fissure veins indicate that they formed under conditions of moderate temperature and pressure, perhaps in the lower Epithermal zone of Lindgren. No definite evidence bearing on the extent of supergene enrichment was found in these deposits. The various mineral deposits of the quadrangle are thought to represent different stages in the intrusive history of the surrounding crystalline igneous rocks. In the first stage of intrusion, blocks of sedimentary rocks from the walls of the magma chamber were engulfed by the intrusive and metamorphosed to form contact metasomatic deposits. Continued differentiation of the magma resulted successively in a pegmatitic rest-magma containing small amounts of metals and hydrothermal solutions containing abundant metals; these solutions moved upwards thru the crystalline and surrounding rocks to form various types of mineral deposits. The present concentration of roof pendants, pegmatite dikes, and hydro-thermal deposits in a belt along the northern edge of the mountains is thought to reflect the original limits of the batholithic chamber.