LOS FILOS

GEOLOGY

Regional Geology | Local Geology

The Los Filos Project is located in the Morelos-Guerrero Basin in southern Mexico.  The roughly circular basin is occupied by a thick sequence of Mesozoic platform carbonate sediments comprising the Morelos, Cuautla and Mezcala Formations, and has been intruded by a number of granitoid bodies.

Regional Geology

The region of southern Mexico is underlain by a cratonic assemblage, approximately 2,000 metres thick of moderately to strong folded Mexozoic platform carbonate adn clastic rocks.  The deposits lie near the centre of a large circular-shaped feature known as the Morelos- Guerrero Basin.  The basinal sequence is composed of the Morelos, Cuautla and Mezcala Formations. The carbonate sequence of the Morelos-Guerrero Basin is underlain by Precambrian and Paleozoic basement rocks.  The majority of the metallic mineralization (gold and massive sulphide) is hosted by the Morelos Formation which is a Cretaceous-age medium-bedded to massive fossiliferous limestone up to 900 metres thick.  The Cuautla and Mezcala Formations are made up of shales and thin-bedded limestones. 

The sedimentary formations were subsequently intruded by granodioritic plutons and related phases of dioritic stocks and dike intrusive bodies.  The associated hydrothermal activities lead to the local development of alteration halos in the sediments surrounding the intrusives, including the formation of hornfels in argillaceous formations and, more importantly, marble within calcareous formations.

The Cretaceous sediments and granitoid intrusions are unconformably overlain by a sequence of intermediate volcanic rocks and alluvial sediments (red sandstones and conglomerates) which partially cover the region.

Regional structures in the Morelos-Guerrero Basin include sets of northeast-and-northwest-striking faults and fractures which cut both the carbonate sequence and the intrusives.  The distribution of intrusive bodies in northwest-trending belts is thought to reflect the control on their emplacement by northwest trending faults.

Gold, silver and base metal mineralization in the Nukay area is spatially and temporally related to the emplacement of early Tertiary porphyritic diorites, tonalites and granodiorites into the carbonate sequence of the upper Cretaceous Morelos Formation.  Mineralization is either hosted by or spatially associated with, marble formed during contact metamorphism of the carbonates.  Massive magnetite, hematite, goethite and jasperoidal silica, with minor associated pyrite, pyrrhotite, chalcopyrite and native gold typically occur in the veins and metasomatic replacement bodies that developed at the contacts between the platform carbonates and intrusives.

The Nukay area lies along the crest of an antiform or uplifted ridge, 6 to 8 kilometres wide and trending north-northeast.  The age and genesis of the anticlinal feature has not been established but is believed to be related to compressional forces during the late Cretaceous Laramide orogenic event.

Regional mineralization styles comprise the skarn‑hosted and epithermal precious metal deposits and volcanogenic massive sulphides. In Guerrero, these occur as two adjacent arcuate belts, with the gold belt lying to the east and on the concave margin of the massive sulphide belt.  Both are approximately 30 kilometres wide and over 100 kilometres long, from northwest to southeast, between Mochitlán and Telolapan.  Skarn hosted and epithermal precious metal deposits include Todos Santos, Nukay, Bermejal and Mochitlán.  Volcanogenic massive sulphide deposits (gold-silver-lead-zinc-copper) include Campo Seco, Farallon and Rey de Plata.

Local Geology

In the Los Filos area mineralization, which includes both Los Filos and Bermejal deposits, is associated with three diorite to granodiorite stocks that were emplaced in carbonate rocks of the upper Cretaceous Morelos Formation.  The stocks, known as East, West, and Bermejal are early Tertiary in age and resulted in high temperature calc-silicate and oxide metasomatic alteration (skarn) assemblages that were followed by distinct meso- to epithermal alteration.  The Los Filos deposit formed along the north, east and southern margins of the East stock that geologic evidence and argon dating have indicated is slightly older than the West stock.

The differing morphology of the East and West Nukay stocks is believed to reflect different structural controls during emplacement.  The exposure of the West stock is roughly circular and about 1.3 kilometres in diameter.  The East stock is elongate in a north-south direction.  It is about 1.4 kilometres long and 0.5 to 0.7 kilometres wide in the south but in the north, a western lobe extends for one kilometre in a west-southwest to east-northeast direction.

Similar to West stock, El Bermejal is a circular stock about 600 metres in diameter with an important structural control on the mineralization. The first structural system was regional northwest trending faulting, interpreted as being of deep seated origin. It is of late-Cretaceous age and does not cut Tertiary volcanics. This system appears to control the distribution of the ore-forming stocks at Gudalupe-Bermejal.

Marble beds consistently dip away from the margins of the East stock, indicating that the diorite was emplaced during active doming of the Morelos Formation.  In contrast, the West and Bermejal stocks generally have steep-sided, simple contacts and do not show any sill-like extensions, suggesting that the diorite was passively emplaced during a period of tectonic quiescence.