Boss, Bixby

Missouri, USA

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Boss-Bixby is a Proterozoic volcanic hosted, iron-copper deposits in the Saint Francois Mountains district of South-east Missouri, ~140 km SW of Saint Louis and 65 km SE of Rolla (#Location: 37° 38' 33"N, 91° 10' 44"W).

For details of the regional to local scale setting and geology of the Saint Francois Mountains district see the separate Southeast Missouri Iron District record.

The Boss-Bixby prospect, which contains ~70 Mt of copper mineralisation at an unspecified grade, was discovered in 1956 by drilling a blind magnetic anomaly below Palaeozoic cover. It lies on a major regional, northeast-trending lineament, where it intersects the northwestern margin of a 20 km diameter ring complex structure. The deposit is hosted within a syenite pluton, the Boss Plutonic Complex, part of a ring of intrusions surrounding an inferred central granite pluton. The syenite ring intrusion and its associated mineralisation, which is elongated along the regional lineament over an interval of ~1200 m, intrudes brecciated rhyolitic volcanic rocks and is cut by post-mineralisation diorite, granite and aplite dykes. The host syenite is believed to be the latest of a series of steep, dyke-like, northeast-southwest elongated intrusions on the margin of the ring complex (Seeger, 2000; Kisvarsanyi, 1989; Kisvarsanyi and Kisvarsanyi, 1989).

The deposit is a composite of several pods of higher grade copper mineralisation, separated by low-grade zones, within the iron oxide mass. The mineralisation is disseminated throughout the syenite intrusion, although the main developments are concentrated in breccias and irregular fracturing on the hanging wall of the syenite body and in the adjacent, brecciated, structurally overlying, rhyolitic volcanic rocks. The principal "ore minerals" comprise magnetite, chalcopyrite and bornite, with lesser hematite, ilmenite and rutile as well as pyrite, carrollite, molybdenite, cobaltite, sphalerite, galena, chalcocite, covellite, cubanite and enargite. Boss-Bixby is notable for the presence of ilmenite and titaniferous magnetite in some phases, and the absence of apatite. In contrast, most other magnetite deposits in the district comprise low TiO2 magnetite and include apatite, which has associated REE.

Two varieties of magnetite, blue and brown, are observed, the former of which is found at depth, progressively rimmed by the brown (more oxidised) variety, until a boundary, above which the latter dominates in the upper part of the deposit. The magnetite locally contains some chalcopyrite, bornite and pyrite inclusions, which are interpreted as sulphide crystals trapped or exsolved at the time of crystallisation, although later chalcopyrite replacement of magnetite is common. The distribution of hematite and magnetite is also zoned, with magnetite dominating in the lower parts of the deposit, hematite in the upper, with a mixed transition zone straddling the blue to brown magnetite boundary. A zonation is also evident between upper oxide and lower sulphide dominant assemblages, with the interface being at higher levels on the margins of the deposit, but plunging to depth in the core of the system (Kisvarsanyi, 1989; Kisvarsanyi and Kisvarsanyi, 1989).

Hagni and Brandom (1989) divide the mineralising system at Boss-Bixby into 5 stages, corresponding to a gradual decrease in temperature:
i). an initial magmatic iron oxide stage (800 to 700°C), where titaniferous magnetite, and lesser specular hematite (commonly containing ilmenite exsolution lamellae) occurs as very fine grains and disseminations throughout the syenite, most likely magmatic in origin. This stage was followed by an episode of brecciation and
ii). high-temperature (700 to 600°C), metasomatic-hydrothermal iron oxide (low TiO
2?), occurring as coarse-grained magnetite that coats and separates breccia fragments;
iii). endoskarn alteration, throughout the syenite, producing an assemblage of andraditic garnet, scheelite, quartz, epidote and molybdenite, followed by martitisation of magnetite;
iv). sulphide and precious metals stage, which post-dates all of the magnetite, although it is difficult to determine whether martitisation preceded or accompanied the introduction of sulphides, which occur as pods within the overall iron oxide mass. This sulphide stage commenced with the deposition of early pyrite and carrollite, where carrollite rims zoned, cobaltiferous pyrite, followed by the precious metals, predominantly in the form of electrum and hessite, with lesser galena. This was followed by the main late episode of copper-iron sulphides, principally involving pyrite, chalcopyrite and bornite, with small amounts of enargite and chalcocite. Chalcopyrite occurs as a replacement of host rock and earlier oxide minerals, and locally forms veins. Bornite is found adjacent to, and partially replacing chalcopyrite grains. Both chalcopyrite and bornite rim magnetite, hematite and pyrite;
v). late oxide and sulphide stage, which includes chalcopyrite, and specular hematite that may occur as small vugs within the chalcopyrite and replaces bornite.

Fluorite is a late addition. Hagni and Brandom (1989) show that strong metasomatic K feldspar (orthoclase) alteration was after syenite intrusion and accompanied the brecciation and iron oxide mineralisation, followed by alteration that is principally garnet, calcite, fluorite, gypsum, chlorite and quartz associated with the skarn and sulphide stages (Kisvarsanyi, 1989). Depth of burial (350 m of Palaeozoic sedimentary rocks) and low grades have resulted in the resource remaining unexploited (Seeger, 2000).

This summary is an extract from Porter (2010) in "Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, v.3, Advances in the Understanding of IOCG Deposits", available from PGC Publishing, Adelaide.

The most recent source geological information used to prepare this summary was dated: 2000.     Record last updated: 18/7/2013
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Kisvarsanyi G  1989 - The Boss iron-copper deposit, Missouri: in Brown V M, Kisvarsanyi E, Hagni R (Ed.s),  Olympic Dam Type Deposits and Geology of Middle Proterozoic Rocks in the St Francois Mountains Terrane, Missouri Soc. of Econ. Geol.   Guidebook no. 4 pp 69-81

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Seeger C M, 2000 - Southeast Missouri Iron Metallogenic Province: Characteristics and General Chemistry,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 237-248
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