Several small to medium-sized volcanogenic massive sulfide deposits have been identified within ophiolitic volcanic rocks of Josephine County, Oregon. The most notable occurrence located to date is the Turner-Albright deposit (Fig. 1), which was formed by a combination of sub-seafloor replacement and seafloor exhalative processes within a back-arc rifting environment. As currently defined, the Turner-Albright contains minimum drill-indicated reserves of 6.5 million tons of massive and semi-massive sulfides. Values are reported for gold, silver, copper, zinc and cobalt, with 2 to 4 million tons being potentially economic at current metal prices. Geological and geophysical data suggest that the deposit may be significantly larger, and that the present reserves may be substantially increased by additional exploration adjacent to the known mineralization. This brief discussion is intended to summarize the geologic setting and history of the Turner-Albright, and to relate some of the factors which may affect a future production decision.
The regional geology of southwestern Oregon and northwestern California has been the subject of numerous studies in recent years [Cater and Wells (1953), Wells and Walker (1953), Helming (1966), M.O. Garcia (1976, 1979), Vail (1977), Cunningham (1979), Ramp and Peterson (1979), Harper (1980, 1983), Evans and others (1984), and others]. Pioneer workers including Diller (1914), Winchell (1914), and Shenon (1933), mapped the many mines and prospects which were being actively worked in the region in the early 1900's.
The Turner-Albright deposit occurs in the Western Jurassic Belt (WJB); the westernmost and youngest of four arcuate, north-south trending, litho-tectonic belts which comprise the Klamath Mountains geomorphic province. The lithologies and age relationships within the Klamaths indicate repeated accretion, beginning in the early to middle Paleozoic and continuing through the Mesozoic, of ophiolitic and/or island arc terrains and associated sedimentary units to the western edge of the North American continent. Jurassic and Cretaceous intrusives (gabbroic to granitic) intrude all the units. The WJB is in thrust contact with a similar suite of late Paleozoic and Triassic ophiolitic/arc units to the east, and is under-thrust from the west by the late Jurassic to Cretaceous Franciscan (Dothan) melange.
A prominent feature of the WJB in SW Oregon and NW California is the Josephine Ophiolite (Fig. 2), and coeval volcaniclastics and flows associated with island arc development. The Josephine Ophiolite, dated at 157 m.y. (Harper and Saleeby, 1980), is interpreted to be the product of Jurassic back-arc spreading, with island arc development occurring relatively westward. The ophiolite sequence, which regionally trends NNE with a steep SE dip, is essentially complete, with preservation of all major lithologies associated with classical ophiolite stratigraphy. The basal ultramafic portion (Josephine Peridotite) is comprised predominantly of tectonized harzburgite which has undergone partial to locally complete serpentinization. Cumulate and massive gabbro is well exposed 4 km. southwest of the Turner-Albright in the headwaters of the Monkey Creek drainage. The entire sheeted dike complex, from the lower transition with the gabbro to the upper gradational contact with extrusive volcanic flows and pillows, is preserved essentially intact on both flanks of Monkey Creek Ridge.
In SW Oregon, Jurassic extrusive rocks (both ophiolite and arc derived) have been collectively named the Rogue Volcanics, and include basic to locally felsic flows, tuffs, breccias and agglomerates. Volcanic members associated with the Josephine Ophiolite include basaltic flows and pillows, with interlayered breccias, hyaloclastites, and relatively thin clastic and/or chemical sedimentary horizons. The Josephine Ophiolite, as well as the associated island arc volcanics, are conformably overlain by the Galice Formation, which is composed predominantly of inter-bedded greywacke and shale. Type localities for the Rogue and Galice Formations occur NW of Grants Pass in the Galice District, and as such are associated with island arc development. Harper (1983) has proposed that the WJB be divided into two terrains; a northern Rogue River Terrain which would include the intermediate to locally felsic island arc volcaniclastics and flows typical of the Galice District, and a southern Josephine Terrain, represented by the mafic and ultramafic units of the Josephine Ophiolite.
Precious and base metal mineralization within the WJB is widespread and consists of several varied genetic types. In addition to the Turner-Albright, several other massive sulfide deposits have been located. While a lack of data prohibits a definitive genetic classification for most of the showings, it is probable that several may be associated with ophiolite development (Monumental, Fall Creek, Eagle Group, etc.), while others appear to be related to island arc development (Almeda, Goff, Silver Peak, Yankee Silver Lode, etc.). Numerous very high grade gold/silver/copper/zinc occurrences, commonly associated with mafic to granitic intrusives, occur throughout the Klamath Mountains. Both vein and high-grade gold 'pockets' have eroded to form locally rich placer deposits, many of which have been extensively worked by methods ranging from pick and shovel to large scale hydraulic mining.
The Turner-Albright (T-A) deposit is situated in southern Josephine County, immediately north of the California border and approximately 2 miles west of Highway 199 (Fig. 1). Access to the T-A is via Lone Mountain Road in O'Brien, Oregon, which parallels the West Fork of the Illinois River to the turnoff to the property, a distance of approximately 6 miles. From the turnoff, an extensive system of access and drill roads provide year-round entry to most portions of the deposit.
Relief at the T-A is moderate to locally steep, with elevations ranging from 2000' to 3100'. Area rainfall totals during the winter months are quite heavy, with seasonal averages in excess of 100" to be expected. Snowfall is common above 3000', and can last from December through April. Storms come in groups, with weeks of clear weather common between systems. Summers are hot and dry, with highs above 100°F. not uncommon from July through mid-September. Steep slopes covered with thick stands of brush and timber, old growth poison oak, and yellow-jackets with a special vengeance for geologists all tend to hinder field activities during the summer months.
Mineralization associated with the T-A was originally located in the late 1800's. Early efforts concentrated on developing the potential gold content of several discontinuous gossan outcrops located on or near the ridge separating Blue Creek from the headwaters of the West Fork of the Illinois River. Sporadic exploration and limited development continued through the 1930's but these efforts were not successful in defining an economic reserve. Several short crosscuts driven at the base of the oxide horizon reached primary sulfides which were of sufficient grade to allow three claims to be patented in the late 1950's.
Exploration of the underlying sulfide deposit began in earnest in 1954 with a 1-year program by Granby International. Associated Geologists of Grants Pass continued exploration below the gossans intermittently throughout the 1960's and early 70's with several programs consisting of churn and shallow diamond core drilling. A two year program by American Selco in 1974/75 explored the potential of the 'South Zone' gossans, and resulted in drill-indicated reserves of 150,000 tons of sulfide ore averaging 1.70% copper and 0.03 oz/ton gold across an 8' wide zone of highly siliceous basaltic breccias. Evidence of a large mineralized body north of the 'South Zone' was indicated by an Induced Polarization geophysical survey and three short diamond drill holes; however, American Selco considered the prospects of locating an economic deposit poor and allowed their option to expire at the end of 1975.
Baretta Mines Ltd. of Calgary, Alberta, Canada obtained an option upon termination of the American Selco program. Through August 1981, Baretta Mining Inc., a wholly-owned subsidiary, conducted extensive exploration on the Turner-Albright itself, as well as initial exploration of favorable units to the south and southwest. A total of 30 diamond core holes, with an aggregate length of 35,500', were completed at the T-A, resulting in an indicated in-place geologic reserve of 3 million tons averaging 0.09 oz/ton gold, with additional values in copper, zinc, silver, and cobalt. Drilling by Noranda Exploration (1982) and Rayrock Mines Inc. (1983/84) continued to refine both the geologic and structural characteristics of the deposit. Drilling on the deposit to date exceeds 75,000' in 80 separate drill holes. At the present time, reserve estimates place the Turner-Albright at 2 to 4 million tons averaging approximately 0.12 oz/ton gold, 0.60 oz/ton silver, 1.55% copper, 3.70% zinc, and 0.50% cobalt. The wide range in tonnage reflects the current uncertainty over the full effect that post-mineral faulting may have had on the continuity of portions of the deposit.
Recently, two separate studies of the T-A have been initiated by branches of the U.S. government to study the genetic and metallurgical characteristics of the deposit. A team of geologists, marine geologists, and geochemists from the U.S. Geological Survey are studying the T-A to determine its similarities to active hydrothermal systems which have recently been identified at several venting sites along mid-ocean ridges. In addition, the U.S. Bureau of Mines is beginning a detailed mineralogical study of the cobalt-bearing sulfides at the T-A. Their intent is to help in defining and developing a metallurgical process to treat the complex ores found at the deposit.
The T-A is situated near the base of the extrusive pillow lavas and flows of the Josephine Ophiolite, 50 to 200 meters above their gradational lower contact with the sheeted dike sequence. In the immediate vicinity of the T-A, the majority of ophiolite-related lithologies normally found stratigraphically below the extrusives are missing due to post-ophiolitic low-angle faulting which has juxtaposed the uppermost portion of the extrusive/sheeted dike transition zone against serpentinized mantle peridotite. Compared with the total section as exposed south of the T-A, up to 1.5 km or more of the ophiolite stratigraphy is missing, including the middle and lower sheeted dikes, the entire massive and cumulate gabbro sequence, and an unknown amount of mantle peridotite.
With the exception of numerous mafic pegmatite and rodingite dikes which occur within major shears in the ultramafic mass, all of the lithologies currently exposed at the T-A are interpreted to be associated with the primary development of the Josephine Ophiolite (Fig. 3). Following is a brief description of the major units identified at the T-A.
Extrusive volcanic rocks occurring at the Turner-Albright consist of basaltic flows, pillows, and hyaloclastites which commonly contain plagioclase, clinopyroxene and/or iron titanium phenocrysts. Feldspar microlites and/or calcite veinlets and amygdules occur locally, and individual units may be locally vesicular. Well developed pillow structures are evident, both in outcrop and in drill core. Minor to locally intense alteration occurs, consisting of prehnite/pumpellyite, chlorite, sphene, and albite (+/- silica, hematite, and epidote), with increased alteration being localized within and adjacent to zones of shearing and faulting. Except where associated with mineralization, clinopyroxene is rarely altered to any degree. Regional prehnite/pumpellyite facies metamorphism has overprinted much of the original alteration associated with seafloor and hydrothermal reactions, and it is often difficult to determine the age or cause of specific alteration products.
Recent work by Robert Zierenberg of the U.S.G.S. has defined a second extrusive unit which is of limited extent and is apparently restricted to the mineralized horizons. To date, this unit, which consists of glassy fragments of a relatively primitive mafic magma, has not been found as flows or pillows. The rock typically exhibits phenocrysts of olivine and/or chromium spinel (with occasional plagioclase) in a groundmass of glass and radiating clusters of quenched pyroxene crystals. Extensive hydrothermal alteration within the mineralized horizon commonly masks the nature of many of the fragments; however, where primary textures are still visible, clasts of the regionally dominant plagioclase bearing lava series appear to be restricted to the lowest portions of the mineralized horizons and may represent minor accumulations of rubble on the seafloor prior to the extrusion of the mafic lavas.
Gabbro (Coarse-grained Basalt)
As applied at the T-A, the term gabbro refers to diabasic to micro-gabbroic (locally gabbroic) textures which occur within the cores of thick extrusive basalt flows and/or pillows. These units commonly contain plagioclase and/or pyroxene phenocrysts of up to 5mm in a generally fine grained to aphanitic groundmass. There is no compelling evidence to date to indicate an intrusive origin for the unit, and the gabbro is interpreted to represent coarse grained members of the dominant plagioclase bearing lava series. Thick sections (>50 meters) of gabbroic textured flows commonly occur within 10 meters of the top of the mineralized horizons, and may represent ponding of basaltic lavas within the primary depositional basin.
Very fine grained clastic sedimentary units occur as definable horizons 10cm to 5m thick, minor interpillow and interflow accumulations, and within the matrix of hyaloclastite breccias. Color varies from red (hematitic) to green, brown, grey, and black (carbonaceous). Green and grey mudstones are often indistinguishable from silicified basaltic gouge in drill core. Measurements of bedding from outcrop, as well as sub-surface structural calculations from 3-points, indicate a regular NNE strike to the units (sub-parallel to the regional trend of the ophiolite); however, dips vary from 30° SE to nearly vertical. Composition of individual clasts is difficult to determine, but it is probable that both hemipelagic and local sources contributed to the formation of the muds. Local increases in the silica content of the sediments indicate an exhalative or biogenic source for at least a portion of the material. Radiolarians have been observed in several of the more siliceous mudstones, and confirm an approximate 155 m.y. date for the ophiolite (Harper, verbal comm.). Thin (up to 1m) mudstone layers commonly cap the exhalative horizons, and appear to be laterally more extensive than the sulfide bodies themselves, as several mud horizons extend beyond the known limits of sulfide mineralization. At least two, and possibly three additional clastic horizons have been identified at the T-A which are not known to be associated with sulfide development.
Basin Floor Rubble
From an examination of textures associated with the sulfide bodies located at the T-A, it is apparent that a large portion of the deposit occurs as a replacement of brecciated basalt fragments. The Basin Floor Rubble (BFR) represents accumulations of up to 75 meters of brecciated basalt which covered the original depositional basin prior to the onset of hydrothermal activity and the venting of the sulfide horizons. The majority of the silica stockwork sulfides, as well as a large portion of the massive sulfide horizon, may occur within highly altered portions of this unit. Intense hydrothermal alteration within this section of the T-A stratigraphy obscures the composition of many of the fragments; however, it is apparent that fragments of the mafic lava series form the majority of the unit, with clasts of the regionally dominant plagioclase bearing lava being generally restricted to the base of the rubble pile.
High angle faulting associated with the formation of the T-A resulted in several moderate to high relief fault scarps in the original depositional basin. Brecciation and erosion led to the accumulation of talus deposits at the base of these structures. Individual talus piles can include fragments of basalt, mudstone, and sulfides, with minor amounts of gabbro. The talus deposits commonly have been subjected to a high degree of internal shearing. Sulfides occur as angular to sub-rounded fragments derived from pre-existing, faulted exhalative and stockwork horizons, as well as replacement deposits formed by later fluid movement through the units. As defined at the T-A, the talus deposits are differentiated from the BFR by their stratigraphic position at the top of the sulfide horizons, their lack of extensive hydrothermal alteration, and the presence of mineralized fragments derived from the existing sulfide bodies. It is likely, however, that portions of the BFR may actually represent pre-mineral talus deposits, and may account for mineralized areas containing plagioclase bearing lava fragments within the predominantly mafic BFR.
Ophiolitic sheeted dikes are characterized by sub-parallel diabasic dikes, and are interpreted to represent the conduits for the magma which supplied the overlying extrusive flows and pillows. The upper and lower contacts of the unit as a whole are commonly gradational. The upper transition zone with the extrusive lavas is composed of diabasic dikes with a downward decreasing proportion of basaltic 'screens', while the lower contact zone with the massive gabbro is characterized by extremely erratic and confusing diabase/gabbro textural variations.
Due to faulting which has completely removed the lower portions of the ophiolite, only the uppermost portion of the upper transition zone remains at the T-A. This portion of the T-A stratigraphy is poorly exposed, and has only been identified in several drill holes in the NW portion of the deposit and in extensively weathered outcrops in fault contact with serpentinite. Individual dike margins are marked by chill zones up to 1cm across, and are often brecciated. Moderate to locally intense epidote alteration is common. Textures within the cores of individual dikes and the enclosing basaltic screens are often indistinguishable, which makes identification of this transition zone extremely difficult in outcrop where the chill and/or breccia margins are generally obscured.
Partially to completely serpentinized mantle peridotite occurs immediately west of the T-A in the headwaters of the North Fork of the Illinois River. Pods of serpentinized dunite also occur and may represent primary cumulate differentiation within the upper mantle. Mafic pegmatite and rodingite dikes are common along the faulted contact with the extrusive basalt/sheeted dike transition zone, as well as in shear zones within the peridotites. Thin (1 to 3cm) seams of powdered magnetite also occur locally along the contact. The serpentinites are highly magnetic relative to the other units in the vicinity and can be readily located by their magnetic signature.
Sulfide minerals identified at the T-A include pyrite (+/- marcasite), sphalerite, chalcopyrite and linnaeite, with trace amounts of tetrahedrite, stannite, galena and pyrrhotite. While assumed contributions from multiple vent sources and extensive post-mineral faulting complicate any study of the primary zonation patterns, it appears that the original metal distribution resulted in copper/gold rich centers at depth within the BFR and proximal to the vents, with zinc/silver, and pyrite with cobalt zones occurring with increasing distance from the venting sites. Limited thin and polished section work indicates that the metallurgical characteristics of the deposit are complex and may complicate extraction of the base and precious metals. Fine grained chalcopyrite and sphalerite are tightly intergrown with pyrite and, to a limited extent, with each other. Gold occurs as discrete micron sized blebs within chalcopyrite (and, to a limited extent, sphalerite) and pyrite. This gold/pyrite association results in low to locally moderate gold values (0.02 to 0.07 oz./ton) in the distal pyrite 'halo' in the absence of any base metal credits. Cobaltiferous linnaeite occurs as rims on pyrite grains. Porous colloform marcasite is most abundant in the uppermost portion of the deposit, and may in part be a product of near surface alteration of the primary iron sulfides. It must be stressed that these findings are fragmentary, and that a final metallurgical definition of the deposit remains to be made.
As currently defined, the sulfide bodies at the T-A are composed of three inter-related types of mineralization (Fig. 4). Massive sulfide horizons, containing >50% total sulfide content and up to 30 meters thick, occur at the stratigraphic top of the mineralized section. A silica stockwork stringer zone consisting of highly altered breccias occurs below and lateral to the massive sulfides. Total sulfide content generally decreases to trace amounts in the stringer zone with distance from the main exhalative centers, resulting in a third definable horizon, termed a mineralized basalt, which contains less than 20% total sulfides. The potentially economic portions of the deposit are generally restricted to the massive and stringer horizons, but are not necessarily associated with the greatest sulfide content. A large percentage of the massive horizon at the northern end of the deposit is composed essentially of massive pyrite/marcasite and is of a relatively low economic grade. Where exposed at the surface, all three units oxidize to form gossans or 'llozzans' (derived from the oxidation of semi-massive sulfides) which mark the up-dip western limit of the deposit. Limited outcrop sampling and shallow drilling support the interpretation that the majority of the gossans (and llozzans) were derived from portions of the deposit with a relatively low gold content, and therefore have little potential to develop substantial reserves of leachable ore.
The massive sulfide horizons which occur at the T-A are interpreted to have been formed as either sea-floor exhalites, or by the extensive alteration and replacement of basaltic breccias resulting in nearly complete obliteration of all primary textural features. Evidence of brecciation within the massive horizon commonly increases at lower stratigraphic levels, with ghosts of almost completely replaced basaltic clasts grading into highly mineralized rock with identifiable basalt and chert(?) fragments. It is probable that many or all of the chert fragments, which are commonly non-mineralized, are actually completely silicified basaltic glass. The origin of any given portion of the massive horizon (ie. exhalite or total replacement) may be difficult to determine due to this extensive alteration, and it is often impossible to define the original rock-water interface. From the amount of basaltic fragments within the sulfide horizons, it is clear that the majority of the mineralization at the T-A was the result of partial to complete replacement of basaltic breccias. The uppermost portion of the massive horizon, however, commonly exhibits fragmental textures, and it is possible that some of this sulfide brecciation may represent collapsed chimney structures which were initially built by the venting of sulphide-rich fluids on the sea floor. In addition, several small worm casts (1 to 2mm in diameter) have been tentatively identified. These two bits of evidence support an exhalative seafloor origin for the upper 1 to 3 meters of the deposit.
At the T-A, the silica stockwork stringer zone contains from 20% to 50% primary sulfides, and represents a conformable transition from essentially complete replacement of basaltic breccias to non-mineralized flows, pillows, and hyaloclastites. The contact between the silica stockwork zone and the overlying massive sulfides (as well as with the more distal mineralized basalt) is gradational, and the actual location is somewhat irregular and arbitrary. The silica stockwork is almost certainly a result of the percolation of mineralizing fluids through basaltic rubble, and is characterized by silica flooding of the breccias, with the addition of pyrite (+/- marcasite), chalcopyrite, sphalerite, and accessory sulfide minerals. Mineralized flows and/or pillows have not been identified within the silica stockwork. Hydrothermal penetration of the breccia pile resulted in substantial alteration of the original rock (silica + sulfides + chlorite + albite). From a study of partially altered fragments within the silica stockwork, it is apparent that the majority of the clasts are related to the mafic lavas discussed above. The degree of mineralization and the economic value of the silica stockwork stringer zone are both somewhat erratic. This may in part be due to the original configuration of the rubble pile, with areas of higher mineralization reflecting increased fluid movement along paths of greatest permeability.
Mineralized basalt includes that portion of the volcanic breccias and flows which were subjected to alteration by hydrothermal fluids, but which contain a total sulfide content of less than 20%. Preliminary re-logging of selected drill core indicates that while some of the breccia fragments are related to the mafic lavas, clasts of the plagioclase bearing lava series occur. It is also evident that mineralization within flow units, as opposed to being restricted to altered breccias, occurs to a limited extent at the northern end of the deposit. An increase in hematitic alteration has also been noted within breccias and flows which occur stratigraphically below and lateral to the sulfide rich portions of this unit. The mineralized basalts, which are generally of very low economic grade, are interpreted to represent the most distal effects of the mineralizing fluids.
The majority of the known sulfides at the T-A occur as three vertically stacked horizons which trend NE with a moderate SE dip. These three zones occur in two, and possibly three, separate time-stratigraphic horizons, and have been designated the Upper High-grade Zone (UHZ), the Main Upper Zone (MUZ) and the Main Lower Zone (MLZ). Post-mineral faulting has broken the MUZ and MLZ into somewhat discrete fault-bounded mineralized blocks; however, in many cases the original thickness of the disrupted sulfide horizon was greater than the displacement along the fault, so that when intercepted in drill core no readily discernible lithology change occurs across the structure. This is especially true in the MUZ, which is up to 100 meters thick. A minimum of three generations of faulting (pre-mineral, post-mineral, and emplacement) have been recognized to date. Additional and extensive faulting severely disrupts the stratigraphy associated with the UHZ.
A complex series of pre- and post-mineral high-angle northwest trending normal faults have been partially defined (F-series faults). At least 5 separate structures (F-1 thru F-5) have been identified within the known sulfide horizons, and there is evidence for additional faulting south of the deposit. Outcrop measurements and correlations between drill intercepts indicate that the F-series faults strike N60°W and dip to the north from 65° to 85°. The pattern is complicated by poorly defined branching and interconnecting near vertical E-W splay faults.
The southernmost structure, F-1, is interpreted to have existed prior to the onset of hydrothermal activity and to have controlled the movement of the primary mineralizing fluids. While there is no evidence to indicate that other F-series faults pre-date the mineralization, the possibility of hydrothermal penetration and/or pre-mineral movement along some or all of the remaining F-series faults cannot be ruled out. Post-mineral movement along the F-series faults disrupted the stratigraphy immediately after deposition of the sulfide horizons, with a resulting down-dropping of the overall sulfide horizon to the northeast (Fig. 5). Timing of post-mineral activity along the F-series faults is bracketed by the deposition of the sulfide horizons and the extrusion of the thick gabbroic-textured flows which generally appear to be unaffected by their movement. Post-mineral displacement, calculated by measuring the offset of the tops of adjacent sulfide horizons, averages 30 to 40 meters. Due to the steepness of the structures and their orientation normal to the strike of the sulfide horizons, standard cross-sections are not sufficient to fully define their characteristics. A series of horizontal and longitudinal sections are currently being prepared to aid in reconstruction of the original depositional setting and interpretation of these critical structures.
A later series of low-angle east-west trending post-mineral reverse(?) faults are indicated (R-series faults). Timing of the R-faults is unknown, but it is possible that these structures were associated with the emplacement of the Josephine Ophiolite along the continental margin, as well as with the faulting and removal of the lower portion of the ophiolite at the T-A. At least three R-series faults have been identified to date (R-1, R-2 and R-3). Three-point structural calculations and outcrop measurements indicate that these faults strike generally east-west and have a very shallow north dip (20°). The major impact of these low-angle structures was along R-1, where an apparent 150 to 200 meters of displacement resulted in dislocation of the original single sulfide horizon into the MUZ and MLZ. Offsets along R-2 and R-3, which are located above R-1, are minor and generally do very little to disrupt the MUZ. It is probable that additional R-series faults exist within the MLZ. It is important to note that the R-series faults cut and displace the F-series faults, which greatly complicates any attempt to reconstruct both the original setting of the deposit and the geometry of the depositional basin. The ultimate effect of all these structures on the T-A stratigraphy is still poorly understood, and it is probable that a full understanding will require underground mapping.
The uppermost sulfide horizon (UHZ) is relatively thin (2 to 15 meters), but of very high grade and located within 25 meters of the surface. Drill indicated reserves for the heart of the UHZ total 50,000 tons at an average grade of 0.40 oz/ton gold, 1.70 oz/ton silver, 4.30% copper, 1.35% zinc and 0.08% cobalt. Geologic and structural interpretations indicate that the UHZ is laterally extensive and directly overlies the MUZ toward the southern end of the deposit, with a thickening wedge of basalt, mudstone, and/or hyaloclastite separating them to the north. There is also evidence that mineralization associated with the UHZ overlies portions of the MLZ as well, which supports the theory that a single sulfide horizon was faulted into the MUZ and MLZ along R-1. Extensive post-mineral faulting at the north end of the deposit has severely disrupted the UHZ. Drilling to date has delimited the known zone both along strike and up-dip. The potential is good for locating additional mineralization to the southeast (down-dip), as well as faulted portions to the north.
The relationship between the MUZ and MLZ, which contain the bulk of the reserves, is uncertain at this time. Drill indicated reserves for the combined MUZ and MLZ total 3 million tons at an average grade of 0.12 oz/ton gold, 0.60 oz/ton silver, 1.55% copper, 3.70% zinc and 0.05% cobalt.
The majority of mining engineers who have examined the T-A have indicated that the MUZ may be amenable to surface mining methods. Preliminary estimates of waste to ore ratios are fairly high, however, and without a substantial increase in pittable reserves the MUZ would probably have to be developed from underground. The possibility of significantly expanding reserves associated with the MUZ is relatively poor. The general boundaries of the MUZ have been well defined, and consist of surface outcrop (up dip), R-1 (down dip), F-1 (SW),and the serpentinite (NE) (see Fig. 3). From the suggested fault pattern, several unexplored wedges still exist within these limits which could increase reserves in the MUZ by as much as 250,000 to 300,000 tons of potentially economic ore. In addition, approximately 1 to 1.5 million tons of mineralized basalt, containing from 0.02 to 0.07 oz. gold and 2 to 5 pounds of cobalt to the ton, occur within the northern part of the deposit. This portion of the T-A, while of sub-economic grade at current metal prices, would represent a substantial cobalt/gold reserve given the proper economic conditions. The inclusion of these units as minable reserves would help in reducing the waste:ore ratio to a level acceptable for surface mining methods.
Because of its depth below the surface, the MLZ would require underground mining methods. The base of the known orebody could be accessed by a 3500' crosscut driven from the east. The possibility of doubling or tripling the MLZ reserves is considered excellent. Structural, geologic and geophysical evidence all indicate that mineralization may extend to the SE down the plunge of F-1 for a considerable distance. Detailed Pulse-EM geophysical work during the summer of 1985 was successful in defining the known mineralization, and indicates that a two to three fold increase in MLZ reserves is probable. Due to the increasing depth of the mineralization, however, the Pulse-EM system was unable to define the SE limit of the MLZ. Because of this depth of penetration limit, the suggested increase in the size of the zone is considered a minimum value. Recent drilling by Rayrock Mines (prior to the Pulse-EM survey) also indicates that substantially greater thicknesses of high grade mineralization occur down dip from the deepest intercepts of either Baretta or Noranda. Mineralization encountered in these deep holes is characterized by very high zinc values at the stratigraphic top of the sulfides (>10% over 25m true widths), and increasing copper/gold values near the base of the section. Due to the high conductivity of chalcopyrite, and as sphalerite is non-conductive and therefore generally invisible to EM readings, it has been interpreted that the indicated additional reserves may contain a relatively large percentage of chalcopyrite (Crone, 1985).
Approximate Drill Indicated ReservesZone Tonnage Gold Silver Copper Zinc Cobalt UHZ 50,000 0.40 1.70 4.30 1.35 0.08 MUZ 1,600,000 0.13 0.50 1.40 3.00 0.05 MLZ 1,400,000 0.10 0.75 1.75 4.50 0.04
The Turner-Albright is interpreted to be an ophiolitic, volcanogenic sulfide deposit which was the product of replacement and exhalative processes within a back-arc rifting environment. Convection of oceanic waters, superheated by ascending (and differentiating) magmatic mantle material, resulted in the leaching of S, Fe, Cu, Zn, Au, Ag, Co, etc. from the mafic and ultramafic pile, with precipitation of silica and sulfides at or near the rock/water interface (Fig. 6). Additional mineralization commonly occurs distal to the venting sites as a silica stockwork, which represents zones of large scale hydrothermal penetration and replacement, and to a limited extent the plumbing system which supplied the exhalative horizon. The actual portion of the massive sulfide horizon which formed as a result of exhalative seafloor venting may be fairly small. The original depositional basin at the T-A apparently had an extensive cover of basaltic rubble, which could have had a direct effect on the ultimate strength of the exhalative process. The highly permeable nature of the breccias probably resulted in the spreading out of the mineralizing fluids into this clastic horizon prior to actual venting on the seafloor. Some of the mineralized breccias are compositionally different from all other lavas identified to date, and are characterized by phenocrysts of olivine and/or chromium spinel in a groundmass of glass and radiating clusters of quenched pyroxene crystals. Mudstones (locally siliceous) occur at the stratigraphic top of the sulfide horizons. It is probable that the cherty muds are in part the result of the addition of excess silica into the seawater in the vicinity of the sulfide vents.
It is reasonable to infer a period of relative inactivity with respect to volcanism both during and immediately following the formation of the sulfide bodies at the T-A. This quiescent period would allow time for the formation of the existing sulfide horizons and the accumulation of the mudstones. From recent work in the Josephine and other ophiolites, Harper (verbal comm.) postulates that magma chambers which feed the extrusive basalts experience cyclical periods of activity, followed by periods when they are essentially frozen and inactive. Partial crystallization of the upper differentiated portions of the magma chamber would possibly allow deeper, less mature lavas to escape, and may account for the extrusion of the mafic lava series at the T-A. It is important to note that the introduction of the sulfides at the T-A immediately followed the accumulation of the mafic BFR, and it is reasonable to assume that the same series of events contributed to both of these apparently anomalous (in a time-stratigraphic sense) events.
Subsequent to the formation of the deposit, the T-A was subjected to regional emplacement metamorphism and at least two generations of post-mineral faulting. This structural break-up of the deposit complicates a full geologic and genetic understanding of the T-A, and will certainly have an effect on any future mine plans.
Contemporary geologic studies often categorize mineral deposits by genetic 'type', and much effort has been made over the past several years to mold the Turner-Albright into a Cyprus Type deposit. While there are many similarities in gross geological features, it is the writer's opinion that there are significant differences between the T-A and the classic Cyprus Type deposit and that they represent different sub-types of ophiolite hosted massive sulphide deposits. The writer proposes that a separate classification be considered for the Turner-Albright, and that it be classified as a Josephine Type deposit based upon the following characteristics:
1) The T-A is ophiolite hosted, and occurs intimately associated with seafloor volcanism and extensional tectonics. Mineralization is structurally controlled, and is restricted to the lowest portions of the extrusive lava series immediately above the extrusive/sheeted dike transition zone.
2) Features common to the Cyprus deposits, including umbers, ochres, "vertically extensive stringer zones", and "extensively altered footwall rocks" (Franklin and others, 1981) have not been identified at the T-A. Iron-poor locally siliceous mudstones occur at the T-A in the same relative stratigraphic position as the Cyprus ochres.
3) Greater than 90% of the known sulfide mineralization at the T-A is the result of large-scale replacement of basaltic breccia and talus. The original depositional basin had a cover of up to 75 meters of highly permeable basaltic rubble, which is compositionally different from both the footwall and hangingwall basalts. The permeable nature of the breccias had the effect of dissipating the mineralizing fluids into this clastic horizon prior to venting on the seafloor, with only a minimal amount of the hydrothermal fluids reaching the rock-water interface to form exhalative sulfides.
4) A true 'silica stockwork' zone, in which hydrothermally altered flows and pillows stratigraphically below the massive horizon represents the feeder system for the overlying exhalative sulfides, does not occur at the T-A. The silica stockwork at the T-A is the result of extensive hydrothermal penetration and replacement of basaltic breccias, and only in a very limited sense does it represent the feeder system for the exhalative horizon.
5) The sulfide bodies at the T-A have anomalously high gold values. As currently defined, the T-A contains 6.5 million tons (including the mineralized basalt) with an average gold content of 0.055 oz/ton. Significantly greater values (up to 0.462 oz/ton over a 15m true width) are found within the higher grade portions of the deposit, and the potentially economic portion averages 0.12 oz/ton.
The writer would like to thank Karen Comstock for the drafting used in this paper, and Len Ramp, Pat Shanks, and Lloyd Frizzell for critically reviewing the manuscript. Thanks are also due to Baretta Mines Ltd. and Rayrock Mines Inc. for their permission to prepare this article.
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Fig. 1: Location Map - Turner-Albright Massive Sulfide Deposit
Fig. 2 : Schematic diagram showing the gross stratigraphy of the Josephine Ophiolite and the relative positions of the various units which occur in ophiolite suites
Fig. 3 : Generalized surface geology of the Turner-Albright and vicinity
Fig. 4 : Stratigraphic X-section during the late stages of the development of the Turner-Albright deposit, and prior to the deposition of the clastic (± silica) mudstone horizon
Fig 5 : Cross section through Turner-Albright deposit prior to dislocation along the R-series faults and emplacement along the continental margin
Fig. 6 : Generalized diagram depicting the development of exhalative and stringer sulfide deposits associated with extensional tectonics along a spreading center ridge axis
Plate 1 : Simplified schematic block diagram of the Turner-Albright massive sulfide deposit