Western Sierra Nevada/Foothills Metamorphic Belt

Field Trips Stops for Days 1 & 2 Cal Poly—Geology Club 2005 Field Trip Western Sierra Nevada/Foothills Metamorphic Belt GEOLOGY CLUB FIELD TRIP WESTERN SIERRA NEVADA - MOTHER LODE Introduction ject. In the northern one-third of the MLB the gold seems to occur within the quartz veins. However, to the The Mother Lode Belt (MLB) produced in excess south the gold lies at the contact between the quartz of 40 million ounces of gold through 1964 and with veins and footwall metamorphics; often finely reactivation of several mines in the 1980 and 1990’s, disseminated in rocks some distance from the quartz produced an additional 10-20 million ounces of gold. veins. As such, it ranks as one of the largest gold producing districts in the United States. During this field trip we Most of the gold produced from the MLB prior to 1980 will examine the MLB and adjacent Sierra Nevada ba- was free gold. Gold-bearing pyrite was known to be a tholith. Our objective will be a better understanding of major constituent both of the quartz veins and footwall the geology of the Mother Lode region and some rocks adjacent to the ore zone, but it was rejected dur- feel for models to explain both gold emplacement and ing processing. In the 1980’s and 90’s significant pro- the evolution of the Sierra Nevada. duction came from these low grade auriferous pyrite zones. Other sulfide minerals reported from the California can be divided into a series of tectonic prov- ore include arsenopyrite, sphalerite, galena, chal- inces (Fig. 1) each playing an important role in the geo- copyrite and tetrahedrite. The gold telluride, petzite, logic evolution of the western United States. While is also a rare constituent of some deposits. important from a purely academic standpoint, eco- nomic geologists are concerned with integrating paleo- Several models have been proposed for the Mother tectonic reconstructions into some sort of framework Lode deposits, but they all fall under three general um- that will aid in the exploration for gold deposits. brellas: Figure 2 shows the distribution of various metallic min- 1. Syngenetic, based on the general sug- eral deposit types in California. On this trip we will be gestion that the gold was deposited examining the gold-quartz vein deposits. They occur in originally in the sedimentary rocks which many diverse localities in California from the northern host the quartz veins and perhaps remo- Mojave Desert to the Klamath Mountains of the north- bilized during subsequent metamorphic ern Coastal Ranges. By far the most important, however, are the veins of the Mother Lode and adjacent Grass and/or igneous events. Valley District. 2. Hydrothermal/magmatic, with the It is noteworthy that the most comprehensive paper on gold related to and derived from the Mother Lode was published in 1929 (Knopf, Mesozoic intrusives (i.e. the Sierra Ne- 1929). Subsequent to that time, little of significance vada batholith). has been written on the district. Knopf described 3. Metamorphic (more recently the term the Mother Lode as a complex system of quartz veins amagmatic has been employed), with the and mineralized rock 195 km long and about 1.5 gold being emplaced by some unspeci- km wide localized along an east dipping fault with fied mechanism related to continent-arc offsets of 10’s of kilometers. The quartz is not a continu- collision. ous mass, but rather forms discontinuous lenticular pods as much as 20 meters thick. Ore bodies seem to be We will have the opportunity to discuss each of these associated with the thickest portions of the quartz models during the field trip, however, constraints veins and are often localized where the veins in- to any model must be kept in mind. tersect or branch. They typically have short strike lengths, but may be continuous in the down dip ♦ Limited fluid inclusion work indicates the direction for thousands of meters. All ore zones dip presence of CO2 rich inclusions. This is consistently eastward, but according to Knopf dip angle not surprising given the alteration of the is highly variable and changes generally as a function host rock to ankerite. The source of the of lithology. CO2 is difficult to establish due to the complexity of the MLB geol- The quartz veins consist of essentially pure milky white ogy, but sedimentary carbonates which is coarsely crystalline, often containing inclu- are rare to nonexistent along strike. sions of the host rocks. Alteration is common and perva- In other less structurally complex dis- sive with ankerite dominating. Knopf argued that the tricts, CO2 rich inclusions have been alteration of host rock to ankerite (a carbonate) taken to indicate a deep (possibly mantle) must have released large quantities of silica which source for the ore fluids. 2 then recrystallized as the quartz veins. Localiza- ♦ Stable isotopic studies have shown that tion of gold is a more complex and debatable sub- much of the water was of meteoric origin. Figure 1. Simplified tectonic map of California. 3 Figure 2. Distribution of metallic mineral deposits in California. 4 Of course, metamorphism has been exten- eastward dipping consistent with deposition in the Ne- sive so the question might be asked, is this vadan synclinorium. It consists of a metamorphosed merely an overprint on the original iso- (low-mid greenschist facies) basal pillow lava and argil- topic values. lite overlain by additional argillites and chert. Fossil ♦ Age dates on alteration assemblages preservation is better with conodonts, fusulinids and (mariposite) indicate the veins were em- radiolaria indicating Permian through latest Triassic placed approximately 115 Ma ages. (Cretaceous). Again are these age dates accurate or have they been reset by Both the Shoo Fly and Calaveras have been intruded by nearby igneous intrusions? stocks and batholiths of the Sierra Nevada orogen. In- trusions range in age from 140MY (Jurassic) to 80MY ♦ Curiously, the ores appear to set on oceanic basement rocks (i.e., generally rocks (Cretaceous) and in general are younger from west to of basaltic affinity). Gold is absent to the east. Provenance studies indicate both rock units are east where the basement has a more conti- derived from or related to the North American craton. nental character. The western boundary of the Calaveras Complex is General Geology-Stratigraphy marked by the Melones Fault Zone (MFZ). West of the MFZ (to be discussed below), are a series Any attempt to construct a generalized stratigraphic of rocks of markedly different character. Stratigra- column for the Mother Lode can be an exercise in fu- phy for these rocks is only poorly known. In general, tility. The paucity of fossils in the sedimentary rocks, the basement consists of a true ophiolite sequence with metamorphic overprint and chaotic nature of the pillow lavas, gabbro and serpentinized ultramafics rock units make interpretation problematic. occurring in a chaotic assemblage (mélange). The Widely disparate ages have been often placed on ophiolite sequence is in turn overlain by a 4 km thick the same formation by different authors. Further- sequence of volcanic rock of largely basaltic to more, the is no general agreement as to which units are andesitic composition. Capping the sequence is a se- part of which terrain (compare for example, Figures 3 ries of weakly metamorphosed (zeolite-prehnite/pumpellyite and 6). facies) fine-grained clastics with minor intercalated Figure 3 is a generalized geologic map of the MLB. graywacke and conglomerate, and volcanics often col- Note it divides the foothills into three distinct lectively termed the Mariposa Formation. Trace fossils belts, termed the Western, Central or Mother Lode, and have placed the age of the Mariposa Formation at early Eastern. Unfortunately, although most researchers now Jurassic. adopt the concept of three “belts” there is no general Rocks of the Western Belt lie to the west of the Bear consensus as to which rock units constitute each belt. Mountain Fault Zone (BMFZ). These rocks are poorly For the purpose of this discussion we will treat the rock exposed and little studied. In general, the stratigraphy units from east to west. suggests similarities to rocks east of the fault zone, The oldest rocks in the region are a part of the Shoo Fly leading to the widely held belief that the BMFZ repre- Complex of lower Paleozoic age. They are a sequence sents a splay of the MFZ with the two merging at of eastward dipping sandstones, shales, cherts and depth. Thus, rocks of the Western Belt would be minor volcanics apparently deposited on the west- analogous to those of the Central Belt and perhaps are ern limb of a major synclinorium (the Nevadan). repeated section. Further to the west, Western Belt These are in turn unconformably overlain by a thick rocks are unconformably overlain by the submarine fan (11 km) sequence of volcanics and volcaniclastic deposits of the Cretaceous Great Valley Sequence. rocks of dacitic to rhyolitic composition. Shoo Fly General Geology-Structure rocks have been metamorphosed to mid-high greenschist facies. Few fossils have survived the metamor- The Melones Fault Zone marks a major crustal phism, but recently Silurian and Devonian fossils have suture separating dominantly oceanic rocks to the been discovered. These fossils, in conjunction with west from largely continental rocks to the east. Its detrital zircons (?), suggest that Shoo Fly rocks are strike length is at least 350 km and may exceed that Ordovician to Mississippian in age. distance considerably. On most regional scale maps such as Figure 4 the MFZ appears as a thin line, but in The Calaveras Complex lies to the west of the Shoo reality the fault zone varies from a narrow zone a few Fly Complex and is thought to be in fault contact (the hundred meters wide (such as our stop at Bullion Calaveras-Shoo Fly Thrust).

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