Homestake Mining Company -- McLaughlin Mine
Geology of the McLaughlin Deposit
By Dean Enderlin, 2002

Geology of the McLaughlin Deposit continued . . . Page 3 of 5

The fluid pathways for the hot-spring system were localized along a northwest-striking segment of the complexly deformed Stony Creek fault.  The intrusives, along with brittle and/or silicified lithons in the ophiolite and adularized horizons in the Knoxville (Sulfur Creek) formation of the Great Valley Sequence, served as the flow paths for the ascending hot-spring fluids.  The system was driven by a paleothermal anomaly related to regional magmatism of the Clear Lake Volcanics. The image below shows the relative positions of the orebody to the key geologic units of the deposit. USGS graphics.
   
The formation of the gold vein stockwork was characterized by syntectonic multistage vein filling of extensional fractures.  With the sealing of the system in the course of each vein filling cycle, the system would fracture in response to overpressuring and tectonic dilation.  As a result, pervasively hydrofractured wallrock and hydrothermal eruption breccias were common near the surface of the system. The image at right shows a fist-sized specimen with at least 5 generations of veining. D. Enderlin photo.
   
Precious-metal-bearing veins were dominantly crustiform - colloform quartz, chalcedony and opal, up to tens of centimeters in width.  Veins typically occurred in mutually crosscutting swarms, having orthogonal relationships to nearby fault shears.  The veins tended to be steeply dipping, with varying orientations corresponding to complexly evolving local strain fields. The most impressive of all the vein swarms was a massive zone of vein filling known as the "sheeted vein." It formed the root of the San Quentin sinter, and accounted for approximately 25% of the total gold production at the McLaughlin mine. The image below shows the sheeted vein as it appeared at the mining face in 1987. The yellow line marks the upper boundary of the vein zone. To the left of the boundary is unaltered polymictic melange (cataclasite). D. Enderlin photo.
   
The "sheeted vein" zone plunged eastward at an angle of about 25 degrees from horizontal. Its outcrop was adjacent to the San Quentin sinter terrace. It formed on the southeast edge of a large tectonic block (lithon) of metabasalt. A complex zone of strain, where convergent faults deflected around this large block, produced a "pressure shadow" or dilatent zone where the sheeted vein zone formed. The repeated tearing apart of rock along this edge, drew fluids into the resulting cracks and voids… occasionally with high enough velocity to produce fluid "throttling." Where evidence of this was found, the vein matter often contained coarse gold and vein textures indicative of extreme supersaturation of silica with respect to quartz. Syneresis cracks and colloform bands in some of the veins give us clues that silica was precipitating at such a rapid rate near the throttling zones that it initially deposited in a gelatinous state. With time, it dehydrated and recrystallized. The picture below shows syneresis (shrinkage) cracks in chalcedony from the sheeted vein (right) and a comparative sample with similar syneresis textures from the Linn vein in the Calistoga Mining District (a nearby precious metals mining district in the Coast Ranges). The cracks appear as small, arcuate fissures on the surfaces of the chalcedony veins. D. Enderlin photo.
 
 
The enormity of the sheeted vein zone was especially striking when viewed in cross-section. The image below is a profile of average gold concentrations in the zone. This model is based on assay results of blastholes collected during the course of mining. The bold blue line (marked SCF) to the right of the sheeted vein is the Stony Creek Fault. Surficial volcanics are marked Qv. D. Enderlin graphics.
   
A large example from the sheeted vein zone is preserved in Homestake's interpretive rock display ("Stonehenge") at the core library. The boulder at right measures about two meters tall, and is representative of the upper portion of the sheeted vein. The boulder was collected from a zone that assayed about 0.40 oz. Au per ton.

The sheeted vein was the oldest of the orebodies at the McLaughlin mine, and by far the most spectacular. The long-lived and highly permeable fracture system that hosted the sheeted vein, provided the ideal conditions to convey an enormous volume of water to the surface. The high quantity of silica-saturated water that emerged from the outflow point of the sheeted vein is responsible for the formation of the San Quentin sinter terraces that deposited nearby. Photo courtesy J. Farmer, NASA-Ames Research Center.

   
The hot springs that formed the McLaughlin deposit were by no means passive. With temperatures well above 200 degrees Celsius in the heart of the sheeted vein, the springs frequently sealed, overpressured, and exploded. Each hydrothermal explosion event would fragment the surrounding rock (hydrofracturing), producing new pathways for water to ascend to the surface. The image at left shows hydrofractured Jurassic metabasalt (greenstone) from the vicinity of the San Quentin. The web of veinlets is pervasive throughout the rock (larger veinlets are about 1 mm in width). The green coloration is a chlorite-montmorillonite-celadonite clay assemblage that formed by chemical interaction of boiling hot springs water and the rock. D. Enderlin photo.
   
In a hydrothermal steam eruption, rock capping the hot springs would be torn loose and often hurled through the air for short distances. Accumulations of these explosion fragments were called hydrothermal explosion breccias. The largest clasts observed at McLaughlin were over 2 meters in diameter. Smaller ones (such as those at right) had dimensions in centimeters. The bluish-white clasts are shards of sinter that were dislodged in the hydrothermal explosion. D. Enderlin photo.
   
The cycle of open hot springs flow, punctuated by hydrothermal explosion events, produced a complex stratigraphy in the sinter pile. Dozens, if not hundreds, of explosion events were recorded in the layers of sinter. Each cycle would appear as a fragmental layer of hydrothermal explosion breccia capped by laminated sinter terrace deposits. These alternating rhythmic beds built up layer upon layer, to produce the San Quentin Hill. The image at left shows a hydrothermal breccia bed overlain by thinly laminated sinter. The hand points to the contact between the two. McLaughlin mine photo.
   
Thermal springs were not limited to the San Quentin Hill. The photo at right shows an ancient carbonate spring south of the McLaughlin mine. The mineral deposits that form these spring terraces are tufa, rather than sinter. Their relative age with respect to the McLaughlin deposit is not known. Small saline seeps still emanate from this site, but the flow that built the terraces is no longer present. A spring terrace can be seen in the right foreground of the photo. In the distance are modern seeps emanating from a fault zone where metasediments (grassland) meet serpentinized peridotite (chaparral). D. Enderlin photo.
   
Although the local geothermal gradient has returned to near the average for the region, springs and seeps still emerge from highly localized fluid pathways in the district. These springs are highly mineralized, and represent the last vestiges of the processes that formed the McLaughlin deposit. The spring-seep shown at left, emerges from the ophiolite a short distance south of the McLaughlin mine pit. Such spring seeps often host unusual plant communities that take advantage of the peculiar water chemistry to avoid competition from exotic species.  More information on spring seep chemistry can be found in Appendix 11 of the McLaughlin Mine Closure Plan. D. Enderlin photo.
   
The sheeted vein zone in the South Pit of the McLaughlin mine serves as a modern pathway for small amounts of upwelling mineral water. The image at right shows bicarbonate-rich water violently effervescing in a shallow borehole at the bottom of the pit. N. Lehrman photo.
Animated globe courtesy NOAA NESDIS National Geophysical Data Center 
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